Abstract
Methods of dissection of the heart are described and illustrated. Simulated echocardiographic views are described and illustrated, and advice is given on how best to obtain them and in what circumstances they are most useful. Sequential segmental analysis is described in the context of the normal and malformed heart. Sampling for histology is described for the post-mortem heart and for cardiac specimens submitted to the surgical pathologist. A short guide to photography is given. The tables provide a summary of histological features of the normal heart and their significance and also summarise the application of immunohistochemistry to the heart.
2.1 Introduction
Armed with knowledge of the structure of the normal heart we are now in a position to dissect the heart. Sequential segmental analysis is the tool used for assessing the components of the heart and how they relate to one another to form a coherent whole. However, before we can get there, we must take the heart apart. That is to say we must open it up to inspect its features so that individually they can be examined.
2.2 Dissection
There is, in essence, no difference between hearts obtained at autopsy and those explanted during cardiac transplantation, any difference lying largely in the indications for their removal and in the usual absence of much of the atria in the explanted heart. A detailed discussion of examination of the explanted heart is given in Chapter 14. The spectrum of abnormality at post-mortem is of course much greater, ranging from the normal to any abnormality imaginable.
At post-mortem, the heart is examined in situ and its position in the chest and the position of its apex noted. The pericardium is opened anteriorly and its contents, if any, noted. The vascular connections should be examined, particular care being given to the venous system, noting the presence of the innominate vein once the thymus has been removed and whether there is a persistent left superior caval vein. The pulmonary venous connection should be established. This can usually be done by lifting up the apex of the heart. The left pulmonary venous attachments will prevent the heart frombeing elevated far. The right pulmonary veins are more difficult to assess. The atrial appendages should be inspected to confirm usual situs, and the relation of the great vessels noted in relation to each other. The relative size of the ventricles and of the atria can also be noted. The site of the aortic arch should be recorded – whether to the right or to the left. The branching pattern should be inspected, taking particular care to confirm the origin of the right subclavian artery from the brachiocephalic artery – if not, a retro-oesophageal right subclavian artery could easily be missed. The arterial duct should be inspected by folding forward the left lung, and the size of the aortic isthmus and juxta-ductal area inspected – coarctation can very easily be missed if not specifically sought.
Ideally, and particularly where cardiac abnormality is known or suspected, the heart together with the lungs should be removed and fixed in formalin before further dissection. The tissues are easier to handle and retain their relationships better when fixed, and histological sampling is infinitely easier and more exact. This is not always possible, but even in cases where, for whatever reason, the heart cannot be retained, overnight fixation in 20% formalin will fix it sufficiently well for detailed examination and histological sampling.
Following fixation, the soft tissues can be dissected from around the heart and lungs. This is tedious and not essential, but it does permit exact relationships to be inspected and recorded. Particular care is needed to avoid cutting small vessels, especially collateral vessels. Photographic records are essential. Increasingly, I also use video recording to permit demonstration. Once the lungs have been detached, a more detailed analysis of the heart can begin. The method of dissection depends on the nature of the abnormality to be demonstrated, but, to some extent, the methods are arbitrary and depend as much on personal preference as on scientific validation. Also, if the specimen is to be retained for further study, demonstration or research, the extent of dissection and removal of tissue must be correspondingly limited.
There are multiple methods of opening the heart which in itself indicates that none is overwhelmingly superior to the others. My own prejudice in the uncomplicated case is to adopt the traditional method of following the flow of blood. The right atrium is opened by cutting from the inferior caval vein to the tip of the right atrial appendage. It is better not to open the superior caval vein: the superior caval junction with the right atrial appendage marks the site of the sinoatrial node, and cutting this area risks loss of landmarks should it become necessary to sample the node for histology. The superior caval vein should be probed, however, to ensure that it is not thrombosed. Opening from the inferior caval vein displays all the right atrial structures; there is a good view of the atrial septum, and the appendage and coronary sinus can easily be inspected. The tricuspid valve can be inspected from above before cutting down the lateral border of the right ventricle through the atrioventricular junction to the apex (Figure 2.1). A further cut from the apex to the pulmonary artery completes the opening of the right heart. This method does destroy the continuity of the valves but allows close inspection of their constituent parts. The right ventricular aspect of the interventricular septum is exposed, as is the pulmonary infundibulum (Figure 2.2). The incision may be extended through the arterial duct into the aorta. Alternatively, the duct may be probed and sectioned transversely for histology.
Figure 2.1 Right heart opened in the line of the blood flow. The atrium was opened from inferior caval vein to the tip of the atrial appendage and the ventricle on its acute margin. The septal structures of the right atrium and right ventricle are well displayed by this technique. The tricuspid valve is also readily inspected.
Figure 2.2 The same heart viewed with the right ventricular outflow tract exposed by a cut from the ventricular apex to the pulmonary trunk. This view permits inspection of the free wall of the right ventricle, the subpulmonary infundibulum, the pulmonary valve and the proximal pulmonary artery.
The left atrium is opened between the pulmonary veins, and the incision can be extended into the atrial appendage to give better visualisation of the chamber. The orifices of the pulmonary veins should be probed, because even in pulmonary vein stenosis the vessels may appear of good calibre externally. The left ventricle is opened by cutting down the free wall of the ventricle through the atrioventricular junction to the apex (Figure 2.3). To expose the outflow tract, one of two approaches may be used, neither of which is without disadvantage. The aortic outflow can be exposed by cutting up through the aortic leaflet of the mitral valve. This destroys the integrity of the valve leaflet, but exposes the coronary arteries well and does not transect the proximal left coronary artery (Figure 2.4). Alternatively, the incision may be made through the anterior wall of the ventricle close to the septum leaving the anterior leaflet of the mitral valve intact but transecting the left coronary artery near its origin (Figure 2.5). The incision is extended along the length of the aortic arch on its convex aspect to permit inspection of the junction of the arterial duct and aorta. The coronary artery ostia should be inspected carefully, and the arteries may be probed with a very fine probe. Their course on the epicardium should be noted and the artery supplying the posterior interventricular artery identified. It is rarely necessary to make transverse cuts every few millimetres, as in the adult. It can be advantageous to dissect the epicardial course of the coronary arteries and their major branches. The epicardial veins are especially difficult to dissect, particularly at the crux and at the junction of the anterior interventricular and atrioventricular grooves, where arteries and veins cross each other in a random pattern, but their overall configuration can be checked by inspection.
Figure 2.3 The left heart exposed by opening the left atrium between the pulmonary veins and then cutting down to the left ventricular apex. The left atrium is well seen as is the mitral valve and its supporting structures.
Figure 2.4 The left ventricular outflow tract exposed by cutting through the anterior leaflet of the mitral valve. The two halves of the valve leaflet have been separated to demonstrate the structures of the outflow tract. This cut passes through the non-coronary cusp of the aortic valve, and the origins and epicardial course of the coronary arteries are not affected. It does, however, cut through the atrial wall to access the aorta.
In addition, where there has been surgical intervention, it is necessary to define the surgical procedure, the presence of cannulae, lines and pacing wires, assist devices, conduits, grafts and valves.
This approach works well in the neonatal and older heart, although marked ventricular hypertrophy can make internal inspection difficult. For smaller hearts and particularly for small macerated hearts with abnormality, formalin fixation of the heart is essential, followed by examination with some form of magnification system. I prefer a dissecting microscope because of the clarity of detail and the range of magnification available. For embryos, especially those from ruptured ectopic pregnancies, embedding the embryo whole and serial sectioning has until now been the only sure way of thorough examination. The advent of microfocus CT holds promise that these very small specimens can now be examined in exquisite detail (Figure 2.6) [1].
(A) The heart weighs 0.1 g and is close to the limits of comfortable dissection with the aid of a dissecting microscope. The ascending aorta and its branches are small.
(B) The corresponding micro-CT shows the vascular anatomy because the iodine used to obtain tissue contrast attaches to blood cells and acts as a vascular contrast medium. It demonstrates the large calibre arterial duct and the thread-like aortic arch. The left common carotid artery and the left subclavian artery are also clearly outlined. The arterial duct is 0.5 mm in internal diameter
Figure 2.6 Termination of pregnancy at 13 weeks for hypoplastic aortic arch.
2.3 Sequential Segmental Analysis
The near-universally followed method of describing the heart relies upon sequential segmental analysis [2]. This system recognises the heart to comprise three sets of segments:
Atrial segments
Ventricular segments
Arterial segments
The segments are identified by their most consistent feature. For the atria this is the morphology of their appendages; for the ventricles it is the morphology of the septal aspect of their apical trabecular component; and for the arterial trunks it is the branching pattern of the vessels. The atrial situs is determined, and, following identification of the segments, their morphology is noted and their connections to each other assessed. Associated abnormalities can then be described. For most hearts this can be done very quickly. With complex malformations, the system comes into its own.
2.3.1 Situs
Situs applies only to the atria; the atrial situs can be usual (situs solitus), reversed (situs inversus) or mirror image (isomerism). With usual atrial situs, there is a morphologically right atrium on the right side and a morphologically left atrium on the left side. In situs inversus, the morphologically left atrium is situated on the right side and the morphologically right atrium on the left side (Figure 2.7). In isomerism, both atria are of similar morphology, either both of right-sided morphology or both of left-sided morphology (Figure 2.8).
Figure 2.8 Isomerism of the atrial appendages. A fetus of 13 weeks with left atrial isomerism. There are bilateral morphologically left-sided atrial appendages (long and narrow with a hooked end). The heart is left sided with a left-directed apex. Cases of atrial isomerism usually have multiple other abnormalities. In this case they included: bilateral superior caval veins, double inlet left ventricle with common atrioventricular valve, rudimentary right ventricle with ventricular septal defect (VSD) and lacking an inlet, discordant ventriculoarterial connections with anterior aorta, right-sided stomach with multiple small spleens and intestinal malrotation.
2.3.2 Topology
A further feature that must be assessed is the topology of the ventricles. This refers to the left-handedness or right-handedness of a ventricle and is assessed by placing the palm of an imaginary hand of the interventricular septum with the thumb in the atrioventricular junction, the wrist at the ventricular apex and the fingers in the outflow tract [3]. For a normal right ventricle, only the right hand can be so placed. Thus, the right ventricle is said to have right-handed topology (Figure 2.9).
Figure 2.9 Topology. The right hand is placed palm down on the right aspect of the interventricular septum with the thumb in the atrioventricular junction and the remaining fingers in the outflow tract. This indicates right-handed topology. It is not possible to do this with the left hand. Similarly, in the left ventricle, only the left hand can be so deployed. For most practical purposes it is not necessary to invoke topology, but in complex arrangements of ventricles and connections it becomes a very useful descriptor.
2.3.3 Segmental Connections
The connection of the segments is assessed at the atrioventricular level and at the ventriculoarterial level (Figure 2.10). The connection at either of these levels can be concordant, discordant or show absence of one of the connections. Additionally, at the atrioventricular level there may be ambiguous connection or double inlet to one ventricle; at the ventriculoarterial level there may be double outlet from one ventricle.
2.3.4 Atrioventricular Connections
1. With concordant atrioventricular connection the morphologically right atrium is connected to the morphologically right ventricle and the morphologically left atrium to the morphologically left ventricle. This can occur with usual atrial arrangement or with situs inversus.
2. In discordant atrioventricular connection the morphologically right atrium is connected to the morphologically left ventricle and the morphologically left atrium to the morphologically right ventricle. This also can occur with either usual or inverted atrial arrangement.
3. Ambiguous connection relates to the atrioventricular connection in atrial isomerism, whether left or right. Clearly, one atrium is going to be connected to the right ventricle and one to the left, but since both atria are of the same morphology, the connection cannot be described as concordant or discordant and must, therefore, remain ambiguous. This can occur where the ventricular topology is either right-handed or left-handed.
4. Both atria may connect to a single ventricle – so-called double inlet ventricle, in which case the other ventricle is usually small and connected to the dominant ventricle via a VSD. The atrial arrangement may be usual, inverted or isomeric (right or left).
5. There may be absence of either right or left atrioventricular connection. A further layer of complexity is added by the fact that the valves may override or straddle the interventricular septum.
2.3.5 Ventriculoarterial Connections
1. With concordant ventriculoarterial connections, the morphologically right ventricle is connected to the pulmonary trunk and the morphologically left ventricle is connected to the aorta.
2. In discordant ventriculoarterial connections, the aorta arises from the right ventricle and the pulmonary trunk from the left ventricle.
3. There may be double outlet from one ventricle, both great arteries arising from that chamber with absent arterial outlet from the remaining chamber.
4. Finally, there may be a single outlet from the heart. This group encompasses not only a common arterial trunk (truncus arteriosus), but also either pulmonary artery or aorta when the other artery is atretic and cannot be identified to connect to the heart.
2.4 Simulated Echocardiographic Views
In clinical practice it is now unusual to receive a heart that has not undergone some form of imaging, such that the cardiac defect is already known or suspected. It is, therefore, worth considering cutting the heart in one of the standard echocardiographic planes following fixation, better to demonstrate the particular abnormality. Attempting to do so in the unfixed heart is not recommended. The standard views that are easy to obtain are the four-chamber view, the long-axis view and the short-axis view. The four-chamber view is easy to obtain, cutting as it does from base to apex through the central parts of both atrioventricular valves. It is best to make an incision in the roof of the atria first. A pair of forceps can then be introduced through each of the atrioventricular junctions to the apex of the heart, and these are then used as guides for the knife cut (Figure 2.11). The four-chamber view is especially useful in displaying variations in ventricular or atrial size (Figure 2.12). It is my preferred cut of the heart in cardiomyopathy where it demonstrates relative chamber dimensions.
(A,B) The formalin-fixed heart (in this case an explanted heart with dilated cardiomyopathy) is stood on its apex. A pair of forceps is placed through each atrioventricular valve with their tips at the ventricular apices. These forceps serve as a guide for the knife. A long-bladed, sharp knife is then placed between the blades of both pair of forceps and the heart is cut from base to apex.
(C) The resultant specimen is cut through the centre of each of the atrioventricular valves. It displays the relative sizes of the atria and ventricles.
(A) Dilated cardiomyopathy. The dilated left atrium and ventricle are readily appreciated, together with the endocardial fibrosis in both.
(B) Hypertrophic cardiomyopathy. The gross thickening of the left ventricular wall and interventricular septum is evident. The lead of an implantable cardioverter defibrillator is visible in the right atrium.
(C) Restrictive cardiomyopathy. This specimen is viewed from behind, so the right ventricle is on the right of the picture. The marked atrial dilatation characteristic of restrictive cardiomyopathy is present.
Figure 2.12 Examples of hearts cut in a simulated four-chamber view.
The simulated long-axis view takes a bit more practice to obtain but essentially runs through the central parts of the mitral and aortic valves. It cuts through the right ventricular outflow tract, which remains as a muscular oval. Again, a cut is made first in the left atrium and in the aorta, and forceps are introduced (with care) through the aortic valve, and separately through the mitral valve, which are then used as guides for the knife cut (Figure 2.13). The long-axis view is particularly suited to demonstrating left-sided structures and is useful, for example, in aortic stenosis or any other form of left ventricular outflow obstruction (Figure 2.14).
(A) The formalin-fixed heart is stood on its apex. The aorta has been transected in its ascending part and the roof of the left atrium has been incised. Forceps are introduced through the aortic valve and through the mitral valves to the left ventricular apex and their blades are aligned.
(B) A sharp long-bladed knife is introduced between the blades of the forceps, and the heart is cut from base to apex, using the forceps as guides.
