Dissection Methods Of The Heart

Many older methods (7) are impractical for routine diagnostic pathology. Only the inflow-outflow and short axis (bread slice) methods have withstood the test of time; the latter technique is applicable to virtually any form of heart disease. In addition, some recently described methods are useful for teaching purposes and correlations with current cardiac imaging (8-14).

INFLOW-OUTFLOW METHOD OF CARDIAC DISSECTION This technique is suitable primarily for normal hearts. For each side of the heart, the atrium is opened first, and then the ventricle is opened along its inflow and outflow tracts, following the direction of blood flow (Fig. 3-1). Valves are cut between their commissures.

Using scissors, the initial cut is made from the inferior vena cava to the right atrial appendage, sparing the superior vena cava with the region of the sinus node. The right ventricular inflow tract is opened with a knife or scissors from the right atrium, through the posterior tricuspid leaflet, running parallel to and about 1 cm from the posterior ventricular septum. The outflow tract is opened in a similar fashion, approx 1 cm from

Outflow And Inflow Tracts Ventricles

Fig. 3-1. Inflow-outflow method of cardiac dissection. The method is shown in a normal heart. (A) Opened right atrium and right ventricular inflow tract. (B) Opened right ventricular outflow tract and pulmonary artery. (C) Opened left atrium and left ventricular inflow tract. (D) Opened left ventricular outflow tract and aorta.

Fig. 3-1. Inflow-outflow method of cardiac dissection. The method is shown in a normal heart. (A) Opened right atrium and right ventricular inflow tract. (B) Opened right ventricular outflow tract and pulmonary artery. (C) Opened left atrium and left ventricular inflow tract. (D) Opened left ventricular outflow tract and aorta.

the anterior ventricular septum, extending through the anterior pulmonary cusp and into the main pulmonary artery.

The left atrium is opened with scissors between the right and left upper pulmonary veins and then between the upper and lower veins on each side. The incision can be extended into the left atrial appendage to assess for mural thrombus. The left ventricular inflow tract is opened with a long knife along the inferolateral aspect through the left atrial wall near its appendage, through the midportion of the posterior mitral leaflet, between the two mitral papillary muscles, and through the apex. The outflow cut travels parallel to the anterior ventricular septum and about 1 cm from it. This curved cut is best accomplished with a scalpel; care should be taken not to cut into either the anterior mitral leaflet or the ventricular septum. Scissors can be used to extend the cut across the left aortic cusp and into the ascending aorta, to one side or the other of the left coronary ostium. Further slicing into the myocardium is not recommended.

SHORT-AXIS METHOD OF CARDIAC DISSECTION This is the method of choice not only for the evaluation of ischemic heart disease (2,5,15) but for virtually any other cardiac condition, because the slices expose the largest surface area of myocardium. They correspond to the short-axis plane produced clinically by two-dimensional echocardiography (8-14).

For this method, the flat diaphragmatic aspect of the heart is placed on a paper towel to prevent slippage, and cuts 1.0-1.5 cm thick are made with a sharp knife, parallel to the atrioventricular groove. One firm slice should be used, or two slices in the same direction, avoiding sawing motions that leave hesitation marks. Each slice is viewed from the apex toward the base (Fig. 3-2), analogous to echocardiography imaging. The basal third of the ventricles is left attached to the atria. The basal portion is then opened according to the inflow-outflow method, as described earlier.

OTHER TOMOGRAPHIC METHODS OF DISSECTION AND REPAIRING MISTAKES For teaching purposes, the

Left Ventricular Hypertrophy Dissection

Fig. 3-2. Short-axis method of cardiac dissection. (A) Normal heart, with ventricular cross-section oriented for evaluation. (B) Old transmural myocardial infarct, involving inferior wall of left ventricle, with secondary left ventricular dilation. (C) Right ventricular hypertrophy and dilation due to chronic pulmonary hypertension. (D) Complete atrioventricular septal defect, showing the common atrioventricular valve.

Fig. 3-2. Short-axis method of cardiac dissection. (A) Normal heart, with ventricular cross-section oriented for evaluation. (B) Old transmural myocardial infarct, involving inferior wall of left ventricle, with secondary left ventricular dilation. (C) Right ventricular hypertrophy and dilation due to chronic pulmonary hypertension. (D) Complete atrioventricular septal defect, showing the common atrioventricular valve.

short-axis, long-axis, and four-chamber planes are ideally suited for demonstrating cardiac pathology (10,11). Additional planes that have proven useful clinically and at autopsy include right ventricular long axis, left-sided two-chamber, right-sided two-chamber, transverse (horizontal, or foreshortened four-chamber), frontal (or coronal), lateral (or parasagittal), and others. Hearts should first be fixed in a distended state, either by perfusion fixation (see Chapters 4 and 5) or by chamber distention with cotton or paper towels.

Repairing Mistakes If mistakes are made in attempting tomographic dissection, pieces can be glued back together and then recut in a more desirable plane of sectioning. For most purposes, any of the commercially available cyanoacrylate glues (such as Superglue® or Krazy Glue®) will suffice (12,13).

Four-Chamber Method Using a long knife and beginning at the cardiac apex, a cut is extended through the acute margin of the right ventricle, the obtuse margin of the left ventricle, and the ventricular septum (Fig. 3-3). Cutting is then extended through the mitral and tricuspid valves and through the atria. This will divide the heart into two pieces, both of which show all four chambers. The upper half can then be opened along both ventricular outflow tracts, according to the inflow-outflow method previously described.

Long-Axis Method For this cut, the plane is best demarcated with three straight pins before making the cut. The first pin is placed in the cardiac apex, the second in the right aortic sinus (adjacent to the right coronary ostium), and the third near the mitral valve annulus, between the right and left pulmonary veins. The heart can then be cut along this plane, from the apex toward the base (or in the opposite direction), passing through both the mitral and aortic valves (Fig. 3-4).

Base of Heart Method This method displays all four valves intact at the cardiac base and thus is ideal for demonstrating anatomic relationships between the valves themselves and between the valves and the adjacent coronary arteries and the atrioventricular conduction system. The technique is best

Normal Heart Specimen

Fig. 3-3. Four-chamber method of cardiac dissection. (A) Normal heart, showing both atrioventricular valves and all four cardiac chambers. (B) Idiopathic dilated cardiomyopathy, with dilatation of all four chambers, compared with a normal heart (to the right). (C) Hypertrophic cardiomyopathy, with disproportionate ventricular septal hypertrophy. (D) Restrictive cardiomyopathy, with biatrial dilatation.

Fig. 3-3. Four-chamber method of cardiac dissection. (A) Normal heart, showing both atrioventricular valves and all four cardiac chambers. (B) Idiopathic dilated cardiomyopathy, with dilatation of all four chambers, compared with a normal heart (to the right). (C) Hypertrophic cardiomyopathy, with disproportionate ventricular septal hypertrophy. (D) Restrictive cardiomyopathy, with biatrial dilatation.

applied to hearts with prominent valvular disease, including prosthetic valves (Fig. 3-5) (10,11).

The left anterior descending coronary artery can be evaluated before dissecting the base of the heart, but the right and circumflex arteries are best left uncut until afterward. The ventricles are sliced in the short-axis plane before the cardiac base is dissected, and slices can extend above the level of the tips of the mitral papillary muscles. With the cut surface of the ventricles placed on a paper towel, the atria are removed. Begin at the inferior vena cava with scissors and cut into the right atrium, staying about 0.5-1.0 cm above the tricuspid valve annulus. Cut only through the atrial free wall, taking care not to injure the adjacent right coronary artery. End the cut at the upper aspect of the atrial septum, adjacent to the ascending aorta.

For the left atrium, first locate the ostium of the coronary sinus, near the inferior vena cava, and cut in a retrograde fashion along the outer wall of the coronary sinus in the left atrio-ventricular groove. Then, use scissors or a scalpel to cut through both the inner wall of the coronary sinus and the adjacent left atrial free wall. This cut should extend from the lower aspect of the atrial septum to the level of the left atrial appendage. Continue the cut between the mitral valve annulus below and the appendage above, dissecting the left atrial wall away from the ascending aorta. At the upper border of the atrial septum, the left atrial cut should meet that from the right atrium. Cut through the atria septum, from its upper to lower aspects, and remove the two atria from the cardiac base.

Transsect the two great arteries along their sinotubular junctions, at the level of the valve commissures. After removing the ascending aorta and pulmonary artery, the arterial sinuses can be trimmed away with scissors to better demonstrate the two semilunar valves. The aortic valve is located centrally and abuts against the other three valves. After photographs have been taken, the right and circumflex coronary arteries can be evaluated for obstructions.

Window Method This method is useful for the preparation of dry cardiac museum specimens, using paraffin and other materials (5,16,17) or plastination, which is the currently favored method (see Chapter 15) (18). Hearts should be perfusion-fixed, as described in Chapters 4 and 5. Windows of various sizes can be removed from the chambers or great vessels with a scalpel (Fig. 3-6). The blocks of tissue that are removed in this manner can be used for histologic study. Windows should initially be made small. Then, by looking inside the heart, one can determine how much to enlarge the opening to best demonstrate the lesion of interest.

Normal Right Ventricular Outflow Tract

Fig. 3-4. Left ventricular long-axis method of cardiac dissection. (A) Normal heart, showing left ventricular inflow and outflow tracts, left atrium, ascending aorta, and right ventricular outflow tract. (B) Myxomatous mitral regurgitation, with prolapse of the posterior leaflet. (C) Old transmural myocardial infarct, with a large apical anteroseptal aneurysm. (D) Membranous ventricular septal defect.

Fig. 3-4. Left ventricular long-axis method of cardiac dissection. (A) Normal heart, showing left ventricular inflow and outflow tracts, left atrium, ascending aorta, and right ventricular outflow tract. (B) Myxomatous mitral regurgitation, with prolapse of the posterior leaflet. (C) Old transmural myocardial infarct, with a large apical anteroseptal aneurysm. (D) Membranous ventricular septal defect.

Single Plane Angiography Images
Fig. 3-5. Base-of-heart method of cardiac dissection. (A) Normal heart. (B) Myxomatous mitral valve disease.
Myxomatous Mitral Valve Leaflets
Fig. 3-6. Window method of cardiac dissection. Window in the great arteries, showing a widely patent ductal artery.

Unrolling Method This technique can be used to demonstrate opacified epicardial arteries in a single plane. Following postmortem coronary angiography, the ventricular septum and free walls are unrolled by one of three techniques (7). The method of Rodriguez and Rainer (19) is the simplest and is best accomplished on fresh hearts. All unrolling techniques cause considerable mutilation of the heart and should be reserved for research studies.

Partition Method Partitioning techniques are used to weigh each ventricle separately for detailed assessment of ventricular hypertrophy (6,7,20). Because these techniques also mutilate the specimen, it is recommended to first evaluate the heart diag-nostically by the short-axis method, as described earlier. Partitioning begins with the stripping of epicardial fat and coronary vasculature from the specimen. Next the atria and great arteries are removed. Excision of the valves is optional. Finally, the ventricular free walls are separated from the ventricular septum. The weight of each cardiac segment can now be compared to tables of normal values (6,7).

Injection-Corrosion Method Plastic or latex is injected into the coronary vasculature or into the cardiac chambers and great vessels (5,7,21-24). Casts made from silicon rubber are resilient and nonadhesive and can therefore be extracted from the coronary arteries or cardiac chambers without resorting to corrosion of the specimen (23).For further details on injection-corrosion methods, see Chapter 15.

Dissection of the Cardiac Conduction System In situ demonstration of the glycogen-rich left bundle branch with Lugol's iodine solution is possible but only within 90 min after death (5). The atrioventricular (AV) bundle and proximal portion of the right bundle lie too deep to be shown by this technique. However, the AV node, AV (His) bundle, and right bundle branch can be observed by gross dissection (5,25,26) although the procedure is of no practical diagnostic value. The sinus node cannot be identified in this manner.

Many descriptions exist of the microscopic evaluation of the conduction system in normal and abnormal hearts (2,5,6, 25,26). In practice, such an examination is rarely necessary, except for cases of nontraumatic death in which toxicologic studies are negative and no anatomic cause of death can be found. Another example is complete heart block. In such cases, the sinus node and the atrioventricular conduction tissues should be evaluated microscopically.

To remove a block of tissue that consistently contains the sinus node, the first cut should be made with scissors just anterior to the terminal crest, cutting through the numerous pectinate muscles (Fig. 3-7). This cut should extend to the upper border of the right atrial appendage. The second cut, perpendicular to the first, courses along this upper border and into the superior vena cava. The third cut, roughly perpendicular to the second and parallel to the first, travels along the right atrial wall, where it joins the atrial septum, and is directed from the superior vena cava toward the inferior vena cava. This cut should be about 2 cm long. The fourth cut completes the rectangular shape of the tissue block.

From this block, 6-8 sections are made with a scalpel, parallel to the second and fourth cuts. This cuts the sinus node artery, which usually can be seen grossly, in cross section. All sections can usually be submitted in 2 or 3 (consecutively labeled) cassettes. Because the node contains substantially more collagen than the adjacent myocardium, a trichrome or Verhoff-van Gieson stain will aid in its identification. Between the ages of 10 and 90 yr, the percentage of collagen normally expected in the sinus node is approx the same as one's age (25).

To remove a tissue block that consistently contains the AV conduction system, the dissection should commence from the right side of the heart. The AV node is located just above the tricuspid valve annulus, between the coronary sinus ostium and the membranous septum, within the triangle of Koch. First, orient the heart with the right-sided chambers opened such that the right atrium is positioned above and the right ventricle below (Fig. 3-8). Using a scalpel, remove a rectangular block of tissue, approx 2.0 cm in height, that extends laterally from the coronary sinus ostium to the far right side of the membranous septum. Within the tissue block, the tricuspid annulus should

Node Dessection
Fig. 3-7. Dissection of the sinus node for microscopy. (A) Right lateral view of the right atrium, showing the rectangular region (black lines) to be removed. (B) Excised tissue block, showing the endocardial aspect. (C) Sections cut for microscopy.

be skewed upward, such that about 1.5 cm of atrial septum is included at the side near the coronary ostium and 1.5 cm of ventricular septum is present at the side of the membranous septum.

The excised tissue block will contain much of the septal tricuspid leaflet and portions of the mitral and aortic valves; only the pulmonary valve should remain uncut. Valves can be trimmed back to within 0.5 cm of their annuli. For right-handed cutting, rotate the specimen 180°, with the right atrium closer to the prosector than the right ventricle and the left-sided cham bers against the cutting board. Using a scalpel, cut 6-10 sections about 3 mm thick, beginning at the side nearest to the coronary sinus ostium and progressing toward the side with the membranous septum. Place tissues, in that order, into cassettes labeled AV-1, AV-2, and so on. Depending on the thickness of the ventricular septum, each cassette may hold 1-3 specimens.

Generally, each paraffin block from the conduction system needs to be cut only at one level. Trichrome or Verhoff-van Gieson stains are most suitable to identify the conduction sys

Black Dissection Board
Fig. 3-8. Dissection of the atrioventricular (AV) conduction tissues for microscopy. (A) Opened right atrium (above) and right ventricle (below), showing the rectangular specimen (black lines) to be removed. (B) Excised tissue block, showing its right-sided aspect. (C) Sections cut for microscopy.

tem because it is insulated with collagen. In rare instances, such as iatrogenic injury to the conduction system, one or two blocks may be cut at several levels to better delineate the damage, but exhausting the block to make slides from every 10th to 40th section is indicated only for detailed research investigations.

Perfusion Fixation of the Heart The method is recommended for dissections that are to be used for teaching purposes, including both museum specimens and photographs (10,11). In general, perfusion-fixation is indicated for the tomographic and window types of dissection, and several methods have been described (27-29). Even without perfusion apparatus, hearts can be fixed in an apparent distended state by filling the chambers and vessels with cotton (30). For more detailed descriptions of perfusion fixation techniques, see Chapters 4 and 5.

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