Mechanisms of improved ischemic regional dysfunction by bradycardia. Studies on UL-FS 49 in swine

Effect of coronary hyperperfusion on regional myocardial function and oxygen consumption of stunned myocardium in pigs

The "Gregg phenomenon" implies that myocardial function and oxygen consumption (MVO2) increase when coronary perfusion is enhanced within or above the autoregulatory range. We have recently demonstrated that the "Gregg phenomenon" has no significance for regional myocardial function and MVO2 in anesthetized swine in situ. There is, however, some evidence that the "Gregg phenomenon" may exist within stunned myocardium. To test whether coronary hyperperfusion increases regional myocardial function and MVO2 in stunned myocardium, in six anesthetized swine the left anterior descending coronary artery (LAD) was cannulated and perfused at constant pressure (CAP) using an extracorporal circuit. The coronary vein which parallels the LAD was cannulated to allow measurement of regional MVO2 and regional systolic wall thickening (WT%) of the anterior myocardium was assessed using sonomicrometry. Blood flow (CBF) to the LAD was increased by increasing CAP within the extracorporal circuit or by intracoronary adenosine infusion (150 micrograms/min). In normal myocardium, increasing CBF from 71.4 +/- 19.7 (SD) to 156.7 +/- 48.8 ml/min/100 g by increasing CAP from 100 +/- 10 to 190 +/- 10 mm Hg or increasing CBF from 75.1 +/- 29.1 to 189.2 +/- 45.8 ml/min/100 g by intracoronary adenosine infusion did not increase WT% (34.3 +/- 12.2% vs 32.1 +/- 10.6% and 32.3 +/- 10.7% vs 30.1 +/- 13.2%, respectively). MVO2 was not changed during enhanced CAP (6.94 +/- 1.05 vs 8.10 +/- 1.08 ml/min/100 g) and during intracoronary adenosine infusion (6.67 +/- 1.45 vs 7.30 +/- 2.23 ml/min/100 g). Twenty min of hypoperfusion followed by 30 min of reperfusion depressed WT% by 47% (p less than 0.05). However, MVO2 was only decreased by 23% (NS). In the stunned myocardium, increasing CBF from 62.1 +/- 36.4 to 157.1 +/- 60.0 ml/min by increasing CAP was not associated with an increase in WT%. MVO2, however, increased from 5.14 +/- 1.07 to 8.88 +/- 1.83 ml/min/100 g (p less than 0.05). Comparable results were achieved when CBF was increased from 60.3 +/- 28.7 to 176.9 +/- 48.5 ml/min by intracoronary adenosine infusion. WT% was unaffected, while MVO2 increased from 4.69 +/- 0.92 to 9.46 +/- 3.39 ml/min/100 g (p less than 0.05). Thus, increasing coronary perfusion within or above the autoregulatory range increases MVO2 in stunned myocardium, but without a simultaneous increase in regional myocardial function.

The relationship between regional blood flow and contractile function in normal, ischemic, and reperfused myocardium

The prevailing paradigm of coronary physiology and pathophysiology is that a balance between blood flow (i.e., supply) and function (i.e., demand) exists under normal conditions and that an imbalance between supply and demand occurs during ischemia. However, this paradigm is derived largely from studies relating changes in total coronary inflow to global ventricular function. The present article examines the relationship between myocardial blood flow and function on a regional level and proposes that a change may be needed in the current paradigm of coronary pathophysiology. In normal myocardium, considerable heterogeneity of regional blood flow exists, indicating either similar heterogeneity of metabolic demand and function or questioning the precision of metabolic coupling between flow and function. After the onset of ischemia, a transient imbalance between the reduced blood flow and function may exist. However, myocardial function rapidly declines and during early steady-state ischemia regional myocardial blood flow and function are once again evenly matched. Such supply-demand balance may persist over prolonged periods of ischemia enabling the myocardium to remain viable through reduction of energy expenditure for contractile function, i.e., to "hibernate". Whereas in "hibernating" ischemic myocardium, regional myocardial blood flow and function are both reduced but appropriately matched to one another, flow and function appear to be largely uncoupled in reperfused "stunned" myocardium. The clinical identification of viable but ischemic (hibernating) and postischemic (stunned) myocardium is of utmost importance in patients undergoing reperfusion procedures. A new paradigm of coronary and myocardial pathophysiology, encompassing a regional as well as a global view of perfusion and function, will have to include explanations for phenomena such as myocardial hibernation and myocardial stunning.

Acute effects of parenteral beta-blockade on regional ventricular function of infarct and noninfarct zones after reperfusion therapy in humans

Although the mechanism is unknown, clinical trials have suggested that intravenous beta-adrenergic blockade may prevent early cardiac rupture after myocardial infarction. Previous studies have examined effects of beta-blockers on global left ventricular function after myocardial infarction; however, few data exist regarding their immediate effects on regional function or in patients after successful reperfusion. Therefore, 65 patients in whom thrombolysis with or without coronary angioplasty achieved reperfusion at 4.6 +/- 1.7 h from symptom onset were studied. Low osmolarity contrast ventriculograms were obtained immediately before and after administration of 15 mg of intravenous metoprolol (n = 54) or placebo (n = 11). Intravenous metoprolol immediately decreased heart rate (from 92 to 76 beats/min, p less than 0.0001), increased left ventricular diastolic volume (from 150 to 163 ml, p less than 0.001) and systolic volume (from 72 to 77 ml, p less than 0.0005) but did not change systolic and diastolic pressures. Although there was no difference in ejection fraction after metoprolol, centerline chord analysis revealed reduced noninfarct zone motion (from 0.41 to 0.12 SD/chord, p less than 0.05), improved infarct zone motion (from -3.1 to -2.9 SD/chord, p less than 0.01) and smaller circumferential extent of hypokinesia (from 30 to 27 chords, p less than 0.05). Patients with dyskinesia of the infarct zone had the most striking improvement in infarct zone wall motion. Because these changes occurred immediately after beta-blockade, they could not be attributed to myocardial salvage. No significant changes in heart rate, left ventricular volumes or regional wall motion were apparent in the control group.(ABSTRACT TRUNCATED AT 250 WORDS)

Heart rate and beta-adrenergic mechanisms in acute myocardial infarction

Increased heart rate is an independent predictor of mortality in patients with acute myocardial infarction. Elevated heart rate is due to increased sympathetic activity and/or decreased parasympathetic activity. In placebo-controlled trials beta-blockers are known to reduce mortality as well as morbidity and these effects are most evident among patients with elevated heart rate. Studies on circadian variation have demonstrated that there is an increased sympathetic activity in the morning as well as a more frequent onset of ischemic attacked including acute myocardial infarction and sudden death. There seems to be a close relationship between increased sympathetic activity and the onset of ischemic events which can be prevented by prophylactic institution of a beta-blocker.

Mechanisms of benefit in the ischemic myocardium due to heart rate reduction

The studies reviewed here examine the role of heart rate reduction in the beneficial effect observed following beta-adrenoceptor blockade during exercise-induced ischemia in conscious dogs. To further study the effects of heart rate reduction on regional blood flow in an ischemic bed without collateral circulation, anesthetized swine with controlled coronary perfusion were also studied. Measurements of regional myocardial blood flow (microspheres) and contractile function (sonomicrometers) during steady state exercise in dogs with chronic coronary artery stenosis indicated the existence of severe regional contractile dysfunction and subendocardial ischemia. The administration of beta-adrenoceptor blockade (1.0 mg/kg atenolol) improved regional contractile function when heart rate was reduced from 220 to 165 beats/min. Atrial pacing during exercise to prevent the bradycardia following beta-adrenoceptor blockade eliminated the improved regional function and blood flow. Thus, the beneficial effect of beta-blockade was only apparent when exercise heart rate was reduced. In anesthetized swine with constant inflow coronary perfusion, two levels of coronary hypoperfusion were examined at heart rates of 91 beats/min or 55 beats/min. Bradycardia was produced using the bradycardic agent UL-FS 49 (0.3 mg/kg). Regional contractile function and subendocardial blood flow were markedly improved at the lower heart rate for either level of reduced coronary perfusion, indicating a redistribution of blood flow towards the subendocardium. The improvement in contractile function was larger than predicted on the basis of the improvement in blood flow per min to the subendocardium. Independent relationships between regional contractile function and the subendocardial blood flow per min were observed for each heart rate. Thus, the studies in conscious exercising dogs indicated that heart rate reduction is an essential mechanism for the improvement of ischemic regional myocardial contractile function during exercise by beta-blockade. This is likely the result of the marked improvement in subendocardial blood flow per beat which accompanies the reduced heart rate; regional myocardial blood flow per beat appears to be a predictor of regional contractile function during ischemia both at rest and during exercise.

Influence of heart rate on mortality after acute myocardial infarction

Elevated heart rate (HR) during hospitalization and after discharge has been predictive of death in patients with acute myocardial infarction (AMI), but whether this association is primarily due to associated cardiac failure is unknown. The major purpose of this study was to characterize in 1,807 patients with AMI admitted into a multicenter study the relation of HR to in-hospital, after discharge and total mortality from day 2 to 1 year in patients with and without heart failure. HR was examined on admission at maximum level in the coronary care unit, and at hospital discharge. Both in-hospital and postdischarge mortality increased with increasing admission HR, and total mortality (day 2 to 1 year) was 15% for patients with an admission HR between 50 and 60 beats/min, 41% for HR greater than 90 beats/min and 48% for HR greater than or equal to 110 beats/min. Mortality from hospital discharge to 1 year was similarly related to maximal HR in the coronary care unit and to HR at discharge. In patients with severe heart failure (grade 3 or 4 pulmonary congestion on chest x-ray, or shock), cumulative mortality was high regardless of the level of admission HR (range 61 to 68%). However, in patients with pulmonary venous congestion of grade 2, cumulative mortality for patients with admission HR greater than or equal to 90 beats/min was over twice as high as that in patients with admission HR less than 90 beats/min (39 vs 18%, respectively); the same trend was evident in patients with absent to mild heart failure (mortality 18 vs 10%, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)