An experimental model with left ventricular hypertrophy caused by subcoronary aortic stenosis in dogs

An experimental model with left ventricular hypertrophy caused by subcoronary aortic stenosis in dogs S. R. K. Iyengar, F.R.C.S.(C.), E. J. P. Charrette, F.R.C.S.(C.), C. K. S. Iyengar, F.R.C.P.(C.), and R. B. Lynn, F.R.C.S.,

Kingston, Ontario, Canada

The detrimental effects of anoxic cardiac arrest on normal canine hearts are well documented.':" Levitsky and associates" have shown that left ventricular hypertrophy secondary to long-standing supracoronary stenosis of the ascending aorta renders the myocardium more susceptible to anoxic injury. In human subjects with aortic valvular stenosis, it is believed that there is interference with coronary blood flow, in addition to left ventricular hypertrophy, in view of the subcoronary position of the stenosis. To our knowledge there is no experimental model in which left ventricular hypertrophy has been produced by subcoronary stenosis. As a preliminary project prior to investigating the effects of anoxic cardiac arrest on the hypertrophied, as compared to the normal, canine left ventricle, we attempted to produce subcoronary stenosis which would result in gross and microscopic myocardial changes similar to those described in patients with aortic valvular stenosis.

From the Division of Cardiovascular and Thoracic Surgery, Queen's University, Kingston, Ontario, Canada. Supported by The Ontario Heart Foundation (Grant No. 2-4).

Received for publication April 6, 1973. Address for reprints: Dr. S. R. K. Iyengar, Etherington Hall, Queen's University, Kingston, Ontario, Canada.

Methods and material Several methods were found to be unsatisfactory: Under inflow occlusion of 3 minutes, (1) the aortic cusps were painted with a 25 per cent carbolic acid solution, (2) hemoclips were applied over the commissures, and (3) the cusp margins were plicated with a suture. However, none of these techniques was found suitable in our laboratory. Placing a Teflon or Dacron tape around the aortic root below the left coronary artery and reducing the aortic circumference by 15 to 20 per cent did permit survival and consistently produced left ventricular hypertrophy, Details of this technique are as follows, Twenty adult mongrel dogs weighing 20 to 33 kilograms were anesthetized with intravenous Nembutal, 0.5 gr. per kilogram, intubated, and placed on the Harvard respirator. The chest was opened through a left thoracotomy. The electrocardiogram and arterial pressure were continuously monitored. With patience and meticulous care, the left coronary artery was dissected and a tunnel was made between the pulmonary and aortic roots. A Teflon or Dacron tape, 2 to 4 mm. wide, was placed around the aortic root below the left coronary artery, and the circumference of the aorta was estimated (Fig. 1). Response of the heart to

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Fig. 1. Diagrammatic illustration of technique of left subcoronary aortic constriction with a 2 to 3 mm. wide Teflon tape (A-B). Origin of the left coronary artery (LCA) is shown high for clarity. In actual dissection, the interval between the left coronary artery origin and the aortic annulus was about 4 to 5 mm. RCA, Right coronary artery. AO, Aorta. Aortic cirAB - (AC + BC). Extent of constriction (AC, + BC,) - (AC + BC). cumference

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Fig. 2. Simultaneous left ventricular pressure, aortic pressure, and electrocardiographic tracings showing changes with acute 50 per cent left subcoronary constriction of the aorta.

varying degrees of aortic constriction was then assessed. Acute reduction of the circumference by 50 per cent produced dilatation of the ventricles and irreversible fibrillation. Sequential electrocardiographic tracings and arterial and ventricular pressures are shown in Fig. 2. With 30 to 50 per cent constriction, varying degrees of arrhythmia developed, and irreversible ventricular fibrillation super-

vened within 1 hour of the procedure. Next, over a period of 30 minutes, the circumference was reduced gradually by 15 to 20 per cent, with care taken to avoid constriction of the coronary artery or occlusion of the septal branches. This resulted in a decrease of 28 to 33 per cent in the aortic cross-sectional area, which was well tolerated, and produced a palpable thrill over the aortic root.

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Fig. 3. A, Left ventricular and aortic systolic pressures were equal before subcoronary constriction. B, No appreciable systolic gradient is seen across aortic valve with Isuprel drip before subcoronary constriction (intraoperative). C, Gradient of 5 to 15 mm. Hg immediately after the subcoronary constriction (intraoperative). D, With Isuprel drip, a gradient of 40 to 50 mm. Hg is seen immediately after subcoronary aortic constriction (intraoperative).

Table I. Left ventricular hypertrophy produced by subcoronary aortic stenosis

No.

(Kg.)

Interval after stenosis (days)

1 2 3 4 5 6 7 8 9

20 22.2 25 27.3 23.6 32.8 26 22 21.3 29 24 27.6

63 41 55 57 61 28 42 36 48 68 61 89

Weight

10 II

12

Actual heart weight

Estimated normal heart weight" (Gm.)

180 228 229 275 225 350 290 268 202 267.5 233 278

162 179 202 221 191 267 210.6 178.2 172.5 235 194.5 223.5

Per cent increaset 11

27 13

24 18 24 25 50 17

14 20 24

LV thickness (em.) 1.1

Gradient (mm. Hg)

Not measured

1.6 1.8 1.6

33

1.5

10 Not measured Sudden death Sudden death Not measured

1.9 1.9 2.0 1.6 1.7 1.9 1.7

Not measured

10

IO

35

Not measured

Legend: LV, Left ventricle . • Estimated normal weight was determined on the basis of the ratio of heart weight to body weight reported by Hermann." tp < 0.0001.

After thoracotomy in 2 dogs, a cardiac catheter was introduced into the left ventricle through the left atrial appendage, and simultaneous aortic and left ventricular pres-

sures were recorded (Fig. 3, A). Prior to constriction, a slow Isuprel drip (0.2 mg. in 250 ml. normal saline) produced no appreciable gradient (Fig. 3, B). After

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Fig. 4A. Six weeks after left subcoronary stenosis. Left ventricular and papillary muscle hypertrophy. Aortic stenosis seen in the inset.

Fig. 48. Six weeks after left subcoronary stenosis. Microinfarct with fibroblastic proliferation, lymphocyte, plasma, and neutrophil infiltration (hernatoxylin-phloxin-saffron xI25 ) .

gradual constriction by 15 to 20 per cent, a gradient of 5 to 15 mm. Hg was noted (Fig. 3, C). In dogs in which an Isuprel drip was started at this stage, a gradient of 40 to 50 mm, Hg was observed (Fig. 3, D). The hearts were examined at necropsy. Total heart weight and left ventricular thickness 2 em. below the mitral annulus were measured. The average ratio of the heart weight to the body weight as reported by Hermann" was used to determine the estimated normal heart weight and the per cent increase due to hypertrophy. All tissues were examined by light microscopy. Results. Data on 12 dogs 4 to 12 weeks after subcoronary stenosis are summarized in Table I. Two of the dogs (Nos. 7 and 8) which were quite active and healthy in the vivarium died suddenly after 6 and 5 weeks, respectively. Autopsy revealed no abnormality except left ventricular hypertrophy and acute pulmonary edema. Apparently the cause of death was a sudden

arrhythmia. The remaining 10 dogs were electively placed on cardiopulmonary bypass to study the effects of anoxic cardiac arrest on the hypertrophied left ventricle as compared to the normal canine heart. The results are being reported separately. The gross excess in observed heart weight over the expected weight ranged from 11 to 50 per cent. By evaluating the differences between the observed and expected average heart weights according to Student's t test, we found the excess to be highly significant (p < 0.000 I) . The left ventricle was hypertrophied and the right ventricle appeared normal in the specimens examined. Microinfarcts, fibrosis, and ischemic changes were seen on histologic examination. Representative gross and microscopic changes are shown in Figs. 4A and 4B. The gradient across the stenotic area was measured in 5 anesthetized dogs prior to anoxic cardiac arrest. It varied from 10 to 35 mm. Hg at a 4 to 10 week interval

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after creation of the subcoronary stenosis. The hemodynamic effects in similar models at intervals longer than 12 weeks is under evaluation in our laboratory. Discussion

Levitsky and co-workers- have described models with left ventricular hypertrophy produced by constriction of the ascending aorta. McLaughlin and associates' have used the same technique to produce hypertrophic subaortic stenosis which persists even after correction of the coarctation of the ascending aorta. The experimental model we have described in the present report comes closest to the clinical situation by fulfilling the following criteria: (1) The stenosis is in the subcoronary position; (2) there is a significant increase in heart weight due to left ventricular hypertrophy; (3) the microscopic changes show microinfarcts, fibrosis, and ischemic changes similar to those that have been described in patients with acquired aortic valvular stenosis; and (4) sudden death of 2 dogs which were healthy and active is consistent with the natural history of aortic stenosis. Although it is technically feasible to place the constricting tape below both the right and left coronary arteries, we have used the left subcoronary position in the present series because 85 per cent of the canine myocardial blood flow is contributed by the left coronary artery.' The natural history and the long-term hemodynamic effects of subcoronary aortic stenosis in dogs is being evaluated. Summary A technique of producing left ventricular hypertrophy in dogs by subcoronary stenosis

has been described. An 11 to 50 per cent degree of hypertrophy (p < 0.0001) was noted at 4 to 12 weeks. Ten of these dogs were subjected to anoxic cardiac arrest, and the findings will be reported separately. We gratefully acknowledge the help of Winston Offord and Stephen Chin for technical assistance, Dr. S. Ramchand for review of pathology, and Dr. A B. Kraus for statistical analysis. REFERENCES

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Buja, L. M., Levitsky, S., Ferrans, V. 1., Souther, S. G., Roberts, W. C., and Morrow, A G.: Acute and Chronic Effects of Normothermic Anoxia on Canine Hearts, Circulation 43: 44, 197 I. Goldman, B. S., Trimble, AS., Sheverini, M. A, Teasdale, S. J., Silver, M. D., and Elliott, G. E.: Functional and Metabolic Effects of Anoxic Cardiac Arrest, Ann. Thorac. Surg. 11: 122, 1971. Levitsky, S., Sloane, R. E., Mullin, E. M., McIntosh, C. L., and Morrow, A. G.: Normothermic Myocardial Anoxia, Ann. Thorac. Surg. 11: 228, 1971. Waldhausen, 1. A., Braunwald, N. S., B1oodwell, R. D., Cornell, W. P., and Morrow, A G.: Left Ventricular Function After Elective Cardiac Arrest, J. THORAC. CARDIOVASC. SURG. 39: 799, 1960. Iyengar, S. R. K., Ramchand, S., Charrette, E. P., and Lynn, R. B.: An Experimental Study of Subendocardial Hemorrhagic Necrosis After Anoxic Cardiac Arrest, Ann. Thorac. Surg, 13: 214, 1972. Hermann, G. R.: Experimental Heart Disease, Am. Heart J. 1: 213, 1925-1926. McLaughlin, J. S., Morrow, A G., and Buckley, M. J.: The Experimental Production of Hypertrophic Sub aortic Stenosis, J. THORAC. CARDIOVASC. SURG. 48: 695, 1964. Gregg, D. E., and Fisher, L. C.: Circulation, vol. 2, in Field, J., editor: Handbook of Physiology, American Physiological Society, Baltimore, Md., 1963, The Williams & Wilkins Company, p. 1530.