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

Anatomic and physiologic heterogeneity in patients with syndrome X: an intravascular ultrasound study

OBJECTIVES

We used intravascular ultrasound imaging of the epicardial vessels to assess coronary morphology, vasomotor response to exercise and exercise-vasomotion after beta-adrenoceptor blockade in patients with syndrome X.

BACKGROUND

Syndrome X is defined as chest pain, abnormal exercise test results and normal coronary angiographic findings. Because of the limitations of coronary angiography, intravascular ultrasound was used to define coronary pathophysiology.

METHODS

Thirty patients with syndrome X were studied with intravascular ultrasound imaging (30 MHz, 4.3F catheter) of all three major epicardial vessels. Supine arm exercise was performed during coronary imaging. Lumen area was assessed at rest and during peak exercise. The exercise-imaging protocol was repeated after loading with 0.1 mg/kg body weight of intravenous propranolol.

RESULTS

Three morphologic groups were identified using intravascular ultrasound: normal coronary arteries (no plaque, intimal thickness < 0.25 mm, n = 12), atheromatous disease (mean [+/- SD] area stenosis 37.9 +/- 7.2%, n = 10) and marked intimal thickening (0.73 +/- 0.11 mm, n = 8). Patients with normal coronary arteries displayed a vasodilatory response to exercise (+16.9% area increase); patients with abnormal coronary arteries displayed a vasoconstrictive response to exercise (-17.4% in the group with plaque; -17.6% in the group with intimal thickening). Propranolol loading attenuated the vasodilatory response in the group with normal coronary arteries (+6.4% area increase) and attenuated the vasoconstrictive response in the two groups with abnormal coronary arteries (-8.0% in the group with plaque; -8.8% in the group with intimal thickening).

CONCLUSIONS

Most patients with syndrome X have abnormal coronary arteries by intravascular ultrasound. Intravascular ultrasound identifies three distinct morphologic groups: normal coronary arteries, atheromatous plaque and intimal thickening. Exercise-vasomotion is normal in patients with syndrome X who have normal coronary arteries by ultrasound; patients with abnormal arteries (plaque or intimal thickening) have an abnormal (constrictive) response to exercise. Propranolol loading attenuates vasoreactivity in all subgroups, suggesting divergent therapeutic utility.

Effect of intravenous propranolol on coronary vasomotion at rest and during dynamic exercise in patients with coronary artery disease

Coronary vasomotion was studied at rest and during bicycle exercise with biplane quantitative coronary arteriography in 28 patients with coronary artery disease. Patients were divided into two groups; the first 18 patients served as controls (group 1), and the next 10 patients were treated with propranolol 0.1 mg/kg, which was infused intravenously before exercise (group 2). Luminal area of a normal and a stenotic vessel segment was determined at rest, during supine bicycle exercise, and 5 minutes after sublingual administration of 1.6 mg nitroglycerin after exercise. In group 1, the normal vessel showed vasodilation (+16%, p less than 0.001) during exercise, whereas the stenotic vessel segment showed vasoconstriction (-31%, p less than 0.001). After sublingual administration of nitroglycerin, there was coronary vasodilation of both normal (+36%, p less than 0.001 vs. rest) and stenotic (+20%, p less than 0.001) vessel segments. Patients with angina pectoris during supine exercise (n = 10) had significantly (p less than 0.05) more vasoconstriction (-36%) than patients without angina (-23%). In group 2, intravenous administration of propranolol at rest was associated with a decrease in luminal area of both normal (-24%, p less than 0.001) and stenotic (-43%, p less than 0.001) vessel segments; however, during subsequent exercise, both normal (-2%, p = NS vs. rest) and stenotic (-3%, p = NS vs. rest) vessel segments dilated when compared with the measurements after propranolol. Administration of nitroglycerin further increased luminal area of both vessel segments (normal segment, +23%, p less than 0.001; stenotic segment, +46%, p less than 0.001 vs. rest). It is concluded that dynamic exercise in patients with coronary artery disease is associated with coronary vasodilation of the normal and vasoconstriction of the stenotic coronary arteries. Patients with exercise-induced angina had significantly more stenosis vasoconstriction than patients without angina although minimal luminal area at rest was similar. Intravenous administration of propranolol is accompanied by a significant decrease in coronary luminal area of both normal and stenotic vessel segments at rest, which is overridden by dynamic exercise and sublingual nitroglycerin. The reduction in myocardial oxygen consumption and the prevention of exercise-induced stenosis vasoconstriction might explain the beneficial effect of beta-blocker treatment in most patients with coronary artery disease.

Relationship between beta-adrenoceptors and coronary blood flow heterogeneity

The purpose of this study was to investigate the hypothesis that the heterogeneous distribution of beta adrenoceptors contributes to the control of flow heterogeneity in the canine myocardium. beta adrenoceptor density and affinity were measured simultaneously with coronary blood flow in multiple sections of the left ventricle of 14 anesthetized open chest dogs. Radioactive microspheres were used for the measurement of blood flow. The left ventricle was cut into 15 subepicardial (EPI) and 15 subendocardial (ENDO) sections. Receptor density (Bmax) and dissociation constant (Kd) were measured using [125I]-iodopindolol. The average control myocardial blood flow (MBF) was 86 +/- 15 ml/min/100 g. Isoproterenol (0.5 micrograms/kg/min) increased MBF by 82%, whereas propranolol (2 mg/kg) reduced MBF by 13%. The mean value for Bmax was unaltered by either treatment. Under control conditions, a significant positive correlation (r = 0.26, p less than 0.0001) was observed between Bmax and blood flow. In the isoproterenol treatment group, this correlation was enhanced (r = 0.49, p less than 0.0001). Beta adrenoceptor blockade led to a negative correlation. Kd showed no overall correlation with blood flow. Kd but not Bmax was significantly higher in the EPI than in the ENDO and in the base compared to the apex. There appears to be a direct linear relationship between the distribution of beta adrenoceptors and MBF distribution which is enhanced under conditions of high beta adrenergic activity. There is a correlation between beta adrenoceptor activity and blood flow distribution in the canine myocardium.

Improved distribution of regional oxygenation in denervated ischemic dog myocardium

The role of the adrenergic nervous system in the response to coronary artery occlusion has been examined using surgical and chemical denervation techniques. Experiments were conducted on four groups of dogs (n = 18): 1) untreated controls; 2) intrapericardial denervation immediately prior to coronary ligation; 3) surgical denervation 2 weeks prior to the experiment; and 4) chemical sympathectomy 5 days prior to the experiment with 6-hydroxydopamine (50 mg/kg). Small artery and vein O2 saturations were obtained microspectrophotometrically and combined with radioactive microsphere blood flow determinations to calculate regional myocardial O2 consumption in open chest dogs. Denervation significantly reduced the preocclusion heart rate from 165 +/- 16 beats/min in the control to 114 +/- 13 in the chronic surgically denervated and to 137 +/- 15 in the chemically sympathectomized groups. After 2 hours of occlusion, the O2 consumption and flow were similar in the nonischemic area except for lower values in the surgically denervated group. Total coronary blood flow and O2 consumption in the occluded regions were not significantly affected by chronic denervation. However, significant elimination of areas with low venous O2 saturation (less than 20%) were found in the ischemic myocardium of the chronically denervated groups as compared with the control or with the acutely denervated dogs. The mean venous O2 saturation was found to be significantly higher in all regions of these two groups as compared with the control. The O2 extraction was also lowered. Thus, chronic denervation reduced microregional heterogeneity of oxygenation in the ischemic myocardium.(ABSTRACT TRUNCATED AT 250 WORDS)

Myocardial oxygen consumption, oxygen supply/demand heterogeneity, and microvascular patency in regionally stunned myocardium

Although oxygen consumption closely parallels mechanical work in the normal heart, previous studies have found that stunned myocardium may have normal or even increased oxygen consumption despite depressed function. In this study we used microspectrophotometry to measure the oxygen saturations within arteries and veins of less than 100 micron diameter in quick-frozen biopsy samples from normal and regionally stunned myocardium of 10 open-chest anesthetized dogs. Regional myocardial blood flow, measured by radioactive microspheres, was similar in stunned and normal regions, as was mean arteriolar oxygen saturation. However, mean venous oxygen saturation was lower in the stunned region (epicardium 38.0% vs 43.8%, p less than .02; endocardium 36.2% vs 39.5%, p = .12), indicating increased oxygen extraction and consumption, despite a marked reduction in mean systolic segmental shortening from 14.4% to 0.5%. In addition, there was greater vein-to-vein heterogeneity of oxygen saturation in the stunned region, with an excess of veins having low saturations (statistically significant in epicardium, nonsignificant trend in endocardium). Microvascular injections studies with Microfil or drafting ink revealed filling of over 95% of arterioles and 85% of capillaries in the stunned region, similar to the findings in the normal region. Our results are consistent with an inefficient transfer of energy into myocyte contraction or an increased use of energy for noncontractile activities in stunned myocardium. In addition, the finding of increased heterogeneity of oxygen extraction suggests that the injury to stunned myocardium may not be uniform to all contractile elements, but instead may be focally and irregularly distributed.

The effect of diltiazem on ST-segment elevation and myocardial blood flow distribution during pacing-induced ischemia

The purpose of this study was to determine if diltiazem can reduce the severity of pacing-induced ischemia independently of increases in overall and microregional ischemic blood flow. Sixteen anesthetized dogs were subjected to atrial pacing and had their left anterior descending coronary arteries (LAD) occluded until significant ST-elevation occurred. Cessation of pacing resulted in abolition of ST-segment elevation. ST-elevation as well as hemodynamics were measured during 5 min periods of pacing + LAD stenosis before, and 10, 40 and 70 min after treatment with intracoronary (i.c., just distal to the stenosis) diltiazem (1.8 micrograms/kg), i.v. diltiazem (180 micrograms/kg) or saline. Myocardial blood flow was measured using radioactive microspheres under baseline conditions, pacing, pacing + stenosis, and pacing + stenosis + drug (70 min post-drug). Both i.c. and i.v. diltiazem significantly and similarly reduced pacing-induced ST-elevation at 40 and 70 min post-drug with the highest measured reductions occurring for both at 70 min (50-60% reduction). Overall ischemic regional myocardial blood flow was unaffected by i.c. and i.v. diltiazem. Diltiazem given i.v. resulted in reduced flow in the lightly ischemic region and increased flows in the subepicardial half of the severely ischemic region. Diltiazem given i.c. resulted in a reduced subepicardial flow in the lightly ischemic region with no other changes occurring in the other regions. Thus, both i.c. and i.v. diltiazem can reduce the severity of pacing-induced ischemia and, in the doses given, in an equivalent fashion. Diltiazem also seems to be able to reduce severity of ischemia in a manner independent of increases in ischemic region flow and in fact can reduce flow in marginally ischemic tissue.

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.

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.

Selective enhancement of function of stunned myocardium by increased flow

Although augmentation of flow does not improve the performance of normal myocardium, the hyperemic response after brief coronary occlusion is associated with transient hyperfunction in the previously ischemic region. In this study we assessed the effect of vasodilator-enhanced coronary blood flow on the systolic function of postischemic stunned myocardium. In 18 open-chest, anesthetized dogs the anterior descending artery was occluded for 5 min, followed by a 10 min period of reflow, repeated 12 times with a final 90 min recovery period. After the recovery period, either 0.06 mg/min dipyridamole (n = 6), 1 mg/min papaverine (n = 6), or 1.5 micrograms/kg/min nitroglycerin (n = 6) was infused intravenously for 15 min. Regional myocardial blood flow, which had returned to normal before administration of vasodilator, was increased 150% above baseline by dipyridamole and 80% by papaverine, but was unchanged by nitroglycerin. Segmental shortening decreased after repeated occlusions: from 17.5% to 0.9% in the group later treated with dipyridamole, from 18.6% to 6.7% in the papaverine group, and from 19.2% to-1.9% in the nitroglycerin group (p less than .005 for all groups). Segmental shortening increased to 8.8% after dipyridamole, 13.6% after papaverine, and 5.1% after nitroglycerin (p less than .05 for all groups), although the load-independent end-systolic pressure-length relationship (ESPLR) showed a significant shift to the left, reflecting enhanced performance, only after dipyridamole and papaverine. For all dogs combined, the percent improvement in ESPLR was correlated with the percent increase in flow (R = -.73, p less than .001). Performance was unchanged in the control region despite similar augmentation of flow.(ABSTRACT TRUNCATED AT 250 WORDS)

Control of the heterogeneity of O2 saturation in small myocardial veins

There is a wide variability in the measured O2 saturations (SvO2) of small myocardial veins (20-100 microns). The purpose of the present study was to examine several factors that contributed to this variability. Studies were conducted on anesthetized open-chest dogs. The role of beta-adrenoceptors and flow in the control of the heterogeneity of SvO2 was assessed. The hearts were quick-frozen in liquid N2 and the SvO2 in the subepicardial and subendocardial (ENDO) regions of the left ventricule was determined by microspectrophotometry. Under control conditions, ENDO SvO2 averaged 35% with a variance of 337. The control variance was much more than that of the methodological errors. With propranolol or practolol, variance was reduced by about half, e.g. ENDO SvO2 variance was 145 with propranolol. This reduction in SvO2 variance could be prevented by isoproterenol. When flow is reduced by partial occlusion the SvO2 values were skewed toward lower saturations. When nitroglycerin is administered during this partial occlusion, the SvO2 distribution is restored toward normal. It can be concluded that some of the variability of SvO2 in the heart is control by beta 1-adrenoceptors. There also appears to be an inverse relationship between coronary blood flow and SvO2 heterogeneity. Further work is necessary to determine what other factors control the variability of SvO2.

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.

Prevention of ventricular fibrillation during acute myocardial infarction by intravenous propranolol

A trial of intravenous followed by oral propranolol, started within 4 h of onset of suspected myocardial infarction and continued over 27 h, was carried out in 735 patients; 364 received propranolol, 371 were controls. Ventricular fibrillation during the first 48 h after entry to the trial occurred in 2 treated patients and in 14 controls (p = 0.006). Rates of hospital mortality, complications other than ventricular fibrillation, and progression from threatened to completed infarction did not differ between treated and control patients. Ventricular fibrillation was not apparently prevented by prior beta-blocker treatment, which was not a reason for exclusion from the trial. This intravenous/oral propranolol regimen seems to prevent ventricular fibrillation due to evolving myocardial infarction.