The effects of nitroglycerin on the coronary blood flow and oxygen consumption of the myocardium in anesthetized dogs

THE EFFECT OF NITROGLYCERIN ON LEFT VENTRICULAR WALL TENSION IN FIXED ORIFICE AORTIC STENOSIS

Symptoms of myocardial ischemia occur in aortic stenosis in the absence of coronary artery disease. Increased left ventricular wall tension, a principal determinant of myocardial oxygen requirements, may contribute to these symptoms. This investigation was undertaken during the course of transseptal left heart catheterization in order to study the effect of nitroglycerin on left ventricular wall tension in eight patients with fixed orifice aortic stenosis. In seven of the subjects, nitroglycerin significantly reduced tension-time index per beat and per minute. In addition, left ventricular size may have diminished as a consequence of lower diastolic filling pressure. In view of the relationship between wall tension and myocardial oxygen consumption, these effects should favorably influence the balance between oxygen availability and oxygen requirements in patients with fixed obstruction to left ventricular outflow. The eighth patient experienced an appreciable increase in tension-time index, suggesting secondary hypertrophic subaortic stenosis.

Effect of nitroglycerin on coronary collateral function during exercise evaluated by quantitative analysis of thallium-201 single photon emission computed tomography

A noninfarcted, entirely collateral-dependent myocardial region provides an opportunity to assess the effect of nitroglycerin on coronary collateral function during exercise. Stress thallium-201 computed tomography was performed in seven patients with effort angina and no history of myocardial infarction, both before and after nitroglycerin (0.3 mg). All patients had single-vessel disease with total or subtotal (99% with delay) occlusion of proximal left anterior descending coronary artery and well-developed collaterals. The pressure-rate product, mean blood pressure, and heart rate at peak exercise did not differ before and after nitroglycerin. The size of the perfusion defect and the severity of ischemia during exercise estimated by quantitative analysis of thallium-201 single photon emission computed tomography were significantly less after nitroglycerin administration (extent score: 23 +/- 17 vs 7 +/- 9, p less than 0.01; severity score: 20 +/- 22 vs 3 +/- 4, p less than 0.05). The pressure-rate products at peak exercise did not differ before and after nitroglycerin, which suggested that the reduction in perfusion defect size was unlikely to be the result of decreased myocardial oxygen consumption. These results suggest that nitroglycerin improved coronary collateral function during exercise and thus prevented exercise-induced myocardial ischemia.

Beneficial effects of perfluorochemical artificial blood on cardiac function following coronary occlusion

This study compares the effects of perfluorochemical artificial blood versus whole blood on the systolic and diastolic function of regionally ischemic myocardial preparations. Regional ischemia was produced by ligation of the circumflex coronary artery in isolated, blood-perfused rabbit hearts. Three minutes after occlusion, half the hearts were switched from the blood perfusate to perfluorochemical artificial blood; the other half continued to be perfused with blood. Isovolumic left ventricular (LV) developed pressure, dP/dt and resting pressure were monitored before, and for 2 hours after coronary occlusion. After 90 minutes of regional ischemia, perfluorochemical-treated hearts exhibited significantly greater developed pressure than those perfused with blood (78 +/- 6% versus 61 +/- 5% of preligation values; P less than 0.05). At the end of the experiment, LV dP/dt was 21% greater in the perfluorochemical-perfused group than in the blood-perfused group (74 +/- 8% versus 53 +/- 10%; P less than 0.01). Perfluorochemical perfusion also preserved diastolic function by preventing the 58% increase in left ventricular chamber stiffness (i.e., resting pressure; P less than 0.01) associated with circumflex ligation. Thus, in the present model of regional ischemia, perfluorochemical artificial blood is significantly better than blood at maintaining both systolic and diastolic myocardial function after a major coronary artery has been occluded.

Evaluation of the effects of systemic nitroglycerin on perfusion of ischemic myocardium in coronary heart disease assessed intraoperatively by antegrade blood flow through intact saphenous vein bypass grafts

To evaluate systemic nitroglycerin (NTG) effects on reduced perfusion of ischemic myocardium in coronary disease, intravenous NTG actions on coronary circulatory dynamics were directly assessed intraoperatively during aortocoronary bypass surgery in 24 patients. Thus metered antegrade blood flow was measured through 56 separate intack saphenous vein bypass grafts to analyze NTG perfusion response in the obstructed native coronary artery (CA). In 34 bypassed CA with proximal luminal diameter narrowing greater than 50% to 90%, NTG reduced (p less than 0.01) graft flow (GF) 82 to 63 cc/min, thereby indicating that NTG dilated proximal stenoses with resultant increased CA flow. In 11 bypassed CA obstructed greater than 90% to 100% with well developed collaterals distally, NTG decreased (p less than 0.05) GF 64 to 53 cc/min, thus indicating enhanced collateral flow. In contrast, in 11 bypassed CA obstructions greater than 90% to 100% without collaterals, that NTG increased (p less than 0.02) GF 91 to 100 cc/min indicated NTG nonresponsiveness of the severely diseased CA. Thus systemic NTG improves perfusion to ischemic myocardium subserved by diseased coronaries with less than 90% stenosis or by greater than 90% obstructed vessels with substantial collaterals distally.

The role of vasodilator therapy in heart failure

This article has attempted to summarize the current status of the therapeutic use of vasodilator drugs in acute and chronic heart failure. It is apparent from the increasing number of publications in this area that this alternative to more standard forms of therapy is likely to find a permanent and important place in the management of patients with heart disease. It should also be apparent that ideal drugs for the therapy of chronic heart failure are not yet available. Nevertheless, it is probable that such drugs will emerge and become at least as important as the routine use of digitalis in such patients.

Influence of vasodilators upon function and metabolism of ischemic myocardium

Although the systemic hemodynamic effects of vasodilators such as nitroprusside, phentolamine and nitrates are well known, relatively little information is available regarding their effects upon the function and metabolism of ischemic myocardium. Experimental and clinical studies indicate that vasodilators improve the mechanical performance of regional ischemic myocardium, probably by simultaneous reduction of peripheral resistance and reduction of the degree of ischemia. The majority of evidence, although still controversial, seems to indicate that myocardial perfusion can also be increased, particularly when coronary collateral vessels are present. Concomitant reduction in preload contributes to reduced oxygen demand, as evidenced by findings of reduced oxygen extraction. Thus, the balance of the oxygen supply and demand may be improved as indicated by decreases in lactate production. In addition, limited evidence in experimental animals and man suggests that vasodilators may also reduce the extent of myocardial injury as measured by S-T segment mapping and the creatine phosphokinase (CPK) release technique. However, these effects are contingent upon the arterial pressure response, and directionally opposite results may be anticipated if hypotension occurs. Since the mechanism of action of vasodilators is reasonably well understood, vasodilator therapy can be administered safely in anticipation of both improvement in total cardiac performance and a decrease in severity of ischemia.

Pentaerythritol trinitrate and glyceryl trinitrate on myocardial oxygen consumption and haemodynamics in the dog

1. The effects of pentaerythritol trinitrate (pentrinitrol) and glyceryl trinitrate on myocardial oxygen consumption and myocardial and systemic haemodynamics were studied in anaesthetized open-chest dogs. An in vivo oximeter in the coronary sinus permitted continuous determination of arteriovenous oxygen difference and myocardial oxygen consumption. All parameters were determined simultaneously at various intervals after drug administration. 2. Myocardial oxygen consumption was diminished by both nitrates for more than 16 min. Changes in arteriovenous oxygen difference and coronary sinus oxygen content were variable between drugs. Following an initial transient increase, coronary blood flow was reduced by both nitrates. Aortic blood pressure, aortic blood flow, left ventricular end-diastolic pressure and left ventricular dP/dt were also reduced. Heart rate and contractile force were not appreciably altered by either nitrate. 3. The decrease in myocardial oxygen consumption appears to be associated with the haemodynamic profile of these drugs. Both nitrates produced comparable reductions in preload (left ventricular end-diastolic pressure) and afterload (aortic pressure) as well as dP/dt.

Hemodynamic and metabolic effects of sodium nitroprusside on the performance and metabolism of regional ischemic myocardium;

To assess the effects of sodium nitroprusside (5-10 mug/min) on total and regional cardiac performance, energetics, and lactate metabolism during acute ischemia, studies were performed in 21 open-chest dogs. For studies of regional function and metabolism, length gauges were sutured to the epicardial surface and an epicardial vein adjacent to the artery to be occluded was cannulated. Following occlusion of the left anterior descending coronary artery, cardiac output, mean arterial pressure, epicardial vein blood flow, and systolic shortening of the ischemic segment decreased significantly, In the blood samples from the ischemic zone, but not in those from the coronary sinus, lactate extraction shifted to production. In seven control dogs these alterations persisted throughout the experiment. In 14 animals treated with nitroprusside, cardiac output increased while peripheral resistance and mean arterial pressure decreased. Systolic shortening in the ischemic segment increased from 1.10 +/- 0.24 (SEM) to 1.77 +/- 0.30 mm (P less than 0.005). In eight dogs, regional venous outflow increased from 1.9 +/- 0.1 to 3.0 +/- 0.4 ml/min despite a slight reduction in mean arterial pressure. Concomitantly, regional negative lactate balance was reduced from -61.0 +/- 20.0 to -23.2 +/- 5.7% (P less than 0.05). These results indicate that nitroprusside significantly improves both total cardiac performance and the mechanical performance of regional ischemic myocardium. Moreover, this improvement in mechanical function occurred concomitantly with apparent increase in regional perfusion and reduction in lactate production, suggesting that nitroprusside simultaneously alleviates ischemia.

The effects of nitroglycerin on coronary collaterals and myocardial contractility

Nitroglycerin (TNG) causes a prolonged dilatation of coronary collaterals. To demonstrate a functional significance of this dilatation we measured the effect of TNG on myocardial contractile force in dogs 2(1/2)-4 wk after the left anterior descending coronary artery (LAD) had been embolized in closed-chest animals. Development of collaterals was documented by angiography. Via a left thoracotomy the main left coronary artery (LCA) and LAD distal to the embolized plug were cannulated. Coronary flow and perfusion pressure were recorded. Contractile force was measured with gauges sutured to epicardial areas supplied by the left circumflex coronary artery (LCf) and occluded LAD. Coronary perfusion pressure in the LCA was gradually decreased until the contractile force recorded by the LAD gauge diminished while the LCf gauge was unaffected. Under these conditions, with coronary perfusion pressure held constant with the aid of a Starling resistance, TNG (18 mug) injected into the LCA increased peripheral LAD pressure by 3-12 mm Hg and contractile force in the LAD region by 36% (range 20-90%), returning it to near-normal levels, while having minimal effect in the LCf area. These changes persisted for 5 min. When LCf and LAD areas were both ischemic, intracoronary TNG had minimal effect on peripheral LAD pressure and contractile force. Thus, TNG causes prolonged dilatation of coronary collaterals and presumed increased collateral flow with subsequent enhancement of myocardial contractile force in ischemic areas. This effect is seen only when ischemia is limited to an area supplied by the collaterals. When the whole heart is ischemic, collaterals are unresponsive to TNG, suggesting that these collaterals dilate fully when the regions from which they originate become ischemic.

Direct and reflex effects of nitroglycerin on coronary and left ventricular dynamics in conscious dogs

The effects of intravenous and sublingual glyceryl trinitrate (nitroglycerin), 40 mug/kg, were studied on coronary blood flow and resistance, left ventricular (LV) pressures (P) and diameters (D), rate of change of pressure (dP/dt), (dP/dt)/P, and on the velocity (V) of myocardial fiber shortening in conscious dogs. Nitroglycerin i.v. caused substantial coronary vasodilatation prior to any changes in systemic hemodynamics. Mean coronary flow increased by a maximum of 47 ml/min and coronary sinus P(o2) rose from 16 to 26 mm Hg while pressure and diameter began to fall, and heart rate began to rise. After the maximal fall in mean arterial pressure (-26 mm Hg), a secondary peak in coronary flow occurred which was associated with increases in heart rate (100 beats/min), (dP/dt)/P (22%), and isolength V (12%). Beta blockade prevented the reflex increases in contractility but only a part of the reflex tachycardia; the remainder was prevented by cholinergic blockade. Maintaining heart rate constant minimized the decreases in LV D and increases in contractility. When the reflex inotropic and chronotropic effects were prevented by a combination of atrial pacing and beta blockade the early coronary vasodilatation was unaltered, but the later coronary vasodilatation was minimized. Thus i.v. nitroglycerin in the conscious dog exerts a potent direct coronary vasodilating action and also a secondary coronary vasodilation caused by reflex increases in contractility and heart rate. The decreases in diameter are largely the result of tachycardia. Sublingual nitroglycerin produced directionally similar, but quantitatively lesser effects on coronary flow and resistance, LV D, LV P, and contractility.

Effect of ischemia and antianginal drugs on the distribution of radioactive microspheres in the canine left ventricle

Radioactive microspheres were used to estimate the changes in regional myocardial blood flow occurring during acute myocardial ischemia. Carbonized 15-µ spheres were injected into the left atrium of 28 open-chest dogs and the radioactivity of selected areas determined after sacrifice. Acute occlusion of the left circumflex coronary artery produced a significant diminution in the proportion of microspheres reaching the circumflex area. In addition, there was a disproportionate decrease in endocardial radioactivity in the ischemic area (endocardial/epicardial radioactivity ratio falling from 1.17 to 0.76, P < 0.001) but not in the nonischemic area. Both nitroglycerin (0.4 mg) and propranolol (1 mg/kg) failed to cause a significant change in the ratio of circumflex to descendens radioactivity during ischemia. They did, however, cause a significant increase in the ratio of endocardial to epicardial radioactivity in both ischemic and nonischemic areas.

Some effects of nitroglycerin upon the splanchinic, pulmonary, and systemic circulations

Splanchnic, pulmonary, and systemic hemodynamics were studied in 18 patients after the sublingual administration of nitroglycerin. The drug, contrary to expectations, produced an over-all vasoconstrictive effect on the splanchnic circulation rather than vasodilatation. There was no evidence of venous pooling in this bed, and indeed the data may indicate a splanchnic supportive role in augmenting venous return to the heart with disengorgement of its own volume. In contrast, there was vasodilatation and pooling of blood in the pulmonary vascular bed. The systemic circulation probably sustains several effects by nitroglycerin, including arterial vasodilatation. A direct change in large artery distensibility probably explains the modest fall in systolic blood pressure seen. Further decline in arterial pressure may depend on venous pooling of a small or large degree. Probably the fall in systemic and specific organ flows is also linked to decreased venous return and the vascular readjustments provoked thereby. Pulsus alternans was produced by nitroglycerin, a previously unreported effect of the drug, but the mechanism by which it arose could not be defined.

The effect of nitroglycerin on the systemic and coronary circulation in man and dogs: myocardial blood flow measured with xenon

The pharmacodynamics of nitroglycerin have been studied in dogs and man.

In dogs, intravenous nitroglycerin (TNG) caused a decrease in left ventricular work, myocardial blood flow (MFB), and myocardial oxygen consumption, and no significant change in coronary vascular resistance.

Intracoronary artery TNG in dogs caused an immediate increase in MBF and a decrease in coronary vascular resistance that persisted until arterial pressure fell.

In man, sublingual TNG (0.4 mg.) caused a decrease in left ventricular work, MBF, and myocardial oxygen consumption, and no significant decrease in coronary vascular resistance in patients with and without arteriographically proven coronary artery disease.

The injection of doses of 0.1 to 0.2 mg. of TNG directly into the coronary artery in man caused an immediate increase in MBF and a decrease in coronary vascular resistance in patients with and without arteriographically proven coronary artery disease.

The hypothesis proposed for the mechanism of action of TNG in the relief of angina pectoris is first, a decrease in coronary vascular resistance due to its effect on the coronary circulation, and, secondly, a decrease in cardiac work due to its effect on the systemic circulation.

THE EFFECTS OF DIPYRIDAMOLE AND NITROGLYCERIN ON CORONARY BLOOD FLOW, MEAN ARTERIAL PRESSURE, AND HEART RATE IN DOGS

The effects of the coronary dilators dipyridamole and nitroglycerin were studied in 21 dogs, either under conditions of constant pulmonary artery inflow (constant PAI experiments) or under conditions of constant arterial pressure as well (constant PAI and AP experiments). The two drugs had different effects on coronary flow in the constant PAI experiments, although both reduced peripheral arterial pressure. Nitroglycerin caused a brief biphasic response, consisting of a peak increase followed by a fall below control, whereas dipyridamole maintained coronary flow above control levels and for a much longer period.In the constant PAI and AP experiments, the biphasic response after nitroglycerin was not observed, and a significant increase in the peak flow over that noted in the constant PAI experiments occurred. With dipyridamole the peak coronary flow was not significantly changed by holding the arterial pressure constant.The implication from the significantly greater coronary flow response after nitroglycerin in the constant PAI and AP experiments compared with that in the constant PAI experiments is that in the latter experiments reduced coronary resistance secondary to nitroglycerin would seem to have been masked by the rapid fall in peripheral pressure. Dipyridamole caused a progressive increase in coronary flow in the face of a reduction in peripheral pressure.