Myocardial infarction in dogs with acute and gradual occlusion of the circumflex or right coronary arteries.код для вставкиСкачать
THE ANATOMICAL RECORD 204:113-122 (1982) Myocardial Infarction in Dogs With Acute and Gradual Occlusion of the Circumflex or Right Coronary Arteries JACK L. WILSON AND KONRAD W. SCHEEL Departments of Anatomy (JL.WJ and Physiology and Biophysics (K.W.S.), University of Tennessee Center for the Health Sciences, Memphis, TN 38163 ABSTRACT This study was designed to quantitate and describe the incidence and magnitude of myocardial infarction in the canine heart following acute and gradual occlusion of the circumflex or right coronary arteries. In animals with acute occlusion, the circumflex artery was ligated just distal to the bifurcation of the left coronary artery for 4 hr (seven dogs). Gradual occlusion was produced by placing an Ameroid occluder on the circumflex artery for 1 month (nine dogs), 3 months (nine dogs), and 5 months (eight dogs) and on the right coronary artery for 3 months (nine dogs). Ten dogs served as controls. At the end of the experiments the dogs were sacrificed, and identification of myocardial infarction was made with an enzyme-mapping technique in dogs with acute occlusion and with histological methods in dogs with gradual occlusion. The volume of ventricular infarction was determined with the use of an Apple I1 Computer and graphics tablet. After 3 months, gradual occlusion of the right coronary artery produced a 22% incidence of infarction which was significantly less (P <.01, x2) than the 67% incidence observed with 3 months of gradual circumflex occlusion. The average infarct volume produced by gradual right coronary occlusion was 0.94 + 0.69%. The average volume of left ventricular infarction in animals with circumflex acute occlusion was 15.6%+ 6.6 and the incidence of infarction was 100%.With gradual occlusion of the circumflex artery for 1,3, and 5 months, average left ventricular infarction was 2.02 ? 1.01%, 3.13 f 1.53%, and 2.96 2 1.35%, respectively. There were no significant differences in the amount of damage observed among the three groups with gradual occlusion, and the average incidence of infarction for these three groups was 76%.In the 1-, 3- or 5-month animals with circumflex occlusion, no additional areas of necrosis subsequent to the original damage were found, indicating that infarction is a single event in this model of gradual occlusion. These results suggest that infarct size is determined primarily by factors at the time of total occlusion and that gradual occlusion allows sufficient time for collateral growth, thereby limiting the extent of myocardial injury. Although acute coronary occlusion and the ensuing development of myocardial infarction have been studied extensively (Baughman et al., 1981; Jugdutt et al., 1979; Lowensohn et al., 1976; Marcus et al., 1976; Miura et al., 1979; Rose et al., 1976; Willerson et al., 19771, this type of occlusion is not typical of atherosclerosis in man, which causes a gradual occlusion over a long period of time. Gradual coronary occlusion may allow time for preexisting collaterals to grow and compensate partially or completely for the diminished blood flow to the myocardium of the impaired vascular bed 0003-276W82/2042-0113$03.000 1982 ALAN R. LISS, INC (Bloor, 1974;Cohen, 1978; Schaper et al., 1967; Schaper, 1971; Scheel et al., 1976). Thus, gradual occlusion may result in an absence o r a considerable reduction of myocardial necrosis as compared to acute coronary occlusion, which consistently produces myocardial infarction (Miura et al., 1979; Rivas et al., 1976). Therefore, in the present investigation, our first purAddress reprint requeststo Dr. Jack L. Wilson, Department of Anatomy, University of Tennessee Center for the Health Sciences, 875 Monroe Avenue, Memphis, TN 38163. Received June 5,1981; accepted June 22,1982. 114 J.L. WILSON AND K.W. SCHEEL pose was to determine the incidence and magnitude of myocardial infarction with gradual occlusion and compare them with infarction following acute occlusion. The hemodynamic aspects of gradual occlusion and coronary collateralization have been reported previously (Schaper, 1971; Scheel et al., 1976,1979). Our purpose was to provide further evidence concerning the role of coronary collaterals in myocardial damage. In clinical studies, 17-22% of patients with myocardial infarction have shown additional myocardial necrosis 2-17 days following the original damage (Hutchins and Bulkley, 1978; Reid et al., 1974). The atherosclerotic disease process develops slowly with simultaneous involvement of numerous coronary vessels. Therefore, it is difficult to determine if this additional myocardial necrosis develops as an extension from the originally injured myocardium or is due to a progressive occlusion of other vessels involved in the disease process. The model of gradual coronary occlusion used in this study (Ameroid technique) is a pure form of single-vessel occlusion without involvement of other arteries. Therefore, it was possible to determine if this single-vessel type of coronary occlusion produces time-delayed infarctions. Previous studies have shown that collateralization in dogs is an active process up to 5 months followingAmeroid occlusion (Scheel et al., 1976, 1980). In the second aspect of this study, we observed animals after 1, 3, and 5 months of occlusion to determine whether, during this time when collaterals are growing,there are any additional areas of myocardial infarction that might develop following the original injury. In the final aspect of this study, we investigated the extent of infarction in right versus left gradual coronary occlusion. Because the work load of the right ventricle is less than that of the left ventricle, which pumps against a higher pressure a t the same cardiac output (Lowensohn et al.,1976; Peter et al., 1972;Ram0 et al., 1970; Ratliff et al., 19701, this comparison was expected to reveal whether a difference in work load could alter the susceptibility to infarction. METHODS In this study, 53 young adult male dogs weighing between 15 and 18 kg were used. All chronic dogs were examined for a normal electrocardiogram (leads I, 11,111)and negative microfilariae before being accepted into a conditioning program in which they were treated for intestinal parasites and vaccinated for distemper and hepatitis. Gradual Circumflex and Right Occlusion The basic protocol for animals with gradual occlusion was to place an Ameroid constrictor on the circumflex or right coronary artery of the dogs and allow them to remain caged for either 1, 3, or 5 months. In these groups the Ameroid constriction was placed close to the origin and proximal to any branches of the respective vessels. The Ameroid constrictor consists of a plastic material encased in a stainless steel ring that is slipped onto the artery. The Ameroid plastic is a hygroscopic material that swells with the absorption of tissue fluids and results in total occlusion of the vessel in 17-18 days (Schaper, 1971). Surgical placement of the Ameroid constrictor on the circumflex artery followed careful sterile procedures previously described in detail (Scheel et al., 1976, 1977). At the termination of the study the animals were sacrificed for histopathological examinations. The animals were divided into five groups. Group I consisted of ten control dogs. Gradual circumflex occlusion was produced by placing an Ameroid constrictor on the circumflex artery in group I1 (nine dogs) for 1 month, in group I11 (nine dogs) for 3 months, and in group IV (eight dogs) for 5 months. In group V gradual occlusion of the right coronary artery was produced by placing an Ameroid occluder on the right coronary artery (nine dogs) for 3 months. Following the above periods of Ameroid occlusion, the hearts were removed from the animals, and the aorta was attached to a constant-pressure (100 mmHg) reservoir filled with normal saline. In animals with Ameroid occlusion, a cannula was placed in the circumflex or right artery just distal to the occluder and connected by tubing to the reservoir. The coronary vessels were flushed with normal saline and then perfused with 1 liter of McDowell’s fixitive for 20 min (McDowelland Trump, 1976). After fixation, each heart was examined for complete coronary occlusion by the Ameroid. The hearts were cut into 7.5-mm thick transverse slices with a Hobart meat slicer. The slices were stored in 1 liter of fresh McDowell’s fixative for an additional week, dehydrated, and embedded in paraffin. From each slice, 7 p.m sections were cut with a sledge microtome. The sections were stained with hematoxylin-eosin, Movat’s pentachrolhe (Russell, 1972), and Gomori’s one-step trichrome (Wells, 1966) con- MYOCARDIAL INFARCTION FOLLOWING CORONARY OCCLUSION nective tissue stains to identify areas of myocardial infarction. Using whole heart histologic sections from each of the transverse slices described above, the endocardial outlines of the left ventricle including the septum were traced on drawing paper. The free wall of the right ventricle was traced separately. The necrotic regions from each slice were projected with a B&L microprojector and traced separately on drawing paper. Depending on the size of the infarct, a x 2.7, x 5, or x 12 low-power objective was used. A careful effort was made to include all necrotic and infarcted regions in the drawings. This was accomplished by constantly changing to higher-power magnification to determine the borders and location of infarcts of all sizes. The extent of myocardial infarction was quantitated utilizing an Apple I1 computer and its associated graphics tablet. After the tracings were completed, the following calculations were made: (1) The area of the left ventricle, including the septum, was calculated as the area within the borders circumscribed by the epicardial surface of the ventricular wall and the right side of the septum (A1)and the ventricular endocardial border (A2). The total area of the left ventricle in each slice was defined as A1-A2. The area of each slice was multiplied by the slice thickness (7.5 mm) to obtain volume measurements (Ginks et al., 1974). The volumes of the slices were summed to give the total volume of the left ventricle, VT. (2) The total volume of infarction, VYI,for each heart was determined separately as the sum of all necrotic regions traced for each slice, corrected for magnification factors, and multiplied by slice thickness. (3)The magnitude of infarction was expressed in percent volume of the left ventricle, and calculated as the ratio of total infarct volume and total left ventricular volume (VMnINT x 100). Right ventricular areas and volumes were determined in a similar manner. 115 age was determined by the TTC (triphenyl tetrazolium chloride) enzyme-mapping technique (Lie et al., 1975). The hearts were sliced transversely at a thickness of 1 cm. The slices were washed in cold water and incubated at 37°C in a solution of 'M'C for 45 min. The ischemic myocardium was grayish in color and normal myocardium was bright red. The quantitation of acute myocardial damage followed the technique described for animals with gradual circumflex occlusion. The normal and damaged areas of the slices were traced on clear mylar, and the calculations were made with the use of the Apple graphics tablet as described above. RESULTS This study was designed to describe and quantitate myocardialinfarction followingacute and gradual occlusion of the coronary arteries. In this section the histopathological changes observed with these two types of occlusion will be presented separately. Control Animals Group I There was no evidence of myocardial infarction or ischemia observed in any of the control animals. Gradual Circumflex and Right Coronary Occlusion Group I1 (circumflex occlusion, 1 month) There was a 77% incidence of myocardial infarction of the left ventricle in this group of dogs (Table 1).The average volume of damaged left ventricle in animals with infarction was 2.02%,with most animals (five dogs) showing less than 1.0%volume of infarction (Table 1). Infarcts in the five animals with least damage consisted of small, focal (1-2 mm2) areas of fibrosis located in the endocardial and midAcute Circumflex Occlusion myocardial levels at the posterior papillary and A 4-hour acute occlusion of the circumflex posteriolateral aspeds of the left ventricle (Fig. artery was produced in eight dogs (group VI) 1). Histologically, these infarcts were a t least and the extent of myocardial necrosis was de- 1 week old, and the necrotic muscle had been termined with an enzyme-mapping technique. replaced by collagen tissue with elongated fiThe general surgical procedure was similar to broblasts scattered evenly throughout. In the two animals with greater than 1%left that used in Ameroid application. A suture was placed around the circumflex artery and li- myocardial damage, infarction involved the gated for a period of 4 hr. No drugs or external portion of the left ventricle typically supplied methods were used to control fibrillation. One by the circumflex artery. This included the posanimal died from ventricular fibrillation dur- terior papillary muscle, adjacent septum, and the posteriorlateral and lateral wall of the left ing the experiment. At the end of 4 hr the dogs were sacrificed, ventricle (Fig. 2). The areas of damage were and the anatomical extent of myocardial dam- found in the sections taken midway between 116 J.L. WILSON AND K.W. SCHEEL TABLE I . Myocardial infarction Infarction volume as percent' of total left or right ventricle Number of dogs within infarct range Group n Incidence II (circumflex, 1mo Ameroid) I11 (circumflex, 3-mo Ameroid) IV (circumflex, 5-mo Ameroid) Total V (right 5-mo Ameroid) VI (circumflex acutd-hr) 9 77 9 - x S.E. 0.03-1.0% 1.0-5.5C 5.5-10.0% 2.02 * 1.01 5 1 1 - - 67* 3.13 1.53 2 3 1 - - 8 88 2.96 k 1.35 4 2 1 - 26 9 76 22 2.70 2 0.35 0.94 2 0.69 11 1 6 1 - 3 - - - 7 100 15.6 f 6.6** - - 2 3 I 2 2 10.&20% 20-26% 'Percent = values based only on do with infarction. *Different from group V, P < .01 **Differentfmm groups XI, 111, IV, P < ,001, (analysis of variance). (,$ base and apex of the hearts. They were limited to the inner half of the myocardium with no epicardial involvement. In both animals histological examination indicated that a large core of necrotic muscle was in the process of being removed. Surrounding the necrotic muscle was a large cellular band of numerous fibroblasts and macrophages with few lymphocytes and eosinophils (Fig. 3). Peripheral to the necrotic core, we observed a large area of collagen fibers and fibroblasts with an active vascular proliferation typical of reparative tissue at this stage. These vessels were very numerous and evenly dispersed throughout the infarcted area (Fig. 3). Group I11 (circumflex occlusion, 3 months) In this group the incidence of infarction was 67%, and the average volume of infarction was 3.13% (Table 1). The small infarcts consisted of areas of subendocardial fibrosis older than that observed at one month (group11).The larger infarcts also showed further aging changes from those observed at 1 month and were found in the inner half of the myocardium. All remnants of necrotic muscle within the infarct had been removed at 3 months, and the collagen fibers and fibroblasts were beginning to arrange themselves into parallel bundles (Fig. 4). No recent infarcts were observed. There was a general reduction in the degree of vascularization within the infarct compared to that seen in group 11. However, there were areas within the infarcts in which vascularization was still prominent and concentrated. Group IV (circumflex occlusion, 5 months) The incidence of left ventricular infarction in this group was 88% with a 2.96%average volume of ventricular damage (Table 1). Microscopically, the infarcts were located in the subendocardium and inner half of the myocardium and were older than those observed in the previous groups. The smaller lesions (four dogs) were comprised of compact bundles of collagen with few scattered fibroblasts. The larger infarcts (three dogs) consisted of an almost acellular scar tissue of dense collagen in the septal, posterior, and posteriolateral areas of the left ventricle. The cellular content was limited to fibroblasts, and the increased vascular growth observed in earlier infarcts had disappeared except at the periphery of some of the large scar areas (Fig. 5). Group V (right coronary occlusion, 3 months) Myocardial infarction was found in 22% of the animals (Table 1).The average volume of infarction was 0.94%of the right ventricle. The infarcts were found exclusively in the free wall of the right ventricle; and although the right ventricular free wall was thin, the pathological changes were located in the inner half of the MYOCARDIAL INFARCTION FOLLOWING CORONARY OCCLUSION Fig. 1. Group 11 (circumflex, 1-moAmeroid). An example of a small, focal area of subendocardial fibrosis (F)frequently found in the posterior papillary muscle. Hematoxylin and eosin. x 40. 117 Fig. 2. Group I1 (circumflex, 1-mo Ameroid). A midlevel, transverse section through the entire heart showing a large myocardial infarct in the posterior and posteriolateral aspects of the left ventricle (arrows). Note large core of necrotic muscle (N) in posterior papillary muscle. Right ventricle (R).Movat’s pentachrome stain. x 1. 118 J.L. WILSON AND K.W. SCHEEL myocardium. The infarcts consisted of small (approximately 1 mm2)collagen scars with few fibroblasts and, histologically, appeared similar to the small lesions observed with three months of circumflex occlusion. There were no lesions in the septum or the left ventricle. Acute Occlusion Group VI Infarcts were found in all of the seven animals that survived 4 h r of acute circumflex occlusion (Table 1).The average volume of left ventricular infarction was 15.6%,ranging from 8% to 26%. In each animal, the infarcts involved very large areas of the lateral, posterior, and the adjacent septa1 aspects of the left ventricle. The lesions were found in all slices from the apex to the base of the hearts (Fig. 6). The color intensity of the TTC staining consistently was prominent after 4 hr of occlusion, and infarcts could be outlined by the unaided eye. Statistical Observations Using an analysis of variance, no significant differences were found between the volumes of infarction with 1, 3, or 5 months of circumflex occlusion (groups 11,111, IV, see Table 1).Also, there was no significant difference in the volume of infarction between animals with right occlusion (group V) and animals with 3 months of circumflex occlusion (group 111). However, there was a significant difference (P <.01, x2) between the incidence of infarction found in animals with right coronary occlusion (group V) compared to animals with gradual circumflex occlusion (group 111).There was also a significant difference (P <.001) between the volume of infarction found in animals with acute circumflex occlusion (group VI) and animals with gradual circumflex occlusion (groups 11, 111, or IV). DISCUSSION In this study, we were able to compare myocardial damage following acute and gradual coronary occlusion using similar quantitative methods. The results show a large reduction in the extent of myocardial infarction of the left ventricle following gradual circumflex occlusion (2.7%)when compared to acute circumflex occlusion (15.8%), and a lower incidence of infarction with gradual occlusion (70%)than acute occlusion (100%).Following gradual right occlusion, the average amount of right ventricular infarction (0.94%)was less than with left myocardial infarction but was not statis- tically different from 3 months of gradual circumflex occlusion. However, the incidence of infarction was considerably less with right occlusion (22%)as compared with circumflex occlusion (67%, P <0.01, x2). To our knowledge, this is the first study to quantitate and compare myocardial infarction following acute and gradual occlusion of the circumflex artery using similar procedures. Other studies with gradual occlusion referred to the incidence of infarction but have not quantitated the magnitude of infarction (Flameng et al., 1973, 1975a,b, 1979; Schaper et al., 1967). Myocardial Znfarction With Circumflex Occlusion Magnitude of Infarction Acute occlusion. In a number of recent studies, investigators have reported a wide range in magnitude of infarction with acute coronary occlusion. Reimer and Jennings (1979) reported that 31% of the left ventricle was infarcted with permanent circumflex occlusion in dogs after 4 days. In conscious dogs, average left ventricular infarct sizes of 10.6%(Jugdutt et al., 1979),19%(Rivaset al., 1976),and 21.3% (Baughman et al., 1981) have been reported 2-6 days after permanent acute occlusion of the circumflex artery. In animals with acute occlusion of the circumflex artery followed by reperfusion, infarction of 10%(Lowe et al., 1978) and 15.4%(Baughman et al., 1981) of the left ventricle has been observed. Other studies that investigated acute occlusion of the anterior descending artery, reported infarction of 21% (Willerson et al., 1977) and 22% (Miura et al., 1979) of the left ventricle. In this study, 4 hr of acute circumflex occlusion produced a 15.6% infarction of the left ventricle. This result is within the range observed by other investigators. The variability of infarct sue between the various studies could be due to the conditions and methodologiesunder which the experiments were performed (consciousdogs, anesthetized dogs, permanent occlusion, occlusion followed by reperfusion, and site of ligation). Gradual Occlusion. The results from the animals in this study indicate that a considerably smaller volume of myocardial infarction (2.7%) occurs with gradual occlusion. The size of myocardial damage could be limited because collaterals are given sufficient time to grow and provide blood flow to the impaired vascular bed during the time the gradual occlusion occurs (Bloor, 1974; Schaper, 1971). Fig. 3. Group 11(circumflex 1-moAmeroid). Higher magnification at edge of necrotic core of muscle (N) observed in Figure 2. Note the large cellular band ( C ) of fibroblasts and macrophages surrounding the necrotic muscle and the adjacent large area of scar tissue and vascular proliferation. Hematoxylin and eosin. x 40. Fig. 4. Group 111 (circumflex, 3-mo Ameroid). Infarcted area from animal showing condensation and parallel arrangement of collagen fibers and fibroblasts with a reduction of vascularization within the infarct from that observed in group II. Hematoxylin and eosin. x 40. Fig. 5. Group N (circumflex, 5-mo Ameroid). Portion of a large infarct showing a dense area of scar tissue with vascularization restricted to the periphery (p) of the infarct. Hematoxylin and eosin. x 40. Fig. 6. Group N (circumflex, 5-mo Ameroid). Transverse slices from a heart of an animal with acute circumflex coronary occlusion for 4hr. Slices are labeled a-g from apex to basal end of the heart. Posterior edges of the hearts are to the right of the illustration. Infarcted areas appear white and normal myocardium appears dark with the TTC staining technique. Infarcted areas were consistently found in the septa], posterior, and posteriolateral aspects of the left ventricle. 120 J.L. WILSON AND K.W. SCHEEL Our results also showed that with gradual occlusion of a single vessel no additional areas of infarction were found subsequent to the original damage. Although the exact time of myocardial infarction can not be pinpointed, the histological evidence suggests the occurrence to be coincident with total Ameroid occlusion. Using the classical, sequential, and characteristic pathological events of infarction described in dogs (Barrie and Urback, 1957; Bing, et al., 19561, our careful examinations of the slides made it possible to differentiate new infarcts that might have occurred after the initial time of occlusion in the 1-, 3-, and 5-month animals. In group 11, studied a t 1 month (12-14 days after total occlusion), no recent necrotic areas were observed. In groups I11 and IV, studied a t 3 and 5 months, the infarcts were found in various stages of healing and repair, which became progressively more advanced in the 5month group. This indicates that the extent of infarction was related to the hemodynamics of the vasculature a t the time of occlusion. In this single-vesseltype of gradual occlusion, no new areas of infarction developed beyond the original injury. This suggests that the additional myocardial necrosis reported in clinical studies of atherosclerosis (Hutchins and Bulkley, 1978; Reid et al., 1974) probably develops from the simultaneousprogression of the disease in other vessels and/or branches of the same vessel. Incidence of Infarction Acute Occlusion. Infarction was found in all animals with acute (4-hr) occlusion of the circumflex, which is consistent with previous studies (Miura et al., 1979; Rivas et al., 1976). Gradual Occlusion. Schaper and colleagues have reported an incidence of infarction ranging from 15%to 50%following gradual (Ameroid) occlusion of the circumflex artery of the dog (Flameng et al., 1973, 1975a,b; 1979 Schaper et al., 1967; Schaper, 1971). The lesions were described as small, subendocardial infarcts in the posterior left ventricular wall. Schaper (1979)has reported recently that, with improved techniques, a higher incidence of infarction was observed than previously reported. In other studies, the incidence of patchy subendocardial fibrosis of the posterior papillary muscle was reported in 17% of the dogs after 6 months of Ameroid occlusion (Lambert et al., 1977) and in 28% of dogs with 5 and 8 weeks of Ameroid occlusion (Neil1 and Oxendine, 1979). The present results indicated a higher incidence of left ventricular infarction (6748%).The higher incidence observed in this study probably is due to the fact that smaller infarcts were found by using ~ 4 3 0magnification. Incidence and Magnitude of Myocardial Infarction With Gradual Right Coronary Occlusion The present study found the magnitude of infarction to be less in right coronary artery occlusion (0.94%)than in circumflex occlusion (3.13%)studied after a n equivalent time (Table 1). Because only two animals developed infarctions after right coronary occlusion, it was not possible to demonstrate that the difference in magnitude of damage was statistically significant. There was a significantly greater incidence of left ventricular infarction when compared to right (P <.01, x2), as shown in Table 1. A possible explanation for this observation is that ventricular work output for the left heart is greater than for the right, resulting in a decreased susceptibility of the right myocardium to infarction following coronary occlusion. This proposal is supported by the observation that when right ventricular hypertension, induced by pulmonary stenosis, was produced prior to right Ameroid occlusion, right infarction was found in 80%of pigs (Peter et al., 1972; Ratliff et al., 1970). In the absence of hypertension, these investigators did not find right ventricular infarction. It could be argued that the work loadigm of right coronary perfusion territory is identical to that of the left circumflex; however, it recently has been shown (Scheel et al., 1982) that the perfusion territory of the right coronary artery (22.5gm) is about one half that of the circumflex (52.9gm in 16-18 kg dogs), while the average pressure in the right ventricle is one fifth to one sixth that of the left ventricle (Guyton, 1981). Factors Influencing Infarct Size Three basic factors that can influence or limit the size of a n infarct following gradual coronary occlusion have been proposed: (1) the rate of development of a collateral network from pre-existing or innate collaterals within the heart, (2) the number and caliber of collaterals available a t time of occlusion, and (3) the size of the occluded vascular bed. The rate of collateral development stimulated by gradual occlusion is critical in limiting infarct size, since this will determine the status of the collateral circulation at the time of total occlusion. This is evidenced by the greatly reduced infarct size seen in gradual versus acute occlusion. MYOCARDIAL INFARCTION FOLLOWING CORONARY OCCLUSION In dogs with gradual occlusion, collateral growth continues beyond the time of total occlusion. There is a very rapid and significant epicardial collateral growth a t one month which plateaus after 5-6 months of circumflex occlusion (Schaper, 1971; Scheel et al., 1976). Also, intramyocardial collateral growth from the septa1 artery has been shown to contribute approximately 25% of the collateral circulation to the major coronary arteries (Scheel et al., 1980). Since the amount of necrosis appears to be dictated by the rate of growth and number of collaterals available at the time of total occlusion, continued collateral growth would serve to increase the coronary reserve to the remaining viable myocardium. The second factor reported to influence infarct size is the number and caliber of collateral vessels available to provide blood flow to the impaired vascular bed at the time of occlusion (Maroko et al., 1971; Schaper, 1979; White et al., 1978).In Ameroid dogs, the number and size of preexisting collaterals was sufficient to reduce the volume of infarcts to about one sixth of those observed after acute occlusions. A third influence on the extent of myocardial infarction has been reported to be the size of the perfusion territory of the occluded vascular bed (Lowe et al., 1978). Lowe et al. (1978) examined why occlusion of the same coronary artery at identical anatomical sites in dogs resulted in a significant variation in the size of infarcts. Their results indicated a direct correlation between the amount of necrosis and size of the occluded circumflex bed (myocardium a t risk). Since coronary flow to the occluded vessel is supplied via collaterals, the collateral circulation is in series with the impaired vascular bed. Because of this series connection, flow to the impaired vascular bed is a function of the sum of the resistances of the collateral and coronary vasculature. Therefore, it would seem more reasonable to propose that infarct size is determined by the relationship between the amount of collateral flow available and the size of the vascular bed supplied by the collaterals. For example, if there was a large collateral blood supply perfusing a small vascular bed, then infarction might not develop. However, if one assumes the same bed size but a significantly reduced collateral flow, infarction might occur. Thus, it is not the size of the perfusion territory, nor the collateral circulation alone, that determines the magnitude of infarction, but the ratio of the collateral resistance to perfusion-territory resistance. This principle could 121 apply to animals with either acute or chronic occlusions.However, with gradual coronary occlusion, the ratio decreases due to collateral development which decreases the probability of infarction (Bloor, 1974; Schaper, 1971). The results of this study support this concept, since infarct size with gradual occlusion was considerably less than that seen with acute occlusion. ACKNOWLEDGMENTS This study was supported by USPHS grant HL24323 and Tennessee Heart Association grant R07-3005-92. We would like to thank Mrs. Rhonda Williams and Mrs. Hue-ly Guan for excellent technical assistance and Ms. Bequi Krell for secretarial assistance. 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