Acute coronary occlusion in the pig: effect of nitroglycerin on regional myocardial blood flow

Heat stress causes oxidative stress but not inflammatory signaling in porcine skeletal muscle

Heat stress is associated with death and other maladaptions including muscle dysfunction and impaired growth across species. Despite this common observation, the molecular effects leading to these pathologic changes remain unclear. The purpose of this study was to determine the extent to which heat stress disrupted redox balance and initiated an inflammatory response in oxidative and glycolytic skeletal muscle. Female pigs (5-6/group) were subjected to thermoneutral (20 °C) or heat stress (35 °C) conditions for 1 or 3 days and the semitendinosus removed and dissected into red (STR) and white (STW) portions. After 1 day of heat stress, relative abundance of proteins modified by malondialdehyde, a measure of oxidative damage, was increased 2.5-fold (P < 0.05) compared with thermoneutral in the STR but not the STW, before returning to thermoneutral conditions following 3 days of heat stress. This corresponded with increased catalase and superoxide dismutase-1 gene expression (P < 0.05) and superoxide dismutase-1 protein abundance (P < 0.05) in the STR but not the STW. In the STR catalase and total superoxide dismutase activity were increased by ~30% and ~130%, respectively (P < 0.05), after 1 day of heat stress and returned to thermoneutral levels by day 3. One or 3 days of heat stress did not increase inflammatory signaling through the NF-κB pathway in the STR or STW. These data suggest that oxidative muscle is more susceptible to heat stress-mediated changes in redox balance than glycolytic muscle during chronic heat stress.

Regional cardiac tissue oxygenation as a function of blood flow and pO2: A near-infrared spectroscopic imaging study

Near-infrared spectroscopic imaging (NIRSI) is useful to assess cardiac tissue oxygenation in arrested and beating hearts, and it shows potential as an intraoperative gauge of the effectiveness of bypass grafting. The purpose of this study was to determine whether NIRSI can reliably differentiate among a range of cardiac oxygenation states, using ischemia and hypoxia models independently. An ischemia-reperfusion model was applied to isolated, beating, blood-perfused porcine hearts, in which the left anterior descending (LAD) artery was cannulated. LAD flow was decreased stepwise to approximately 50, 20, and 0% of normal flow and was completely restored between ischemic episodes. Upon completion of the ischemia-reperfusion protocol, the hearts were further subjected to periods of increasingly severe global hypoxia. Regional oxy- and deoxy-hemoglobin (myoglobin) levels were derived from spectroscopic images (650 to 1050 nm) acquired at each step. Oxygenation maps vividly highlighted the area at risk for all degrees of ischemia. Oxygenation values differed significantly for different LAD flow rates, regardless of whether intermediate reperfusion was applied, and oxygenation values during progressive hypoxia correlated well with blood oxygen saturation. These results suggest that NIRSI is well suited, not only to identify ischemic or hypoxic regions of cardiac tissue, but also to assess the severity of deoxygenation.

Influence of coronary venous retroinfusion and vasodilatation on regional myocardial blood flow measurement with microspheres. An analysis of 'microsphere loss' from ischaemic and reperfused porcine hearts

The influence of coronary venous retroinfusion and a vasoselective calcium antagonist felodipine on the microsphere loss in a porcine model of myocardial ischaemia and reperfusion was studied. Sixteen open-chest pigs underwent 45 min of myocardial ischaemia induced by occlusion of the left anterior descending coronary artery followed by 4 h of reperfusion. Either felodipine (felo-retro group, 7 nmol kg-1, n = 6) or the corresponding amount of vehicle (vehicle-retro group, n = 5) was infused retrogradely into the coronary veins over 30 min, starting 5 min before reperfusion. In a third group, the same amount of felodipine was administered intravenously (felo-iv group, n = 5). Myocardial regional blood flow was measured with radiolabelled microspheres (phi = 15 microns) injected before ischaemia to investigate a possible loss during ischaemia. In the felo-retro group, the apparent blood flow in the ischaemic areas, expressed as a percentage of the corresponding values in the non-ischaemic areas (%-flow), were 73 +/- 15, 73 +/- 11 and 75 +/- 19 in the subendocardial, midmyocardial and subepicardial layers, respectively. The corresponding percentage flows were 64 +/- 11, 70 +/- 11 and 62 +/- 9 in the vehicle-retro group and 75 +/- 18, 77 +/- 15 and 76 +/- 11 in the felo-iv group. The differences between the groups were not statistically significant. It is concluded that in this open-chest preparation microsphere loss observed in the ischaemic and reperfused myocardium is not increased by coronary venous retroinfusion or by a concomitantly administered vasodilative agent like felodipine.

Cardiovascular profile of 5 novel nitrate-esters: a comparative study with nitroglycerin in pigs with and without left ventricular dysfunction

1. Four cumulative 10 min intravenous infusions of 0.05, 0.2, 0.5 and 2.0 mg min-1 were used to compare the cardiovascular profile of 5 novel nitrate-esters dissolved in Intralipid 10% to that of nitroglycerin (GTN) in conscious pigs. 2. Infusion of Intralipid 10% alone had no effect on any of the systemic haemodynamic parameters. GTN infusions decreased mean arterial blood pressure dose-dependently from 94 +/- 2 mmHg to 79 +/- 3 mmHg (P less than 0.05) and raised cardiac output from 2.74 +/- 0.09 l min-1 to 3.40 +/- 0.18 l min-1 (P less than 0.05) due to an increase in heart rate (by up to 43 +/- 3%), as stroke volume decreased slightly. Systemic vascular resistance decreased (by 32 +/- 3%) and left ventricular end-diastolic pressure fell from 5.2 +/- 0.4 mmHg to 2.2 +/- 0.5 mmHg (both P less than 0.05). 3. The novel compounds CEDO 8811, CEDO 8834 and CEDO 8901 increased cardiac output only at the highest dose (7%, 8% and 9%, respectively). There was no change in mean arterial blood pressure as the increase in cardiac output was counterbalanced by arterial vasodilatation. All three compounds reduced left ventricular end-diastolic pressure slightly. 4. CEDO 8816 was a more potent arterial and venodilator than the aforementioned CEDO compounds, as the decreases in systemic vascular resistance and left ventricular end-diastolic pressure were already significant at lower doses. The fall in stroke volume was fully compensated by the increase in heart rate and as a result cardiac output increased by 11 +/- 3% (P less than 0.05) at the highest dose. 5. CEDO 8956 was the most potent vasodilator of the novel compounds and exhibited a cardiovascular profile similar to that of GTN. Left ventricular end-diastolic pressure decreased significantly during infusion of 0.2mgmin-'. Mean arterial blood pressure decreased by 11 +/- 2% (P < 0.05) in spite of an increase in cardiac output by up to 20 +/- 2% (P < 0.05), due to a decrease (by 27 +/- 1%, P <0.05) in systemic vascular resistance. The increases in heart rate (20 +/- 5%, P < 0.05) and LVdP/dtmax (38 +/- 4%, P < 0.05) were, however, considerably less after CEDO 8956 than after GTN. 6. The potential of CEDO 8956 in the treatment of chronic left ventricular dysfunction was evaluated during administration to conscious pigs (21-23 kg), in which the left circumflex coronary artery was ligated 4 weeks earlier. In these animals, baseline values for cardiac output and LVdP/dtx were lower and those of systemic vascular resistance and left ventricular end-diastolic pressure were higher than in the first group of experiments. 7. Both GTN and CEDO 8956 in doses of 0.05 to 2.0 mg inm increased cardiac output dosedependently (by up to 34% and 19%, respectively). The decrease in systemic vascular resistance was larger with GTN (35%) than with CEDO 8956 (17%), which resulted in a 13% decrease in mean arterial pressure during infusion of GTN, whereas there was no change in mean arterial pressure during infusion of CEDO 8956. Both compounds increased LVdP/dt,,,,X (by 48% and 30%, respectively) and lowered left ventricular end-diastolic pressure to normal levels. 8. At a dose of 1.0Omg min- 1, both GTN and CEDO 8956 increased left ventricular blood flow parallel to the increase in myocardial oxygen demand. At this dose, GTN also caused vasodilatation in the vascular beds of the brain, kidneys and adrenals. With CEDO 8956 no significant changes were achieved. 9. We conclude that the cardiovascular profile of CEDO 8956 in both normal animals and in animals with chronic left ventricular dysfunction warrants further study on its usefulness in the treatment of a number of cardiovascular disorders.

Detection of myocardial infarction in the mini-pig using NMR imaging

Spin-echo images of 10 myocardial infarcts in nine mini-pigs were obtained at 30 h, 3 days, and approximately 10 days postinfarction. Infarcts were not detected at all at 30 h in five out of five cases examined. At 3 days postembolization (six cases) one infarct was certainly detected, whilst at 10 days (nine cases) all infarcts were seen as high-signal areas in long TE spin-echo sequences. After 2 weeks no further infarct signal change was detected (three cases), but myocardial thinning became more evident. Using techniques similar to those reported here, early postinfarct changes in the dog have been detected by other authors. Possible reasons for this difference between pig and dog are discussed.

Amelioration by nitroglycerin of left ventricular ischemia induced by percutaneous transluminal coronary angioplasty: assessment by hemodynamic variables and left ventriculography

Increasingly longer balloon inflation times during coronary angioplasty can create significant left ventricular ischemia, amelioration of which was attempted in this study using nitroglycerin. Hemodynamic variables were assessed during inflation of an angioplasty balloon in the proximal left anterior descending coronary artery of 10 patients. Regional wall motion was assessed by left ventriculography during a separate balloon inflation. Nitroglycerin (200 micrograms) was then administered intravenously, and hemodynamic and ventriculographic assessments during balloon inflations were repeated. Balloon inflation resulted in a marked increase in left ventricular end-diastolic pressure (from 9.2 +/- 2.1 to 19.4 +/- 2.9 mm Hg) and time constant of left ventricular relaxation (from 44.2 +/- 6.2 to 62.3 +/- 11.3 ms) and a decrease in distal coronary artery perfusion pressure (from 54 +/- 9 to 33.1 +/- 4 mm Hg). Time to onset of angina was 29 +/- 3 seconds and time to ST segment depression of 1 mm or greater was 30 +/- 3 seconds. Regional wall motion analysis 30 seconds after onset of balloon inflation revealed marked hypokinesia and akinesia in the anteroapical segments with graduated depression of inferior wall motion, greatest at the apex. After the administration of nitroglycerin, balloon inflation resulted in a smaller increase in end-diastolic pressure (from 5.0 +/- 2.7 to 8.3 +/- 2.6 mm Hg) and time constant (from 47.9 +/- 4.7 to 54.4 +/- 9.2 ms; both p less than 0.01 versus standard balloon inflation). Distal coronary artery pressure remained similar to standard balloon inflation (32 +/- 3 mm Hg) despite lower mean arterial pressure (89 +/- 5 mm Hg, p less than or equal to 0.01).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of nitroglycerin on regional myocardial blood flow following an experimental coronary spasm

This study was designed to evaluate the effect of nitroglycerin (30 micrograms given as an i.v. bolus) on regional distribution of myocardial blood flow in conscious dogs, following an acute coronary occlusion similar to a coronary spasm. The left anterior descending (LAD) coronary artery was acutely occluded with a balloon cuff occluder. The distribution of blood flow between the endocardium and the epicardium of both the normal and ischemic area of the left ventricle was determined by means of the radioactive microsphere technique. Acute occlusion of the left anterior descending coronary artery produced a significant decrease of blood flow reaching the area irrigated by this artery; this decrease was of a lesser magnitude after administration of nitroglycerin. In addition, ischemia produced a disproportionate decrease in endocardial blood flow. This decrement was also of a lesser magnitude following administration of nitroglycerin. Blood perfusion to the non-ischemic myocardium was not altered. These results indicate that an intravenous bolus of nitroglycerin, given after a brief coronary occlusion simulating a coronary spasm, increases blood flow to the ischemic myocardium, induces a favorable redistribution of blood flow toward the ischemic endocardium and does not produce any decrement of blood perfusion to the non-ischemic myocardium.

A new method for clinical assessment of the negative inotropic action of anaesthetics--with special reference to isoflurane and its effect on myocardial oxygenation

A new method for maintaining the peripheral determinants of myocardial oxygen demand constant is described. Intravenously administered phenylephrine and nitroglycerin were used to control afterload and preload. Heart rate was kept constant with atrial pacing. The method was used to establish the in vivo negative inotropic effect of 1% end-tidal isoflurane in eight patients with ischaemic heart disease during positive pressure ventilation (IPPV). Stroke volume measured during steady-state isoflurane anaesthesia and IPPV with preload, afterload, and heart rate kept constant was 23% below awake control. The decrease in myocardial oxygen consumption was 22% and correlated well (r = 0.891) with the fall in left ventricular performance (stroke volume).

Distribution of the collateral blood flow at the lateral border of the ischemic myocardium after acute coronary occlusion in the pig and the dog

Residual blood flow in pigs (n = 8) and dogs (n = 11) was measured by tracer microspheres (85Sr) 1 hour after occlusion of the left anterior descending coronary artery (LAD). Collateral blood flow was distinguished from overlap flow, defined as the blood flow of non-ischemic myocardium interdigitating into the ischemic area, by direct LAD injection of isotope-labelled microspheres (125I) prior to ligation. In the center of the acutely ischemic pig myocardium the residual blood flow, i.e., the myocardial perfusion remaining after LAD occlusion, was 0.01 +/- 0.01 ml/min/g subendocardially and 0.02 +/- 0.01 ml/min/g subepicardially , as estimated with 85Sr-labelled microspheres. These values were significantly lower than the corresponding values for the dog, 0.13 +/- 0.05 ml/min/kg (p less than 0.05) subendocardially and 0.28 +/- 0.08 ml/min/g (p less than 0.01) subepicardially . In the lateral aspects of the ischemic area, calculations of overlap flow were made with the aid of the distribution of the microspheres injected into the LAD. Values of the residual blood flow were normalized and non-ischemic myocardial perfusion was set to 100 units. In subepicardial layers of the myocardium with calculated overlap flows corresponding to 20, 50 or 80 units, respectively, the residual blood flow (overlap flow + collateral flow) actually measured in the pig was 31 +/- 4, 55 +/- 4 and 75 +/- 7 units and in the dog 65 +/- 6, 79 +/- 5 and 91 +/- 2 units. The values for the dog were significantly different from the respective value for the pig (p less than 0.01). In the subendocardial layers the difference between the two species regarding residual blood flow was similar, although the difference was statistically significant only for myocardium with a calculated overlap flow of 80 units. When the calculated overlap flow was subtracted from the measured residual blood flow, the collateral blood flow was found to be extensive in the dog and virtually absent in the pig. When, in the dog, the collateral blood flow across the lateral border of the ischemic area was related to the amount of myocardium it supplies, it was found to be homogeneously distributed. Thus neither subendocardially nor subepicardially could a gradient of collateral blood flow be detected. It is concluded that in the pig the collateral blood flow is almost nil throughout the acutely ischemic myocardium, both in subendocardial and subepicardial layers. In contrast, the dog has an extensive collateral flow. No lateral gradient of this collateral blood flow could, however, be detected.

Glucocorticoid accelerates renal functional maturation in fetal lambs

Renal function was measured in seven premature lambs delivered spontaneously after a fetal injection of betamethasone, four near term lambs delivered via cesarean section after a fetal injection of a placebo and in ten spontaneously delivered full term lambs. Glomerular filtration rates were significantly higher in the premature betamethasone treated than in the near term placebo treated and the full term lambs. Fractional sodium excretions were significantly lower in the betamethasone treated preterm than the placebo treated near term lambs. This suggests that glucocorticoid accelerates renal glomerular and possibly tubular maturation. As in the lung, fetal administration of glucocorticoid stimulates functional maturation of the kidney.

Intravenous nitroglycerin in acute myocardial infarction

Until recently, the use of nitroglycerin in patients with acute myocardial infarction has been contraindicated primarily because of possible hypotension or tachycardia which, with sublingual or oral preparations, would be difficult to control. The introduction of intravenous nitroglycerin and its use during experimentally induced infarction paved the way for its use in patients with acute myocardial ischemia. It was shown that nitroglycerin, administered intravenously during experimentally induced coronary artery occlusion, redistributed flow to the subendocardium of the ischemic area while it maintained flow to the normal areas and was associated with decreased S-T segment elevation. Preliminary studies in patients with acute infarction have confirmed the beneficial effect of intravenous nitroglycerin on S-T segment elevation, and a subsequent study in 31 patients with acute infarction showed a decrease in infarct size with improved hemodynamics. In a more recent randomized prospective study in 85 patients with acute myocardial infarction, we found that intravenous nitroglycerin was associated with a decrease in infarct size in patients with inferior infarction (36 percent reduction, p less than 0.05) but that it had no effect on infarct size in patients with anterior infarction. Whether or not nitroglycerin should be used routinely for reduction of infarct size remains to be determined, but results indicate that it is effective and safe for the treatment of cardiac failure in patients with acute myocardial infarction.

Reduction of infarct size in patients with inferior infarction with intravenous glyceryl trinitrate. A randomised study

Glyceryl trinitrate was previously said to be contraindicated in patients with acute myocardial infarction. Its intravenous administration during acute infarction, however, was associated with a beneficial effect as determined by ST segment mapping. Most recently in a selected group of patients with acute infarction and abnormal haemodynamics, intravenous glyceryl trinitrate was shown to reduce infarct size estimated by enzymes. The present study was performed to verify the safety of intravenous glyceryl trinitrate in patients with infarction under conventional clinical conditions without invasive monitoring and to determine its effect on infarct size in a prospective randomised trial involving 85 patients with infarction (43 treated and 42 control). Treated patients received glyceryl trinitrate within 10 hours of the onset of symptoms (mean 6.0 hours), and the dose was titrated to preset limits for changes in heart rate and blood pressure. In patients with inferior infarction, infarct size estimated by enzymes in the treated was only 12.2 +/- 1.8 versus 19.1 +/- 3.6 CK gram equivalents per metre squared in the placebo group. A similar but statistically insignificant trend was observed for subendocardial infarction but no difference was observed for anterior infarction. Ventricular arrhythmias determined from 24 hour tapes were more frequent in treated patients though this was not statistically significant. Lignocaine requirements in treated and control (1692 +/- 250 vs 1512 +/- 232 mg/24 h) were similar, as were the requirements for morphine (11.4 +/- 1.8 vs 12.2 +/- 2.2 mg/24 h). Results indicate that intravenous glyceryl trinitrate can be administered safely during evolving infarction without invasive monitoring and reduces infarct size in patients with inferior infarction.

Intravenous nitroglycerin for the treatment of angina at rest unresponsive to standard nitrate therapy

Thirty-five patients who had angina at rest that was unresponsive to standard therapy comprised of oral or topical nitrates and beta-blocking drugs were treated with a continuous infusion of intravenous nitroglycerin (IVNTG). The infusion was started at 10 micrograms/min and increased by 10 micrograms/min increments every 5 minutes until an infusion rate of 50 micrograms/min was reached. After each episode of rest angina, the infusion was increased by 50 micrograms/min in the same stepwise manner. Data from a 24-hour baseline control period were compared with those from a 24-hour IVNTG endpoint period at which time the highest IVNTG infusion rate was administered. The average IVNTG infusion rate was 140 +/- 15 micrograms/min. With IVNTG therapy, the number of episodes of angina at rest decreased from 3.5 +/- 0.4 to 0.3 +/- 0.1, sublingual nitroglycerin use decreased from 1.9 +/- 0.3 to 0.4 +/- 0.1 mg/day, and morphine sulfate administration decreased from 5.5 +/- 1.3 to 0.4 +/- 0.2 mg/day (all p less than 0.001). When each patient's response on the endpoint day was analyzed, 25 were defined as complete (no rest angina), 8 as partial (greater than 50% decrease in the number of episodes/day from control values), and 2 as nonresponders. No significant drug-induced adverse effects occurred. IVNTG appears to be effective therapy for angina at rest refractory to standard oral and topical medications.

Comparison of the effects of nitroprusside and nitroglycerin on coronary size

Nitroglycerin and nitroprusside are known to differ in their relative degree of systemic arterial and venous dilation. Nitroglycerin has been shown to be a potent large-vessel coronary dilator, but the effects of nitroprusside on coronary artery size are unclear. Accordingly, we studied coronary artery angiographic responses to both nitroprusside and nitroglycerin in 12 patients. Diameters Of left coronary artery segments were measured by quantitative angiography before and during an intravenous infusion of nitroprusside and after sublingual nitroglycerin when both drugs were administered in doses adjusted to achieve reductions in aortic pressure. Dilation of the left coronary artery was observed after nitroprusside and after nitroglycerin. Degrees of dilation were similar in the various left coronary artery segments after either nitroprusside or nitroglycerin. In general, segments located more proximally dilated less than those located more distally after either agent. We conclude that both nitroprusside and nitroglycerin are potent dilators of large epicardial and of smaller intramuscular coronary artery segments. The magnitude of dilation of all measured left coronary artery segments appeared remarkably similar with nitroprusside and nitroglycerin given in doses that produced a similar reduction in aortic pressure.

Regional blood flow and oxygenation of the rabbit heart subjected to acute coronary occlusion - effect of phenoxybenzamine

The effect of phenoxybenzamine (2 mg/kg) on subepicardial (EPI) and subendocardial (ENDO) blood flow and oxygenation was determined in ischemic and non-ischemic areas of hearts subjected to acute coronary occlusion in 24 pentobarbital-anesthetized open-chest New Zealand white rabbits. Ten minutes after occlusion, blood flow measure with radioactive microspheres was 57% lower in the occluded region compared to the non-occluded area. There were no EPI vs. ENDO differences at this time. Phenoxybenzamine was then administered and one hour after occlusion, flow in all areas was significantly reduced. Flow in the occluded area fell to a lesser extent than in the non-occluded area. ENDO flows were lower than EPI in both areas. After occlusion, relative tissue PO2 fell significantly in the affected areas. No other changes were significant. Phenoxybenzamine lowered relative tissue PO2 in the occluded ENDO but had no other effect. This indicates that the relationship of O2 supply to consumption is preserved in the heart after phenoxybenzamine except in the occluded ENDO. In this region, the fall in relative tissue PO2 indicates that the reduction in flow is large compared to any change in metabolism. The reduction in this relationship may be due to reduced perfusion pressure for collateral flow and/or "coronary steal".

Magnitude of dilatation of large and small coronary arteries of nitroglycerin

Vasodilatory responses of segments of large epicardial left coronary artery (CA), small intramyocardial CAs (0.3-1.0 mm), coronary stenoses and CAs filled by collaterals were determined in 34 patients. Measurements were made before and after nitroglycerin (0.4 mg, sublingual) by means of quantitative magnification coronary angiography using photospot film and a calibrated 6-power viewing device. The left main CA, proximal, middle and distal anterior descending and circumflex segments, and small CAs showed dilatation that varied in magnitude. When magnitude of dilatation was compared with control diameter of the vessel and its location, control diameter proved to be the significant independent variable. CAs with the smallest control diameter showed the greatest magnitude of vasodilatation. CA branches filled by collaterals had vasodilatation similar in magnitude to that of CAs of comparable control diameter. Although coronary stenoses dilated, the magnitude of dilatation was less than that observed in nonstenosed arterial segments of similar control diameter. When areas of stenosis were excluded, however, results were similar regardless of whether the patient had CA disease. These data indicate that a principal determinant of the CA vasodilatory response to nitroglycerin is the size of the artery before nitroglycerin.

The effects of intravenous nitroglycerin on hemodynamics, coronary blood flow and morphologically and enzymatically estimated infarct size in conscious dogs

Nitroglycerin (TNG) decreases ST-segment elevation accompanying myocardial ischemia, but its effect on morphometrically and enzymatically estimated infarct size (IS) has not been defined. Accordingly, coronary occlusion was produced in 92 conscious dogs; 65 survived for 24 hours. Thirty-three received TNG (200-300 microgram/min i.v. for 8 hours) and the results were compared with those in 32 untreated dogs. Coronary blood flow (CBF) was measured with tracer microspheres (141Ce, 85Sr and 95Nb) 5 minutes after occlusion before TNG, 20 minutes after TNG and again at 8 hours. Mean blood pressure decreased from 103 to 84 mm Hg with TNG, vs 99 to 94 mm Hg in controls (p > 0.02). Nitroglycerin increased CBF in the subendocardium of ischemic areas by 45% (0.09 to 0.13 ml/min/g). The dogs were sacrificed after 24 hours and IS was estimated morphometrically (25 +/- 1% vs 27 +/- 1% of left ventricular weight) and from myocardial CK depletion (23 +/- 1% vs 24 +/- 1%) were similar for the two groups. Thus, despite increased subendocardial CBF, prolonged i.v. TNG did not decrease infarct size, although a 15% difference would have been detected with this sample size. TNG may relieve coronary spasm but does not appear to be beneficial with sustained coronary occlusion.