This article deals with a detailed analysis of the dissection of the heart at autopsy. Since most of causes of death are cardiac it is essential that all pathologists are familiar with the approach to dissection of the heart, taking of blocks for histology and possible analysis of the conduction tissue.
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Most deaths are due to coronary artery disease and knowledge of the heart is essential to all pathologists who do autopsy work. In the UK and the USA, most autopsies are performed at the request of the coroner or medical examiner.1 Twenty-three per cent of all deaths in England have an autopsy which is 124 000 per year.2 The rate is much lower in Scotland and Northern Ireland due in part to acceptance of only external examination of the deceased combined with a clinical history.3 In Europe, the autopsy rate is lower still being <5% of all deaths.4 The majority are carried out by a local pathologist working in a general hospital or a forensic pathologist. There is marked variation in the quality of autopsies and in particular the way the cardiovascular system is examined.2 This is despite published guidelines for pathologists investigating sudden death.5 6
Diseases of the brain and heart account for the majority of deaths in developed countries, so a study of the excised heart recorded photographically, measured carefully and studied histologically is essential in most sudden deaths. Exact measurements are needed to confirm cardiac hypertrophy or dilatation, critical valve stenosis, or the outcome from previous surgery. Retention of tissues and organs from a coronial autopsy without relatives' consent is permissible under Coroner's Rule 9 to confirm the cause of death. In the USA, the medical examiner has complete discretion over tissue retention. It is essential that the autopsy is carried out by a skilled pathologist who will carry out a detailed examination of the heart to include taking of histological sections. If the death is sudden and the pathologist finds no cause of death or considers a cardiac cause to be the main finding, he/she may refer the heart to a specialist cardiac pathologist with taking of ultrastructural and frozen material for microbiology and genetic studies. Obvious coronary artery cases need not be referred but where there is the possibility of cardiomyopathy, referral is essential since many pathologists are not familiar with the whole spectrum of cardiomyopathies, particularly arrhythmogenic right ventricular cardiomyopathy'. Forethought on the part of the pathologist is essential in the approach to the cardiac autopsy. Pathologists must prepare in advance especially in cases of young sudden deaths and be proactive in retaining heart tissue in order to provide as accurate a cause of death as possible. Pathologists must communicate via the coroner's officer with the family before undertaking the postmortem to prepare the relatives for the retention of the heart and other tissues. The help of a well-trained considerate coroner's officer is essential in these situations.
All autopsy practitioners should be able to perform a basic examination of the heart and its connecting vasculature, which is akin to the minimum dataset for a cancer report. Minimal information with limited formulaic descriptions of the heart with no measurements is to be avoided but is still all too frequent even in the 21st century. There is a balance to be derived between the majority of autopsy cardiac cases recognised to be routine and those requiring greater detailed descriptions. Most cases of ischaemic heart disease can be reported without recording every detail of the dissection, but a detailed description of the coronary anatomy and ischaemic damage within the myocardium is essential.
I have set out below my approach to the examination of the heart showing how a thorough examination can be done by taking 10 blocks of tissue to include the atria, several sites of both ventricles as well as coronary artery and aorta and the conduction system if indicated. This number of blocks is required in order to get several samples particularly of the ventricles where there can be variation in pathological findings. I must emphasis this is for cases where there is a cardiac cause of death suspected. This is a straightforward plan which I believe can be followed in all cardiac autopsies, particularly where the person has died without cardiac interventions. I call it the Sheppard slicing technique. Obviously if a non-cardiac cause of death is found, this number of blocks is not required and only a sample from the right and left ventricle (in one block) needs to be taken in these circumstances. In postoperative cases, other blocks may be required depending on the procedure done and the autopsy findings.
The pathologist must approach the heart armed with prior information about the patient's background and circumstances of death. Information from the general practitioner, family and witnesses is obtained usually from coroner's office or medical examiner's office. Communication with relevant cardiac centres and access to clinical records may also be important when the patient has had previous interventions or surgery. In postprocedure/postoperative cases, details of the procedures performed and cardiac examination will be guided more specifically by the procedures and their complications, so the technique used to examine the heart will be at the discretion of the individual pathologist.
Consideration of family consent is essential before the autopsy and critical if considering retaining the heart and other tissues for research and teaching. Under UK coronial rules, pathologists can retain tissue for diagnostic purposes but it has to be disposed once the coroner has finished with the case. Consent from the next of kin has to be obtained if the pathologist needs retention beyond this under the human tissue authority rules.7 The taking of tissue for genetic analysis is now allowed under coronial rules with approval of the Human Tissue Authority for 3 months after the coroner has finished his investigation. Consent for specific genetic analysis has to be obtained from the next of kin or else the tissue must be disposed of. Specialist investigation including culture/transport media for electron microscopy, microbiology and DNA extraction should be taken into account prior to the commencement of the dissection in order to optimise sampling. Pathologists argue that these facilities are not available in many public mortuaries where the bulk of autopsies are carried out. I believe a pathologist approaching a postmortem in circumstances where the dead person has previously had no medical history is failing in their duty if they do not approach the case as paediatric pathologists approach a sudden infant death where there are established protocols to be followed. Just as paediatric cases are referred to regional centres, cardiac cases should be referred to specific regional/national centres for specialist examination.8
A digital image of mid-low ventricular transverse sections and other views of the heart are very helpful as a permanent record and for referral when the heart cannot be retained. In sudden cardiac death, retention of the heart and specialist referral should be regarded as the ‘gold standard’, with many cardiac pathologists being prepared to examine, block and turn around cases within a few days. Families can be reassured that the bulk (usually >90% of the cardiac tissues) can be reunited with the body in such circumstances.
The pericardium forms a tough fibrous sac with an outer thick parietal layer and an inner transparent serosal layer firmly adherent to the heart forming the visceral layer. A thin film of fluid lies between the two surfaces with 20–50 ml of clear straw-coloured fluid being normal. After removal of the sternum, it is extremely important to examine the pericardium in the intact state to assess for tamponade. The pericardium will be distended and full to touch if tamponade is present (figure 1). A longitudinal cut is made through the anterior aspect of the pericardial sac and the amount of blood, collected by suction into a container, should be in the region of 500–1000 ml. If the blood has clotted it should be weighed. The mere presence of blood in the pericardium does not indicate tamponade. The blood must distend the sac. There is always blood present after open heart surgery but the pericardium is usually left open (figure 2). If the blood accumulates rapidly it will usually be 300–500 ml while if the accumulation is slower as with serous effusions it can amount to 1000 ml. If indicated, a pericardial fluid sample is taken by needling through an area of pericardium which has been seared for sterilisation. The surface of the visceral as well as parietal pericardium is examined for pus, exudates, adhesions, tumour nodules or dense fibrosis associated with constrictive pericarditis which can follow infections such as tuberculosis or previous cardiac operations or may be linked to collagen vascular disease or often is idiopathic (figure 3). Usually the samples show dense fibrosis and non-specific chronic inflammation. A short longitudinal incision 2 cm above the pulmonary valve will enable a check for thromboemboli in the main pulmonary trunk and two main branches in situ. Needle the right atrium after searing at its junction with the inferior vena cava to obtain a sample of heart blood for culture if sepsis is suspected. Also check the ascending aorta for a thickened wall in aortitis, entry tear in dissection especially in the first 3 cm as well as for dilatation/aneurysm formation and intramural haematomas.
Removal of the heart
The heart is removed by first cutting the great vessels, the aorta and the pulmonary trunk, transversely 2 cm above the semilunar valves. Always be mindful of antemortem thrombi within the main pulmonary artery. If aortic dissection is suspected, leave the aorta intact and dissect it out complete with abdominal aorta down to the iliofemoral junction. Then, cut the inferior vena cava just above the diaphragm and lift the heart by the apex, reflecting it anteriorly and upwards to facilitate exposure of the pulmonary veins at their pericardial reflection. After confirming that the left and right pulmonary veins enter normally into the left atrium, the pulmonary veins are cut. Then the superior vena cava is opened to check for thrombosis or occlusion and opened up into the left and right brachiocephalic veins before being cut across. Following removal of the heart from the pericardial cavity and before weighing the specimen, postmortem blood clot should be removed manually. If one sees aneurysmal dilatation of the right ventricular wall, be aware of the possibility of pulmonary hypertension or right ventricular cardiomyopathy. Left ventricular hypertrophy may be obvious externally. Aneurysm formation in the left ventricle is usually associated with a previous infarct. Right ventricular hypertrophy calls to mind pulmonary disease and look carefully in the pulmonary arteries for atheroma as indicating pulmonary hypertension. Flexibility is called for when dissecting the heart since each disease process requires a different approach. When one notes left ventricular hypertrophy, check for a history of hypertension, check for coarctation of the aorta and check the aortic valve carefully.
The epicardial surface of the heart normally contains fat which varies with the nutrition of the person and increases with age. Normally it fills the atrioventricular groove and extends with the coronary arteries along the anterior and posterior interventricular sulci towards the apex (figure 4). When a patient is obese it may completely envelope the epicardial surface of all chambers especially along the course of the coronary blood vessels. Fat also spreads into the myocardium along the intramyocardial vessels, particularly in the right ventricle and the interatrial septum. The right ventricular wall is the thinnest and contains most fat where it meets the interventricular septum, particularly in elderly patients. It is this area which is most likely to be ruptured during catheterisation procedures. Epicardial fat ranging from 1 to 10 mm usually involves the anterior wall and lateral wall of the right ventricle but rarely the posterior wall. The wall needs to be completely replaced by fat, dilated and attenuated with scarring to consider right ventricular cardiomyopathy, usually in younger patients who do not have coronary artery disease. Fat may also be seen partly replacing the left ventricular wall, particularly on the epicardial surface in this condition. Fat never infiltrates or replaces the full thickness left ventricular wall in the normal ageing heart. Discolouration and haemorrhage on the surface will point to acute infarction or possible rupture. Evidence of rupture may be subtle with a small area of haemorrhage into fat on the surface and not a gaping hole. Careful probing in this area is required. Transverse sections through the myocardium in this area may well show the tract which often is irregular and snakes through the wall from the epicardium to the subendocardium. White patches of epicardium (soldier's patches) are common, especially over the anterior surface of the right ventricle and the apex of the left ventricle. These have been attributed to mechanical trauma or to healed pericarditis, but there is no definite histological evidence for either of these processes and they are frequently found at autopsy. Histologically, there is non-specific fibrosis with a few lymphocytes noted. Remember also that small haemorrhages will be seen on the epicardial surface of the pericardium with resuscitation particularly along the coronary arteries. Swelling around the root of the aorta may point to a sinus of Valsalva aneurysm, a root abscess in aortic valve endocarditis, or an aneurysm of the proximal aorta/coronary arteries. At this stage in the fresh heart, a detailed examination of the coronary arteries is undertaken in routine autopsies.
Examination of coronary vessels
A cut across the aorta above the cusps of the aortic valve will usually expose the valve architecture in order to look for abnormalities, and also to show the ostia of the coronary arteries. Always check the origin of each artery within the respective sinus. While the majority arise within the sinus, there is great variation in the exact location as we have shown in normal individuals with the ostia being at, above and below the sinotubular junction. The majority are below the junction, but do not exceed 2 mm below it.9 Usually there is only one ostium on the left, but in 1% of hearts an additional ostium for the circumflex can be identified. In the right coronary sinus, multiple ostia are common (74% of cases in our study) and give rise to branches supplying the right ventricle with a branch to the conus or infundibulum being particularly frequent. The shape of the ostium can be round, elliptical or crescentric. A probe 2 mm in diameter will easily pass into both vessels in an average adult. With a crescentric shape especially at the origin of the right coronary artery, there is a small intramural course of the vessel usually only a millimetre in length. It is essential to look at the ostia and probe the origins of both coronary arteries in the coronary sinuses. Thus anomalous origin of the coronary arteries will not be missed. Do not probe beyond the first 2–3 mm into vessels in order not to dislodge thrombi in the proximal vessels or miss coronary artery dissection.
In routine autopsies, the pathologist will simply cut across the main vessels at 3 mm intervals in the fresh heart and assess the presence of dilatation, atherosclerotic plaque formation, thrombosis, stenosis or dissection (figure 4). It is generally agreed that cutting the vessels longitudinally can destroy thrombi/emboli and make the estimation of stenosis impossible. The vessels that are usually examined in all hearts include the four major epicardial coronary arteries, the left main, the left anterior descending, the left circumflex and the right coronary arteries. However, attention must also be directed to smaller branches, such as the left diagonals, the left obtuse marginals, the intermediate and the posterior descending coronary arteries. If the coronary arteries are heavily calcified, they will be difficult to cut across. Calcification bears no relation to the severity of coronary artery disease and is very common in the elderly. Crushing of the vessel wall during the struggle to cut the vessel will exaggerate stenosis. If in doubt or impossible to cut, take off the vessel, trim off the adherent fat and decalcify in decalcifying agent for 24 h after which it can easily be cut transversely. The areas of maximal narrowing are noted by specifying the degrees of reduction of the cross-sectional area of the lumen (eg, 0–25%, 26–50%, 51–75%, 76–90% and 100%). Most pathologists agree that, in the absence of other cardiac disease, significant coronary artery narrowing is that exceeding 75%. Remember that coronary arteries collapse after removal and will give an erroneous impression of narrowing. When one is faced with this problem, probing of the vessels with a 2 mm probe will open up the vessel to its rounded configuration as during life and avoid overcalling even when an eccentric atheromatous plaque is present (figure 5A,B).
Although the left coronary artery always supplies a greater mass of muscle than does the right, it is not usually dominant. Left dominance (posterior descending branch is the continuation of the circumflex) is found in only 15% of people. The right coronary artery lies deep within the fat of the right atrioventricular groove and will not be visible on external examination especially in older, more obese subjects. Therefore, deep transverse cuts are necessary to find it. Tracing it below the right atrial appendage is useful. Trace it carefully from its origin to the posterior descending vessel. It is usually large in right dominance but smaller in left dominance.
The left coronary artery originates in the left (anterolateral) aortic sinus and passes undivided for up to 2.5 cm as the left main coronary artery between the aorta and the left atrial appendage. With the vessel wedged between the aorta, pulmonary artery and the left atrial appendage, it may be difficult to visualise deep within fat. I usually identify the left anterior descending vessel in the anterior interventricular groove first close to the apex and work my way back to the dividing vessels and then to the left main stem. The left main stem generally bifurcates into anterior descending and circumflex branches 10–15 mm beyond the ostium. In about a third of individuals, it trifurcates. The branch between the anterior descending and circumflex branches is called the intermediate branch. The anterior descending branch passes in the anterior interventricular sulcus towards the apex. During its course, it gives a variable number of branches (diagonal branches) to the left ventricle. These, together with their parent branch, are important for arterial and vein grafting. The first diagonal branch is a major vessel which originates in the proximal third of the anterior descending branch. It may reach the apex of the heart, and is often submerged in muscle for part of its length. When the left coronary artery trifurcates, this first diagonal branch is replaced by the intermediate branch. In addition to the diagonal branches passing to the left ventricle, there are smaller branches passing to the right ventricle. These infundibular branches to the outflow component of the right ventricle often anastomose with branches from the right coronary artery. In addition to the diagonal and right ventricular sprigs, the anterior descending branch of the left coronary artery also gives a number of branches passing from its underside (epicardial aspect), vertically downwards into the anterior ventricular septum. These important arterial branches are the septal branches, sometimes also called the septal perforators. They are variable in number and site of origin, except for the first branch. The ‘first septal’ artery is a relatively large branch (1–2 mm in diameter) which takes origin from the anterior descending branch close to the origin of the first diagonal branch. This branch can become greatly enlarged in coronary artery disease. It is also the branch which is selectively occluded by alcohol injection to induce infarction in the upper septum in left ventricular outflow obstruction associated with hypertrophic cardiomyopathy. The anterior descending coronary artery becomes smaller as it descends down in the interventricular groove and identification of it may be impossible as the vessel mingles with the diagonal branches in the lower half of the groove. Fortunately, most pathology is in the proximal part of the vessel. Pathologists may also dissect from the aorta out between it and the left atrial appendage to trace distally the left main stem and left anterior descending coronary artery where it divides and extends down on the anterior interventricular groove.
The course of the circumflex artery is more variable than the other coronary arteries. In some hearts, the circumflex vessel terminates almost immediately and often gives off the atrial circumflex artery which runs in the atrial myocardium around the mitral orifice. More usually, the circumflex artery continues to the obtuse margin of the left ventricle and breaks up into the obtuse marginal arteries which are often embedded within the muscle of the left ventricle. The artery runs deep in the fat of the left atrioventricular groove underneath the left atrial appendage and identification is aided by tracing it back from the anterior descending coronary artery and following it from the bifurcation under the left atrial appendage. The circumflex is not a site for coronary artery bypass surgery because of its position deep in fat, similar to the right coronary artery, and it is the obtuse marginal branches that are the sites for vein grafting. In 70% of people, the circumflex branch terminates as an obtuse marginal branch at or near the obtuse margin of the heart. In a small proportion of hearts, the circumflex artery continues all the way around the mitral orifice leading into the posterior descending coronary artery. Rarely both the right and circumflex arteries may supply the diaphragmatic surface without there being a prominent posterior interventricular artery. This arrangement is termed a balanced circulation.
The coronary veins run with the major arteries and return the blood to the coronary sinus. These veins drain into the posteroinferior right atrium above the tricuspid valve (figure 6). The veins form wide thin channels in both the interventricular and the atrioventricular grooves. A large vein called the great cardiac vein is formed in the anterior interventricular groove. It runs around the mitral orifice and expands to form the body of the coronary sinus in the left atrioventricular groove. When the circumflex coronary artery is small, the larger wider vein can sometimes be mistaken for the circumflex coronary artery in the left atrioventricular groove. The coronary sinus receives the blood from the middle cardiac vein, which runs up the posterior interventricular groove and also from the small cardiac vein which accompanies the right acute marginal artery and then runs round the orifice of the tricuspid valve in the right atrioventricular groove before terminating in the coronary sinus at the crux (where the right coronary artery turns downwards to become the posterior descending coronary artery). It is important to examine these veins when retrograde cardioplegia (perfusion of veins) is used in cardiac operations to look for complications such as rupture or thrombosis. The coronary sinus and veins are closely associated with the mitral annulus and are increasingly used for catheter ablation studies and cardiac pacing with electrophysiological studies.10
Dissection of the heart
Dissection methods are learnt by personal experience and vary with the individual pathologist. Many people still use the flow of blood method but I prefer serial sections along the short axis of both ventricles up to the tip of the papillary muscles. This reveals abnormalities of the chambers such as acute infarction, hypertrophy, scarring, thinning, fatty replacement, nodules, pericardial or endocardial thickening as well as papillary muscle abnormalities (figures 4 and 7). Slicing the ventricles in this bread-loaf manner correlates also well with cardiac imaging during life with ECHO and MRI. The cuts are made parallel to the atrioventricular groove at 1–1.5 cm intervals from the apex of the heart to a point approximately 2 cm caudal to the atrioventricular groove with the atrioventricular valve apparatus left intact in the remainder of the specimen. At mid-ventricular level halfway between the atrioventricular groove and the apex, I measure the thickness of the anterior, lateral and posterior right ventricular wall, interventricular septum, anterior, lateral and posterior left ventricular wall excluding trabeculae and papillary muscles. Right ventricle measures 3–5 mm in thickness while the left ventricle varies from 10 to 15 mm. In addition, for assessing cardiac hypertrophy/dilatation, the transverse chamber diameter for the right and left ventricles is also done to include trabeculae. Both should measure 30–35 mm in transverse diameter. Beware of overinterpretation of wall thickening with reduction in chamber diameter which occurs with death in systole. The heart weight will be normal unlike true hypertrophy where the weight is increased. Also be aware that the left ventricle wall also thickens below the mid-ventricular level and so avoid lower cuts. The left ventricular wall thins at the apex also. Location of pathology such as hypertrophy, thinning and scarring may be stated using terms relating to the standard anatomic frame of reference (eg, anteroseptal, posterolateral, basal). The extent of disease should be described in terms of circumference of the ventricle and location in the longitudinal portion of the ventricle involved (eg, basal third, middle third and apical third). The distribution within the wall is also described (eg, transmural or subendocardial). One must also look carefully at the right ventricle to check for focal involvement with complete replacement of the outflow tract, anterior, lateral and basal wall beneath the tricuspid leaflet by fat when one is looking for evidence of arrhythmogenic right ventricular cardiomyopathy. Be aware that there is usually 3–5 mm of epicardial fat covering the anterior and lateral right ventricle (figure 7) but not usually the posterior wall of the right ventricle. This can be 10–20 mm in thickness in obese subjects.11
When examining the right side of the heart, you must ask first before going any further do you need to take the conduction system. If there is a history of heart block then it is important but for other arrhythmias such as atrial fibrillation and supraventricular tachycardia with re-entry pathways, it is impossible to locate the source of the arrhythmias unless one sections the whole atrioventricular junction which is impossible in routine cases. Electrophysiological studies in living patients are more informative.
Before removing the conduction tissues, look from below at both atrioventricular valves to check that they are normal. Check the papillary muscles and attached chordae. The right atrium is opened from the inferior vena cava to the tip of the right atrial appendage. This gives good exposure to the right atrium, right atrial appendage, coronary sinus and interatrial septum, which is mainly fossa ovalis, and inspect the intact tricuspid valve from above. This incision is preferable to one that joins the superior and inferior vena cavae, which will often destroy the sinoatrial (SA) node which lies just to the right of the entry of the superior vena cava into the atrium at the top of the atrial crest. The right atrium contains the fossa ovalis and the mouth of the coronary sinus, two important landmarks. Look for Chiari network which is the enlarged and possible fenestrated valve of the inferior vena cava which is of no significance generally. Look into the fossa ovalis for atrial septal defects at this stage. The fossa membrane can be probe patent usually at the anterior rim in up to 5% of people but this closes against the atrial septum from the left side and is usually of no significance. Note the mouth of the coronary sinus which will be prominent in cardiac failure. It is useful to palpate the right ventricle at this stage and measure the diameter of the tricuspid valve which usually admits middle three fingers in an adult and is usually 110–130 mm in the average adult. If there is tricuspid valve pathology, keep the valve intact and look at the right ventricle below the valve. If vegetations are seen they should be sampled in a sterile manner for culture. Once the valve is determined to be normal then make a lateral right incision down to the short axis cut already made. Then inspect the right atrium and right ventricle as well as the tricuspid valve which you note has three ill-defined leaflets, anterior, septal and posterior with poorly developed papillary muscles, particularly the septal and posterior muscles while the anterior is usually more developed (figure 6). Pathology of the tricuspid valve is rare but slight floppy change is common in cardiac failure. Endocarditis is looked for carefully if intravenous drug abuse is recorded. Pacemakers are usually inserted into the right atrium and right ventricle through the tricuspid valve. The pacemaker wire can be attached to the tricuspid leaflets but rarely causes problems with thrombosis, infection or fibrosis. The apex of the leads is embedded in the wall of the right atrium and right ventricle in dual pacing systems. Rupture due to lead insertion is extremely rare.
Alternatively, you may also, following the short axis transverse cuts, open up the right atrium and right ventricle by incising the posterior wall of the right atrium into the right ventricle just parallel to the septum. I prefer the lateral cut since I can then take a block to include the right atrium, right coronary artery, tricuspid valve and right ventricle.
The muscular shelf which separates the tricuspid and pulmonary valves in the roof of the ventricle is called the supraventricular crest. Because the pulmonary outflow tract is surrounded by a ring of muscle before merging into the pulmonary valve, the pulmonary valve can be removed and used as an autograft to be inserted into the aortic position, while a homograft replaces the pulmonary valve (Ross procedure). The moderator band in the right ventricle is also a unique structure carrying the right bundle branches from the interventricular septum to the free wall of the right ventricle.
Examination of the right ventricular outflow tract (RVOT) involves cutting up and looking into the main pulmonary artery. Insert two fingers to go up through the pulmonary valve out into main pulmonary artery. Then cut through the anterior right ventricular wall at this point to lay open the pulmonary valve and main pulmonary artery. The circumferences of the pulmonary valve should be 50–60 mm in the normal adult. You will already have checked for pulmonary emboli in the main pulmonary artery on cutting across it at initial dissection of the heart. But one should also look in the RVOT to check for impacted emboli. The pulmonary valve is similar in structure to the aortic valve. It has three semilunar shaped cusps separated by three commissures. They are the right, left and posterior leaflets. Each cusp is thinner and more transparent than the aortic valve, but has basically the same structure. Always check for anomalous origin of a coronary artery from the pulmonary sinuses; usually it is the left coronary artery which will be hugely dilated.
Check the thickness of the RVOT. One will already have measured the thickness of the right ventricle anterior, lateral and posterior wall at mid-ventricular level as well as the diameter of the chamber which will include the papillary muscles and trabeculae. One should look carefully for transmural fatty infiltration in the anterior wall of the RVOT since this is the site of earliest localised change in arrhythmogenic right ventricular cardiomyopathy. If right ventricular hypertrophy is noted always look carefully in the lungs for chronic obstructive airways disease and pulmonary hypertension.
On the left side, the left atrium is opened by cutting across the roof of the atrium between the left and right pulmonary veins with extension to the left atrial appendage to look for thrombi. The wall of the left atrium is smooth with irregular ridges at the interatrial wall side often near the fossa ovalis which are a normal finding (figure 8). The left atrial appendage should be examined in all cases with atrial dilatation, history of atrial fibrillation or where there is a stenotic or floppy mitral valve. Thrombi should be looked for carefully within the blind ending recesses within the appendage. Look down and check the intact mitral leaflet which should admit two fingers. The left ventricle is opened laterally between the anterior and posterior papillary muscles down as far as the transverse cuts already done. Alternatively you can cut close to the posterior interventricular septum as in the right ventricle. The circumference of the mitral valve is 70–90 mm in the average adult. It is possible to lift the anterior leaflet of the MV at this point and inspect the aortic valve. The aortic valve should admit two fingers and if it is normal then one can dissect out into the outflow tract. Cut up through the anterior leaflet of the MV up into the outflow tract and ascending aorta. Alternatively, if you want to preserve the anterior leaflet of the mitral valve, make an anterior incision into the wall of the LV to run parallel to the left anterior descending and the interventricular septum and cut between the left atrial appendage and pulmonary artery, through the main stem of the left coronary artery and root of the aorta and putting the lower jaw of the scissors into the aortic outflow tract. This will open into the left ventricle outflow tract and aorta keeping the anterior leaflet of the mitral valve intact (figure 9). The circumference of the aortic valve at the sinotubular junction is usually <60 mm. One can see both ostia of the coronary arteries in their sinuses and the non-coronary sinus. Photography of the chambers and valves is done as one progresses through the dissection. Ventricular septal defects must be looked for as they mainly occur around the area of the membranous septum, which lies in the triangle between the right coronary sinus and non-coronary sinus. Once the heart has been fully opened in this fashion it is usual to weigh the heart, with subsequent cross-comparison against standard charts for body mass and sex. All excess tissue such as attached lung, excess aorta and pulmonary artery should be removed before weighing the heart. Fixation may increase heart weight by up to 5%. With regard to dissected ventricle weights (Fulton method), I never do this unless there is a medico-legal issue about hypertrophy. The original description of individual ventricular weights was on fixed tissue with all the fat stripped from the surface myocardium which is extremely tedious and impractical for most pathologists.
The mitral valve has two leaflets of markedly dissimilar shape and circumferential length (figure 8). The most anteriorly located leaflet is square in shape and takes up only one-third of the circumference. Its most characteristic feature is its fibrous continuity with the leaflets of the aortic valve (figure 9). The posterior leaflet is long and thin, making up two-thirds of the annular circumference. It is attached throughout its length to the diaphragmatic wall of the ventricle. The commissure between the leaflets is orientated in a posteromedial and anterolateral position. Its two ends are traditionally described as separate commissures, which are supported by prominent anterior and posterior papillary muscles which have their origins very close together. A characteristic feature is that the mitral valve never possesses chordal attachments to the septum in contrast to the tricuspid valve. The apex of the left ventricle has fine trabeculae and the septal surface is smooth. The left ventricular wall thickness is best measured on mid-ventricular slice as already described. Around 12–15 mm thickness is normal and should not include trabeculae. The outlet from the left ventricle leads to the aortic valve. There is no muscular ventricular-infundibular fold in the left ventricle which separates the mitral and aortic valve so that they are in direct continuity and disease of one can easily spread to the other (figure 9).
Examination of the heart valves
Both atrioventricular valves are inspected from above and below in order to assess the degree of stenosis or floppy change that may be present, as well as the possibility of vegetations and perforations with infection. Inspect closely the valves and record ring circumferences. Annular dilatation, leaflet fibrosis, distortion, calcification or other pathology is noted. If chordal or papillary muscle rupture is suspected which is usually seen in the mitral valve, first cut transversely at the apex and locate the origin of both papillary muscles, then cut upwards on the obtuse margin between the papillary muscles in order to expose the whole tensor apparatus and assess for chordal rupture. If a valve abnormality requires closer inspection, the atria, including interatrial septum, may be removed 1 cm above the atrioventricular valves. The ventricular aspects of the atrioventricular valves can be viewed following removal of the serial slices of ventricle as described above.
The semilunar valves are best studied after removal of the aorta and main pulmonary artery above the coronary ostia or valve annulus. Measurements of the circumference of valves are not useful in valve stenosis, but can be useful for incompetence. For histology, the leaflets are sectioned together with a portion of the adjacent atrium and ventricle/or vessel walls. The posterior leaflet of the tricuspid and portion of right atrium, right coronary artery and lateral right ventricle are included in the lateral cut described above (figures 4 and 15). The posterior mitral valve leaflet is sectioned including a portion of left atrium, left atrioventricular groove, great vein with circumflex coronary artery and left ventricular free wall in that lateral incision as described above (figures 8 and 15). Histological sections of aortic and pulmonary valves should include artery above and ventricular muscle below.
In aortic stenosis, there is usually extensive nodular calcification of all three leaflets with nodules obvious on the aortic side of the valves. The leaflets will be rigid and not allow one finger insertion. Remember that aortic valve leaflets thicken with age and become stiffer (aortic sclerosis) but will still be freely mobile.
In floppy mitral valve there is usually a mild degree of ballooning between chordae in patients over the age of 40 but for significant floppy change, note ballooning above the atrioventricular junction on opening up the left atrium and inspecting the valve from above. One can also fill the intact left ventricle with water and note the closing of the valve to look for ballooning also. The normal valve will oppose both leaflets as a smooth floor with no ballooning into the left atrium.
After examination of the heart, the blood clot is removed and the heart should be weighed, usually 200–500 g in male subjects and 150–400 g in female subjects. Exclude ascending aorta and pulmonary artery attachments as well as fibrous pericardium and hila of the lungs with attached fat or lymph nodes.
The conduction tissues
Where conduction disturbances are suspected clinically such as heart block, histological examination of the cardiac conduction tissues can be rewarding. Many pathologists are intimidated by the prospect of doing conduction system studies because the conduction tissues cannot be visualised grossly. Yet, with practice and careful attention to anatomical landmarks, these structures can be dissected and removed for histological examination.
The SA node is invisible. It is a saddle shaped structure lying immediately subepicardially within the terminal groove at the junction of the superior vena cava and the right atrial appendage. Because the sinus node is not visible grossly, the entire block of tissue from the area where the superior caval vein meets the right atrial appendage should be taken as a rectangular longitudinal section to include the proximal superior vena cava and atrial appendage wall (figure 10). The block is divided into two longitudinally and embedded face down where divided. Sections are cut parallel to the long axis of the superior vena cava. Microscopically the node consists of relatively small diameter, haphazardly orientated atrial muscle cells admixed with connective tissue around a nodal artery (figure 11). The myocytes are altered electrically to conduct electrical signals and look paler than the surrounding myocytes. Collagen increases with age so that the myocytes become smaller and the SA node is rich in fibrous tissue in all cases over age of 50. Thus, scarring or fibrosis of the SA node is impossible to tell in older patients and histology in the sick sinus syndrome which is usually due to ischaemic heart disease will not reveal any abnormality. There is also no anatomical evidence for the existence of specialised pathways between the sinoatrial and atrioventricular node but electrophysiologically, fast and slow fibre pathways are identified.
Atrioventricular conduction tissue
The atrioventricular node extends from the subendocardium of the right atrium, penetrates the atrioventricular membranous septum and divides on the crest of the muscular septum to extend onto the subendocardial surface of the interventricular septum of the left ventricle. It thus has a complex trajectory from the right side of the heart to the left as an isolated cluster of altered myocytes similar to those in the SA node. The atrial component of the atrioventricular axis is contained within the triangle of Koch which lies between the mouth of the coronary sinus and the septal leaflet of the tricuspid valve including the membranous septum (figure 6). The atrioventricular node of the conduction system is a microscopic structure lying in the subendocardium within this triangle of Koch which is defined at its base by the attachment of the septal leaflet of the tricuspid valve, posteriorly by an imaginary line from the posterior mouth of the coronary sinus down to the septal leaflet and anteriorly by line from coronary sinus down to the membranous septum with overlying part of septal leaflet and underlying septal myocardium. The atrioventricular nodal tissue passes through the atrioventricular membranous septum at the apex of the triangle of Koch, as the penetrating atrioventricular bundle (of His). The apex of the triangle anteriorly is the membranous septum and denotes the point at which the bundle of His penetrates the membranous septum to reach the left ventricle. It then emerges in the subaortic outflow tract beneath the commissure between the non-coronary and right coronary leaflets of the aortic valve (figure 9). The axis branches immediately in the normal heart, usually on the crest of the muscular septum but sometimes to its left side. The left bundle branch then fans out on the smooth aspect of the septum in a continuous cascade, splitting into three divisions: anterior, septal and posterior towards the ventricular apex (figure 9). The right bundle branch turns back through the interventricular septum as a cord-like structure before crossing in the moderator band and ramifying into the right ventricular myocardium.
I prefer to take the atrioventricular node from the right atrial aspect of the heart. I locate the triangle of Koch between the mouth of the coronary sinus and the septal leaflet of the tricuspid valve. By putting the area up to strong light coming from a window or shining a torch behind it shows up the membranous septum. The area is then flattened against the aortic outflow tract. I put a longitudinal cut from the mouth of the coronary sinus down to the interventricular septum 5 mm below the attachment of the tricuspid valve. I make a parallel longitudinal cut beyond the membranous septum from 10 mm above the septum and 10 mm below. Two transverse cuts above in the right atrium and below in the interventricular septum complete the square to be removed (figure 12). I remove 5 mm of excess right atrial and ventricular muscle so that the atrioventricular septal area and membranous septum can be laid flat with the aortic outflow tract flat to the cutting surface on the bench. The block to be excised reaches from the anterior margin of the coronary sinus to the medial papillary muscle of the right ventricle. It is often quite difficult to dissect out, because the membranous septum is thin and cutting it evenly as a thin membrane between two muscle blocks can be challenging with twisting and oblique cuts. Four even and parallel cuts are done longitudinally to include the membranous septum through which the bundle of His passes, the septal leaflet of the tricuspid valve and non-coronary leaflet of the aortic valve (figure 13). Use a very sharp scalpel with fresh blade and even parallel cuts. One can also approach the atrioventricular conduction system from the left ventricle. The left ventricular outflow tract is laid flat with removal of excess cardiac muscular tissue; the block can be cut with aorta above and interventricular septum below to include membranous septum which is in the triangle between the non-coronary sinus and right coronary sinus. Again cut a square to include this area with aorta above and interventricular septum below including the crest of the muscular interventricular septum. The block should include the non-coronary cusp of the aortic valve and the crest of the ventricular septum. In either case the block of tissue removed should be divided in the plane, from posterior to anterior, and should be marked with India ink, so that orientation can be maintained throughout the embedding process. Mark the non-cutting surface of each block, as the block is embedded, to give uniformity of face for each block.
I usually divide the block into four pieces and embed them in blocks 9 and 10 as shown below. In the adult heart the entire tissue should be ideally step-sectioned, and every 25th or 50th section stained with Masson's trichrome but this is totally impractical in routine practice and a detailed anatomical knowledge of the variation in nodal, bundle and dividing branch morphology is lacking. We usually cut one section from each block which will generally enable you to view the atrioventricular node, the penetrating bundle, the dividing bundle, and proximal right and left bundle branches. Obviously if there is a prior history of heart block, serial sectioning is required to detect a local lesion. The Masson trichrome stain is the most useful for delineating the myocardium, nodal tissues and membranous septum (figure 14A,B).
The tissue excised must include the atrioventricular septum, the membranous septum and the crest of the interventricular septum. Removal of the conduction system involves taking a large square block, so prior detailed study of the cardiac chambers and coronary arteries must be done before removal of the SA and atrioventricular nodes, which are usually the last blocks taken and are labelled as 8, 9 and 10 in my block list.
The aorta is distinguished from the pulmonary trunk by its branching pattern. It takes origin usually behind and to the right of the pulmonary trunk at the base of the heart. The origin at the ventriculoarterial junction is characterised by the three sinuses which support the semilunar attachments of the aortic valve. In the normal heart, it is the sinuses facing the pulmonary trunk that usually give rise to the coronary ostia, the so-called right and left facing sinuses. The ascending portion of the aorta in the normal heart gives rise to the brachiocephalic (innominate) artery followed by the left common carotid and the left subclavian arteries. Beyond this is the isthmus which is the junction of the aortic arch with the arterial duct (ductus arteriosus). The duct is a wide channel in the fetus and the newborn, but closes rapidly shortly after birth. It is represented subsequently by the attachment of the arterial ligament to the underside of the arch. Here one looks for coarctation which may be missed during life. The circumference of the aorta is usually less than 60 mm in adult and coarctation usually less than 40 mm in the adult. The aorta then continues as the descending thoracic aorta, which gives rise to the bronchial and intercostal arteries before piercing the diaphragm to become the abdominal aorta. Always check the ascending aorta for the entry tear of acute dissection, the ridges of chronic healed entry tears, dilatation, aneurysm formation, wall and intimal thickening in aortitis as well as atheroma.
The pulmonary trunk
The pulmonary trunk arises in front of and to the left of the aorta and has a very simple branching pattern, dividing into the left and right pulmonary arteries. The trunk in the fetus continues as the arterial duct into the descending aorta, the right and left pulmonary arteries being side branches from the flow pathway from the duct to the aorta. After birth with closure of the arterial duct, the site is marked by the arterial ligament with the recurrent laryngeal nerve passing around it. Always check the pulmonary trunk for thromboemboli and atherosclerosis pointing to pulmonary hypertension.
Taking of blocks
I usually take 10 blocks which will include (see figure 15 dissection blocks):
Block 1: two pieces of the RVOTs below the pulmonary valve as well as a lateral slice at the cut right margin to include the right atrium, right coronary artery, posterior leaflet of the tricuspid valve and lateral right ventricle as described above.
Block 2: anterior, lateral and posterior wall of the right ventricle taken at mid-ventricular level.
Blocks 3 and 4: anterior and posterior interventricular septum.
Block 5: anterior wall of the left ventricle with anterolateral papillary muscle and left anterior descending coronary artery;
Block 6: lateral cut to include left atrium, posterior mitral leaflet, circumflex coronary artery and lateral left ventricle.
Block 7: posterior wall of the left ventricle to include posteromedial papillary muscle and also a transverse piece of ascending aorta,
Blocks 8, 9 and 10: SA and atrioventricular node as described above.
I usually do a H&E stain as well as an Elastic Van Gieson on each block. A Masson trichrome is useful for delineating the conduction system in blocks, 8, 9 and 10.
Immunostaining is only required for myocarditis to see the number of T lymphocytes (CD3) and macrophages (CD68) within the section.
Other immunostains for tumours such as myxoma (calretinin) may rarely be required. There are no good immunomarkers for evidence of early irreversible infarction within the myocardium.
Stepwise plan for cardiac examination
Inspect heart in situ in the chest
Note pericardium and any fluid within
Acute accumulation: 300 ml/chronic: 1000 ml plus
Remove heart by cutting great vessels and veins
Inspect exterior of heart
Cut coronary arteries and branches transversely
Cut ventricles transversely up to tips of papillary muscles
Measure thickness of anterior, lateral and posterior right ventricle, interventricular septum, anterior, lateral and posterior left ventricle at mid-ventricular level
Cut into both atria and inspect both atrioventricular valves
Make lateral cuts between atria and ventricles including atrioventricular valves
Inspect ventricles and valves
Open up into right and left ventricular outflow tracts and inspect valves and coronary arteries.
Inspect aorta and pulmonary arteries
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Competing interests None.
Provenance and peer review Commissioned; externally peer reviewed.
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