Limitation of myocardial infarct size after surgical reperfusion for acute coronary occlusion

J

THORAC CARDIOVASC SURG

84:353-358, 1982

Limitation of myocardial infarct size after surgical reperfusion for acute coronary occlusion We investigated the effect of different forms of myocardial protection on infarct size and on the necrotic myocardial process after reperfusion for acute occlusion of the left anterior descending coronary artery (LAD) in dogs. Three control groups were formed: a J hour, 2 hour, and 6 hour locally ischemic control. Three experimental groups were locally ischemic for J hour and then reperfused after an additional hour of local ischemia on cardiopulmonary bypass with the heart protected by intermittent ischemia. cold potassium cardioplegia. or blood cardioplegia. To delineate the area at risk. the LAD was temporarily occluded 30 seconds before the 6 hour sacrifice time. and monastral blue dye was injected through a polyvinyl catheter placed in the left atrial appendage. The LAD area at risk (ATJ was not stained. After 6 hours the heart was excised and treated with triphenyltetrazolium chloride (ITe) to define the area of myocardial necrosis (AN). The AN/AR ratio was determined for each animal by planimetry. Mean values were then computed in each of the six groups and evaluated by the Student's t test for paired data. The J hour control group had an AN/AR ratio of 64% ± 5%; the 2 hour control group. 80% ± 6%; and the 6 hour control group. 92% ± J%. The intermittent ischemia group had an AN/AR ratio of 83% ± 2%; the crystalloid cardioplegic group (2 hours of ischemia) had a ratio of 69% ± 4%. similar to the J hour control but significantly smaller than the 2 hour control (p < 0.05); and the blood cardioplegia group had an AN/AR ratio of 48% ± 8%. significantly better than any other group. These data demonstrate that myocardial necrosis after coronary occlusion is a time-related phenomenon and will increase to encompass a large fraction of the area at risk unless there is physical or pharmacologic modification during reperfusion, such as crystalloid or blood cardioplegia.

Douglas Wood, B.A., Charles Roberts, B.A., Stephen H. Van Devanter, M.D., Robert Kloner, M.D., Ph.D.,* and Lawrence H. Cohn, M.D., Boston, Mass.

Myocardial necrosis following acute coronary occlusion is an evolving lesion consisting of a central necrotic area surrounded by a border zone of reversibly injured myocardium. J-5 Mortality and morbidity, including the incidence and severity of ventricular tachyarrhythmias after myocardial infarction, directly reflect the amount of myocardial damage.v"? Medical

From the Departments of Medicine and Surgery, Harvard Medical School and the Brigham and Women's Hospital. Boston, Mass. Supported in part by grants from the SunLife Assurance Company, National Institutes of Health Grant HL-23140, and SCORE Grant 26215. Received for publication Dec. 7, 1981. Accepted for publication Feb. 24, 1982. Address for reprints: Lawrence H. Cohn, M.D., Department of Surgery, Brigham and Women's Hospital, 75 Francis St., Boston, Mass. 02115. *Established investigator of the American Heart Association. 0022-5223/82/090353+06$00.60/0

© 1982 The C. V. Mosby Co.

treatment of acute myocardial ischemia is directed at limiting the size of the infarction by preventing extension of necrosis into this border zone. 11. 12 Surgical treatment by coronary revascularization improves oxygen supply to the ischemic area by reperfusion and salvages myocardium otherwise destined for infarction, provided that reperfusion occurs before capillary damage has ensued and myocardial edema and hemorrhage have resulted. With increasing attention of some surgical units to the reperfusion of acute myocardial infarction, these problems are becoming increasingly pertinent. 13- 20 Operative therapy for myocardial infarction is indicated when the patient may be reperfused within 4 to 6 hours and protection of the nonischemic myocardium and limitation of myocardial infarct size can be accomplished. In the experiments reported here, we investigated the possibility that certain forms of myocardial protection, in addition to protecting nonischemic myocardium, might also limit infarct size and 353

The Joumal of Thoracic and Cardiovascular Surgery

354 Wood et al.

EJ ~

LAD Occlusion Cordiopulmonory Byposs

rn Time Post·Byposs

L' Sacrifice

~ , LAD Reperfusion

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GROUPIT (2hr Control) I

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GROUP lIT (6hr Control)

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GROUP lJ1 I GROUP J!

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120

180

240

300

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MINUTES Fig. 1. Schematic representation of six experimental groups of dogs undergoing LAD occlusion and reperfusion. LAD. Left anterior descending coronary artery.

perhaps even arrest the necrotic myocardial process. Conversely, we also wished to investigate some standard types of myocardial protection that might worsen the ischemic process.

Methods Retired racing greyhounds with hypertrophied left ventricles were used as experimental subjects. Anesthesia was induced with thiopental sodium (30 mg/kg, given intravenously) and maintained with L-chloralose and urethane (4 ml/kg, 6.67 em L-chloralose and 80 gm urethane in 500 ml 0.9% sodium chloride). Animals were intubated and ventilated with 100% oxygen. After cannulation of the left femoral artery to monitor pressure and obtain blood samples and cannulation of the left femoral vein to monitor central venous pressure and administer drugs, a left lateral thoracotomy was made in the fifth intercostal space and the heart was suspended in a pericardial cradle. Procainamide hydrochloride (50 mg/min, IV) was administered for 20 minutes prior to occlusion of the left anterior descending coronary artery (LAD) distal to the second diagonal branch in all dogs. The occlusion was made with a Schwartz vascular clip. Lidocaine hydrochloride (5 to 10 mg/kg, IV) was used to control any resulting ventricular tachyarrhythmias. If ventricular fibrillation ensued, the animal was excluded from the study (10 animals).

All animals had LAD occlusion for 1 hour and then were randomized to one of six groups, all of which were placed on cardiopulmonary bypass (Fig. 1). Dogs were placed on cardiopulmonary bypass for 90 minutes (at flows of 100 ml/kg) by cannulating the right atrium and the right femoral artery. All experimental periods from time of initial occlusion to sacrifice lasted 6 hours. There were three normothermic control groups subjected to a total of 1, 2, or 6 hours of ischemia. In the 1 hour control group (Group I, five animals), the LAD occlusion was released with the initiation of cardiopulmonary bypass. In the 2 hour control group (Group Il, five animals), each animal had 2 hours of ischemia and then was placed on cardiopulmonary bypass; the occlusion was released with initiation of bypass. In the 6 hour control group (Group III, six animals), each animal was placed on cardiopulmonary bypass after 60 minutes of LAD occlusion, but the occlusion was not released for the duration of the experiment. The three experimental groups were also placed on cardiopulmonary bypass after 1 hour of LAD occlusion and cooled to 30° C to simulate clinical reperfusion. In Group IV (intermittent ischemia, five animals), the aorta was intermittently cross-clamped for periods of 15 minutes and released for 5 minutes to allow partial reperfusion while the LAD was still occluded over a I hour period. The LAD occlusion was then released, the subjects were rewarmed to 38° C, subjected to electri-

Volume84 Number3 September. 1982

Surgical reperfusion for acute coronary occlusion

cal defibrillation, and weaned off bypass. In Group V (cold cardioplegia, five animals), total ischemic arrest by cross-clamping of the aorta was maintained for another 60 minutes and the heart was topically cooled with 40 C lactated Ringer's solution and intra-aortic crystalloid cardioplegia (30 mEq potassium chloride and 5 mEq sodium bicarbonate per liter of 2.5% dextrose and 0.45% sodium chloride). In Group VI, blood cardioplegia plus topical hypothermia was used in seven animals. The blood cardioplegic solution was composed of 800 mllactated Ringer's solution, 200 ml blood from the oxygenator, 28 mEq potassium chloride, 10 ml citrate-phosphate-dextrose, and sodium bicarbonate titrated to pH 7.75 at 8 C. A total of 300 ml of each cardioplegic solution was injected at the outset of ischemia and midway through the 45 minute crossclamp period. Throughout the cross-clamp period the septal temperatures were recorded every 5 minutes and maintained at 140 to 190 C. Ten minutes after the cross-clamp was removed in Groups V and VI, the LAD occlusion was released, the subjects were rewarmed to 380 C, subjected to electrical defibrillation, weaned off cardiopulmonary bypass. Thus the total LAD occlusion period in the three experimental groups was 120 minutes: 1 hour before cardiopulmonary bypass and 1 hour during cardiopulmonary bypass. In order to delineate the in vivo area at risk of becoming necrotic in all groups 6 hours after the original LAD occlusion, the LAD was temporarily reoccluded for 30 seconds, and monastral blue dye (15 to 20 ml) was injected through a polyvinyl catheter placed in the left atrial appendage. The area perfused stained blue and the LAD area at risk was not stained. Previous studies have shown that regional myocardial blood flow within this area at risk is less than 0.4 ml/min/gm tissue. 21- 23 The heart was then immediately excised and placed in ice. The right ventricle was dissected away from fat and valve tissue, and the left and right atria were separated from the left ventricle and the atrioventricular ring. The isolated left ventricle and was then frozen with liquid Freon and sliced into 5 mm transverse sections from the apex to the atrioventricular groove with a rotary blade. The slices of left ventricle were allowed to thaw and then placed between two clear glass plates. Endocardial and epicardial outlines of the nonperfused area at risk of each slice were traced on a clear plastic sheet, and each slice was subsequently photographed with Kodak Tungsten 160 color film. The areas were determined by planimetry. The heart slices were incubated in triphenyltetrazolium chloride (TTC) at room temperature for 20 minutes to define the area of myocardial necrosis.": 25 TIC combines with lactic dehydrogenase to stain nor0

35 5

Table I Group Group I: One-hour control

Dog No. 2

75 55

3

56

I

4

5 Mean ± SEM Group II: Two-hour control

I 2

3 4 5 Mean ± SEM Group III: Six-hour control

I 2

3 4 5 6 Mean ± SEM Group IV: Intermittent ischemia

Mean ± SEM Group V: Cold cardioplegia

76 64 ± 5*t 85 93 90 64 69 80 ± 6 93 89 93 91 94

92 92 ± I

I 2 3 4 5 I

2 3 4 5 Mean ± SEM Group VI: Blood cardioplegia

60

82 88 79 81 86 83 ± 2 70 59 61

72

82 69 ± 4*tt

I 2 3 4 5 6 7

Mean ± SEM

66 55

47 20 80

37 30

48 ± 8*tt§

Legend: AN / Aa ratio, Area of necrosis/area at risk ratio.

*p < tp < *p < §p <

0.05 versus Group II. 0.005 versus Group III. 0.01 versus Group IV. 0.05 versus Group V.

mal myocardium red; necrotic cells containing lactic dehydrogenase remain pale yellow. Traces were again made between two glass plates and evaluated by planimetry. The endocardial and epicardial outlines and areas of necrosis were traced on clear plastic sheets, slices were again photographed, and the areas were determined by planimetry. Values for the area at risk, the area of necrosis, and the respective total muscle area of each slice were obtained by the photographyplanimetry technique. The total area at risk divided by its corresponding total muscle area and the total area of

The Journalof Thoracicand Cardiovascular Surgery

356 Woodetal.

Fig. 2. A, Area at risk, cold cardioplegic solution. B, Area of necrosis after cold cardioplegic solution (AN/A R = 61%). necrosis divided by its corresponding total muscle area were determined for each dog. A ratio of the area of necrosis and the area at risk (ANIA R) was determined for each dog. Mean values were then computed for each of the six groups and evaluated by the Student's t test for paired data.

Results The results of area at risk and the ANIA R ratio are shown in Table I. Area at risk varied from 24% to 36% of the left ventricle. In Group III (6 hour permanent occlusion), 92% ± 1% of the area at risk became necrotic. In comparison to Group III, dogs subjected to 1 hour of coronary occlusion plus reperfusion (Group I) and dogs subjected to 2 hours of ischemia plus reperfusion (Group II) had a significantly smaller percentage of the area at risk become necrotic (64% ± 5% in Group I and 80% ± 6% in Group II, p < 0.05 versus Group III). In Group IV, the intermittent ischemia group, the percentage of the area at risk to become necrotic (83% ± 2%) was similar to that in the 2 hour control group (Group II). In contrast, in Group V (cold crystalloid hyperkalemic cardioplegia), the ANIA R ratio was 69% ± 4%, whereas in Group VI) (blood cardioplegia), the ANIA R ratio was 48% ± 8%, significantly better than with crystalloid cardioplegia. Thus, despite 120 minutes of total ischemia, these two cardioplegia groups were similar to the I hour control group and had significantly less necrosis than the 2 hour (Group II) or 6 hour (Group III) control groups. Blood Po 2 , blood pH, and arterial perfusion pressure were similar in all groups. Representative samples of the myocardium after cold cardioplegic protection are shown in Fig. 2.

Discussion This study demonstrates that hypothermic ischemic arrest with the heart protected by cold crystalloid cardioplegia and cold blood cardioplegia is effective, not only in protecting the entire heart during ischemic arrest, but also in limiting the amount of myocardial necrosis after coronary occlusion. The crystalloid and blood cardioplegia groups were subjected to 2 hours of LAD occlusion (1 hour of normothermic ischemic arrest before bypass and 1 hour of hypothermic ischemic arrest during bypass) and had AN/A R ratios similar to that of the 1 hour normothermic control; in the case of the blood cardioplegia group, the ratio was significantly less than even the 1 hour normothermic control group. Both crystalloid and blood cardioplegia groups had considerably less necrosis than the 2 hour control group, in which the ANIA R ratio was about 80%. Thus it appears that cardioplegic solutions, especially blood cardioplegia, may not only protect the nonischemic myocardium but also serve to limit extension of myocardial necrosis during coronary reperfusion following acute coronary occlusion. Blood cardioplegia has been shown experimentally to reverse global ischemia produced by normothermic arrest," but this phenomenon has not been demonstrated after segmental coronary occlusion. In this era of increasing emergency surgery for revascularization of acute myocardial infarction, the timing of the operation and the attempts to minimize reperfusion injury are critical." These animals were operated upon within a period of time that some surgeons have considered possible from the time of occlusion to the time of reperfusion." This study demonstrates that myocardial necrosis after coronary occlusion is time

Volume 84 Number 3 September. 1982

related and will increase unless there is physical or pharmacologic modification. Because of the enormous variability in the occluded coronary bed size and coronary collateral flow, we chose to use an in vivo area-at-risk technique to delineate zones of ischemic myocardium. The technique of injecting blue dyes into the left atrium while the coronary occlusion is maintained has previously been shown to be reliable and to minimize the variability in coronary bed size and collateral flow among experimental groups." The dye circulates to tissue receiving flow but does not reach tissue with flow rates lower than 0.4 ml/min/gm. When in vivo area-at-risk techniques are used and no intervention is applied, a high percentage of tissue becomes necrotic (90% to 100%) within the risk region, as in Group III (92% ± 1%) of this study. The technique has the advantage of delineating a readily visible zone of ischemia without relying on radioactivity to measure coronary blood flow or to delineate ischemic zones on autoradiographs. The use of TIC to delineate zones of necrosis after only brief periods of ischemia has recently been shown to be an effective mode of assessing early infarct size. 24 • 29 The advantage of the in vivo area-at-risk and TIC techniques for necrosis is that each heart slice serves as its own control. Whereas 90% to 100% of the area at risk usually becomes necrotic following a 6 hour untreated coronary occlusion, beneficial interventions result in considerably less necrosis.P Since so few patients have undergone emergency coronary bypass for acute myocardial infarction, no "best" form of myocardial protection has evolved to protect the additionally vulnerable ischemic myocardium distal to the complete coronary occlusion. The results of this paper would suggest that very cold cardioplegic solutions and probably oxygenated cardioplegic solutions, such as blood or fluorocarbons, might be the agents of choice to protect this highly vulnerable area of myocardium in addition to preserving global myocardial function.

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