The experimental induction of myocardial infarction

The experimental

induction

of myocardial

infarction

A. L. Myasnikov, Ph.D. N. IV. iYipschidse, M.D. E:. I. Tchazov, M.D. Moscow, 1:. s. S.li.

M

yocardial infarction may be induced experimentally bJ- ligating one of the coronary arteries, or by embolization of a coronary artery with a foreign material, such as liquapodium or glass beads. It is evident, however, that these methods do not correspond to the actual conditions under which myocardial infarction occurs in man. In man, myocardial infarction generally develops as a result of arteriosclerotic changes in the wall of the coronary arteries. In addition, functional factors, such as myocardial arteries, stress, spasm of the coronary and biochemical changes, play a role in the pathogenesis of myocardial infarction. We have undertaken to produce experimental myocardial infarction in rabbits without resorting to the mechanical methods generally utilized to obstruct the supply of blood to cardiac muscle. Instead, those factors were utilized which are generally considered to contribute to the development of myocardial infarction in man. Experimental

method

Coronary atherosclerosis was induced ill rabbits by the administration of cholesterol in the diet for 6 months according to the method of Anichkov. These cholesterol-fed animals were subjected to additional factors which, from the clinical point of view, were considered to contribute to the development of myocardial infarcReceived

for publication

May

16. 1960.

tion. These additional factors included myocardial stress, altered blood coagulability, and coronary arterial spasm. 1. Twenty-two animals were made arteriosclerotic by feeding them cholesterol but were not subjected. to any other influences considered to produce myocardial infarction. 2. Twenty-five cholesterol-fed animals were subjected to strenuous physical exertion so as to produce myocardial stress. Eight animals were subjected to the same physical stress but were not fed cholesterol. 3. Fifteen animals which had been fed cholesterol for 120 days were given single small doses of thrombin intravenously. Each of 8 animals which had not been fed cholesterol received a single small dose of thrombin intravenously. 4. To each of 10 healthy animals a single intravenous dose of Pituitrin alone was given, whereas an additional 21 healthy animals were given single doses of intravenous Pituitrin and thrombin in combination. 5. Animals which had been fed cholesterol for 2 months were given single doses of Pituitrin and thrombin separately and in combination. Results

Cholesterol feeding abne. None of the animals which were fed diets high in cholesterol, but which were not subjected to additional influences, developed acute myocardial infarction (Table I). These animals developed marked atherosclerosis of the

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aorta and coronary vessels, but in spite of narrowing of the lumens of the coronary arteries, myocardial infarction did not occur. Only small areas of fibrosis were found in the myocardia, especially in the intramural branch of the left coronary artery (Figs. 1 and 2). Cholesterol feeding plus physical stress. Twenty-five cholesterol-fed animals were subjected to intensive physical exercise in order to impose a stress on the myocardium. The animals were made to run on a treadmill for 1 hour daily during the entire period of cholesterol feeding. Serial electrocardiograms showed progressive signs of coronary failure. In most of the animals, episodes of cardiac asthma developed periodically and persisted to the end of the experiment. Some of the animals died in the fourth and fifth months, i.e., they did not survive the 6-month experimental period. At necropsy, large areas of myocardial necrosis (Fig. 3) and scarring (Fig. 4) secondary to arteriosclerotic changes were found in all of the animals. Inflammatory reaction in the perinecrotic areas, a characteristic of myocardial in-

induction

of myocardial

infarction

77

Fig. 1. Cholesterol feeding only. Pu’ote that arteriosclerotic plaques almost completely obliterate the lumens of the intramuscular branches of the left coronary artery (Sudan III).

Table 1

Experimenta conditions

1. Cholesterol feeding alone 2. Cholesterol feeding plus physical stress Physical stress alone 3. Cholesterol feeding (120 days) and administration of thrombin Administration of thrombin to healthy animals 4. Administration of Pituitrin to healthy animals Combined administration of Pituitrin and thrombin to healthy animals 5. Short-term cholesterol feeding (70 days) and combined administration of Pituitrin and thrombin

Number Of animals

Number with

of animals

myocardial infarction (necrobiosis of heart muscle)

22

0

25 8

2.5 0

15

10

8

0

10

0

21

17

16

14

Fig. 2. Cholesterol feeding only. Note the connective tissue scars (myocardial fibrosis) in the anterior wall of the left ventricle (Mallory’s method).

Fig. 3. Cholesterol feeding plus physical stress. Note the large focus of myocardial necrosis and the round cell infiltration in the perinecrotic area (hematoxylin-eosin) .

7x

Myasnikov,

Kipschidze,

clnd Tchazov

farction, was noted (Fig. 3). In several rabbits, aneurysm of the left ventricle had developed (Fig. 5). Thrombosis was not found in any animal. The areas of necrosis were distributed throughout the anterior, lateral, posterior, and septal walls of the left ventricle and in the papillary muscles. (See Fig. 6.) In the control group of 8 animals subjected to the same physical stress, but without cholesterol feeding, moderate left ventricular hypertrophy was noticed at necropsy, but no areas of necrosis were found in the myocardium in any of the animals (Table I). COMMENT. According to this data, physical stress in the presence of coronary atherosclerosis is important in the pathogenesis of myocardial infarction.

Fig. 6. Coronal multiple sent the

Cholesterol feeding ~111s physical stress. section of the entire heart of an animal with myocardial infarcts. The black areas represites of mvocardial infarction.

Fig. 7. Cholesterol feeding plus administration of thrombin. Electrocardiographic changes before and after the administration of a single dose of thrombin. Note the depression of the S-T segment in Leads I, II, and CR,, and the elevation of the S-T segment in Lead III after thrombin was given.

Fig. 4. Cholesterol feeding plus physical stress. Note the dense scar found in tissue removed from the posterior wall of the left ventricle (pycrofuchsin).

Fig. 5. Cholesterol feeding plus physical stress. section was taken from the wall of an aneurysm the left ventricle (pycrofuchsin).

The of

Cholesterol feeding plus administration of thrombin. Cholesterol-fed animals were administered a single small dose of thrombin intravenously. The animals were given 3-4 C.C. of thrombin (activity, 12 seconds*), a dose which in 8 healthy animals did not produce coronary thrombosis (Fig. 10, Table I). In the rabbits made arteriosclerotic by cholesterol feeding, such small doses of thrombin led to formation of thrombosis and myocardial infarction in 10 of 15 animals (Figs. 7, 8, and 9). COMMENT. These experiments indicate that changes in the coagulability of blood play a definite role in the pathogenesis of myocardial infarction. *The

activity of tbrombin solution was to the time which was necessary oxalated blood by 0.1 ml. thrombin clotted in 1 second.

determined according for clotting: 0.1 ml. solution at 32’ was

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Experimental

Fig. 8. Cholesterol feeding plus thrombin. Note the thrombus obliterating one of the branches of the left descending coronary artery in the same animal whose electrocardiogram is shown in Fig. 7. (Objective 20X, ocular 7X.)

Fig. 9. Cholesterol feeding plus thrombin. The myocardial necrosis and lipoidosis of the blood vessels occurred after the administration of 3.0 C.C. of thrombin intravenousIy. (Objective 20X, ocular 7X.)

Combined administration of Pituitrin and thrombin to healthy animals. In another group of animals the role of coronary spasm in the pathogenesis of myocardial infarction was studied. Pituitrin (and in 8 experiments, barium chloride) was used to produce coronary arterial spasm. The intravenous administration of 0.3 to 0.5 cc. of Pituitrin did not produce any significant changes in the electrocardiograms of 10 healthy animals, and at necropsy no areas of necrosis were found in these animals. Additional experiments were carried out in which Pituitrin and thrombin were administered together to healthy animals. This was done in order to investigate the possibility of producing

induction

of myocardial

infarction

79

acute coronary failure in the absence of coronary atherosclerosis under the influence of two factors, namely, spasm and altered coagulability of the blood. The combined administration of Pituitrin and thrombin did result in electrocardiographic changes typical of an acute disturbance of the coronary circulation in 17 of 21 animals (Fig. 10, Table I). At necropsy there were thrombi in the coronary vessels (Fig. 11)) myocardial ischemia, and, in animals which lived long enough (3 to 5 days), histologic changes such as are seen in myocardial infarction (Fig. 12).

Fig. 10. Combined administration of Pituitrin and thrombin to a healthy animal. Note that the administration of thrombin alone resulted in essentially no changes in the electrocardiogram, whereas the administration of Pituitrin and thrombin in combination resulted in electrocardiographic changes consistent with acute posterior myocardial infarction.

Fig. 11. Combined administration of Pituitrin and thrombin to a healthy animal. Note the thrombus obliterating one of the branches of the left descending coronary artery. The animal was given 4.0 cc. of thrombin and 0.5 C.C. of Pituitrin intravenously. (Objective 20X, ocular 7.5X.)

COMMENT. These results intlicate that coronar) arterial spasm in association with altered coagulability of the blood can lead to the formation of thrombus and myocardial infarction without organic involvement of the arterial wall by arteriosclerosis. Administration of thrombin a.rul,~or Pituitrin to slightly arteriosclerotic animals. The administration of thrombin or Pituitrin alone to animals made slightly arteriosclerotic by short-term cholesterol feeding (2 months) did not result in myocardial infarction. However, the combined atlministration of thrombin and Pituitrin

Fig. 14. Combined administration of f’ituitrin and thrombin to a slightly arteriosclerotic animal. Fresh specimen of the heart, showing a ventricular ancurysm.

Fig. 12. Combined administration of Pituitrin and thrombin to a healthy animal. Myocardial necrosis found at necropsy in an animal to whom 4.0 C.C.of thrombin and 0.5 C.C.of Pituitrin had been administered intravenously.

Fig. 13. Combined administration of Pituitrin and thrombin to a slightly arteriosclerotic animal. Longitudinal section of coronary blood vessels. The lumen of the vessel on the right is obliterated by a thrombus. The vessel on the left is patent. The animal was fed cholesterol for 3.5 days and received 3.0 C.C. of thrombin and 0.3 C.C.of Pituitrin intravenously.

produced electrocardiographic and morphologic evidence of myocardial infarction (Fig. 13) and aneurysmal formation (Fig. 14, Table I).

1. -4 method for inducing myocardial which more closely approximates the clinical conditions under which myocardial infarction develops in man has been described. 2. It has been shown that atherosclerosis of the coronary arteries is the most significant but not the sole factor in the pathogenesisof m>.ocardial infarction. 3. The ilnportant role played 115’ three additional factors in the development of myocardial infarction has been confirmed experimentally. These factors are: (a) myocardial stress, (b) altered coagulability of blood, and (c) spasm of the coronary arteries. infarction