Acute coronary occlusion: Prolonged increase in collateral flow following brief administration of nitroglycerin and methoxamine

EXPERIMENTAL STUDIES

Acute Coronary Occlusion: Prolonged Increase in Collateral Flow Following Brief Administration of Nitroglycerin and Methoxamine NORINE L. CAPURRO, PhD KENNETH M. KENT, MD, PhD HOWARD J. SMITH, MB ROGER AAMODT, PhD STEPHEN E. EPSTEIN, MD, FACC

Bethesda, Maryland

From the Section on ExperimentalPhysiologyand Pharmacology, CardiologyBranch, NationalHeart, Lung, and Blood Institute, Bethesda, Maryland. Manuscript received November8, 1976,accepted December 14, 1976. Address for reprints: Norine Capurro, PhD, National Institutesof Health, National Heart, Lung, and Blood Institute, Building 10, Room 7B-15, Bethesda, Maryland 20014.

Regional coronary blood flow was determined with the radioactive microsphere technique 10 and 70 minutes and 2 1/2 and 5 hours after abrupt occlusion of the left anterior descending coronary artery in 12 closed chest sedated dogs. In six dogs, nitroglycerin, 200 to 400 #g/min, was infused intravenously 10 to 70 minutes after occlusion. Methoxamine was administered to return blood pressure and heart rate to prenitroglycerin levels. Ten minutes after occlusion (before treatment) collateral flow values and ischemic zone endocardial/epicardial flow ratios were equivalent in untreated (0.11 -I- 0.03 ml/min per g; 0.31 -t- 0.05) and treated dogs (0.14 -I- 0.02 ml/min per g; 0.29 -I- 0.03). In untreated dogs, collateral flow did not change over 5 hours; the endocardial/epicardial flow ratio was decreased at 5 hours (0.21 Jr 0.05, P <0.05). In contrast, in treated dogs, collateral flow and the endocardial/epicardial flow ratio were increased at 70 minutes (0.27 -4- 0.04 ml/min per g, P <0.05; 0.53 4- 0.10, P <0.05). Most importantly, collateral flow remained elevated 5 hours after occlusion (0.26 -I- 0.03 ml/min per g, P <0.05) although treatment was discontinued 70 minutes after occlusion. Hence, collateral flow was unchanged over 5 hours of occlusion in untreated dogs, but short-term treatment with nitroglycerin and methoxamine resulted in a sustained increase in collateral flow. These findings may be a result of stimulation by nitroglycerin and methoxamine of the spontaneous rate at which intrinsic collateral function increases after ischemia. Alternatively, nitroglycerin and methoxamine may maintain cell viability until collateral vessels develop spontaneously.

The ultimate fate of ischemic myocardium after acute coronary occlusion probably depends in large measure upon the rate at which the coronary collateral circulation develops and the magnitude of the resulting collateral flow. Recent evidence demonstrates that collateral flow increases during nitroglycerin infusion after acute coronary occlusion in closed chest sedated dogs I and that collateral function is enhanced after intracoronary administration of nitroglycerin to patients undergoing coronary bypass surgery for coronary artery disease. 2 The purposes of this investigation were (1) to define the collateral flow patterns in untreated dogs during the first 5 hours after coronary occlusion, (2) to confirm the previously demonstrated acute increase in collateral flow produced by nitroglycerin, and (3) to determine whether collateral flow returns to pretreatment levels when the drug is discontinued or whether the acute increase in collateral flow induced by nitroglycerin promotes a sustained augmentation of collateral flow. The effect of nitroglycerin and methoxamine, rather than of nitroglycerin alone, was evaluated because previous studies from our laboratory 1,3have shown that such combined therapy, in the absence of left ventricular failure, produces a more pronounced reduction in S-T segment elevation. than does nitroglycerin alone:

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Methods Preparation: At an initial operation, 12 dogs weighing 18 to 23 kg received general anesthesia, and the heart was exposed through a left thoracotomy. An inflatable cuff was placed around the left anterior descending coronary artery just below the first major diagonal branch, an intramyocardial electrode was implanted in the area of potential ischemia, a pacing wire was attached to the left atrium and a catheter was inserted in the left atrium. The chest was closed and the ends of the wires, cuff and catheter were left in a subcutaneous pouch. One week later, at the time of study, the dogs were sedated with morphine (1.5 mg/kg body weight) and diazepam (2 mg/kg initially, plus supplemental doses as required over the 5 hours of study). While the dogs were under local anesthesia, the ends of the cuff, catheter and wires were exposed from their subcutaneous site, and a cannula was inserted in a femoral artery. Arterial blood pressure, left atrial pressure and the electrocardiogram were continuously monitored. The animals were mechanically respired with room air only when necessary to maintain arterial oxygen saturation above 90 percent and pH between 7.35 and 7.45. Heart rate was maintained constant between 80 and 100 beats/min with pacing when necessary. Permanent occlusion of the left anterior descending coronary artery was accomplished with inflation of the balloon cuff. Regional myocardial blood flows: Tracer microspheres (15 4- 5 u) labeled with cerium-141, ytterbium-169, strontium-85 and scandium-46 (3M Company) were injected through the left atrial catheter, one isotope at each of the following times after occlusion: 10 minutes, 70 minutes, 2 1/2 hours and 5 hours. Approximately 1 million spheres per isotope injection were administered. For each microsphere injection, arterial reference sampling was begun approximately 10 seconds before injection and continued for exactly 3 minutes. Blood was withdrawn from the femoral artery with a Harvard pump at a rate. of 7.6 ml/min. Two 5 ml aliquots of each blood sample were transferred to counting tubes. After the final microsphere injection 5 hours after occlusion, the animal received pentobarbital and the heart was excised. Maintenance of occlusion over the 5 hours was verified at autopsy. Full thickness samples (1 to 2 g) of myocardium were dissected from the left ventricle: four from the normal zone, three from the border zone (defined anatomically as the myocardium between diagonal branches of the left anterior

descending coronary artery just proximal and distal to the occluder) and four to seven from the ischemic zone. Each sample was bisected to divide the epicardium and endocardium and placed in preweighed counting tubes. The radioactivity of the myocardial and blood samples was measured in a Packard Autogamma Spectrometer (model 5220). Windows were set over the main photo peaks of each isotope. Myocardial blood flow values were determined from the reference and myocardial sample counts, using four simultaneous equations to correct for overlap. Endocardial, epicardial and transmural flow values (in ml/min per g) and endocardial/ epicardial flow ratios were determined in normal, border and ischemic zone myocardium for each animal. Statistical analyses (Student's t test for paired data where appropriate) of regional flow were performed with each dog contributing one sample (no. = 12). Values are reported as mean 4- standard error of the mean. Treatment: In six dogs, treatment for 1 hour with nitroglycerin and methoxamine was begun 10 minutes after occlusion (immediately after the first microsphere injection). Nitroglycerin, 200 to 400/zg/min, was administered with a constant intravenous infusion. The dose was sufficient to produce a 20 percent decrease in mean arterial pressure or a 25 percent increase in heart rate. Methoxamine was admin-

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FIGURE 1, Time course of changes in regional myocardial blood flow after acute coronary occlusion: effect of nitroglycerin and methoxamine. Points show the mean (verUcal bars show standard error of the mean) blood flow values in six untreated (closed circles) and six treated (open circles) dogs. The heavy bla~ck bar at bottom indicates the duration of treatment.

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NITROGLYCERIN AND COLLATERAL FLOW--CAPURRO ET AL.

istered intravenously in 0.5 mg bolus injections as needed to return pressure and rate to prenitroglycerin values. Treatment was discontinued 70 minutes after occlusion (after the second microsphere injection). Results

Pretreatment values: Myocardial blood flow and hemodynamic measurements were equivalent in the two groups of dogs (untreated and treated) in the pretreatment period (Table I). Ten minutes after occlusion myocardial blood flow in the untreated group averaged 1.15 ± 0.24 ml/min per g in the normal zone, 0.49 ± 0.06 in the border zone and 0.11 ± 0.03 in the ischemic zone. Flow values measured 10 minutes after occlusion in the treated group were not significantly different from values in dogs in the untreated group. Similarly, the two groups of dogs did not differ significantly in heart rate or systemic arterial pressure. Transmural flow values: In untreated animals myocardial blood flow in the normal, border and ischemic zones (collateral flow) did not change significantly with time (Fig. 1). In dogs treated for I hour (10 to 70 minutes after o c c l u s i o n ) w i t h nitroglycerin and methoxamine, normal and border zone flow values did not change significantly over the 5 hours. However, flow in the ischemic zone of the treated animals was significantly (P <0.05) increased from the 10 minute value 70 minutes and 2 1/2 and 5 hours after occlusion. Flow values at 70 minutes and 2. 1/2 and 5 hours did not significantly differ from each other. Hence, although drug infusion was discontinued at 70 minutes, collateral flow remained elevated 2 1/2 and 5 hours after occlusion. The stability of collateral flow in untreated animals and the consistent augmentation of flow in treated animals are demonstrated in Figure 2, which shows the

temporal changes in collateral flow in the individual dogs. In the untreated group, only one of six dogs showed a definite increase in collateral flow in the 5 hours after occlusion. In contrast, only one of six treated animals had no substantial increase in collateral flow 70 minutes after occlusion, and even this animal had elevated collateral flow 5 hours after occlusion. Differences in collateral flow between the two groups were not related to differences in hemodynamic variables. Heart rate and systemic arterial pressure were similar for the two groups in the 5 hours after occlusion (Fig. 3). Heart rate, which was maintained above 80 beats/rain by pacing when necessary, did not change significantly for either group during the 5 hours. Mean systemic arterial pressure was Significantly reduced (P <0.05) in both groups at 5 hours (from 104 ± 6 to 89 + 5 mm Hg in the untreated animals and from 96 ± 4 to 88 + 4 in the treated animals). Epicardial and eudocardial flow values: The nitroglycerin-induced increase in transmural collateral flow was the result of increases in both epicardial and endocardial flow. Temporal changes in epicardial and endocardial flow in the ischemic zone of untreated and treated animals are shown in Figure 4. Ten minutes after occlusion, the respective epicardial flow values in the two groups averaged 21 ± 6 and 32 ± 8 percent of the normal zone epicardial flow. Epicardial flow in the untreated group did not change during the 5 hours of occlusion. In the treated group, epicardial flow in the

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May 4. 1977

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and treated groups averaged 1.28 + 0.06 and 1.37 ± 0.07, respectively, in the normal zone, 0.81 ± 0.08 and 0.79 ± 0.08 in the border zone and 0.31 ± 0.05 and 0.29 ± 0.03 in the ischemic zone. Ratios in the normal and border zones did not change with time in either group. In untreated animals, endocardial/epicardial ratios in the ischemic zone tended to decrease with time. At 5 hours, the ratio was significantly reduced (P <0.05) from the 10 minute value. Treatment with nitroglycerin and methoxamine significantly elevated endocardial/epicardial ratios in the ischemic zone at 70 minutes (P <0.01) and at 2 1/2 hours (P <0.05). At 5 hours, the ratio was greater than at 10 minutes, but not significantly so; however, the 5 hour endocardial/epicardial ratio was significantly greater (P <0.05) in the treated group than in the untreated group.

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TI ME- offer occlusion FIGURE 4. Time course of changes in epicardial (upper panel) and endocardial (middle panel) flows and endocardial/epicardial flow (ENDO/EPI) ratio (lower panel) in the central ischemic zone: effect of nitroglycerin and methoxamine. Epicardial and endocardial flow values are expressed as percent of the 10 minute normal zone values for epicardial and endocardial flow. Points show the mean values (vertical bars show the standard error of the mean) in six untreated (closed circles) and six treated (open circles) dogs. The heavy black bar at bottom indicates the duration of treatment.

ischemic zone increased to 50 percent of normal flow by 70 minutes after occlusion and was maintained at more than 50 percent of normal flow up to 5 hours after occlusion. Endocardial flow in the ischemic zone averaged 5 ± 2 and 7 ± 2 percent of normal zone flow in the untreated and treated groups, respectively, 10 minutes after occlusion. In the untreated group, endocardial flow did not change with time. Endocardial flow 5 hours after occlusion was less than, but not significantly different from, flow 10 minutes after occlusion. In contrast, the treated group had a three-fold increase in endocardial flow at 70 minutes. This increase was maintained at 2 1/2 and 5 hours. E n d o c a r d i a l / e p i c a r d i a l flow ratio in ischemic z o n e : In addition to increasing both epicardial and endocardial flow, treatment with nitroglycerin and methoxamine improved the endocardial/epicardial flow ratio in the ischemic zone (Fig. 4). Ten minutes after occlusion, endocardial/epicardial ratios in the untreated

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Until recently, the time course of changes in collateral flow, particularly during the first few hours after abrupt coronary occlusion, had not been clearly defined. Earlier studies utilized a variety of techniques and provided conflicting information. Rees and Redding<5 reported a transient increase in xenon-133 clearance at 1 1/2 to 2 hours after occlusion in open chest or closed chest anesthetized dogs, followed by a decrease in clearance over the next 4 hours. However, Haft and Damato 6 found no change in krypton-85 clearance 1 1/2 hours after occlusion in either closed chest or open chest dogs. Pasyk et al. 7 demonstrated an increase in peripheral coronary pressure in the first hours after occlusion in closed chest conscious dogs, but Bloor and White s found that the increase in peripheral coronary pressure was not accompanied by an increase in retrograde flow. They suggested that peripheral coronary pressure is not a reliable index of collateral flow early after occlusion. Smith et al., 9 using the microsphere technique in open chest anesthetized dogs, found an increase in collateral flow 2 hours after occlusion. These disparate results could well be attributed to the technique used to measure flow, the state of the animals (that is, open chest versus closed chest) or the depth of anesthesia. Recent studies, including our own, utilizing the microsphere technique in closed chest conscious or sedated dogs, have demonstrated that collateral flow is essentially unchanged for the first 6 hours after abrupt coronary occlusion. Cox et al. 1° showed that neither epicardial nor endocardial flow in the central ischemic zone had changed 6 hours after occlusion. Epicardial flow was subsequently increased 12, 18 and 24 hours after occlusion, whereas endocardial flow remained decreased. Bishop et al. n found no change in collateral flow 6 hours after occlusion, followed by an increase in flow at 24 and 96 hours. Again, epicardial flow increased substantially between 6 and 24 hours whereas flow to the endocardium of the central ischemic zone was relatively unchanged. Rivas et al} 2 also reported no change in collateral flow between 2 and 6 hours after occlusion but a substantial increase at 24 hours. They did document an increase in collateral flow 2 hours after occlusion from that measured 45 seconds after occlusion. How-

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NITROGLYCERIN AND COLLATERAL FLOW--CAPURRO ET AL.

ever, it is quite unlikely that collateral flow had stabilized so early after occlusion. In our study, collateral flow was measured at more frequent intervals during the first 5 hours of acute coronary occlusion: at 10 minutes, 70 minutes, 2 1/2 hours and 5 hours. We found, in agreement with the other recent microsphere studies, that transmural collateral flow was unchanged over 5 hours after acute coronary occlusion in untreated dogs. Epicardial flow was constant, endocardial flow tended to decrease (but not significantly) and the endocardial/epicardial ratio was significantly reduced at 5 hours. However, in dogs treated with nitroglycerin and methoxamine transmural collateral flow was increased significantly. Both epicardial and endocardial flows increased acutely in response to treatment arid the endocardial/epicardial ratio was improved. The increase in collateral flow was sustained over the 5 hours studied even though treatment was discontinued at 70 minutes. The nitroglycerin-induced changes in collateral flow were not related to changes in heart rate or systemic arterial pressure, both of which are known to affect collateral flow. 13,14 Although heart rate increased and systemic arterial pressure decreased after administration of nitroglycerin, both of these variables returned to prenitroglycerin values with administration of methoxamine; thus, when collateral flow measurements were made, heart rate and arterial pressure had returned to prenitroglycerin levels. Border zone flow values did not change significantly with time or with treatment with nitroglycerin and

methoxamine. This latter finding is probably related not to the efficacy of nitroglycerin, but rather to the selection of border zone biopsy sites. Recent studies now suggest that the so-called border zone is probably very narrow and physiologically unimportantZ5,16 and that samples of border zone probably include both normal and ischemic myocardium. This concept would explain our observation that the effect of treatment on border zone flow appeared to be intermediate between that on normal and ischemic tissue; that is, the net result probably reflects the relative amounts of normal and ischemic tissue contained in the border zone samples. Clinical implications: Previous studies have demonstrated that nitroglycerin acutely increases collateral flowI and improves the endocardial/epicardial flow ratio in the ischemic zone 17 after coronary occlusion. In this study, flow was improved not only during drug administration, but also up to 4 hours after discontinuation of drug administration. Further studies, at later intervals after occlusion, are necessary to assess the full significance of this finding. However, our data suggest that short-term treatment with nitroglycerin and methoxamine might (1) increase the rate at which intrinsic collateral function develops after ischemia, or (2) maintain cell viability until spontaneous development ofcollateral circulation occurs.

Acknowledgment We are grateful for the valuable technical assistance of Mr. Richard McGill and Mr. William Parker.

References 1. Smith HJ, Goldstein RA, Kent KM, et ah Reduction of myocardial ischemia by nitroglycerin and methoxamine: mechanisms of action (abstr). Am J Cardiol 37:174, 1976 2. Goldstein RE, Stinson EB, Scherer JL, et ah Intraoperative coronary collateral function in patients with coronary occlusive disease. Circulation 49:298-308, 1974 3. Smith ER, Redwood DR, McCarron WE, el ah Coronary artery occlusion in the conscious dog: effects of alterations in arterial pressure produced by nitroglycerin, hemorrhage, and alpha-adrenergic agonists on the degree of myocardial ischemia. Circulation 47:51-57, 1973 4. Rees JR, Redding VJ: Experimental myocardial infarction by a wedge method: early changes in collateral flow. Cardiovasc Res 2:43-53, 1968 5. Redding VJ, Rees JR: Early changes in collateral flow following coronary artery ligation: the role of the sympathetic nervous system. Cardiovasc Res 3:219-225, 1968 6. Haft JI, Damato AN: Measurement of collateral blood flow after myocardial infarction in the closed-chest dog. Am Heart J 77: 641-648, 1969 7. Pasyk S, Bloor CM, Khouri EM, et ah Systemic and coronary effects of coronary artery occlusion in the unanesthetized dog. Am J Physiol 220:646-654, 1971 8. Bloor CM, White FC: Functional development of the coronary collateral circulation during coronary artery occlusion in the conscious dog. Am J Pathol 67:483-500, 1972 9. Smith HJ, Singh BN, Norris RM, et ah Changes in myocardial blood

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flow and ST segment elevation following coronary artery occlusion in dogs. Circ Res 36:697-705, 1975 Cox JL, Pass HI, Wechsler AS, et ah Coronary collateral blood flow in acute myocardial infarction. J Thorac Cardiovasc Surg 69:117-125, 1975 Bishop SP, White FC, Bloor CM: Regional myocardial blood flow during acute myocardial infarction in the conscious dog. Circ Res 38:429-438, 1976 Rivas F, Cobb FR, Bache RJ, et ah Relationship between blood flow to ischemic regions and extent of myocardial infarction. Circ Res 38:439-447, 1976 Kattus AA, Gregg DE: Some determinants of coronary collateral blood flow in the open-chest dog. Circ Res 7:628-642, 1959 Smith HJ, Goldstein RA, Kent KM, el ah Determinants of collateral blood flow and ST segment elevation (abstr). Clinical Res 24:421, 1976 Fischl SL, Sonnenblick EH, Kirk ES: Collateral blood flow in the border zone following acute coronary occlusion (abstr). Am J Cardiol 35:136, 1975 Marcus ML, Kerber RE, Ehrhardt J, et ah Three dimensional geometry of acutely ischemic myocardium. Circulation 52:254-263, 1975 Chiarello M, Gold HK, Leinbach RC, el al: Comparison between the effects of nitroglycerin and nitroprusside on ischemic injury during acute myocardial infarction (abstr). Am J Cardiol 37:127, 1976

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