Effect of coronary artery occlusion on regional arterial and venous O2 saturation, O2 extraction, blood flow, and O2 consumption in the dog heart

Progressively heterogeneous mismatch of regional oxygen delivery to consumption during graded coronary stenosis in pig left ventricle

In normal hearts, myocardial perfusion is fairly well matched to regional metabolic demand, although both are distributed heterogeneously. Nonuniform regional metabolic vulnerability during coronary stenosis would help to explain nonuniform necrosis during myocardial infarction. In the present study, we investigated whether metabolism-perfusion correlation diminishes during coronary stenosis, indicating increasing mismatch of regional oxygen supply to demand. Thirty anesthetized male pigs were studied: controls without coronary stenosis ( n = 11); group I, left anterior descending (LAD) coronary stenosis leading to coronary perfusion pressure reduction to 70 mmHg ( n = 6); group II, stenosis with perfusion pressure of about 35 mmHg ( n = 6); and group III, stenosis with perfusion pressure of 45 mmHg combined with adenosine infusion ( n = 7). [2-13C]- and [1,2-13C]acetate infusion was used to calculate regional O2 consumption from glutamate NMR spectra measured for multiple tissue samples of about 100 mg dry mass in the LAD region. Blood flow was measured with microspheres in the same regions. In control hearts without stenosis, regional oxygen extraction did not correlate with basal blood flow. Average myocardial O2 delivery and consumption decreased during coronary stenosis, but vasodilation with adenosine counteracted this. Regional oxygen extraction was on average decreased during stenosis, suggesting adaptation of metabolism to lower oxygen supply after half an hour of ischemia. Whereas regional O2 delivery correlated with O2 consumption in controls, this relation was progressively lost with graded coronary hypotension but partially reestablished by adenosine infusion. Therefore, coronary stenosis leads to heterogeneous metabolic stress indicated by decreasing regional O2 supply to demand matching in myocardium during partial coronary obstruction.

Myocardial O2 consumption in porcine left ventricle is heterogeneously distributed in parallel to heterogeneous O2 delivery

Myocardial blood flow is unevenly distributed, but the cause of this heterogeneity is unknown. Heterogeneous blood flow may reflect heterogeneity of oxygen demand. The aim of the present study was to assess the relation between oxygen consumption and blood flow in small tissue regions in porcine left ventricle. In seven male, anesthetized, open-chest pigs, local oxygen consumption was quantitated by computational model analysis of the incorporation of 13C in glutamate via the tricarboxylic acid cycle during timed infusion of [13C]acetate into the left anterior descending coronary artery. Blood flow was measured with radioactive microspheres before and during acetate infusion. High-resolution nuclear magnetic resonance 13C spectra were obtained from extracts of tissue samples (159 mg mean dry wt) taken at the end of the acetate infusion. Mean regional myocardial blood flow was stable [5.0 ± 1.6 (SD) and 5.0 ± 1.4 ml·min−1·g dry wt−1 before and after 30 min of acetate infusion, respectively]. Mean left ventricular oxygen consumption measured with the NMR method was 18.6 ± 7.7 μmol·min−1·g dry wt−1 and correlated well ( r = 0.85, P = 0.02, n = 7) with oxygen consumption calculated from blood flow, hemoglobin, and blood gas measurements (mean 22.8 ± 4.7 μmol·min−1·g dry wt−1). Local blood flow and oxygen consumption were significantly correlated ( r = 0.63 for pooled normalized data, P < 0.0001, n = 60). We calculate that, in the heart at normal workload, the variance of left ventricular oxygen delivery at submilliliter resolution is explained for 43% by heterogeneity in oxygen demand.

Superiority of C-11 acetate compared with F-18 fluorodeoxyglucose in predicting myocardial functional recovery by positron emission tomography in patients with acute myocardial infarction

In patients with chronic coronary artery disease, preservation of myocardial oxidative metabolism measured by positron emission tomography (PET) with 11C-acetate is a more accurate predictor of subsequent myocardial functional recovery than is maintenance of glucose metabolism estimated with 18F-fluorodeoxyglucose. However, whether measurements of myocardial oxidative metabolism are more accurate than measurements of glucose metabolism in predicting functional recovery in patients with recent myocardial infarction is unknown. Myocardial oxidative metabolism was measured within 10 days of infarction in 19 patients by analysis of the rate of myocardial clearance of 11C-acetate. Metabolism of glucose was assessed by analysis of the uptake of 18F-fluorodeoxyglucose. Criteria for prediction of the recovery of function based on measurements of oxidative metabolism and glucose metabolism were compared. Threshold criteria with 11C-acetate exhibited superior positive and negative predictive values (89% and 73%, respectively) compared with the criteria of 18F-fluorodeoxyglucose (65% and 57%, respectively) (p <0.025). In addition, the magnitude of functional recovery after revascularization correlated with the severity of the metabolic abnormality present initially. In patients with recent myocardial infarction, the extent of functional recovery can be predicted accurately by measurement of regional oxidative metabolism by PET with 11C-acetate, and these measurements are superior to those of 18-fluorodeoxyglucose.

Cerebral microregional oxygen balance during chronic versus acute hypertension in middle cerebral artery occluded rats

This study was performed to compare microregional 0(2) supply and consumption balance in spontaneously hypertensive rats (SHR), normotensive Wistar Kyoto rats (WKY), and in phenylephrine-induced acutely hypertensive WKY (WKY + ph) rats. Under isoflurane anesthesia, a middle cerebral artery (MCA) of SHR (n = 7) and WKY (n = 14) rats was occluded. Seven of the WKY rats were infused with phenylephrine (WKY + ph) to keep the mean arterial pressure (MAP) at the same level as that of the SHR. In all animals, 1 h after MCA occlusion, regional cerebral blood flow (rCBF) was determined using an autoradiographic technique, and microregional arterial and venous 02 saturations were determined using microspectrophotometry. MAP was 76 +/- 4 (SD), 136 +/- 15, and 132 +/- 12 mm Hg for the WKY, WKY + ph, and SHR groups, respectively. All variables describing regional O2 balance and rCBF were similar between the SHR and the WKY groups in the ischemic cortex as well as in the contralateral cortex. With phenylephrine infusion, rCBF of both the ischemic cortex and the contralateral cortex were increased in the WKY group. The average 02 supply-to-consumption ratio in the ischemic cortex was higher in the WKY + ph than in the WKY or SHR group. In the ischemic cortex, heterogeneity of venous 02 saturation (SvO2), expressed as a coefficient of variation (CV = 100 X SD/mean), was significantly lower in the WKY + ph (18.3 +/- 2.4) group than in the SHR (30.5 +/- 11.8) or in the WKY (31.3 +/- 9.0) group. The number of veins with low 02 saturation (SvO2 < 40%) in the ischemic cortex was significantly lower in the WKY + ph than in the SHR or in the WKY group. Our data suggest that in chronically hypertensive animals, cerebrovascular adaptations enable the microregional 02 balance in focal ischemia to be maintained at a level similar to that of normotensive animals. However, in normotensive animals with focal cerebral ischemia, an acute increase of MAP improves microregional O2 balance.

Protection of evolving myocardial infarction and failed PTCA

Acute myocardial infarction is caused by acute coronary occlusion and is the major cause of death in Europe and the United States. In-hospital mortality is due principally to cardiogenic shock because of extensive ischemic muscle damage. Previous surgical results of coronary artery bypass grafting for left ventricular power failure have been disappointing because intraoperative ischemic injury is superimposed on severe damage already sustained by the myocardium. Surgical revascularization has, in general, been restricted to patients with acute occlusion after elective percutaneous transluminal coronary angioplasty with or without thrombolytic therapy. During the last years new knowledge has been gained in the pathophysiology of acute coronary occlusion on ischemic and nonischemic (remote) myocardium that has evolved in a new surgical strategy for revascularization of patients with evolving myocardial infarctions and failed percutaneous transluminal coronary angioplasty. Studies of the natural history of acute regional ischemia have shown that acute occlusion of a coronary artery not only affects the ischemic myocardium but causes structural, functional, and metabolic alterations in the remote and adjacent myocardium. These changes in the remote myocardium are even more severe if the remote myocardium is supplied by a stenotic coronary artery. Furthermore, many experimental and clinical studies have shown that normal blood reperfusion of myocardium injured previously by ischemia leads to additional damage (reperfusion injury).(ABSTRACT TRUNCATED AT 250 WORDS)

Hemodynamic vascular forces contribute to impaired endothelium-dependent vasodilation in reperfused canine epicardial coronary arteries

OBJECTIVES

We studied canine coronary arterial vasoreactivity after occlusion and reperfusion to examine whether reduced flow or pressure contributed to the abnormalities observed.

BACKGROUND

Ischemia and reperfusion alter endothelial and myocardial function. Causative factors may include altered flow, complement activation or free radical production by endothelial or white blood cells after reoxygenation and neutrophil activation.

METHODS

The coronary arteries of anesthetized, open chest dogs were subjected to 90-min occlusion +/- 2 h of reperfusion. The effect of reperfusion on arterial responses to intracoronary acetylcholine, nitroprusside and phenylephrine was studied using in vivo ultrasound. Arterial segments were also harvested, perfused ex vivo with cell-free buffer and exposed to potassium chloride, nitroprusside, acetylcholine and bradykinin. The effect of ex vivo flow cessation with or without maintained intralumen pressure was also studied.

RESULTS

Results are expressed as mean value +/- SEM. In vivo arterial cross-sectional area increased during infusion with acetylcholine (10(-5) mol/liter [18.5 +/- 9%]) and nitroprusside (10(-5) mol/liter [22.5 +/- 10%]) and decreased with phenylephrine (10(-5) mol/liter [7.6 +/- 7%]). After reperfusion, acetylcholine caused 13.5 +/- 9% vasoconstriction. Nitroprusside and phenylephrine responses were unchanged. Reperfused arterial segments also showed impaired vasodilation in response to 10(-6) mol/liter of acetylcholine (10.6 +/- 5.1% vs. 47.1 +/- 4.9% in control vessels) and 10(-8) mol/liter of bradykinin (4.4 +/- 6.7% vs. 27.9 +/- 8% in control vessels). Ex vivo flow cessation impaired acetylcholine-mediated vasodilation, but this abnormality was prevented when high intralumen pressure was maintained during the no-flow period.

CONCLUSIONS

Reduction in flow and intralumen pressure contribute to the impaired acetylcholine-mediated vasodilation seen after coronary occlusion. This is prevented by maintaining high intralumen pressure during the no-flow period, suggesting that hemodynamic forces may change endothelial function independent of circulating complement or blood cell elements.

Comparison of carbon-11-acetate with fluorine-18-fluorodeoxyglucose for delineating viable myocardium by positron emission tomography

OBJECTIVES

This study was designed to determine in patients with advanced coronary disease whether prediction of recovery of mechanical function after coronary revascularization could be accomplished more effectively by positron emission tomography (PET) with carbon-11 (11C)-acetate than by PET with fluorine-18 (18F)-fluorodeoxyglucose.

BACKGROUND

Results of previous studies have demonstrated that preservation of myocardial oxidative metabolism (measured by PET with 11C-acetate) is necessary for recovery of systolic function after coronary revascularization.

METHODS

Myocardial oxidative metabolism was quantified before revascularization in 34 patients by the analysis of the rate of myocardial clearance of 11C-acetate. Metabolism of glucose was assessed by analysis of uptake of 18F-fluorodeoxyglucose. Receiver operating characteristic curves for predicting functional recovery were derived for the measurements of oxidative metabolism and glucose metabolism. In addition, criteria for prediction of recovery of function based on measurements of oxidative metabolism and glucose metabolism were developed and compared.

RESULTS

Analysis of receiver operating characteristic curves indicated that estimates of oxidative metabolism were more robust in predicting functional recovery than were estimates of glucose metabolism (p < 0.02). Moreover, threshold criteria with 11C-acetate exhibited superior positive and negative predictive values (67% and 89%, respectively) than did the criteria with 18F-fluorodeoxyglucose (52% and 81%, respectively), p < 0.01. In segments with initially severe dysfunction, estimates of oxidative metabolism tended to be more robust than estimates of glucose metabolism in predicting functional recovery. Moreover, in such segments, the threshold criteria with 11C-acetate tended to exhibit superior positive and negative predictive values (85% and 87%, respectively) than did the criteria with 18F-fluorodeoxyglucose (72% and 82%, respectively), although statistical significance was not achieved.

CONCLUSIONS

In patients with advanced coronary artery disease, the extent to which functional recovery can be anticipated after coronary revascularization can be delineated accurately by quantification of regional oxidative metabolism by PET with 11C-acetate.

Alteration in regulation of myocardial blood flow in one-vessel coronary artery disease determined by positron emission tomography

The behavior of myocardial blood flow (MBF) regulation in territories supplied by angiographically normal vessels of patients with coronary artery disease (CAD) has been poorly investigated. Resting MBF and coronary reserve were evaluated in 32 patients with stable angina, no previous myocardial infarction, and isolated left anterior descending or left circumflex coronary artery stenosis (> or = 50% diameter narrowing). MBF was measured, in the absence of any medical therapy, by means of dynamic positron emission tomography and 13N-ammonia. MBF measurements at baseline and after intravenous dipyridamole (0.56 mg/kg administered over 4 minutes), were obtained both in the stenosis-related regions and in contralateral territories. As a control group, 14 normal subjects were evaluated according to the same protocol. At rest, the 32 patients with CAD had similar MBF values in the stenotic and remote regions (0.76 +/- 0.21 and 0.77 +/- 0.19 ml/min/g, respectively, p = NS); both these values were significantly (p < 0.01) reduced with respect to mean MBF in normal subjects (1.03 +/- 0.25 ml/min/g). The dipyridamole study was completed in 30 patients; these patients had lower values of maximal MBF in the stenotic than in the remote regions (1.52 +/- 0.65 vs 1.76 +/- 0.68 ml/min/g, p < 0.05); however, both these values were significantly reduced (p < 0.01) with respect to mean dipyridamole MBF in normal subjects (3.66 +/- 0.92 ml/min/g). Thus, in patients with CAD, resting and maximal MBF can be reduced not only in myocardial territories supplied by stenotic arteries, but also in territories supplied by angiographically normal arteries.

Correlation of heterogeneous blood flow and fatty acid uptake in the normal dog heart

Blood flow heterogeneity in normal myocardium may be caused by heterogeneous metabolic demand. We studied, from 80 tissue samples of the left ventricle (LV) of eight anesthetised, open-chest dogs (with prior beta-blockade (metoprolol) in four dogs), the radioactivity of 201Thallium-chloride (201Tl), an indicator of blood flow, and of the fatty acid 131-Iodine-heptadecanoic acid (131I-HDA), an indicator of metabolic demand, 3 min after intravenous injection. Global LV uptake (in percent of injected dose x 10(-2), per g tissue; mean +/- SD) was 4.94 +/- 0.71 for 201Tl and 4.48 +/- 0.58 for 131I-HDA in the dogs without beta-blockade, and 2.08 +/- 0.26 and 1.69 +/- 0.20, respectively, in dogs with beta-blockade (p < 0.05). Beta-blockade thus decreased the fraction of cardiac output delivered to the LV, concurrently with a decreased heart rate and arterial blood pressure (p < 0.05) and, thus, global metabolic demand and fatty acid uptake. Regional radioactivities per gram were normalized for mean LV radioactivities and heterogeneity was expressed as the coefficient of variation (CV). For pooled data (n = 320) in dogs without beta-blockade, regional 201Tl and 131I-HDA radioactivities varied from a factor of 0.1 to 1.6 and 0.3 to 1.8 of mean radioactivities, with a CV of 22.9 and 19.4%, respectively, and correlated (r = 0.77, p < 0.005). For pooled data (n = 320) in dogs with beta-blockade, regional 201Tl and 131I-HDA radioactivities varied from a factor of 0.2 to 1.5 and 0.2 to 1.6 of mean radioactivity and CV was 23.6% and 24.8%, respectively; r = 0.92 (p < 0.005). The endo/epi ratio for both radioactivities exceeded unity in each dog. In normal myocardium, blood flow and fatty acid uptake are thus heterogeneous, both transmurally and circumferentially, and matched, concomitantly with coupling of global blood flow to global metabolic demand and fatty acid uptake. This supports the idea that heterogeneous myocardial O2 supply reflects heterogeneous metabolic demand.

Effect of isoproterenol on regional myocardial segment work, O2 consumption, and oxygen balance

We tested the hypothesis that positive inotropic stimulation by isoproterenol alters the relationship between regional segment work and regional myocardial oxygen consumption. Regional parameters were compared with external cardiac work and global LV oxygen consumption. In anesthetized open-chest dogs, regional myocardial segment length (ultrasonic dimension crystals) and force development (miniature force transducer) were measured. The integrated multiples of myocardial shortening by corresponding force during an averaged beat expressed segment work (area under the systolic portion of the length-force loop). External cardiac work was calculated from aortic blood pressure and cardiac output. Global and regional myocardial MVO2 were evaluated at baseline and during intravenous infusion of isoproterenol (0.5 and 1.0 micrograms/kg per min). Regional coronary blood flow was measured with radioactive microspheres, and microspectrophotometry of frozen myocardial biopsies was used to evaluate O2 saturation in small arteries and veins. These parameters were used to calculate regional MVO2. Arterial and coronary sinus O2 saturation was used to calculate global LV O2 consumption. Regional myocardial O2 balance was estimated by measurement of NADH redox level using surface fluorometry. It was found that 0.5 micrograms/kg per min isoproterenol increased regional segment work/minute from 4650 +/- 495 to 6750 +/- 750 mm.g/min. Corresponding regional oxygen consumption was disproportionately increased from 5.43 +/- 0.61 to 15.24 +/- 1.37 ml/min per 100 g. External cardiac work was found to decrease from 728 +/- 13 to 562 +/- 25 mmHg.1/min (due to decreased aortic blood pressure), whereas global myocardial O2 consumption increased. Regional myocardial O2 extraction and NADH fluorescence were elevated, indicating impaired tissue oxygenation. Regional MVO2 was increased by 153 +/- 56%, but regional work by only 45.3 +/- 33% (P < 0.05). These results indicate that regional contraction efficiency was markedly reduced by isoproterenol.

Coronary flow reserve

The ability of the coronary circulation to autoregulate is essential for the heart to respond to metabolic demands. Several alterations in function may limit maximal coronary perfusion including atherosclerosis, structural abnormalities of small coronary vessels, extravascular compressive forces, thrombosis, abnormal endothelial regulatory function, and the effect of abnormal myocardium on the coronary circulation. Coronary flow reserve is a unifying concept that examines the limitation in myocardial perfusion that certain disease states impose. At present, even with state-of-the-art technology, the measurement of coronary flow reserve is difficult in routine clinical situations. As the ability to measure regional myocardial perfusion improves, coronary flow reserve may gain more widespread clinical use with perhaps as yet undiscovered therapeutic implications.

Effect of dopamine on regional myocardial function and oxygen consumption in experimental left ventricular hypertrophy

We investigated the hypothesis that the capacity of left ventricular myocardium to respond to an inotropic challenge by dopamine would be diminished in left ventricular hypertrophy induced by plication of the aortic valve. Seven mongrel dogs (LVH group) aged 6-8 weeks and weighing 4-6 kg, were subjected to preliminary surgery in which the noncoronary sinus of Valsalva was plicated. Six months later these animals, as well as a control group of dogs, were subjected to acute experiments in which the effect of dopamine (7.5 and 15 micrograms/kg/min) on regional and global myocardial function and oxygen consumption was studied. Myocardial segment length was measured with ultrasonic dimension transducers, and left ventricular and aortic blood pressures were recorded from catheter-tip transducers. Regional coronary blood flow was determined with radioactive microspheres, and regional oxygen saturation in small arteries and veins was measured using microspectrophotometry. Regional myocardial O2 consumption was calculated from these parameters. Heart weights were significantly elevated in the LVH group, and a pressure gradient of about 25 mm Hg was observed across the aortic valve. In both groups, dopamine infusion produced a dose-dependent increase in heart rate, left ventricular pressure, and LV dP/dtmax. Prior to dopamine infusion, percent shortening per beat was greater in the LVH group (13.97 +/- 1.2%) than in the control group (9.49 +/- 1.07%). Although the maximum speed of segment shortening was elevated by dopamine in both groups, percent shortening was not elevated in the LVH group. Stimulation by the high dose of dopamine produced a threefold elevation in regional coronary blood flow in both groups. Oxygen extraction was unchanged; the proportion of small veins with low O2 saturation was not elevated in LVH hearts, even during dopamine stimulation. Regional myocardial O2 consumption was elevated by dopamine (15 micrograms/kg/min) to about the same extent in both the control and LVH groups (19.1 +/- 2.3 and 17.5 +/- 2.3 ml O2/min/100 g) respectively. It is concluded that, in dogs with six months of aortic stenosis, dopamine does not exhaust functional reserve and the relationship between O2 supply and consumption is not significantly impaired.

Effects of nitroglycerin on regional O2 supply and O2 consumption in reperfused dog myocardium

This study was designed to assess whether nitroglycerin would improve the relationship between O2 supply and O2 consumption in the reperfused ischemic dog myocardium. In 16 dogs the left anterior descending coronary artery was occluded for 2 h, followed by a 4 h period of reperfusion. In 8 of the 16 dogs, an infusion of 10 micrograms/kg per min of nitroglycerin was begun 10 min prior and continued during 4 h of reperfusion. Small artery and vein O2 saturations obtained microspectrophotometrically were combined with regional blood flow measurements using radioactive microspheres to determine regional myocardial O2 consumption. In both groups, 2 h of occlusion lowered the regional flow to a similar level. In the control group, 4 h of reperfusion returned the blood flow towards normal levels, from 15 +/- 20 ml/min per 100 g (mean +/- S.D.) at the end of occlusion to 57 +/- 39 in the affected area compared to 84 +/- 32 ml/min per 100 g in the nonischemic area. In nitroglycerin treated animals, the flow increase with reperfusion was similar to the control group (12 +/- 10 to 65 +/- 33 ml/min per 100 g). O2 extraction was greater in the reperfused than in the unaffected area in both groups. However, reperfused region O2 extraction was lower in the nitroglycerin treated than control group. There was a greater number of arteries and veins with reduced O2 saturations in the control group reperfused area compared to the nonischemic area. Nitroglycerin decreased the number of low O2 saturation vessels in the reperfusion area. Reperfusion alone does not restore the ratio of O2 supply to O2 consumption to control values, while nitroglycerin significantly improves this ratio. Thus nitroglycerin appears to better match the increased flow during reperfusion with microregional O2 consumption.

Effect of reperfusion on O2 supply/consumption balance in ischemic canine left ventricle

This study was designed to assess the effects of reperfusion on regional O2 supply and O2 consumption of ischemic areas of the myocardium in 15 anesthetized open-chested dogs. The left anterior descending coronary artery (LAD) was occluded for 6 h (n = 8), 2 h (n = 5), 2-h occlusion followed by 4-h period of reperfusion (n = 7), and 10-min occlusion followed by 90-min period of reperfusion (n = 3). Small artery and vein O2 saturations obtained microspectrophotometrically were combined with regional flow measurements using radioactive microspheres to determine regional myocardial O2 consumption. Coronary occlusion for 2 or 6 h significantly reduced mean flow to 15 +/- 8 and 13 +/- 14 ml/min/100 g (mean +/- SD), respectively, in the affected LAD areas as compared to 128 +/- 26 and 113 +/- 46 ml/min/100 g in the non-ischemic areas. In the 4-h reperfusion group, reperfusion increased the average flow (60 +/- 42 ml/min/100 g). O2 extraction was greater in the ischemic area than in the unaffected area after both occlusion and 4-h reperfusion. In the affected area, O2 consumption was reduced by 84% after 6-h occlusion. Reperfusion for 4 h increased O2 consumption toward normal values. Coronary artery occlusion produced an increase in the number of arteries and veins with reduced O2 saturations and this was not affected by reperfusion. Short-term occlusion had no significant O2 supply effects after 90 min of reperfusion. It can be concluded that even though there was an increased O2 consumption as a consequence of reperfusion, O2 consumption still appeared to be flow-limited as indicated by the microregions of low O2 supply and/or high O2 extraction.

Beta-adrenoceptor stimulation and blockade during myocardial ischemia in dogs: effect on cardiac O2 supply and consumption

The effect of beta-adrenoceptor blockade and activation on ischemic regional and microregional myocardial O2 supply/consumption parameters was assessed in 28 open chest, anesthetized dogs. Ten minutes after LAD occlusion, dogs were given i.v. saline, 2 mg/kg propranolol, 0.2 mg/kg pindolol, or 1 microgram/kg per min isoproterenol. Coronary blood flow was determined using radioactive microspheres before and 2 h after LAD occlusion while O2 supply/consumption parameters were determined using microspectrophotometry. Ischemia resulted in a 66% reduction in subendocardial flow in controls in the ischemic zone and no experimental treatment significantly altered this flow. Pindolol resulted in a significant improvement in the ischemic regional subendocardial/subepicardial flow ratio (from 0.69 in the control ischemic region to 0.88 during pindolol treatment). O2 extractions were significantly increased and O2 consumptions were significantly depressed in the ischemic regions of all groups. O2 extractions were increased to a lesser degree in the ischemic region with the use of pindolol and propranolol. Propranolol and pindolol both significantly decreased the proportion of veins with low (0-20%) O2 saturations in the ischemic region indicating an improved microregional distribution of blood flow and/or O2 consumption within the ischemic region.

Differences in coronary flow and myocardial metabolism at rest and during pacing between patients with obstructive and patients with nonobstructive hypertrophic cardiomyopathy

Fifty patients with hypertrophic cardiomyopathy underwent invasive study of coronary and myocardial hemodynamics in the basal state and during the stress of pacing. The 23 patients with basal obstruction (average left ventricular outflow gradient, 77 +/- 33 mm Hg; left ventricular systolic pressure, 196 +/- 33 mm Hg, mean +/- 1 SD) had significantly lower coronary resistance (0.85 +/- 0.18 versus 1.32 +/- 0.44 mm Hg X min/ml, p less than 0.001) and higher basal coronary flow (106 +/- 20 versus 80 +/- 25 ml/min, p less than 0.001) in the anterior left ventricle, associated with higher regional myocardial oxygen consumption (12.4 +/- 3.6 versus 8.9 +/- 3.3 ml oxygen/min, p less than 0.001) compared with the 27 patients without obstruction (mean left ventricular systolic pressure 134 +/- 18 mm Hg, p less than 0.001). Myocardial oxygen consumption and coronary blood flow were also significantly higher at paced heart rates of 100 and 130 beats/min (the anginal threshold for 41 of the 50 patients) in patients with obstruction compared with those without. In patients with obstruction, transmural coronary flow reserve was exhausted at a heart rate of 130 beats/min; higher heart rates resulted in more severe metabolic evidence of ischemia with all patients experiencing chest pain, associated with an actual increase in coronary resistance. Patients without obstruction also demonstrated evidence of ischemia at heart rates of 130 and 150 beats/min, with 25 of 27 patients experiencing chest pain. In this group, myocardial ischemia occurred at significantly lower coronary flow, higher coronary resistance and lower myocardial oxygen consumption, suggesting more severely impaired flow delivery in this group compared with those with obstruction. Abnormalities in myocardial oxygen extraction and marked elevation in filling pressures during stress were noted in both groups. Thus, obstruction to left ventricular outflow is associated with high left ventricular systolic pressure and oxygen consumption and therefore has important pathogenetic importance to the precipitation of ischemia in patients with hypertrophic cardiomyopathy. Patients without obstruction may have greater impairment in coronary flow delivery during stress.

Effect of aortic stenosis on oxygen balance in partially ischemic myocardium

Valvar aortic stenosis can result in myocardial underperfusion with or without coronary obstruction. The purpose of this study was to determine how hearts with valvar aortic stenosis without hypertrophy can maintain their oxygen supply/consumption balance with partial left anterior descending coronary artery (LAD) occlusion. Open-chested, anesthetized dogs (n = 9) were subjected to mild valvar aortic stenosis and then to a reduction of LAD flow to 50% of baseline, while controls (n = 9) received partial LAD occlusion without aortic stenosis. Blood flows were determined before and after aortic stenosis and after LAD occlusion using radioactive microspheres. The hearts were then removed for microspectrophotometric analysis of regional venous and arterial oxygen saturation. Aortic stenosis resulted in a pressure gradient of approximately 50 mm Hg, representing mild aortic stenosis. Only a slight increase in myocardial blood flow was seen with aortic stenosis. Ischemia resulted in a significant drop in blood flow in control (40%) and aortic stenosis (55%) animals compared with their own preocclusion values. These ischemic region flows were not different from each other. Aortic stenosis itself did not alter oxygen extraction, although partial occlusion similarly increased extraction for all groups in the ischemic zone. The LAD occlusion resulted in a decreased oxygen consumption in the occluded region of all groups, with no differences noted between control and aortic stenosed animals. Thus, mild, acute aortic stenosis without hypertrophy does not appear to significantly increase the severity of an ischemic episode precipitated by partial LAD occlusion.

Effect of complement depletion on O2 supply and consumption in ischemic dog myocardium

The purpose of this study was to determine whether depletion of serum complement can decrease the severity of an ischemic episode by improving regional O2 supply and consumption parameters in the ischemic region of the heart. Fourteen anesthetized dogs with serum complement intact or depleted (100 U/kg cobra venom factor given 8 hrs before) were subjected to left anterior descending coronary artery (LAD) occlusion for 6 hrs. Myocardial blood flows were determined before and 6 hrs after LAD occlusion using radioactive microspheres. Regional arterial and venous O2 saturations were determined using microspectrophotometry. In control animals, flow decreased from 122 +/- 42 to 13 +/- 14 ml/min/100 g (mean +/- SD) in the occluded LAD region. With complement depletion, LAD occlusion resulted in a flow reduction in the ischemic region (38 +/- 29 ml/min/100 g), but to a lesser degree than seen in the same region in control animals, especially in the subendocardium. O2 consumption was decreased in the ischemic region of both treatment groups, though O2 consumption was higher in this region in complement depleted animals compared to the values in control animals. The O2 supply/consumption ratio was decreased similarly in the ischemic region of control and complement depleted groups. Thus, with complement depletion, flow to the ischemic zone was improved but this region was still flow restricted. The flow increase during complement depletion was sufficient to allow an increased O2 utilization in the ischemic region.

Preservation of myocardial oxygen balance and functional reserve by coronary vasodilators

Reduced myocardial function at very high heart rates may be due to limited coronary blood supply. The effects of the vasodilators nitroglycerin (10 micrograms kg-1 min-1) and elevated CO2 upon regional function during tachycardia were studied. In open-chest anaesthetized dogs, regional contractile force, epicardial tissue blood flow and local NADH redox level were recorded during graded ventricular pacing. It was found that the vasodilating action of nitroglycerin in the unpaced heart was much lower than produced by CO2 (23.6 +/- 5.8% vs. 137.6 +/- 33.5%). Maximal pacing at 275 bpm caused only a moderate flow elevation in control (20 +/- 6.8%) and CO2 conditions (20.3 +/- 4.03%), but marked vasodilation during nitroglycerin infusion (85.2 +/- 14.6%). Regional function during tachycardia was improved similarly by both vasodilators. NADH levels increased with heart rates under all experimental conditions, but the absolute NADH levels were consistently lower following vasodilator treatments. The lowest NADH levels were observed during nitroglycerin treatment at all heart rates. It is suggested that nitroglycerin augments myocardial functional reserve by preserving oxygen balance more than predicted by its vasodilatory effect alone.

Regional O2 supply/consumption in normal and ischaemic rabbit myocardium: effect of nifedipine

The effect of nifedipine infusion on myocardial O2 supply and consumption in flow-restricted and normal regions of the left ventricle was tested in anaesthetized open-chest rabbits after ligation of the left anterior descending coronary artery for one hour. Ten min after occlusion, nifedipine-treated animals were given either a low or high dose of the drug: a 5 micrograms/kg bolus followed by 1 micrograms/kg per min infusion or a 10 micrograms/kg bolus and 10 micrograms/kg per min infusion, respectively. Regional blood flow was measured before and after occlusion using radioactive microspheres and O2 saturation was measured microspectrophotometrically; the Fick Principle was then employed to determine regional O2 consumption. After a 60 min occlusion, blood flow was reduced overall to 51% of pre-ligation flows in the occluded region, and treatment with nifedipine or vehicle did not significantly alter this flow reduction. Blood flow in nonoccluded regions increased 1.6-fold only with the high dose of nifedipine and was unchanged in all other groups. Microspectrophotometric analysis of low dose nifedipine and control hearts showed that O2 extraction was greater in occluded than in normal myocardium (9.0, s.d. = 0.9, ml O2/100 ml blood vs 7.2, s.d. = 0.7, respectively) and that subendocardial extraction exceeded subepicardial. These data suggest that nifedipine administration at this dose had no apparent beneficial effect on O2 supply or O2 consumption in normal or flow-restricted regions of the left ventricle during 1 h of coronary artery occlusion.

Improvement in regional myocardial O2 supply and O2 consumption by nitroglycerin during ischemia

In eight open-chest anesthetized dogs, nitroglycerin (10 micrograms/kg per min) was infused intravenously for 2 h, beginning 10 min following ligation of the left anterior descending coronary artery. Oxygen supply, (radioactive microspheres), extraction (microspectrophotometry) and consumption were determined in subepicardial and subendocardial regions of both ischemic and non-ischemic myocardium, and compared to eight control hearts. In control, coronary occlusion reduced both subepicardial and subendocardial blood flow by 49.5% and 79.5% respectively. In the presence of nitroglycerin, depression of blood flow to the occluded regions was significantly less marked (-79.5% in control and -26.6% in the nitroglycerin group in the subendocardium). O2 extraction was significantly lowered by nitroglycerin in all areas. Regional O2 consumption was significantly lower in the control occluded than non-occluded regions; no regional O2 consumption differences were observed following nitroglycerin. In the occluded regions, nitroglycerin reduced the number of veins with very low O2 saturation. It is concluded that nitroglycerin improves the O2 supply/consumption balance in ischemia by redistribution of blood flow and possibly by alterations in local O2 consumption.

The effect of graded doses of norepinephrine on the O2 supply/consumption balance in ischemic and nonischemic rabbit myocardium

The purpose of this study was to determine the effect of graded doses of norepinephrine on the regional O2 supply/consumption ratio in ischemic rabbit hearts. Open chested, anesthetized rabbits were used and regional flows, O2 extraction, consumption and O2 supply/consumption ratios were determined before and 1 h after occlusion of the left anterior descending coronary artery in controls and animals given 0.1, 1.0 and 10 micrograms/kg per min norepinephrine (NE). After occlusion in controls, mean myocardial blood flow decreased 40% in the occluded region. Blood flow was also depressed in the occluded region for all NE doses compared to their own preocclusion values, but was higher in these regions compared to control animals. O2 extraction was higher in the occluded region compared to the nonoccluded region for all groups; however these values were lower in the NE groups compared to controls. NE increased O2 consumption in the occluded and nonoccluded regions compared to control group values. The O2 supply/consumption ratio was depressed in the occluded region in all groups compared to the nonoccluded region, though no differences were seen between NE and control groups. Thus, increases in the blood flow to the occluded region were proportional to the increases in O2 consumption with infusion of NE, indicating that a reserve exists and can be utilized with NE.

Role of propranolol in improvement of the relationship between O2 supply and consumption in an ischemic region of the dog heart

Several aspects of the myocardial O(2) supply/consumption relationship were determined after coronary artery occlusion and subsequent beta-adrenergic blockade in 16 anesthetized open-chest dogs. Small artery and vein O(2) saturations, and hence extraction, were obtained microspectrophotometrically and combined with radioactive microsphere blood flow determinations to calculate regional myocardial O(2) consumption. Eight dogs remained untreated after coronary artery ligation while another group was given 2 mg/kg propranolol, 10 min after occlusion. Untreated occlusion resulted in decreased arterial and especially venous O(2) saturations, indicating an increased O(2) extraction. Ischemic O(2) consumption was reduced and the subendocardial/subepicardial consumption ratio was reversed (1.26 vs. 0.37) due to the pattern of occluded area flow. Calculated O(2) supply/consumption also decreased. Propranolol produced no significant changes in volume or distribution of flow within the ischemic region while reducing flow, extraction, and consumption in the unoccluded region. The heterogeneity of arterial and particularly venous O(2) saturations within the ischemic region decreased dramatically. Venous O(2) saturations were elevated relative to the control group resulting in a reduced O(2) extraction. The decrease in heterogeneity of arterial and venous O(2) saturations suggest that propranolol eliminates microregions of relatively high O(2) extraction, consumption, and/or a majority of vessels with extremely low flow. This leads to a significant improvement in the O(2) supply/consumption ratio in the ischemic myocardium of the dog. This may be due to a reduction in the heterogeneity and level of beta(1)-adrenergic receptor activity within the heart.