Effects of catecholamines, exercise, and nitroglycerin on the normal and ischemic myocardium in conscious dogs

Newer concepts in the pathophysiology of ischemic heart disease

A model of myocardial ischemia based on the balance of oxygen demand and supply is presented. Ischemia is invariably the result of a limited coronary blood flow (supply), but increased oxygen consumption is often cited as a factor causing an imbalance of demand and supply. The role of contractility in ischemia, however, has been overemphasized, and inotropic agents such as glycosides and isoproterenol frequently have effects on supply that overshadow their effects on myocardial oxygen consumption. With deep ischemia leading to infarction, supply also tends to overshadow demand in determining the extent of transmural necrosis. Moreover, the anatomic pattern of supply precisely delineates both the region at risk following coronary occlusion and the ultimate extent of the infarction. These views are presented in anatomic models of myocardial ischemia and infarction.

Effects of dopamine and dobutamine on the myocardial and systemic circulation during and following cardiopulmonary bypass in dogs

The effect of dopamine (Dp) and dobutamine (Db) on myocardial and systemic blood flow (BF) distribution was compared in dogs being weaned from cardiopulmonary bypass (CPB) after 20 min of normothermic global myocardial ischemia. Drug infusions (10 mcg/kg/min) were begun and BF was measured (radiolabeled microspheres) prior to weaning and 60 min off CPB. On CPB: Pump flows, by design, were similar (100 ml/kg/min) in Dp, Db, and saline control (NS) dogs. Dp and Db significantly (p less than 0.05) increased myocardial BF. Dp did not alter renal, visceral, and skeletal muscle BF and only increased BF to the cervical spinal cord and medulla. Db, however, significantly reduced renal (-49%), splenci (-58%), pancreatic (-27%), and colonic (-47%) BF but increased perfusion in essentially all of the central nervous system. Off CPB: Cardiac output during Dp and Db infusions was significantly greater than NS dogs (107 +/- 7 vs 152 +/- 12 vs 82 +/- 10 ml/kg/min resp.); the greater increase for Db resulting from a larger stroke volume. Dp and Db significantly increased myocardial BF. Dp increased splenic (+77%), gastric (+139%), and gallbladder (+125%) BF but had no effect on renal, hepatic, skeletal muscle, and intestinal BF. Db infusion maintained renal BF similar to NS and elevated BF to most visceral organs. The results of this study show that the myocardium responded to inotropic stimulation despite the previous ischemic insult but the BF changes varied among regional vascular beds with Dp and Db infusions during and following CPB. Of the 2 drugs, Db showed the greater inotropic response off CPB, a similar increase in myocardial perfusion and greater visceral organ bloodflow.

Effects of 12 months of intense exercise training on ischemic ST-segment depression in patients with coronary artery disease

This study was undertaken to determine whether adaptation to 12 months of intense endurance exercise training could alter the relationship between the product of heart rate and systolic blood pressure (double product) and the extent of ischemic ST-segment depression during exercise in patients with coronary artery disease. True (i.e., not symptom-limited) maximum oxygen uptake capacity increased from 25.5 +/- 4.2 ml/kg/min (mean +/- SD) to 35.3 +/- 4.4 ml/Kg/min with training. The maximum degree of ST-segment depression during exercise averaged 0.20 +/- 0.04 mV before and 0.16 +/- 0.08 mV after training despite a 20% increase in maximum double product. The double product at which ST depression (0.1 mV) first appeared was 22% greater after training. The extent of ST-segment displacement at the same double product was less after training. These findings suggest that training, if sufficiently intense and prolonged, can result in a reduction in myocardial ischemia at the same or a higher double product.

Effects of nitroglycerin and nitroprusside on large and small coronary vessels in conscious dogs

The effects of nitroglycerin (NTG) and nitroprusside (NPR) were examined in conscious dogs on measurements of left circumflex coronary blood flow and coronary diameter and on calculations of late diastolic coronary resistance (LDCR) and left circumflex coronary internal cross-sectional area (CSA). The effects of infusions of NTG, 8 microgram/kg/Min for 7 minutes, and NPR, 2.5 microgram/kg/min for 7 minutes, were compared. These doses of NTG and NPR induced similar effects on mean arterial and left ventricular (LV) systolic and end-diastolic pressure, heart rate and LV dP/dt. However, NTG induced significantly greater (p less than 0.05) increases in CSA than NPR, while NPR induced significantly greater (p less than 0.05) reductions in LDCR than NTG. Just before cessation of infusion, CSA rose by 29.2 +/- 4.7% with NTG and by 22.7 +/- 3.9% with NPR, while LDCR fell by 8.8 +/- 3.3% with NTG and by 21.6 +/- 2.7% with NPR. Moreover, the effects of NTG on CSA were significantly more sustained than those for NPR. Thus, NTG and NPR in conscious dogs, in doses that exert similar general hemodynamic effects, elicit qualitatively similar but quantitatively different effects on small vs large coronary vessels. Whereas NTG induced significantly less dilation of small coronary vessels, it exerted significantly larger and longer lasting effects on large coronary arteries.

Dobutamine in chronic ischemic heart failure: alterations in left ventricular function and coronary hemodynamics

Changes in cardiac performance and coronary hemodynamics were evaluated during dobutamine infusion in patients with chronic heart failure associated with ischemic heart disease. At the maximal administered dose (10 micrograms/kg per min) cardiac index increased by 54 percent and stroke work index by 65 percent, indicating improved left ventricular function. Coronary sinus flow and myocardial oxygen consumption increased concomitantly, but myocardial lactate production occurred in only one of eight patients. These findings suggest that improved left ventricular function with dobutamine is associated with increased myocardial oxygen consumption; however, overt myocardial ischemia occurs infrequently.

Noninvasive assessment of left ventricular performance in patients with ischemic heart disease: ear densitographic study during uninterrupted treadmill exercise

Among 738 consecutive patients with chest pain referred for exercise testing, only 17 men fulfilled strict criteria for this pilot study: (1) achievement of at least 90 percent of age-predicted maximal heart rate or electrocardiographic criteria for a positive test; (2) subsequent coronary angiography; (3) exclusion of prior heart disease; and (4) absence of medication. Measurements of systolic time intervals obtained with ear densitography during early exercise differentiated 10 patients with coronary artery disease (Group 1) from 17 men (7 patients plus 10 normal volunteers) with normal coronary arteries (Group 2). Despite nearly identical heart rate and blood pressure responses in both groups, men in Group 1 had a significantly greater reduction of preejection period (PEP) at 1 minute and 4 minutes as well as a greater decrease in PEP/LVET (left ventricular ejection time) ratio. Differences were most significant when expressed as percent change of PEP/LVET ratio from control value (p less than 0.001 at both 1 minute and 4 minutes). After 4 minutes, men in Group 1 had no further decrease in PEP/LVET ratio and in 8 of the 10 men PEP/LVET ratio then increased to peak exercise. By contrast, PEP/LVET ratio continued to decrease to peak exercise in men in Group 2. The early floor in PEP/LVET ratio in Group 1 represents limited functional reserve and the subsequent increase suggests functional deterioration. Thus, densitographic systolic time interval measurements during uninterrupted exercise in unmedicated subjects appear to improve the sensitivity and specificity of the conventional treadmill test.

Abnormal coronary vascular response to exercise in dogs with severe right ventricular hypertrophy

Measurements of right coronary artery blood flow, aortic and right ventricular (RV) pressures and heart rate were radiotelemetered during strenuous, spontaneous exercise in normal dogs and dogs with severe RV hypertrophy induced by chronic (5-6 mo) pulmonary artery stenosis. With fixed pulmonic stenosis, dogs with RV hypertrophy exhibited a decrease (P less than 0.01) in arterial pressure during exercise. Under these conditions, exercise increased right coronary artery blood flow and decreased right coronary vascular resistance less (P less than 0.05) in dogs with RV hypertrophy compared with normal. This attenuated response of right coronary artery blood flow of dogs with RV hypertrophy was not observed when arterial pressures remained at preexercise values during exercise. However, regardless of changes in arterial pressures during exercise, all dogs with RV hypertrophy demonstrated a striking postexercise coronary hyperemia (P less than 0.01), suggesting a perfusion deficit of the hypertrophied right ventricle during exercise. These results imply a fundamental defect in the ability of the coronary circulation of the severely hypertrophied right ventricle to provide sufficient nutrient supply in the face of elevated metabolic demands of exercise.

Comparative peripheral and coronary haemodynamic effects of rimiterol and isoprenaline

1. Rimiterol and isoprenaline produced significant dose-related increases in cardiac output. 2. These changes in cardiac output were accompanied by increases in heart rate and myocardial oxygen consumption which were similar for each drug and dose-related. 3. Isoprenaline in contrast with rimiterol produced direct coronary vasodilation, i.e. coronary vasodilation in excess of that required to meet increases in myocardial oxygen demands. 4. It is suggested that the beta-adrenergic receptors in the human coronary vasculature are mainly of the beta1 type. 5. Rimiterol, because it does not produce direct coronary vasodilation may be preferable to isoprenaline in the treatment of low-cardiac output syndrome where there is regional myocardial ischaemia, since it would be less likely to produce a "coronary steal" effect.

Initial myocardial adjustments to brief periods of ischemia and reperfusion in the conscious dog

The changes in left ventricular (LV) dynamics induced by brief periods of ischemia (100 seconds) and subsequent reperfusion were analyzed in conscious dogs. Global LV ischemia, induced by partially occluding the left main coronary artery, reduced LV flow homogeneously and impaired LV function as reflected by decreases in LV stroke "work" (89 +/- 4% M +/- SE), systolic shortening (72 +/- 4%), velocity of shortening (56 +/- 6%), LV systolic pressure (34 +/- 5%), and dP/dt (59 +/- 6%). Regional LV ischemia, induced by occluding either the left circumflex or anterior descending coronary artery completely, reduced flow to the ischemic segment (82 +/- 3%) while decreasing segment work (96 +/- 5%), shortening (82 +/- 3%), and velocity of shortening (70 +/- 5%), with minimal depression of overall LV function. In both groups the extent of shortening was reduced more rapidly and greater (P less than 0.01) than shortening velocity. Moreover, with localized ischemia, segment work was reduced more (P less than 0.01) than shortening. With reperfusion, a transient overshoot in function above preischemic control levels was observed in both groups (global work increased by 60 +/- 12% and regional work by 28 +/- 4% above control). This overshoot was not dependent on adrenergic mechanisms, but was prevented by inhibiting reactive hyperemia. Thus myocardial ischemia induces a dissociation between extent and rate of myocardial shortening. A further dissociation between shortening and work is apparent with regional ischemia. After reperfusion there is a transient overshoot in function which appears to be dependent upon the associated reactive hyperemia.

Regional myocardial wall thickening response to nitroglycerin. A predictor of myocardial response to aortocoronary bypass surgery

The peak rate of systolic wall thickening (p +/- dTw/dt) in regions of the left ventricle (LV) was determined preoperatively by biplane roentgen videometry in 18 patients before and after sublingual administration of nitroglycerin (NTG) and 3-23 months (median 12) after aortocoronary bypass surgery. The regional LV response to NTG was a reliable predictor for postoperative improvement in regional wall dynamics after successful aortocoronary bypass grafting. The ejection fraction response to NTG or surgery will not predict the regional myocardial response to NTG or surgery, nor will the regional response predict the global response. Subendocardial myocardial infarction is another cause of unimproved regional myocardial function after NTG and aortocoronary bypass surgery.

Myocardial function and coronary blood flow response to acute ischemia in chronic canine diabetes

To examine the influence of preexistent diabetes mellitus on left ventricular performance and coronary blood flow responses to acute ischemia, mild normoglycemic diabetes was induced in nine mongrel dogs after three doses of alloxan, (20 mg/kg, iv), at monthly intervals. Hemodynamic measurements and coronary blood flow (85Kr clearance) were obtained before and after the onset of ischemia. This was produced by occlusion of the proximal left anterior descending coronary artery via a balloon-type catheter in nine intact anesthetized diabetic dogs and 10 nondiabetic dogs. During the 1st hour of ischemia in the diabetic group, the end-diastolic pressure rose from 7 +/- 1.1 (mean +/- SE) mm Hg to 23.8 +/- 2.3 without a significant increase of end-diastolic volume. In controls end-diastolic pressure rose from 8.6 +/- 1.1 mm Hg to 15.3 +/- 1.4, and end-diastolic volume was significantly increased, so that the ratio of end-diastolic pressure and volume was significantly higher in the diabetic group (P less than 0.005). Although indices of contractility did not differ, stroke volume and work reductions were significantly greater in diabetics, despite the fact that coronary blood flow was reduced to a similar extent. Size of the ischemic areas appeared comparable as judged by distribution of dye injected distal to the occlusion. Since potassium loss and sodium gain in the inner and outer layers of ischemic tissue did not differ between the two groups, the intensity of ischemia seemed similar. Glycogenolysis was unimpaired in the diabetic ischemic muscle but triglyceride levels remained elevated. Morphologically the diabetic myocardium was characterized by a diffuse accumulation of periodic acid-Schiff-positive glycoprotein in the interstitium, which was thought to limit diastolic filling of the ischemic ventricle and to contribute to the substantial reduction of ventricular performance.

Hemodynamic effects of dobutamine in patients following open heart surgery

Dobutamine, a new synthetic inotropic agent, was administered by intravenous infusion to 22 patients following open heart surgery. The dosage of dobutamine started from 2 mug/kg/min and was increased stepwise to 4, 6, and 8 mug/kg/min. Cardiac index increased from 2.54 +/- 0.13 to 3.11 +/- 0.19 and 3.50 +/- 0.22 L/min/m2 with 4 and 8 mug/kg/min, respectively. Heart rate increased from 87.6 +/- 2.9 to 96.4 +/- 2.3 and 108.9 +/- 3.2 beats/min with 4 and 8 mug/kg/min, respectively. Systolic and mean aortic pressure and systolic pulmonary arterial pressure were elevated from 126.0 +/- 3.8 to 144.6 +/- 6.0 mm Hg, from 91.8 +/- 2.7 to 96.2 +/- 3.2 mm Hg, and from 33.4 +/- 2.3 to 37.1 +/- 2.5 mm Hg with 8 mug/kg/min, while other pressures were unchanged. Mean left atrial pressure fell significantly from 15.9 +/- 0.6 to 14.8 +/- 0.7 mm Hg. Stroke volume index, stroke work index, and left ventricular work also increased. Premature ventricular beats were observed in only one patient during dobutamine infusion, but no other side effect was observed. We conclude that dobutamine is a potent inotropic agent which increases cardiac output without causing significant tachycardia or arrhythmias, and is useful for the treatment of patients following open heart surgery.

Cardiovascular adjustments to exercise: hemodynamics and mechanisms

The integrated response to severe exercise involves fourfold to fivefold increases in cardiac output, which are due primarily to increases in cardiac rate and to a lesser extent to augmentation of stroke volume. The increase in stroke volume is partly due to an increase in end-diastolic cardiac size (Frank-Starling mechanism) and secondarily due to a reduction in end-systolic cardiac size. The full role of the Frank-Starling mechanism is masked by the concomitant tachycardia. The reduction in end-systolic dimensions can be related to increased contractility, mediated by beta adrenergic stimulation. Beta adrenergic blockade prevents the inotropic response, the decrease in end-systolic dimensions, and approximately 50% of the tachycardia of exercise. The enhanced cardiac output is distributed preferentially to the exercising muscles including the heart. Blood flow to the heart increases fourfold to fivefold as well, mainly reflecting the augmented metabolic requirements of the myocardium due to near maximal increases in cardiac rate and contractility. Blood flow to the inactive viscera (e.g., kidney and gastrointestinal tract) is maintained during severe exercise in the normal dog. It is suggested that local autoregulatory mechanisms are responsible for maintained visceral flow in the face of neural and hormonal autonomic drive, which acts to constrict renal and mesenteric vessels and to reduce blood flow. However, in the presence of circulatory impairment, where oxygen delivery to the exercising muscles is impaired as occurs to complete heart block where normal heart rate increases during exercise are prevented, or in congestive right heart failure, where normal stroke volume increases during exercise are impaired, or in the presence of severe anemia, where oxygen-carrying capacity of the blood is limited, visceral blood flows are reduced drastically and blood is diverted to the exercising musculature. Thus,, visceral flow is normally maintained during severe exercise as long as all other compensatory mechanisms remain intact. However, when any other compensatory mechanism is disrupted (even the elimination of splenic reserve in the dog), reduction and diversion of visceral flow occur.

Influence of dobutamine on hemodynamics and coronary blood flow in patients with and without coronary artery disease

The influence of dobutamine on hemodynamics and coronary blood flow was studied in patients after routine cardiac catheterization. The data demonstrated that dobutamine is a powerful inotropic agent at a dose that has a relatively small influence on heart rate. In patients without coronary artery disease dobutamine greatly increased coronary arterial perfusion. In patients with severe coronary artery diseases dobutamine resulted in a much smaller increase in coronary perfusion, and the pattern of perfusion became more inhomogeneous. The results suggest that dobutamine has a potential inotropic value but raise concern about its influence on regional myocardial perfusion in patients with serious coronary artery disease.

Influence of dobutamine on regional myocardial blood flow and ventricular performance during acute and chronic myocardial ischemia in dogs

The data from this study document that dobutamine is a powerful inotropic agent in anesthetized dogs with acute myocardial ischemia and in awake, unsedated ones with chronic myocardial infarction. Dobutamine significantly increases heart rate at relatively small doses in anesthetized dogs with acute myocardial ischemia but considerably larger amounts of dobutamine are required to significantly increase heart rate in awake, unsedated dogs with myocardial infarction. Dobutamine also significantly increases regional myocardial blood flow to all areas of the heart at 20mug/kg/min in both anesthetized dogs with acute myocardial ischemia and awake, unsedated ones with myocardial infarction. However, in anesthetized dogs 20mug/kg/min of dobutamine significantly increases epicardial ST-segment elevation during acute myocardial ischemia. Propranolol prevents the inotropic and chronotropic effects of dobutamine in both anesthetized and awake, unsedated dogs. This study suggests that during experimental acute myocardial ischemia dobutamine given at doses that significantly increase heart rate and contractility may increase the extent of myocardial damage. The data also suggest that this agent should be of value in the setting of severe myocardial depression without associated severe coronary artery disease to increase cardiac contractility at doses that do not markedly alter heart rate. The hemodynamic and coronary blood flow effects of dobutamine in patients with and without severe coronary artery disease should be evaluated.

Effects of isoproterenol on regional myocardial function, electrogram, and blood flow in conscious dogs with myocardial ischemia

The effects of coronary occlusion and of subsequent isoproterenol infusion were examined in conscious dogs. Left ventricular (LV) function was assessed by measurements of LV diameter, pressure, velocity and dP/dt/P, and regional myocardial function was assessed by measurements of segment length (SL) and velocity of SL shortening in normal, border, and ischemic zones. Regional myocardial function was measured from the same sites, along with intramyocardial electrograms and regional myocardial blood flow as determined by radioactive microspheres. Coronary occlusion resulted in graded loss of function from the normal to severely ischemic zones with graded flow reductions and graded increases in ST segment elevation. Isoproterenol improved overall LV function, and function in the normal zone. Isoproterenol also improved function in 19 of 21 border-zone segments and in all moderately ischemic segments, while elevating further the ST segments. These changes were accompanied by increases in myocardial blood flow. In contrast, in severely ischemic segments, isoproterenol resulted in a deterioration of function, in that paradoxical motion occurred in segments previously akinetic during systole, while paradoxical motion was intensified in those segments in which it was already present. These changes were accompanied by further ST segment elevation but not by concurrent increases in blood flow. In addition, in 2 of 21 border zone segments, myocardial blood flow fell and these segments responded to isoproterenol with complete loss of function; paradoxical motion developed. Thus, in the conscious dog, a strong inotropic agent can improve function, even in the ischemic myocardium, as long as the required additional blood flow can be provided wither through primary or collateral channels.

Metabolism of free fatty acids, glucose and catecholamines in acute myocardial infarction. Relation to myocardial ischemia and infarct size

The myocardial metabolism of free fatty acids, glucose and catecholamines is reviewed in relation to current trends in the therapy of experimental myocardial infarction. Major modifications in the metabolism of free fatty acids, glucose and catecholamines have already been found after acute myocardial infarction in man, and animal experimental data suggest that such metabolic changes might play a role in the modification of infarct size and sometimes in the development of arrhythmias. However, animal experiments often represent extreme situations and the therapeutic use in man of agents to modify the metabolism of free fatty acids, glucose or catecholamines after myocardial infarction needs intensive investigation before general application. The sum total of the evidence from animal experiments suggests that increased circulating concentrations of free fatty acids and catecholamines, if sufficiently high, may be harmful rather than helpful to the outcome of acute myocardial infarction, and that increased provision of glucose (as glucose, insulin and potassium) may be beneficial. Reservations to these conclusions are that the concentrations used appear to be important factors in catecholamine and free fatty acid effects, and that the mechanism of action of glucose-insulin-potassium is more complex than originally thought.