Anatomy of the Heart

The surface electrocardiography (ECG) in both acute and chronic phase of ischaemic heart disease (IHD) may give crucial information about the coronary artery involved and which is the area of myocardium that is at risk or already infarcted. This information jointly with the ECG–clinical correlation is very important for prognosis and risk stratification, as will be demonstrated in this book. Therefore, we will give in the following pages an overview of the anatomy of the heart, especially the heart walls and coronary tree, and emphasise the best techniques currently used for its study.

For centuries, since the pioneering works of Vesalio, Leonardo da Vinci, Lower and Bourgery- Jacob, pathology has been a unique method to study the anatomy of the heart. Since the end of the nineteenth century, the visualisation of the heart in vivo has been possible by X-ray examination. The last 40–50 years started the era of invasive imaging techniques with cardiac catheterisation (angiography and coronary angiography) and modern noninvasive imaging techniques, first with echocardiography and later with isotopic studies, scanner and cardiovascular magnetic resonance (CMR). These techniques open a new avenue to study not only the anatomy of the heart, coronary arteries and great vessels but also the myocardial function and perfusion, and the characterisation of the valves, pericardium, etc.

The coronary angiography (Figure 1.1) is especially important in the acute phase for diagnosing the disease and correlating the place of occlusion with the ST-segment deviations. It is also useful in the chronic phase of the disease. However, in the chronic phase of Q-wave myocardial infarction (MI) the ECG does not usually predict the state of the coronary tree, because the revascularisation treatment has modified, sometimes very much, the characteristics of the occlusion responsible for the MI. Furthermore, the catheterisation technique may give important information for identifying hypokinetic or akinetic areas. The latter may be considered comparable to infarcted areas. Currently, in some cases, the noninvasive coronary multidetector computer tomography (CMDCT) may be used (Figure 1.1).

Figure 1.1 (A) Normal case: coronary angiography (left) and three-dimensional volume rendering of CMDCT (right) showing normal LAD and LCX artery. The latter is partially covered by left appendix in CMDCT. The arrow points out LAD. (B) Normal case: coronary arteriography (left) and three-dimensional volume rendering of CMDCT (right) showing normal dominant RCA. (C) 85-year-old man with atypical anginal pain: (a) Maximal intensity projection (MIP) of CMDCT with clear tight mid-LAD stenosis that correlates perfectly with the result of coronary angiography performed before PCI (b). (D) Similar case as (C) but with the stenosis in the first third of RCA ((a–d) CMDCT and (e) coronary arteriography). (E) Similar case as (C) and (D) but with the tight stenosis in the LCX before the bifurcation ((a) and (b) CMDCT and (c) coronary angiography). (F) These images show that CMDCT may also demonstrate the presence of stenosis in distal vessels, in this case posterior descending RCA ((a–b) CMDCT and (c)) coronary angiography). (G) These images show that CMDCT (a, b) may delimitate the length of total occlusion and visualise the distal vessels (see arrows in (b), the yellow ones correspond to distal RCA retrograde flow from LAD) that is not possible to visualise with coronary angiography (c). (H) A 42-year-old man sports coach with a stent implanted in LAD by anginal pain 6 months before. The patient complains of atypical pain and present state of anxiety that advises to perform a CMDCT to assure the good result and permeability of the stent. In the MIP of CMDCT (a–c) was well seen the permeability of the stent but also a narrow, long and soft plaque in left main trunk with a limited lumen of the vessel (see (d) rounded circle) that was not well seen in the coronary angiography (e) but was confirmed by IVUS (f). The ECG presents not very deep negative T wave in V1–V3 along all the follow-up.

 

Figure 1.2 Echocardiography: see example of volumes, wall thickening and myocardium mass in a normal case and in a patient with post-MI. Above: (A) End-diastolic and (B) end-systolic apical long-axis views of a normal left ventricle. The endocardial and epicardial contours are traced and the built-in computer software of the ultrasound system allows calculation of volumes, wall thickening and myocardial mass. Below: Segmental wall function analysis: post-infarct lateral wall hypokinesis shown in the four view. The left ventricle is dilated. Superposition of the traced endocardial contours at end diastole (A) and end systole (B) shows the hypokinesis and compensatory hyperkinesis of the interventricular septum. (C) It shows the superimposed end-diastolic and end-systolic contours.

The era of modern non-invasive imaging techniques started with echocardiography, which is very easy to perform and has a good cost-effective relation. This technique plays an important role, especially in the acute phase, in the detection of leftventricular function and mechanical complications of acute MI (Figures 1.2). Also, it is verymuch used in chronic ischaemic-heart-disease patients for the study of left-ventricular function and also detection of hypokinetic and akinetic areas. However, echocardiography tends to overestimate the area that is at risk or necrosed, and thus its reliability is good but not excellent. The techniques of echo stress and especially isotopic studies (single-photon emission computed tomography, SPECT) have proved to be very reliable for detecting perfusion defects and necrotic areas. They are very useful in cases where there is dubious precordial pain with positive exercise testing without symptoms.  It has been demonstrated, however, that in some cases (non-Q-wave infarction) the extension of the infarction may be underestimated and that in presence of the left bundle branch block (LBBB) the estimation of some perfusion defects is doubtful.

Figure 1.3 Examples of correlation exercise test – isotopic images (SPECT). (A) Above: Observe the three heart planes (see Figure 1.4B) used by nuclear medicine experts (and other imaging techniques) to transect the heart: (1) short-axis (transverse) view (SA), (2) vertical long-axis view (VLA) (oblique sagittal-like) and (3) horizontal long-axis (HLA) view. Below: Normal case of perfusion of left ventricle. On the middle is (B) the bull’s-eye image of this case. The segmentation of the heart used in this book is shown (Cerqueira, Weissman and Disizian, 2002). On (A) transections of the three axes are shown. The short-axis transections is at the mid-apical level (see Figure 1.8 for segmentation). (B) Above: In the three planes (SA, VLA and HLA) see (A) normal uptake at rest (Re) and during exercise (Ex) can be observed. Middle: Abnormal uptake only during exercise of segments 7, 13 and 17 (see Figure 1.8) in a patient with angina produced by distal involvement of not long LAD. The basal part of the anterior wall of left ventricle is not involved. Below: Abnormal uptake during rest and exercise in a patient in chronic phase of MI produced by distal occlusion of very long LAD that wraps the apex involving part of inferior wall (segments 7, 13 and 17 and also 15) (see Figure 1.8), without residual ischaemia on exercise. In this case the image of abnormal uptake is persistent during rest. See in all cases the ECG patterns that may be found. This figure can be seen in colour, Plate 2.

 

Figure 1.4 Cardiac magnetic resonance imaging (CMR). (A) Transections of the heart following the classical human body planes: (1) frontal plane, (2) horizontal plane and (3) sagittal plane. (B) Transections of the heart following the heart planes that cut the body obliquely. These are the planes used by the cardiac imaging experts: (1) short-axis (transverse) view, in this case at mid-level (see B(1)); (2) horizontal long-axis view; (3) vertical long-axis view (oblique sagittal-like). Check the great difference between the sagittal plane according to human body planes (A(3)) and the heart planes (B(3). (B) It shows the four walls of the heart with the classical names: septal (S), anterior (A), lateral (L) and inferoposterior. Currently, the inferoposterior wall is named for consistency just inferior (I) (see p. 16 and Figure 1.8).

 

Figure 1.5 Hyperenhancement patterns found in clinical practice. If hyperenhancement is present, the subendocardium should be involved in patients with ischaemic disease. Isolated mid-wall or subepicardial hyperenhancement strongly suggests a ‘non-ischaemic’ etiology.

The most recent imaging techniques are CMR (Figure 1.4) and CMDCT (Figure 1.1). The latter is used for non-invasive study of coronary tree. CMR, which may also be used for perfusion and function studies of themyocardium, gives us the best ‘in vivo’ anatomic information about the heart. Thus, this technique, in conjunction with gadolinium injection and contrast-enhanced CMR (CE-CMR), is very useful for identifying and locating MI, as well as for determining its transmurality with extraordinary reliability, comparable to pathological studies. This is why CE-CMR has become the gold-standard technique for studying correlations between ECG findings and infarctedmyocardial areas in the chronic phase of IHD. Also, CE-CMR may distinguish according to location the hyperenhancement areas between ischaemic and non-ischaemic patients (Figure 1.5) and may show in vivo the sequence of the evolving transmural MI. The reproducibility of CE-CMR along time, especially after the acute phase, is very good. It also has the advantage of not producing radiation. The current limitation of CMR, which will probably be solved in the next few years, is the study of coronary tree. Currently, this may be performed non-invasively by CMDCT (see above Fig 1.1).

The heart walls and their segmentation: cardiac magnetic resonance (Figures 1.4–1.14)

The heart is located in the central-left part of the thorax (lying on the diaphragm) and is oriented anteriorly, with the apex directed forwards, and from right to left (Figure 1.4).

 

Figure 1.6 The concept of anterior and posterior infarction

 

Figure 1.7 (A) The left ventricle may be divided into four walls that till very recently were usually named anterior (A), inferoposterior (IP) or diaphragmatic, septal (S) and lateral (L). However, according to the arguments given in this book, we consider that the ‘inferoposterior’ wall has to be named just ‘inferior’ (see p. 16). (B–D) Different drawings of the inferoposterior wall (inferior + posterior walls).

The left ventricle (LV) is cone shaped. Although its borders are imprecise, classically, it has been divided, except in its inferomost part the apex, into four walls, till very recently named septal, anterior, lateral and inferoposterior. In the 1940s–1950s the inferoposterior wall was named just posterior (Figure 1.6A), probably because it was considered opposed to the anterior wall. Later on, only the basal part of this wall, which was thought to bend upwards, was considered really a posterior wall (Figure 1.6B). Therefore, it was named ‘true posterior’ and the rest of the wall just ‘inferior wall’ (Figure 1.6). According to that, for more than 40 years the terms ‘true’ or ‘strict posterior infarction’, ‘injury’ and ‘ischaemia’ have been applied, when it was considered that the basal part of the inferoposterior wall was affected. The committee of the experts of the International Society of Computerised ECG, in accordance with the publications of Selvester andWagner, has named these walls anterosuperior, anterolateral, posterolateral and inferior, respectively. However, this nomenclature has not been popularised, and the classical names (Figure 1.7A) are still mostly used in the majority of papers, ECG books (Figure 1.7B to D), task force and statements.

 

Figure 1.8 (A) Segments into which the heart is divided, according to the transverse (short-axis view) transections performed at the basal, mid and apical levels.

 

Figure 1.9 Images of the segments into which the left ventricle (LV) is divided according to the transverse transections (short-axis view) performed at the basal, mid and apical levels, considering that the heart is located in the thorax just in a posteroanterior and right-to-left position.

Later on, in the era of imaging techniques, the heart was transected into different planes (Figure 1.7) and different names were given to the heart walls by echocardiographists and experts in nuclear medicine. However, recently, the consensus of the North American Societies for Imaging divided the LV in 17 segments and 4 walls: septal, anterior, lateral and inferior (Figures 1.8 and 1.9). This consensus states that the classical inferoposterior wall should be called inferior ‘for consistency’, and segment 4 should be called inferobasal instead of posterior wall. Therefore the word ‘posterior’ has to be suppressed. Figures 1.8 and 1.9 show the 17 segments intowhich the four left-ventricular walls are divided (6 basal, 6 medial, 4 inferior and the apex), and the right side of Figure 1.9 shows the heart walls with their corresponding segments on a polar ‘bull’s-eye’ map, as used by specialists innuclear medicine.Now we will explain, thanks to correlations with CMR, why we consider that this terminology is the best and it will be used further in this book. Page 16 shows the evolution of the terminology given to the wall that lies on the diaphragm.

 

Figure 1.10 (A) The heart, shown out of the thorax by anatomists and pathologists; (B) bull’s-eye image as it is shown by nuclear medicine and (C) transverse transection as it is shown by CMR. In both cases the position of the heart is presented as if the heart was located in the thorax in a strictly posteroanterior position. (D) The injury and infarction vectors (Inj. V and Inf. V) with the same direction but different sense may be seen.

 

Figure 1.11 Magnetic resonance imaging. (A) Thoracic horizontal axial plane at the level of the ‘xy’ line of the drawing on the right side of the figure. The four walls can be adequately observed: anterior (A), septal (S), lateral (L) and inferior (I), represented by the inferobasal portion of the wall (segment 4 of Cerqueira statement) that bends upwards in this case (B).

 

Figure 1.12 (A) The posterior (inferobasal) wall as it was wrongly considered to be placed. With this location an infarction vector of inferior infarction (segments 4 and 10 in case of very lean individuals) faces V1–V2 and explains the RS pattern in these leads. (B, C) The real anatomic position of inferior wall (inferobasal) and lateral wall infarctions. The infarction vector of inferobasal and mid-segment in lean individuals faces V3–V4 and not V1, and may contribute to the normal RS pattern seen in these leads. On the contrary, the vector of infarction of the lateral wall faces V1 and may explain RS pattern in this lead.

 

Figure 1.13 Sagittal-oblique view in case of normal-body-build subject (A) (G shape), in a man with horizontal heart (B) (C shape) and in a very lean subject (C) (U shape). We have found that the inferior wall does not bend upward in C shape (two-third of the cases), and only in very lean individuals with U shape, the largest part of the wall is posterior (5% of the cases) (C).

If we consider that the heart is located in the thorax in a strictly posteroanterior position, as is presented by anatomists and by experts in nuclear medicine, and in the transverse section of CMR images (Figure 1.10A–C), we may understand that in case of involvement (injury or infarction) of basal part of inferior wall (classically called posterior wall) especially when in lean individuals the majority of inferior wall is placed in a posterior position (Figure 1.13C), an RS (R) and/or ST-segment depression in V1 will be recorded (Figure 1.10D). However, now, thanks to magnetic resonance correlations
(Figure 1.11), we have evidence that the sagittal view of the heart is, in respect to the thorax, locatedwith an oblique right-to-left inclination and not in a strictly posteroanterior position, as was usually presented by anatomists, nuclear medicine and the transverse section of CMR (Figure 1.10). This helps us to understand how the RS (R) or predominant ST-segment depression patterns in V1 is the consequence of the infarction of or injury to the lateral, not the inferobasal, segment (classical posterior wall) (Figure 1.12).However,we have to remind that in the majority of cases except for very lean individuals (see Figure 1.13C), the part of the inferior wall that is really posterior just involves the area of late depolarisation (segment 4, or inferobasal). Therefore, in case of MI of this area, there would not be changes in the first part of QRS, because this MI does not originate a Q wave or an equivalent wave.

The CMR technique gives us real information about the in vivo heart’s anatomy (Figure 1.4). In this regard, the following
are important:

(a) CMR patterns of the frontal, horizontal and sagittal planes of the heart following the human body planes are shown in Figure 1.4A. This allows us to knowwith precision the heart’s locationwithin the thorax. In this figure we can observe these transections, performed at the mid-level of the heart.

(b) Nevertheless, bearing in mind the threedimensional location of the heart within the thorax, in order to correlate the left ventricular walls amongst themselves and, above all, to locate the different segments into which they can be divided,
it is best to perform transections following the heart planes that are perpendicular to each other (see Figure 1.4B), as has been already done in nuclear medicine (Figure 1.3; see Plate 2). These planes transect the heart following the heart planes
(Figure 1.4B)andare the following: horizontal longaxis view, short-axis view (transverse) and vertical long-axis view (oblique sagittal-like). In reality the oblique sagittal-like view (Figure 1.11B) presents, as we have said, an oblique right to left and not a strict posteroanterior direction (compare Figure 1.4A(3) with Figures 1.4B(3) and 1.11B). Therefore in the presence of infarction of the inferobasal partof inferior wall (classically calledposterior wall) and especially when the infarction involves the midinferior wall if it is located posteriorly, as happens in very lean individuals (Figure 1.13C), the vector of infarction generated in this area is directed forwards and from right to left and is recorded as RS morphology in V2–V3, but not in V1 where it presents a normal rS morphology (Figure 1.12B). On the contrary, the vector of infarction, in the case of infarction involving the lateral wall, may generate an RS pattern in V1 (Figure 1.12C).

(c) The longitudinal vertical plane (Figures 1.3(2), 1.8C and 1.11B; see Plate 2) is not fully sagittal with respect to the anteroposterior position of the thorax, but rather oblique sagittal, as it is directed from right to left. (The sagittal-like axis follows the CD line in Figure 1.11A.) Compare Figures 1.4B(3) and 1.11B with the true sagittal view – Figure 1.4A(3). The view of this plane, as seen from the left side (oblique sagittal), allows us to correctly visualise the anterior and the inferior heart walls (Figure 1.11B). We can clearly see that the inferior wall has a portion that lies on the diaphragm until, at a certain point, sometimes it changes its direction and becomes posterior (classic posterior wall), now called inferobasal segment. This posterior part is more or less important, depending on, among other factors, the body-build.We have found (Figure 1.13) that in most cases the inferior wall remains flat (C shape) (Figure 1.13B). However, sometimes a clear basal part bending upwards (G shape) (Figure 1.13A) is seen. Only rarely, usually in very lean individuals, does the great part of the inferior wall present a clear posterior position (U shape) (Figure 1.13C).

Therefore, often, the posterior wall does not exist and for this reason, the name ‘inferior wall’ seems clearly better than the name ‘inferoposterior’. On the other hand, the anterior wall is, in fact, superoanterior, as is clearly appreciated in Figure 1.11B. However, in order to harmonise the terminology with imaging experts and to avoid more confusion, we consider that the names ‘anterior wall’ and ‘inferior wall’ are the most adequate for its simplification and also, because when an infarct exists in the anterior wall, the ECG repercussion is in the horizontal plane (HP; V1–V6) and when it is in the inferior wall – even in the inferobasal segment – it is in the frontal plane (FP).

(d) The longitudinal HP (Figures 1.3(3) and 1.8B; see Plate 2) is directed from backwards to forwards from rightwards to leftwards, and slightly cephalocaudally. In Figure 1.8A (arrows), one can appreciate how, following the line AB, the heart can be opened like a book (Figure 1.8B).

(e) The transverse plane (Figures 1.4B(1), 1.3A(1) and 1.8A),with respect tothe thorax, is directedpredominantly cephalocaudally and from right to left, and it crosses the heart, depending on the transectionperformed, at the basal level, mid-levelor apical level (Figure 1.8A). Thanks to these transverse transections performed at different levels, we are able to view the right ventricle (RV) and the left-ventricular septal, anterior, lateral and inferior walls (Figures 1.3(1) and 1.8A; see Plate 2). Thus, the LV is divided into the basal area, the mid-area, the apical (inferior) area and the strict apex area (Figures 1.8A and 1.9).

In order to clarify the terminology of the heart walls, a committee appointed by ISHNE (International Society Holter Non-invasive Electrocardiography) has made the following recommendations :

1. Historically, the terms‘true’ or ‘strictly posterior’ MI have been applied when the basal part of the LV wall that lies on the diaphragm was involved. However, although in echocardiography the term posterior is still used in reference to other segments of LV, it is the consensus of this report to abandon the term ‘posterior’ and to recommend that the term ‘inferior’ be applied to the entire LV wall that lies on the diaphragm.

2. Therefore, the four walls of the heart arenamed anterior,septal,inferior and lateral. This decision regarding change in terminology achieves agreement with the consensus of experts in cardiac imaging appointed by American Heart Association (AHA) (Cerqueira, Weissman and Disizian, 2002) and thereby provides great advantages for clinical practice. However, a global agreement, especially with an echocardiographic statement, is necessary.

The coronary tree: coronary angiography and coronary multidetector computed tomography

In the past, only pathologists have studied coronary arteries. In clinical practice, coronary arteriography, first performed by Sones in 1959, has been the ‘gold standard’ for identifying the presence or absence of coronary stenosis due to IHD, and it provides the most reliable anatomic information for determining the most adequate treatment. Furthermore, it is crucial not only for diagnosis but also forperformingpercutaneous coronary intervention (PCI).Very recently, new imaging techniques, especially CMDCT, are being used more and more with a great reproducibility compared with coronary angiography (Figure 1.1). CMDCT is very useful for demonstrating bypass permeability and for screening patients with risk factors. Recently, it has even suggested its utility in the triage of patients at emergency departments with dubious precordial In chronic-heart-disease patients, there are some limitations due to frequent presence of calcium in the vessel walls that may interfere with the study of the lumen of the vessel. However the calcium score alone without the visualisation of coronary arteries is important in patients with intermediate risk, in some series even better than exercise testing, to predict the risk of IHD.CMDCT has some advantages in case of complete occlusion (Figure 1.1G) and in detecting soft plaques. It is also useful for the exact quantification of the lumen of occluded vessel that is comparablewith intravascular ultrasound (see Figure 1.1H). However, it is necessary to realise the need to avoid repetitive explorations form an economical point of view and also to avoid possible side-effects due to radiation. A clear advantage of invasive coronary angiography is that it is possible, and this is very important especially in the acute phase, to perform immediately a PCI.

The perfusion of the heart walls and specific conduction system
The myocardium and specific conduction system (SCS) are perfused by the right coronary artery (RCA), the left anterior descending coronary artery (LAD) and the circumflex coronary artery (LCX). Figure 1.1 shows the great correlation of coronary angiography and CMDCT in normal coronary tree and some pathologic cases. Figures 1.14B–D show the perfusion that the different walls with their corresponding segments receive from the three coronary arteries. The areas with common perfusion are coloured in grey in Figure 1.14A. Figure 1.14E shows the correlation of ECGleadswith the bull’s-eye image. Themyocardial areas perfused by three coronary arteries are as follows :

• Left anteriordescendingcoronaryartery(LAD) (Figure 1.14B). It perfuses the anterior wall, especially via the diagonal branches (segments 1, 7 and 13), the anterior part of the septum, a portion of inferior part of the septum and usually the small part of the anterior wall, via the septal branches (segments 2, 8andpartof 14, 3and9). Segment14 isperfused by LAD, sometimes shared with the RCA, and also parts of segments 3 and 9 are shared with the RCA. Segments 12 and 16 are sometimes perfused by the second and third diagonals and sometimes by the second obtuse branch of LCX. Frequently, the LAD perfuses the apex and part of the inferior wall, as the LAD wraps around the apex in over 80% of cases (segment 17 and part of segment 15).

 

Figure 1.14 According to the anatomical variants of coronary circulation, there are areas of shared variable perfusion (A). The perfusion of these segments by the corresponding coronary arteries (B–D) can be seen in the ‘bull’s-eye’ images. For example, the apex is usually perfused by the LAD but sometimes by the RCA or even the LCX. Segments 3 and 9 are shared by LAD and RCA, and also small part of mid-low lateral wall is shared by LAD and LCX. Segments 4, 10 and 15 depend on the RCA or the LCX, depending on which of them is dominant (the RCA in >80% of the cases). Segment 15 often receives blood from LAD. (E) Correspondence of ECG leads with the bull’s-eye image. Abbreviations: LAD, left anterior descending coronary artery; S1, first septal branch; D1, first diagonal branch; RCA, right coronary artery; PD, posterior descending coronary artery; PL, posterolateral branch; LCX, left circumflex coronary artery; OM, obtuse marginal branch; PB, posterobasal branch.

• Right coronary artery (RCA) (Figure 1.14C). This artery perfuses, in addition to the RV, the inferior portion of the septum (part of segments 3 and 9). Usually, the higher part of the septum receives double perfusion (LAD + RCA conal branch). Segment 14 corresponds more to the LAD, but it is sometimes shared by both arteries (see before). The RCA perfuses a large part of the inferior wall (segment 10 and parts of 4 and 15). Segments 4 and 10 can be perfused by the LCX if this artery is of the dominant type (observed in 10–20% of all cases), and at least part of segment 15 is perfused by LAD if this artery is long. Parts of the lateral wall (segments 5, 11 and 16) may, on certain occasions, pertain to RCA perfusion if it is very dominant. Sometimes segment 4 receives double perfusion (RCA + LCX). Lastly, the RCA perfuses segment 17 if the LAD is very short.

• Circumflex coronary artery (LCX) (Figure 1.14D). The LCX perfuses most of the lateral wall – the anterior basal part (segment 6) and the mid and low parts of lateral wall shared with the LAD (segments 12 and 16) and the inferior part of the lateral wall (segments 5 and 11) sometimes shared with RCA. It also perfuses, especially if it is the dominant artery, a large part of the inferior wall, especially segment 4, on rare occasions segment 10, and part of segment 15 and the apex (segment 17).

The double perfusion of some parts of the heart explains that this area may be at least partially preserved in case of occlusion of one artery and that in case of necrosis the involvement is not complete (no transmural necrosis).

Both acute coronary syndromes (ACSs) and infarcts in chronic phase affect, as a result of the occlusion of the corresponding coronary artery, one part of the two zones into which the heart can be divided (Figure 1.14A): (1) the inferolateral zone, which encompasses all the inferior wall, a portion of the inferior part of the septum and most of the lateral wall (occlusion of the RCA or the LCX); (2) the anteroseptal zone, which comprises the anterior wall, the anterior part of the septum and often a great part of inferior septum and part of the midlower anterior portion of lateral wall (occlusion of the LAD). In general, the LAD, if it is large, as is seen in over 80% of cases, tends to perfuse not only the apex but also part of the inferior wall (Figures 1.1 and 1.14).

The occlusion of a coronary artery may affect only one wall (anterior, septal, lateral or inferior) or, more often, more than one wall. ACSs and infarcts in their chronic phase, which affect only one wall, areuncommon.Even the occlusionof the distal part of the coronary arteries usually involves several walls. For example, the distal LAD affects the apical part of anterior wall but also the apical part, even though small, of the septal, lateral and inferior wall, and the distal LCX generally affects part of the inferior and lateral walls. In addition, an occlusion of the diagonal artery, although fundamentally affecting the anterior wall, often also involves the middle anterior part of the lateral wall and even the occlusion of the first septal branch artery, or a subocclusion of the LAD encompassing the septal branches involves part of the septum and often a small part of the anterior wall. Probably, the occlusion of oblique marginal (OM) (part of the lateral wall) or distal branches of a non-dominant RCA and LCX (part of the inferior wall) involves only a part of a single wall.

In fact, whetherACSs or established infarctions involve one or more walls has a relative importance. What is most important is their extension, related mainly to the site of the occlusion and to the characteristics of the coronary artery (dominance, etc.). Naturally, on the basis of all that was previously discussed, large infarcts involve a myocardial mass that usually corresponds to several walls, but the involvement of several walls is not always equivalent to a large infarct, as we have already commented. For instance, the apex, although a part of various walls, is equivalent toonly a fewsegments. Therefore knowing what segments are affected allows us to better approximate the true extension of the ventricular involvement. Lastly, although in many cases multivessel coronary disease exists, this does not signify that a patient has suffered more than one infarct.

Consequently, in order to better assess the prognosis and the extent of the ACSs, and infarcts in the chronic phase, it is very important in the acute phase to establish the correlation between the ST-segment deviations/T changes and the site of occlusion and the area at risk (p. 66), and in the chronic phase between leads with Q wave and number and location of left-ventricular segments infarcted (p. 139) (Figures 1.8 and 1.9).

The perfusion of SCS structures is as follows:
(a) The sinus node and the sinoatrial zone by the RCA or the LCX (approximately 50% in each case)
(b) The AV node perfused by the RCA in 90% of cases and by the LCX in 10% of cases
(c) The rightbundle branchand the anterior subdivision of the left bundle branch by the LAD
(d) The inferoposterior division of the left bundle branch by septal branches from the LAD and the RCA, or sometimes the LCX
(e) The left bundle branch trunk receiving double perfusion (RCA + LAD)

This information will be useful in understanding when and why bradyarrhythmias and/or intraventricular conduction abnormalities may occur during an evolving ACS.

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Echocardiography

Introduction
Echocardiography in its current form, has becomean invaluable tool in a modern cardiac intensive care unit environment. Coupled with a clinical examination and monitoring techniques, echocardiography can provide real-time rapid and reliable diagnostic answers that are invaluable to patient care. This noninvasive test can be used to reliably evaluate cardiac anatomy of both normal hearts and those with congenital heart disease and has replaced cardiac angiography for the preoperative diagnosis of the majority of congenital heart lesions. In congenital or acquired cardiac disease, echocardiography may be further used to estimate intracardiac pressures and gradients across stenotic valves and vessels, determine the directionality of blood flow and pressure gradient across a defect, and examine the coronary arteries. Within the realm of critical care, echocardiography is useful to quantitative cardiac systolic and diastolic function, detect the presence of vegetations from endocarditis, and examine the cardiac structure for the presence of pericardial fluid and chamber thrombi. As with all tools, however, a thorough understanding of its uses and limitations are necessary before relying upon the information it provides.

Principles of Echocardiography
Echocardiography uses ultrasound technology to image the heart and associated vascular structures. Ultrasound is defined as sound frequencies above the audible range of 20,000 cycles per second. The primary components of an ultrasound machine include a transducer and a central processor. The transducer converts electrical to mechanical (sound) energy and vice versa. Electrical energy is applied to piezoelectric crystals within the transducer resulting in the generation of mechanical energy in the form of a series of sinusoidal cycles of alternating compression and rarefaction. The energy produced travels as a directable beam which may be aimed at the heart. The sound beam travels in a straight line until it encounters a boundary between structures with different acoustical impedance, such as between blood and tissue. At such surfaces, a portion of the energy is reflected back to the same crystals within the transducer, and the remaining attenuated signal is transmitted distally. Within the ultrasound, machine is circuitry capable of measuring the transit time for the beam to travel from the transducer to a given structure and back again then calculate the distance traveled. A cardiac image is constructed from the reflected energy, or so called ultrasound echoes.

Differing properties of tissues affect the portion of acoustic energy transmitted versus reflected. For example, air reflects the majority of the signal it receives and, therefore, prevents images from being obtained through windows where it is present. Anything hindering or augmenting the reflection of this acoustic signal, such as air, bone, dressings, an open chest, or lines, tubes, or other foreign bodies, will diminish the overall quality of the examination. Therefore, in the intensive care unit, an ultrasound study may be limited by difficulty in finding a good acoustic window to allow for accurate analysis.

The Anatomical Echocardiographic Examination
In order to obtain the best imaging windows, whenever possible, patients are placed in a left lateral decubitus position during a transthoracic echocardiogram. During two-dimensional (2D) echocardiography, all planes are described in reference to the heart and not the heart’s position within the body. For a complete pediatric study, standard views (see Fig.1–5) are obtained from the high left chest just lateral to the sternum (parasternal window), the left lateral chest just inferior and lateral to the nipple (apical window), sub-xyphoid area (subcostal window), and the suprasternal notch (suprasternal window). In patients with more complex anatomy, additional windows, such as the high right parasternal border, may be used to obtain additional information.

 

Fig.1 Standard echocardiographic image planes from the high left chest just lateral to the sternum (parasternal window (a) and (b)), the left lateral chest just inferior to the nipple (apical window (c)), sub-xyphoid area (subcostal window (d)), and the suprasternal notch (suprasternal window (e) and (f)). RA right atrium; RV right ventricle; LA left atrium; LV left ventricle; Ao aortic valve; CS coronary sinus; RVOT right ventricular outflow tract; SVC superior vena cava (drawing from Steven P. Goldberg, MD) 

1.  Parasternal Window
In the anatomically normal heart, the parasternal window allows visualization of the heart aligned along its long axis and short axis. In the long axis (Fig.1a), the left ventricular inflow and outflow tracts can be seen well. As a result, comments can be made from this view regarding the aorta, including its annulus, the sinuses of Valsalva, and the proximal portion of the ascending aorta, as well as its relationship to the mitral valve. Additionally, the ballet-slipper appearance of the left ventricle is featured as the inferoposterior wall and interventricular septum are visualized. The anterior and posterior leaflets of the mitral valve can be visualized. By angulating the transducer and performing a sweep, the right ventricle is brought into focus and an examination of both its inflow including the right atrium and tricuspid valve and its outflow tract, including the pulmonary valve can be performed.

The transducer may be rotated 90° providing a series of short-axis views (Fig.1b) that assist in the evaluation of the chambers of the heart, the semilunar and atrioventricular valves, and the coronary arteries. Sweeping from the apex of the heart toward the base will allow a close cross-sectional examination of the ventricular chambers. The normal left ventricle has circular geometry with symmetric contraction, whether it is visualized at the level of the mitral valve, papillary muscles, or apex. In contrast, the normal right ventricle appears as a more trabeculated crescent-shaped structure when visualized at or below the level of the mitral valve. Sweeping farther toward the base of the heart, the mitral valve’s papillary muscles and the valve itself are viewed. Progressing to the base of the normal heart, the tri-leaflet aortic valve takes the center stage with the right ventricular outflow tract and pulmonary wrapping in an inverted “U” anteriorly and leftward. Additionally a portion of the atrial septum and the tricuspid valve may be profiled. Finally, continuing the sweep allows for the examination of the atrial appendages, ascending aorta in cross-section and branch pulmonary arteries.

 

Fig.1b (continued)

2.  Apical Window
For those not trained in echocardiography, the images obtained with the transducer in the apical position (Fig.1c) are perhaps the most intuitive as it allows for visualization of all four chambers and valves in the heart with a simple left-to-right orientation. Imaging is begun in the four-chamber view, in which the anatomic right and left ventricles may be identified. Sweeps of the transducer from this position identify the posterior coronary sinus and may indicate abnormalities such as a left superior vena cava or unroofed coronary sinus. Proceeding more anteriorly to a five-chambered view, the atrial and ventricular septa may be visualized looking for defects and the left ventricular outflow tract and ascending aorta may be examined. The four chamber view allows for the examination of the anterior and posterior mitral valve leaflets and pulmonary veins as they enter the left atrium. By rotating the transducer to 90° from the four-chamber view, a two-chamber view of the left ventricle and left atrium can be obtained to evaluate the anterior and posterior left ventricular wall function.

 

Fig.1c (continued)

3.  Subcostal Window
For pediatric patients with complex cardiac anatomy, the subcostal position (Fig.1d and Fig.1.e) provides the most detailed information and is often thebest starting place. In order to obtain images in this position, patients are placed supine with the transducer in the subxiphoid position. In larger cooperative patients beyond the infancy period, image quality may be improved by having the patient participate in the examination with held inspiration that allows the heart to move downward toward the transducer. Initial views in this position should determine visceral situs as well as the relationship of the inferior vena cava and aorta. Subsequent views and sweeps will provide detailed analysis of the atrial septum as well as the images related to the ventricular septum, the atrioventricular valves, atrial and ventricular chambers, and drainage of systemic veins. With the rotation of the transducer both ventricular outflow tracts may be visualized. Additionally in some patients the branch pulmonary arteries and the entire aorta may be examined from this position.

 

Fig.1d  (continued)

 

Fig.1e (continued)

4. Suprasternal Window
The views are obtained in this position by placing the transducer in the suprasternal notch (Fig.1.f) with the neck extended. The suprasternal longand short-axis views provide detailed information regarding arch sidedness, anomalies in the ascending and descending aorta and head and neck vessels, the size and branching of the pulmonary arteries, as well as anomalies of systemic and pulmonary venous systems.

 

Fig.1f (continued)

M-Mode Imaging
One of the earliest applications of ultrasound technology that remains an important tool in the evaluation of cardiac function, dimension, and timing, the M-mode echo provides an “ice-pick” view of the heart. An M-mode echo is obtained with the ultrasonic transducer placed along the left sternal border and directed toward the part of the heart to be examined. A single line of interrogation is repeatedly produced and the resultant image is displayed with time along the x-axis and distance from the transducer along the y-axis (see Fig. 2). M-mode obtains an estimate of ventricular function by measuring the short axis shortening fraction and wall thickness.

Fig.2 M-mode echocardiography obtained in the parasternal short axis through the right and left ventricular chambers at the level of the papillary muscles. LVEDD left ventricular end-diastolic dimension; LVESD left ventricular end-systolic dimension

Doppler Evaluation
Frequently in an intensive care setting the clinician is concerned with new or residual flow disturbances from shunt lesions, an abnormal cardiac valve, or narrowing of a blood vessel. While 2D echocardiography determines anatomical relationships, additional information regarding movement of the blood or myocardium is provided by looking for Doppler shifts in the reflected ultrasound waves. The Doppler principle, first described by Johann Christian Doppler, states that for a stationary object, the frequency of ultrasound reflected is identical to the transmitted frequency. Inherently the heart and the blood it pumps do not fit this basic definition. Therefore, when performing a cardiac ultrasound, the moving objects alter the frequency of the reflected signal (the Doppler shift) according to the direction and velocity with which they are moving in relation to the fixed transducer. Additional insights to intracardiac and vascular hemodynamics may be obtained when velocity data is collected. Doppler data are typically displayed as velocity rather than the actual frequency shift. The velocities can then be translated into pressure data using the modified Bernoulli equation: P1 – P2= 4[(V2)2 – (V1)2]. If one assumes that the level of obstruction and therefore the velocity of V1 is negligible compared with the obstruction at V2 the formula becomes even simpler: DP = 4(Vmax)2. Although the modified Bernoulli equation can only be applied in appropriate situations, it does help predict the pressure drop across an abnormal valve or septal defect to give a general estimate of the severity of the lesion which can prove to be valuable information to help manage patients in the intensive care setting.

Of note, during Doppler imaging it is clinically important to recognize the angle of interrogation of blood flow and its impact on the accuracy of our velocity measures. It is important when performing Doppler studies that the line of beam interrogation should be directly in the line of flow, resulting in as little distortion of data as possible. The more off-angle the approach is, the increasingly more severe the underestimation of the true velocity will be. For practical purposes, an angle of interrogation less than 20° is essential to ensure clinically accurate information.

Two commonly used techniques are pulsed and continuous wave Doppler. Pulse wave Doppler allows determination of direction and velocity at a precise point within the imaged cardiac field. However, it is limited in its maximum detectable velocity by the Nyquist limit making it unusable for quantification of high-velocity flow (e.g., as seen with severe obstruction). In contrast, continuous wave Doppler interrogates all points along a given beam. Continuous wave Doppler imaging is not constrained by velocity limits and can hence record velocities exceeding those of pulsed Doppler imaging. The drawback is that while the line of interrogation is identifiable, knowledge of anatomy must already be obtained to identify the precise location of the maximum velocity. Clinically these two techniques are commonly used sequentially to identify the area of interest and then to obtain the maximum velocity.

1. Color Flow Doppler
Color flow Doppler is powerful technique for obtaining additional hemodynamic and anatomic data for patients undergoing echocardiography in the intensive care unit. Color flow Doppler allows velocity information to be overlaid on a 2D anatomic image therefore providing data regarding intracardiac and extracardiac shunts, valvar insufficiency or stenosis, and vessel obstruction. By convention, shades of red are used in identifying blood flowing toward the transducer and blue to indicate blood flowing away from the transducer. Therefore, color flow Doppler defines the presence and direction of shunts and is used to grade the severity of valvar insufficiency.

Current Clinical Applications
Clinical applications of echocardiography within the intensive care unit may be divided into the following major areas:
1. The diagnosis and post-intervention evaluation of anatomic lesions.
2. Evaluation of cardiac function.
3. Diagnosis of intracardiac masses and extracardiac effusions.
4. Guidance of intervention within the intensive care unit

Anatomic Lesions Pre and Post Intervention
Advances in technology have enabled most congenital heart defects to be diagnosed by echocardiography avoiding the risks, time, and cost of invasive cardiac catheterization. In addition, for infants and pediatric patients admitted to an intensive care unit due to being succumbed to shock, echocardiography may be useful for differentiating anatomic causes of shock from functional causes. Patients with obstruction to outflow on the left side of the heart who go undiagnosed at birth frequently present with signs of diminished cardiac output (CO) or frank shock. These lesions including aortic valve stenosis, coarctation of the aorta, and variations of hypoplastic left heart syndrome may be identified and defined by echocardiogram alone.

Following surgical or catheter-based intervention patients convalesce in the intensive care unit. Most patients undergo a postprocedural echo before getting discharged home to document adequacy of the repair and lack of significant complications. In postoperative patients this assessment may prove more complicated as access to the patient and the correct windows may be severely compromised by dressings, intracardiac lines, and chest tubes. Occasionally postoperative patients in the intensive care unit may be found to have unexpected residual lesions (see Fig.3). For example, following repair of septal defects, echocardiography may be useful to screen for the presence of residual shunts which may be less well tolerated secondary to myocardial changes following cardiopulmonary bypass. Often, the presence of a residual lesion is known in the operating room through transesophageal echocardiography or direct discussion with the surgeon. An important role of echocardiography is to distinguish those lesions with hemodynamic consequences from those whose presence has no impact on postoperative care. Transthoracic echocardiography may be used to diagnose and assess the hemodynamic sequelae of shunt lesions, residual stenosis, and function. More complicated is the assessment of coronary flow, right ventricular dynamics, and distal obstruction following intervention. In patients who are experiencing arrhythmias postoperatively, special attention should be paid to the flow within the coronary arteries to ensure that it has not been compromised or that a line or mass in the heart is not causing ectopy.

 

Fig.3 Parasternal short axis image in a patient with pulmonary atresia/VSD who acutely decompensated. White arrows demonstrate the large residual VSD than resulted when a patch dehisced. RA right atrium; RV right ventricle; AV aortic valve

 

Fig.4 (a) and (b): Four chambered view demonstrating color Doppler of tricuspid regurgitation and the corresponding spectral Doppler pattern. The velocity obtained by spectral Doppler may be utilized to estimate pulmonary artery pressures in the absence of downstream obstruction. A complete envelope by pulse wave or continuous wave Doppler provides the velocity of the regurgitant jet which may be translated into pressure data using the equation: DP = 4(Vmax)2. RA right atrium; RV right ventricle; LA left atrium; LV left ventricle.

Unanticipated pulmonary arterial hypertension may slow the progress of a patient in the intensive care unit. In the absence of a Swan Ganz catheter or a direct pulmonary arterial monitoring, echocardiography may be used to estimate the pulmonary artery pressures. There are several methods that may be used to determine the pulmonary artery pressures. In a patient with
tricuspid regurgitation, the velocity of the jet estimates the difference in pressure in the right atrium and the right ventricle (see Fig.4). If there is no stenosis of the pulmonary arteries, pulmonary valve, or right ventricular outflow tract, the difference in pressure between the right atrium and right ventricle plus the right atrial pressure (CVP) provides an estimate of the pulmonary arterial pressures. In the absence of tricuspid valve insufficiency, interventricular septal geometry may be used to help quantify the degree of pulmonary hypertension.

Analysis of Ventricular Function
One of the most frequent uses of echocardiography in the ICU is related to the evaluation of ventricular performance. Improvements in technology allow assessment of both systolic and diastolic function with increasing accuracy.

1. Systolic Function
Accurate and timely assessment of systolic function should be an integral part of the medical management of the hemodynamically unstable critically ill patient. Global assessment of LV contractility includes the determination of ejection fraction (EF), circumferential fiber shortening, and cardiac output (CO). There are several methods that may be used to garner this information. Each has its limitations and assumptions which are paramount to understand prior to clinically applying the information gathered. For assessment of left ventricular function, perhaps the simplest quantitative approach is to use M-mode echocardiography (see Fig.3) in either the parasternal short axis at the level of the papillary muscles or in the parasternal long axis at the tips of the mitral valve leaflets to measure the left ventricular end-diastolic dimension (LVEDD) and left ventricular end-systolic dimension (LVESD) for the determination of the fractional shortening (FS) percentage.

Fractional shortening is derived by the following:

Normal values for fractional shortening in children and infants vary slightly with age, falling typically between 28 and 44%.

Fractional shortening, therefore, provides a method of assessing circumferential change but has several obvious drawbacks. This method assumes that the ventricle being examined has a circular shape in the axis in which it is examined. As a result, changes in diameter may be mathematically related to circumferential fiber-shortening providing an estimate of ventricular function. Therefore anything that alters the circular shape of the left ventricle (anatomic abnormalities intrinsic to congenital heart disease, pre and afterload changes, or ventricular–ventricular interactions) may affect the assessment of fractional shortening by altering the movement of the septum and causing an under or over estimation of the either end-systolic or diastolic dimension.

A second method of assessing ventricular function is via ejection fraction. Ejection fraction is a volumetric appraisal of ventricular fiber shortening. Echocardiographically the most common method of calculating ejection fraction is the biplane estimation of volumes from the apical four-and two-chamber views. One of the more commonly used mathematical algorithms is the Simpson method in which the left ventricle is traced manually at the end diastole and end systole along the endocardium. Using the method of disks the left ventricle is divided into a series of parallel planes and the resultant disks are individually summed to create each volume. Ejection fraction is calculated using the following equation:

Unfortunately, the determination of an accurate ejection fraction is also subject to ventricular shape with the left ventricle assumed to be its normal prolate elliptical shape. Variations from this shape, which occur frequently in pediatrics, significantly alter the relationship between fiber shortening and volume dependence upon when this equation is applied. In addition, patients in the intensive care environment frequently have suboptimal imaging windows making the endocardium difficult to distinguish and trace.

Not infrequently in active pediatric intensive care units, a patient’s heart and/or lung function must be supported for a period of time. Two such modalities of support are extracorporeal membranous oxygenation and ventricular assist devices. Often the pediatric echocardiographer is asked to assist in the management of these patients by providing insight into the recoverability of cardiac function. This request can be one of the more challenging uses of echo in an intensive care setting. As discussed above, many of the techniques commonly used to determine ventricular systolic function and CO are dependent on the loading conditions of the heart as well as contractility. As a result, both of these support systems which unload the heart in an effort to allow recovery time severely limit echo’s utility as a prognostic indicator. Several newer methods of determining myocardial function including Tissue Doppler Imaging (TDI), strain and strain rate, color m-mode, calcium gating, and three-dimensional (3D) echocardiography are entering the realm of echo in the intensive care unit. These newer modalities may prove to be more efficacious than current standard echocardiography is at present.

Diastolic Function
Accurate assessment of diastolic function by echocardiography is an evolving field that has made great strides in the past few years. Diastolic heart failure and its impact on postoperative management also deserve consideration. Spectral Doppler evaluation is a relatively easy and useful method for evaluating diastolic function noninvasively at the bedside. A prominent pulmonary vein atrial reversal wave (a wave) is a marker of diastolic dysfunction. This finding represents marked flow reversal into the pulmonary veins during atrial systole in response to a noncompliant ventricular chamber. The mitral inflow Doppler pattern can also be a useful marker for diastolic dysfunction. Mitral inflow is composed of 2 waves – an E wave representing early passive ventricular filling (preload dependent) and the A wave representing active filling as a result of atrial systole. The E:A ratio, velocity of E wave deceleration and duration of the A wave can be altered in patients with diastolic dysfunction.

Tissue Doppler imaging (TDI) is a newer technique for assessing diastolic ventricular function. TDI allows recording of the low Doppler velocities generated by the ventricular wall motion and directly measures myocardial velocities. In spectral TDI, pulsed Doppler is placed along the myocardial wall (mitral, septal, or tricuspid annulus) recording the peak myocardial velocities. Three waveforms are obtained: a peak systolic wave (Sa), an early diastolic wave (Ea), and an end-diastolic wave (Aa) produced by atrial contraction. The tissue Doppler systolic mitral annular velocity has been shown to correlate with global LV myocardial function [14]. TDI has also been used to estimate diastolic function, and is relatively independent of preload condition. The pulsed Doppler peak early mitral inflow velocity (E) divided by the TD early diastolic mitral annular velocity (Ea) results in a ratio that correlates with the pulmonary capillary wedge pressure. The E/Ea ratio is also useful in estimating mean LV filling pressure. At this time, TDI represents one of the most accurate techniques to assess diastolic function and is therefore of particular interest in the critical care population in whom abrupt changes in preload and afterload are common, making Doppler evaluation of diastolic function less reliable.

Detection of Intracardiac Masses and Extracardiac Effusions
An abnormal area of dense reflectance that is well localized within an echo may represent a mass, thrombus, or calcification. In the postoperative or critical care patient with multiple lines in place, especially in the setting of low flow, care must be taken to evaluate these areas for thrombus formation. Echo is the imaging modality of choice for elucidating and evaluating cardiac mass lesions. Differentiating an area of concern from artifact, can be challenging. Areas that move appropriately throughout the cardiac cycle and the presence of an abnormality in more than a single view, suggest a mass rather than an artifact (see Figs. 5a–d). These findings must in turn be distinguished from such anatomical variations as a prominent Eustacian valve or Chiari network.

 

Fig.5 Demonstrate a thrombus in the right ventricle seen in parasternal short axis (a) and modified four-chamber (b) views. RV right ventricle; LV left ventricle. (c) and (d): Demonstrate a thrombus in the left atrial appendage in both parasternal short axis and a modified four chamber views. RA right atrium; RV right ventricle; AV aortic valve; AO ascending aorta; LV left ventricle.

Major factors that predispose a patient to the development of intracardiac thrombi are the presence of intracardiac lines, diminished CO, and localized stasis in addition to changes within the clotting cascade from sepsis, bypass, intrinsic clotting disorders, or heparin use. Echocardiographic evaluation of patients within the intensive care setting must include an awareness of the increased incidence of thrombus formation and a careful evaluation of areas predisposed to become a nidus for thrombus.

Following cardiac surgery it is not uncommon for patients to develop small collections of fluid in the pericardial space (see Fig.6). Typically, this is of little concern to the clinician; however, in a postoperative patient experiencing tachycardia and/or hypotension, the necessity of recognizing the potential for and screening for cardiac tamponade becomes paramount. In young infants and children, it is frequently difficult to rely on physical exam findings of increased jugular venous pressure or the late finding of pulsus paradoxus. In this instance, a directed and easily performed 2D and Doppler echocardiography can confirm the presence of an effusion and provide accurate assessment of its hemodynamic significance.

 

Fig.6 Subcostal image demonstrating a large circumferential pericardial effusion (green arrows)

The size and extension of a pericardial effusion may be diagnosed from parasternal, apical, or subcostal windows. The apical view is the easiest for obtaining information regarding the effusions hemodynamic significance. From the apical four chamber view both the mitral and tricuspid valve flow patterns are evaluated with the respiratory monitoring in place. Examining the changes in inflow hemodynamics with respiration allows for the evaluation of tamponade physiology. Greater than 25% variability in maximal e wave velocity of the mitral valve with inspiration or 50% of the e wave velocity of the tricuspid valve (see Figs.7a, b) is indicative of significant hemodynamic compromise resulting from the effusion. Additionally, collapse (differentiated from contraction) of the free wall of the right and left atrium (see Figs.8a, b) when the pericardial pressure exceeds the atrial pressure may be seen from this view in a patient with a significant effusion.

 

Fig.7 (a) and (b): Respiratory changes in the mitral and tricuspid valve e wave Doppler patterns consistent with tamponade physiology. The tricuspid valve inflow demonstrates more than 50% variability between inspiration and expiration (a). During mitral valve inflow Doppler, the peak E wave velocity alters more than 30% between inspiration and expiration (b).

 

Fig.8 (a) and (b): Four chambered views demonstrating right atrial and right ventricular collapse (green arrows) as a finding of tamponade physiology. RA right atrium; RV right ventricle; LA left atrium; LV left ventricle.

Echocardiography GuidedProcedures
1.  Pericardiocentesis
Performing “blind” percutaneous pericardiocentesis as a treatment for significant pericardial effusion dates back to the early eighteenth century and it is historically fraught with complications. Improved techniques in the 1970s with the advent of 2D echo allowed more accurate localization of the fluid and the development of echo-guided pericardiocentesis. Echo-guided pericardiocentesis (see Fig.9) has been found to be a safe and effective procedure with insertion of a catheter for drainage used to reduce the rate of recurrence found to complicate simple needle drainage and is considered the primary and often the definitive therapy for patients with clinically significant effusions.

 

Fig.9 Echoguided pericardiocentesis. Green arrow is in the pericardial space demonstrating the large fluid collection. Blue arrow is pointing to the needle that has been advanced into the pericardial space to drain the fluid collection. The large effusion allows the echocardiographer to direct the individual performing the pericardiocentesis away from areas that could lead to complications such as perforation of the myocardium.

2.  Balloon Atrial Septostomy (BAS)
Part of any echocardiographic assessment of a patient with congenital heart disease should include evaluation of the atrial septum. Cardiac lesions such as transposition of the great arteries, hypoplastic left heart syndrome, and tricuspid atresia require an adequate atrial communication. In the setting of a restrictive atrial septal communication or intact septum, a BAS is required to improve mixing and CO. In the past, the procedure, originally described by William Rashkind was performed in the cardiac catheterization laboratory under fluoroscopic guidance. However, during the last decade BAS has been routinely performed at the bedside in the intensive care unit under echocardiographic guidance (see Figs.10a–d). Most commonly either a subcostal view that includes a focused look at the atrial septum, pulmonary vein, and mitral valve or an apical four-chamber view is used. For the echocardiographer, the primary role is to provide continued visualization of the catheters and communicate well with the interventionalist. Advantages of this technique are multifactorial; echocardiography is superior to fluoroscopy during BAS due to a lack of radiation, the ability to perform the procedure at bedside rather than transporting to a catheterization laboratory, and direct, continuous visualization of the atrial septum, pulmonary veins, and mitral valve. The disadvantages of this technique include the potential for interference with maneuverability for both echocardiographer and catheter operator around a small neonate and therefore the risk of contamination of the sterile field. Additionally there is the possibility of poor acoustic windows in an ill neonate who may be mechanically ventilated. However, with proper planning and communication, the limitations of transthoracic echocardiographic guidance of BAS may be minimized.

 

Fig.10 Subcostal images demonstrating echo-guided balloon atrial septostomy (BAS). (a): shows the initial small atrial communication in both 2 dimensional (2D) and color Doppler imaging. (b): reveals the deflated balloon that has been advanced across the atrial communication. It is important during this portion of the procedure for the echocardiographer to ensure that the balloon has not been advanced across the left atrioventricular valve. (c): demonstrates the inflated balloon within the left atrium. It is important to note the balloon’s position away from the mitral valve and pulmonary veins. (d): demonstrates the atrial communication following septostomy using both 2D and color Doppler imaging. RA right atrium; RV right ventricle; LA left atrium; LV left ventricle; Green arrows atrial communication.

Future Directions
There are several areas of advanced imaging that are becoming more commonplace in the practice of pediatric echocardiography. Primary assessment of cardiac mechanics by evaluating myocardial motion, strain, and strain rate has been validated in healthy children and provides additional information regarding myocardial performance. Three-dimensional real-time echocardiography has a growing role in evaluating anatomic defects, valves, and right and left ventricular function independently of geometric assumptions that constrained the previous methods.

1. Myocardial Mechanics
In the past several years, myocardial strain and strain rate have emerged as promising quantitative measures of myocardial function and contractility. Strain (e) is a dimensionless parameter defined as the deformation (L) of an object relative to its original length (Lo), and is expressed as a percentage. Strain rate (SR) is defined as the local rate of deformation or strain (e) per unit of time, and is expressed in 1/s. Strain and strain rate measurements can be obtained from data acquired by Doppler Tissue Imaging or 2D tissue tracking. Strain and strain rate should be of great help in the future in the evaluation of ventricular function, since conventional M-mode and 2D echocardiography have limitations due to complex morphology of the right ventricle and altered left ventricle morphology that occurs in complex congenital heart defects. Left and right ventricular values of strain and strain rate are available for healthy children.

2.  3D Echocardiography
Off-line 3D reconstruction consists of acquisition of sequential 2D slices which are converted to a rectangular coordinate system for 3D reconstruction and provides accurate anatomic information suitable for quantitative analysis. Left ventricular volume, mass, and function can be accurately assessed using RT3D independently of geometric assumption, and ejection fraction can be calculated. The wideangle mode is often used to acquire the entire LV volume, from which further analysis allows determination of global and regional wall motion. Wall motion is evaluated from base to apex with multiple slices from different orientations. The advantage of 3D over 2D is the ability to manipulate the plane to align the true long axis and minor axis of the LV, thus avoiding foreshortening and oblique image planes. LV volume assessment by RT3D is rapid, accurate, reproducible and superior to conventional 2D methods and is comparable to MRI, which represents the gold standard. Three dimensional reconstruction of the tricuspid valve has been shown to be helpful for anatomical assessment of Ebstein’s malformation or after atrioventricular septal defect repair. 3D Echocardiography is a useful adjunct to standard 2D imaging and should be increasingly used in the future.

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