Reduction of exercise-induced regional myocardial dysfunction by propranolol. Studies in a canine model of chronic coronary artery stenosis

Relation between regional function and coronary blood flow reserve in multivessel coronary artery stenosis

In the setting of chronic coronary stenoses, percent wall thickening (%WT) both at rest and during catecholamine stimulation can be abnormal despite normal resting myocardial blood flow (MBF). We hypothesized that this phenomenon is related to abnormal MBF reserve. Accordingly, 15 dogs were studied between 7 and 10 days after placement of Ameroid constrictors around the proximal coronary arteries and their major branches, at a time when collateral development had not yet occurred. %WT and MBF were measured at rest, after 0.56 mg/kg of dipyridamole, and at incremental doses of dobutamine (5-40 microgram. kg(-1). min(-1)). Resting %WT and MBF were normal in all four sham dogs. Resting transmural MBF was normal in all segments in the 11 study dogs, despite reduced (-2 SD of normal) %WT (<30%) in 40 of 82 segments. MBF reserve was reduced (<3) in segments with reduced %WT, and a close coupling was noted between resting %WT and MBF reserve. All segments showed an increase in %WT with dobutamine up to a dose of 20 microgram. kg(-1). min(-1), above which those with abnormal endocardial MBF reserve showed a "biphasic" response. It is concluded that, in the presence of chronic coronary stenoses, abnormalities in resting %WT as well as inducible reduction in %WT during pharmacological stress are related to the degree of abnormal MBF reserve.

Comparative study of dobutamine stress electron-beam computed tomography and exercise thallium scintigraphy in the diagnosis of patients with suspected coronary artery disease

To evaluate the diagnostic value of dobutamine stress electron-beam computed tomography (EBCT) as compared with exercise stress thallium-201 single-photon emission computed tomography (201T1-SPECT) for the detection of myocardial ischemia, 10 patients with proven or suspected coronary artery disease underwent both tests. Nine of the 10 patients also underwent coronary angiography. EBCT images were analyzed objectively to evaluate systolic wall thickening and analyzed segmentally to determine the distribution of the coronary arteries. Dobutamine stress EBCT revealed the presence of ischemia in 59 segments, whereas exercise stress 201T1-SPECT revealed ischemia in 51 segments (agreement = 73%). The advantage of dobutamine stress EBCT was demonstrated in the inferior/posterior segments as compared with the results of exercise stress 201T1-SPECT. The overall sensitivity for detecting ischemic regions supplied by coronary arteries with significant stenosis (diameter stenosis > 50%) was 83% for dobutamine stress EBCT and 79% for exercise stress 201T1-SPECT (p = NS), with specificities of 75% and 82% (p = NS). Thus, dobutamine stress EBCT presents a reasonable alternative to exercise stress 201T1-SPECT for the objective assessment of patients with suspected coronary artery disease.

Factors influencing regional myocardial contractile response to inotropic stimulation. Analysis in humans with stable ischemic heart disease

BACKGROUND

We hypothesized that the response of a myocardial segment to maximal dobutamine reflects not only maximal blood flow but also tethering, metabolic, and beta-blocker status.

METHODS AND RESULTS

Patients with stable ischemic heart disease (n = 27) had positron emission tomographic measurement of blood flow at rest and with adenosine, and echocardiography at rest and with dobutamine. Positron emission tomographic measurement of [18F]fluorodeoxyglucose myocardial distribution also was made. Adenosine blood flow in segments that contracted normally at peak dobutamine was similar to that of segments that became hypokinetic (1.06 +/- 0.72 versus 1.02 +/- 0.77 mL.g-1.min-1). Segments that became akinetic failed to augment blood flow (0.68 +/- 0.30 mL.g-1.min-1). Fluorodeoxyglucose-blood flow mismatch was more common in segments with abnormal wall motion at peak dobutamine (24 of 59, 41%) versus those that contracted normally (63 of 269, 23%; chi 2, 7.40; P < .01). In patients off beta-blockers, segments that contracted normally at peak dobutamine increased blood flow with adenosine (0.70 +/- 0.31 to 0.86 +/- 0.46 mL.g-1.min-1; P < .05), whereas those that became abnormal did not (0.63 +/- 0.24 to 0.65 +/- 0.19 mL.g-1.min-1; P = NS). Segments of patients on beta-blockers that contracted normally at peak dobutamine increased blood flow with adenosine (0.78 +/- 0.31 to 1.10 +/- 0.70 mL.g-1.min-1; P < .05), as did segments that became abnormal (0.74 +/- 0.34 to 1.06 +/- 0.82 mL.g-1.min-1; P = NS). However, segments adjacent to ones with abnormal wall motion at rest had higher frequency of abnormal response at peak dobutamine in groups on (48% versus 16%; chi 2, 14.1; P < .001) and off (51% versus 21%; chi 2, 10.9; P < .01) beta-blockers.

CONCLUSIONS

Augmented contraction at maximal dobutamine depends not only on increased myocardial blood flow but also on tethering, metabolic, and beta-blocker status. Furthermore, impaired flow reserve does not preclude a normal response to maximal dobutamine, since blood flow need not increase greatly to meet demand.

Assessment of right ventricle dimensions with microsonometry in anesthetized rabbits

Dimension measurements of the right ventricle are difficult to obtain because of its complex geometry, thus we evaluated a method of right ventricular dimension measurements. Crystals were placed on the ventral and dorsal side of the right ventricular free wall, approximately one-fourth of the right ventricular semicircle away from the septum, in the middle of a cranio-caudal axis of the ventricles. The effects of an increased (infusion of 20 mL/kg of 5% glucose for 3 min into seven rabbits), as well as decreased preload (nitroglycerin, 5 micrograms/kg/min i.v. n = 6) were measured and compared with changes during a placebo infusion (n = 6). The change in shortening of the right ventricle wall segment correlated with changes in both atrial natriuretic factor (ANF) plasma concentration (r = 0.89, p < 0.05) and central venous pressure (CVP) (r = 0.94, p < 0.05), respectively. Both these variables are influenced by right ventricular function and dimensions in healthy animals. Dimension measurements obtained across the free wall of the right ventricle appear to reflect right ventricular function well and should be useful in assessing the effects of drugs intended for the treatment of angina pectoris or heart failure.

Transmural steal with isoproterenol and exercise in poststenotic myocardium

Transmural coronary steal describes the phenomenon that can occur when coronary narrowing is severe enough to eliminate or nearly eliminate vasodilator reserve in the subendocardial layers. Because blood flow in a maximally vasodilated vascular bed is linearly dependent on perfusion pressure, additional reductions in perfusion pressure will decrease subendocardial blood flow. The subepicardial layers, operating on a different autoregulatory pressure-flow curve, may have vasodilator reserve available and display normal or even elevated blood flow when the subendocardium has reduced perfusion. Therefore, it appears as if subendocardial blood flow has been "stolen" by the subepicardial layers. Blood flow is not actually stolen but redistributed distal to a flow-limiting stenosis and the redistribution tends to favor the subepicardium because it can autoregulate to a lower pressure than the subendocardium. Physiologic interventions such as exercise can alter myocardial oxygen requirements substantially. Vasodilator reserve will be utilized in those parts of the myocardium that have it available, in order to meet the augmented myocardial flow requirements associated with exercise. In poststenotic myocardium, however, decreased vascular resistance in subepicardial layers may reduce poststenotic perfusion pressure which will lead, in turn, to a decrease in blood flow to the subendocardial layers if they are maximally vasodilated. Because transmural systolic function (measured as wall thickening, for example) is largely dominated by changes in subendocardial perfusion, transmural steal during exercise may aggravate the level of dysfunction that occurs by augmenting the subendocardial flow deficit.

Effects of exercise on myocardial catecholamine content and ischemic injury in dogs with gradual coronary occlusion

The effects of exercise on catecholamine content and the extent of myocardial damage in dogs with Ameroid constrictor occlusion of the left circumflex coronary artery were determined. Tissue samples from both the anterior and posterior walls of the left ventricle were obtained for determination of catecholamine content, and the rest of the ventricles were processed for histologic examination. When subjected to treadmill exercise for 40 days after surgery, obstructed animals performed significantly less exercise than sham-operated animals and showed significantly higher percentages of ischemia and necrosis in the left ventricle than sedentary, obstructed, sham-operated, or control dogs. Levels of norepinephrine and epinephrine in the posterior wall of the left ventricle were significantly lower than in the other groups. Our data show that exercise in this well-known model of chronic coronary artery stenosis produced deleterious effects on the myocardium and suggest a marked heterogeneity of catecholamine stores in the myocardium that may have important functional and electrophysiologic consequences.

Comparative effects of arotinolol, labetalol and propranolol on regional myocardial dysfunction induced by flow-limiting coronary stenosis in anesthetized dogs

Effects of arotinolol, a combined alpha- and beta-adrenoceptor blocking agent, on regional myocardial dysfunction produced by severe coronary stenosis in anesthetized dogs were examined and compared with those of labetalol and propranolol. Doses of these three antagonists were selected to produce a comparable degree of the negative chrono- and inotropic effect but a different potency of alpha-adrenoceptor blockade (labetalol greater than arotinolol). Regional myocardial function measured as segment shortening (%SS) was decreased to around 2-3% by constriction of the left circumflex coronary artery (LCX), and then drug or saline was administered i.v. The stenosis of LCX was released 30 min after the administration. No significant alteration in hemodynamic and contractility parameters was seen as compared to the predrug value up to at least 30 min after saline i.v. Arotinolol and propranolol both reduced heart rate and peak positive left ventricular dP/dt (LVdP/dt) without a significant change in LCX flow. Concomitantly, %SS distal to a coronary stenosis was significantly improved by arotinolol and propranolol. On the other hand, labetalol significantly reduced LCX flow probably due to systemic hypotension and failed to improve %SS in the ischemic area, although the agent markedly decreased heart rate and LVdP/dt. These results indicate that arotinolol improves impaired regional myocardial function distal to a coronary stenosis in a similar manner with propranolol.

Mechanisms of regional ischemia and antianginal drug action during exercise

Several mechanisms involved in the production of regional exercise-induced ischemia are described. Each offers the potential for modification using different types of antianginal drugs operating to alter regional O2 demands, improve regional perfusion, or both, leading to reduced ischemia and increased contractile function in the ischemic zone. Evidence is presented for matching of regional subendocardial myocardial blood flow and flow per beat with regional myocardial contraction at various levels of ischemia at rest, during steady-state exercise, and after antianginal drugs, signifying a particularly important role for heart rate control. In addition to reducing myocardial O2 demand per minute, beta-blockers and bradycardic drugs cause improvement of absolute subendocardial blood flow and particularly flow per beat by producing vasoconstriction in the epicardial region of the ischemic zone, with improvement of transmural blood flow distribution. Vasodilator drugs can act at several locations to increase regional blood flow and also to decrease O2 demands. A recruitable vasodilator reserve has been shown to exist during exercise-induced ischemia either in native resistance vessels, collateral channels, or both, which appears to be due at least in part to reduction of increased alpha-adrenergic constrictor tone to the coronary vessels during exercise, even in the presence of severe ischemia. The potential for additive effects using combinations of bradycardic and vasodilating agents are described within a framework relating regional subendocardial blood flow to regional systolic contraction. The experimental findings described suggest some potential new directions for antianginal therapy and, along with recent clinical observations, support the use of combinations of antianginal agents that act by different mechanisms.

Amelioration by beta-adrenergic blockade of regional myocardial dysfunction induced by coronary artery occlusion after, but not before collateral development in conscious dogs

Effects of beta-adrenergic blockade on regional myocardial dysfunction induced by coronary artery occlusion were studied in chronically instrumented, conscious dogs before (n = 8) and after (n = 7) collateral development. Intravenous atenolol or propranolol produced no beneficial effects on systolic shortening in the area rendered ischemic during 2 minutes of circumflex occlusion, before collateral development. After the collateral development by repetitive 2-minute coronary occlusions, regional asynergy recovered to the preocclusive level during 2 minutes of occlusion. Both atenolol and propranolol significantly improved the peak reductions of regional shortening by 19 +/- 9% and 18 +/- 9%, respectively (p less than 0.05 versus without beta-blockade). These beneficial actions of beta-blockade were again noted during atrial tachypacing at matched heart rates of 160 +/- 11 beats/min. Thus the beneficial effects of beta-blockade on regional asynergy during coronary occlusion depend on the level of functional state of the collaterals and cannot be totally accounted for by the reduction in heart rate or by the cardioselectivity.

Comparative electrocardiographic and myocardial blood flow effects of rapid atrial pacing and dipyridamole infusion in dogs with chronic coronary artery occlusion

Altering regional myocardial flow by the use of potent vasodilators as well as increasing oxygen demand by exercise have been employed as diagnostic methods for detecting coronary arterial obstruction. We sought to define the relative capabilities of these two methods to produce abnormal ECG patterns and to alter myocardial blood flow after Ameroid constriction of the left circumflex coronary artery. Atrial pacing to rates of 210 beats/min was performed, followed by intravenous administration of 0.25 and 0.50 mg/kg of dipyridamole. ECGs were recorded to construct body surface isopotential distributions. Flow was measured by serial injections of radiolabelled microspheres. In 15 animals studied after 3 to 5 weeks of Ameroid constriction, pacing increased epicardial flow by 19.14 +/- 7.11% but reduced endocardial flow by 35.69 +/- 12.32%, with a significant reduction (to less than 0.67) in the endocardial/epicardial flow ratio in 12 (80%) dogs; both endocardial and epicardial flows were significantly lower in the ischemic than in the nonischemic bed. Ten dogs developed abnormal ST segment responses of subendocardial ischemia. In contrast, dipyridamole produced a dose-dependent rise in both endocardial (25.33 +/- 8.54% and 55.80 +/- 9.22% after 0.25 and 0.5 mg/kg, respectively) and epicardial (29.13 +/- 7.49% and 66.33 +/- 7.64% after 0.25 and 0.5 mg/kg, respectively) flows without a significant fall in the transmural ratio. These increases were, however, significantly less than those observed in nonischemic bed flow, resulting in a transventricular gradient of blood flow. No dog developed abnormal ECG patterns after dipyridamole infusion. Thus pacing produces both a transmural and a transventricular gradient in blood flow, whereas vasodilation causes only a transventricular flow abnormality.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of atropine and propranolol on the respiratory, circulatory, and ECG responses to high altitude in man

In order to analyze the respiratory, cardiovascular, and ECG responses to acute hypoxic hypoxia, three experimental series were carried out in a randomized manner on 11 healthy, unacclimatized volunteers at rest during standardized stepwise exposure to 6000 m (PAO2 35.2 +/- 2.9 mmHg/4.7 +/- 0.4 kPa) in a low-pressure chamber a) without (control), b) with propranolol, and c) with atropine combined with propranolol. The results show that hypoxic hyperventilation and alveolar gases are not affected by activation of the sympatho-adrenal axis or by parasympathetic withdrawal. Sympathetic activity, however, increases heart rate, stroke volume (pulse pressure), estimated cardiac output and systolic blood pressure, whereas decreased parasympathetic activity increases heart rate and estimated cardiac output, but lowers stroke volume. The fall in peripheral resistance, observed during progressive hypoxia in all three groups, is thought to be due to hypoxia-induced depression of the vasomotor center. At altitude catecholamine secretion and vagal withdrawal synergistically account in the ECG for the R-R shortening, the relative Q-T lengthening, the elevation of the P wave and the ST-T flattening. Probable direct hypoxic effects on the heart are the increase in P-Q duration and the minor but still significant depression of the T wave. It is concluded that at altitude increased sympatho-adrenal and decreased parasympathetic activity is without effect on hypoxic hyperventilation, but accounts for most of the cardiovascular and ECG changes. Diminution of sympathetic activity and imminent vagotonia arising after acute ascent to 6000 m probably reflect hypoxia of the central nervous system.

Role of calcium channel blockers in experimental exercise-induced ischemia

Calcium channel blockers, which induce vasodilation by relaxing vascular smooth muscle cells, have proven effective in the treatment of angina pectoris. To study mechanisms of calcium blockade in ischemic heart disease, conscious chronically instrumented dogs with a single coronary artery ameroid constrictor were studied during steady-state treadmill runs which induced regional myocardial ischemia. During exercise-induced ischemia, regional systolic wall thickening and subendocardial blood flow were both significantly reduced in the ischemic zone. Calcium channel blockade with verapamil, diltiazem, or nifedipine enhanced regional systolic wall thickening. Regional subendocardial blood flow in the ischemic region, measured during diltiazem and nifedipine experiments, improved during exercise. Reduced coronary artery resistance in the native vessels and/or recruitment of collaterals appears to largely explain the increased total myocardial blood supply in the jeopardized area and the increased function. However, after diltiazem, reduced exercise heart rates as well as reduced left ventricular end-diastolic pressure also contributed to the improvement in the oxygen-supply imbalance in the ischemic myocardium. These data provide a basis for understanding the efficacy of calcium channel blocker treatment in patients with coronary artery disease.

Dissociation between regional myocardial dysfunction and subendocardial ST segment elevation during and after exercise-induced ischemia in dogs

The onset and resolution of electrical and functional measures of regional myocardial ischemia were examined in nine conscious dogs during control exercise and exercise after beta-receptor blockade. The dogs had been instrumented with an ameroid constrictor and were studied when no regional dysfunction was evident at rest, although severe coronary stenosis or coronary occlusion with collateral circulation development was present. ST segment elevation was measured on subendocardial electrograms, and regional wall motion was studied by sonomicrometry. During control exercise, subendocardial myocardial blood flow in the ischemic zone, normalized to blood flow in the nonischemic zone, decreased. Subendocardial ST elevation increased slowly, was significantly different from control standing values by 2.5 minutes of exercise and returned quickly to control values within 5 minutes after exercise. Percent systolic wall thickening decreased rapidly, was significantly depressed by 1 minute of exercise and did not return to control values until 30 minutes after exercise. A second, identical exercise stress was performed on the same day after a single oral dose (1 mg/kg body weight) of atenolol. In the ischemic zone during exercise after atenolol compared with control exercise, normalized subendocardial myocardial blood flow was improved and significantly less ST elevation occurred, but the onset and resolution of ST elevation were not altered. Systolic wall dysfunction during exercise was significantly less after atenolol, and function returned toward preexercise values by 1 minute after exercise, even more rapidly than ST segment resolution.(ABSTRACT TRUNCATED AT 250 WORDS)

Can exercise-induced regional contractile dysfunction be prevented by selective bradycardic agents?

Propranolol (0.5 mg X kg-1 X 5 min-1), alinidine (1 mg X kg-1 X 5 min-1) and the benzazepinon UL-FS 49 (0.5 mg X kg-1 X 5 min-1) were investigated in a canine model of exercise-induced transient myocardial dysfunction, mimicking exercise-induced functional impairment during angina pectoris in man. Each drug was infused intravenously, after two control treadmill exercise runs had shown comparable, ultrasonically assessed regional contractile dysfunction in an area supplied by a partly stenosed branch of the left coronary artery. All three drugs abolished exercise-induced regional contractile dysfunction. Propranolol and alinidine comparably decreased heart rate and positive dp/dtmax during exercise. UL-FS 49 showed a marked negative chronotropic effect without affecting positive dp/dtmax. Thus, prevention of exercise-induced regional contractile dysfunction has been shown for the first time using a selective bradycardic agent.

Propranolol ameliorates exercise-induced regional myocardial dysfunction in dogs

The effects of propranolol (0.5 mg/kg) on exercise-induced regional myocardial dysfunction were investigated in an animal model with limited coronary reserve. Limited coronary reserve was accomplished in conscious, chronically instrumented dogs by external filling of a hydraulic cuff occluder placed on a coronary vessel which did not impair resting myocardial function but induced severe dysfunction during treadmill exercise. Propranolol led to a distinct reduction of heart rate, myocardial contractility and systolic blood pressure, thereby ameliorating exercise-induced regional myocardial dysfunction.

Experimental exercise-induced ischemia: drug therapy can eliminate regional dysfunction and oxygen supply-demand imbalance

The purpose of this study was to test the hypothesis that moderately severe exercise-induced regional myocardial ischemia can be prevented by combined pharmacologic intervention. Eight chronically instrumented dogs were studied using an ameroid constrictor to produce critical stenosis of the left circumflex coronary artery. The dogs were studied during steady state treadmill exercise that induced regional myocardial dysfunction (reduced systolic wall thickening; sonomicrometers) and ischemia (reduced subendocardial blood flow; microspheres). During a control exercise run, wall thickening in the ischemic posterior wall decreased from 21.4 to 13.3% whereas subendocardial blood flow failed to increase normally (36% of that in the normal zone). In the control anterior wall, both wall thickening and subendocardial blood flow increased significantly during the control run. Wall thickness-left ventricular pressure loop areas were calculated as an index of regional work; this index increased abruptly with the onset of exercise in both regions but became significantly depressed in the ischemic region during the steady state exercise. Therapy with a combination of atenolol (0.3 mg/kg body weight orally), diltiazem (0.3 mg/kg intravenously) and isosorbide dinitrate (2.0 mg/kg orally) effectively prevented regional myocardial ischemia and regional dysfunction. After drug therapy, wall thickening in the posterior wall increased from 17.3% at rest to 18.8% during exercise, and the regional transmural blood flow pattern was markedly improved. The initial overshoot of the regional work index during exercise was blunted by the drug therapy, and at steady state no differences between the ischemic and control regions were detected. Thus, combined drug therapy can eliminate exercise-induced regional myocardial ischemic dysfunction and appears to normalize the oxygen supply-demand imbalance.

Persistence of regional left ventricular dysfunction after exercise-induced myocardial ischemia

To determine whether regional myocardial dysfunction occurring after exercise-induced ischemic might be caused by continued abnormalities of myocardial blood flow in the post-exercise period, nine dogs were instrumented with ultrasonic microcrystals for determination of circumferential segment shortening, circumflex artery electromagnetic flow probes, and hydraulic coronary artery occluders. Dogs performed treadmill exercise during partial inflation of the coronary artery occluder. When the stenosis was maintained after exercise (persistent stenosis), subendocardial flow = 0.79 +/- 0.42 ml/min per g vs. 1.39 +/- 0.43 ml/min per g control), and this was associated with continued dysfunction in the ischemic zone (segment shortening 45.4 +/- 36.9% of resting control). When the stenosis was released immediately after exercise (temporary stenosis), however, flow was markedly increased 1 min post-exercise (mean transmural flow 4.24 +/- 1.22 ml/min per g; subendocardial flow 4.18 +/- 1.52 ml/min per g), and this was associated with a transient increase in segment shortening to 104.5 +/- 9.3% of resting control. 5 min after exercise, however, moderate reductions in ischemic segment shortening were noted after both temporary stenosis and persistent stenosis runs, and these persisted for 30 min post-exercise. It is concluded that regional left ventricular dysfunction may persist for a significant period of time after exercise-induced ischemia. Furthermore, early after exercise, dysfunction is related to persistent abnormalities of myocardial blood flow, whereas late after exercise it is independent of primary reductions in myocardial blood flow.

The role of beta blockade, with and without intrinsic sympathomimetic activity, in preserving compromised left ventricular function in patients with ischemic heart disease

To determine if intrinsic sympathomimetic activity (ISA) is associated with a hemodynamic advantage over non-ISA beta blockade, 17 patients with ischemic heart disease underwent resting echocardiography and symptom-limited stress radionuclide angiography on three occasions: control, pindolol (with ISA), 10 mg twice a day, and propranolol, 40 mg four times a day, in a nonrandomized design. In six patients with normal resting ejection fraction (EF) (mean = 57 +/- 4%), neither drug was associated with a significant hemodynamic improvement over control. In 11 patients with reduced resting EF (mean = 37 +/- 9%), pindolol was associated with a higher EF and peak velocity of circumferential fiber shortening at rest compared with propranolol but not control. On maximal exertion, pindolol was associated with a higher EF than that during control but was similar to propranolol. These observed beneficial effects on pindolol appear primarily to be related to the partial beta agonist activity which this agent possesses.

Cyclical coronary flow reductions in conscious dogs equipped with ameroid constrictors to produce severe coronary narrowing

In conscious dogs equipped with ameroid constrictors to produce gradual coronary occlusion, coronary flow velocity was monitored prior to complete occlusion when coronary constriction was severe (resting flow velocity reduced by 10-50% from control recordings made days after ameroid implantation). In six of the ten dogs, we observed spontaneous cyclical variations in coronary flow velocity, characterized by gradual reduction in flow followed by very abrupt restoration of flow. The cyclic coronary flow reductions were observed between 20 and 31 days after ameroid implantation. These changes in flow bear striking similarity to those observed by previous investigators using anesthetized, open-chest canine preparations, in which the role of platelets was clearly demonstrated. Consequently, we hypothesize that spontaneous platelet aggregation and de-aggregation within the severely narrowed coronary lumen (enclosed by the ameroid constrictors) could account for our observations.

Effect of ICI 118,587 on left ventricular function during graded treadmill exercise in conscious dogs

Effect of ICI 118,587 (corwin, ICI), a new cardioselective beta-partial agonist, on left ventricular (LV) function was studied and compared with that of isoproterenol and propranolol in 10 conscious normal dogs, instrumented with a micromanometer and pairs of ultrasonic crystals for analysis of LV wall motion. Heart rate, LV dP/dt and mean circumferential shortening velocity were measured at rest and during 3 levels of treadmill exercise (Ex). Before drug administration (control), the heart rate, dP/dt, percent shortening and mean circumferential shortening velocity were enhanced linearly along with the graded Ex. The Ex response curve shifted upward during isoproterenol infusion and downward after the administration of propranolol. With ICI, hemodynamic and contractile indexes were enhanced at rest as with isoproterenol, while these indexes were depressed during maximal Ex as after propranolol. The Ex response curve after ICI crossed the control response curve at a moderate level of Ex. Thus ICI exerts positive inotropic and chronotropic effects at rest when basal sympathetic tone is low, whereas it exerts negative inotropic and chronotropic properties at maximal Ex when sympathetic tone is high. The inherent dual action of this drug is expected to open a new field of treatment for ischemic heart disease with or without heart failure.

Detection of ischemic wall dysfunction: comparison between M-mode echocardiography and sonomicrometry

The quantification of regional ventricular function by M-mode echocardiography was compared to that by sonomicrometry in 10 closed-chest, sedated swine during temporary occlusions of the left circumflex coronary artery. Wall thickening during systole (%WT) was calculated to quantitate regional myocardial function, and percentage of fractional shortening (%FS) was calculated from both sonomicrometer tracings and M-mode echocardiograms. Ventricular dimensions at end diastole and end systole were also compared before and after 2 minutes of coronary artery occlusion. Both techniques detected significant changes in wall thickness, %WT, and %FS after occlusion. Changes in %WT during coronary artery occlusion detected by M-mode echocardiography and sonomicrometry had a significant linear relationship (p less than 0.05). Discrepancy between the two techniques in the measurement of wall thickness at end diastole was attributed to the difficulty in measuring relatively small distances with M-mode echocardiograms. However, we conclude that the clinical M-mode echocardiogram is capable of detecting acute regional wall dysfunction associated with ischemia.

Randomized, placebo-controlled study of the effect of verapamil on exercise hemodynamics in coronary artery disease

At cardiac catheterization, 16 patients with coronary artery disease (14 men and 2 women) were allocated by a random, double-blind method to intervention with placebo (saline solution) or verapamil (0.2 mg/kg total by bolus and by 10-minute infusion). In all patients, resting and exercise (3 minutes with a bicycle at 150 kg X m/min) hemodynamic values were obtained during a control period and after intervention. Subsequent left ventriculography and coronary arteriography revealed a mean ejection fraction of 52 and 53% and the mean number of diseased vessels (3-vessel scale) of 2.1 and 1.5 in the placebo and verapamil groups, respectively. In both groups of patients, exercise induced significant increased heart rate, mean arterial pressure, left ventricular end-diastolic pressure and cardiac index. Verapamil increased the heart rate and decreased the mean arterial pressure at rest and the arterial pressure during exercise. It did not affect exercise-induced increases in left ventricular end-diastolic pressure or cardiac index. These results support a role for peripheral mechanisms mediating the antianginal effects of verapamil.

The ECG changes due to altitude and to catecholamines

In order to distinguish the effects of beta-receptor stimulation on the ECG from other factors during short-term adjustment to hypoxic aerohypoxia, the ECG of 19 volunteers were compared during moderately acute, stepwise exposure to high altitude (6,000 m) in a low pressure chamber, once with and once without beta-receptor blockade (propranolol), and after isoprenaline inhalation at ground level. The results show that beta-receptor stimulation accounts mainly for most ECG changes during altitude exposure, i.e., for the shortening of R-R interval, the lengthening of Q-T and in particular for the ST-T flattening, the latter therefore being only an indirect sign of hypoxia. After exclusion of the catecholamines, the minor but still significant ECG changes at altitude (shortening of R-R interval, increase of P wave, prolongation of P-Q, deviation of the R vector, T wave flattening in the left precordial leads) may be attributed to other, so far undefined factors, such as cardiac hypoxia, vagal withdrawal, or increase of pulmonary resistance.

Electrocardiographic effects of myocardial ischemia induced by atrial pacing in dogs with coronary stenosis. II. Comparison of S-T segment and QRS changes in dogs with left circumflex arterial narrowing

The effects of tachycardia on the QRS complex in the absence and in the presence of coronary arterial constriction are of possible clinical significance. To study the spatial and temporal distributions of these effects, a canine model simulating progressive coronary occlusion was studied. Tachycardia was induced by atrial pacing, coronary narrowing was produced by implantation of an ameroid constrictor about the left circumflex coronary artery and electrocardiographic effects were studied by isopotential mapping techniques at rates of up to 250 beats per minute, one to three weeks after constrictor implant. In five dogs. pacing without coronary constriction demonstrated the spatial and temporal dependence of QRS changes; tachycardia-induced changes varied from one torso site to another at any instant, and from one moment to another at any one torso location. Patterns in ten dogs with constrictors were dependent upon time after ameroid placement. Resting QRS and S-T segment distributions remained unchanged. One week after surgery, QRS and S-T segment patterns during pacing were as in controls. Two weeks after surgery, QRS patterns were as in controls but flat S-T segment depression resulted at high rates. After three weeks, QRS patterns during pacing were abnormal and S-T segment depression was observed. At each of the later times, the QRS response was similar at rates which did and which did not cause S-T depression. Thus, the effects of tachycardia on excitation and recovery forces in the presence of restricted coronary flow are independent of one another. This suggests that the two responses to ischemia may be due to different basic mechanisms and, hence, that their detection may reflect different myocardial abnormalities.

Thallium-201 myocardial imaging during maximal and submaximal exercise: comparison of submaximal exercise with propranolol

Propranolol is an effective drug for patients with angina and has been shown to favorably alter exercise ejection fraction and myocardial perfusion images in patients with coronary disease. A characteristic effect of propranolol is reduction in exercise heart rate (HR). Twenty men with coronary disease (10 with prior infarction), angina-limited exercise tests, abnormal myocardial blood flow distribution images (MBFDI) (201thallium) during exercise, and normal resting ejection fractions underwent treadmill exercise testing with imaging on three occasions. Control maximal exercise was performed initially with measurement of MBFDI. Propranolol, 40 mg by mouth four times a day, was administered for a week with exercise repeated to the same workload. A third study, with men off propranolol, was undertaken with exercise continued only to the HR obtained while the men were taking propranolol (submaximal exercise). All men had improvement in MBFDI while receiving propranolol (men without infarction +780 +/- 88 [average +/- SEM] normalized count rate difference between control and propranolol; men with infarction +724 +/- 73 normalized counts). Greater count differences were noted when control exercise and HR-controlled, submaximal exercise MBFDI were compared with a greater difference in men with infarction (+1094 +/- 89 normalized counts) than for men without infarction (+896 +/- 88 normalized counts). Results suggest that propranolol improves MBFDI during exercise in men with angina, but that submaximal exercise results in more normal MBFDI than does propranolol for exercise to the same HR.

Effects of propranolol on regional myocardial blood flow and function during severe coronary stenosis in dogs

The effects of propranolol alone or associated with atrial pacing were studied on regional myocardial blood flows (RMBF) and regional contractility (sonocardiometry) in non-ischemic, moderately and severely ischemic areas of the canine myocardium. In non-ischemic areas, propranolol reduced both epicardial and endocardial flows, increased the endo/epi ratio and decreased regional contractility. The reductions in subendocardial flow and function were correlated. In moderately and severely ischemic areas, propranolol increased subendocardial flow, reduced subepicardial flow, increased the endo/epi ratio and preserved or even slightly improved regional contractility. There was a good correlation between the propranolol-induced protective effects on regional contractility and the drug-induced increase in subendocardial flow since under atrial pacing subendocardial flow no longer increased and regional function dropped dramatically.

Electrocardiographic effects of myocardial ischemia induced by atrial pacing in dogs with coronary stenosis. I. Repolarization changes with progressive left circumflex coronary. artery narrowing

The electrocardiographic effects of tachycardia in normal dogs and in dogs with progressive coronary stenosis were studied. In five animals, left atrial electrodes were surgically implanted and body surface isopotential patterns were recorded from 84 torso electrodes; pacing at rates of up to 250 beats/min increased recorded voltages registered during the ST segment but did not alter the spatial distribution of these potentials. In 10 dogs, an ameroid constrictor was also implanted about the left circumflex coronary artery. One week after surgery, pacing to rates of 250 beats/min had no effect on ST segment isopotential patterns. However, 2 and 3 weeks postoperatively, when significant coronary constriction was anticipated, pacing-induced ST segment depression was recorded along the inferior anterior and posterior chest surfaces. Abnormal negative voltages were more intense at 3 than at 2 weeks, but the distribution of these abnormalities was unchanged. This pattern was consistent with the location of ischemic myocardium. Calculation of early (10 to 40 ms) ST segment slopes demonstrated that downsloping forms were recorded over most of the area with abnormal ST segment voltages, but that horizontal or upsloping depression occurred at the margins of the abnormal zones. The findings suggest that this canine model may be a useful analog to clinical exercise stress testing to aid in evaluating the significance of clinical electrocardiographic patterns.

Effect of oral propranolol on left ventricular performance in patients with ischemic heart disease. Ear densitographic study during uninterrupted treadmill exercise

The effect of oral propranolol on left ventricular performance during early upright exercise was evaluated by ear densitography in patients with arteriographic coronary artery disease (CAD). Measurements of systolic time intervals differentiated 10 unmedicated patients with CAD (group 1) and 15 patients with CAD taking propranolol (group 3). The patients in group 3 had less shortening of preejection period at 1 minute and 4 minutes of exercise than group 1 patients (p less than 0.001 and p less than 0.005, respectively), with propranolol appearing to prevent the abnormal shortening of preejection period seen in the unmedicated group. Group 3 patients, in contrast to group 1 patients, showed reduction of heart rate and heart rate-blood pressure product both at rest and during exercise. Furthermore, PEP/LVET and percentage change in PEP/LVET from control responses were similar to those of subjects free of CAD (group 2). These results indicate that propranolol effects a favorable change in LV performance by postponing early exhaustion of cardiac reserve, despite significant CAD. There was relatively large overlap in percentage change in PEP/LVET from control between group 2 and group 3 in contrast to the clear separation among unmedicated patients. Thus, the excellent diagnostic accuracy of systolic time intervals recorded during exercise is greatly reduced by beta-adrenoceptor blockade.

Propranolol and the respiratory, circulatory, and ECG responses to high altitude

The role of the sympatho-adrenal system in the acute respiratory and cardiovascular responses to high altitude was studies in 20 volunteers during ascent to 6,000 m in a low pressure chamber, once without (control) and once with beta-adrenergic blockade. Special attention was paid to the hypoxia-induced ECG changes. Propranolol lowered the level of hypoxia-induced cardiovascular reactions, whereas it had no effect on hypoxic hyperventilation and alveolar gases. At altitude, ECG changes during myocardial depolarization occurred in both the propranolol and the control groups, probably due to the direct effects of hypoxia. During the repolarization phase, propranolol led to an almost complete abolition of S-T depression and to significant reduction of T wave flattening. The minor but still significant flattening of the T wave as well as the relative (to the heart rate) lengthening of Q-T is probably due to the direct effects of hypoxia. Propranolol abolishes or diminishes the signs of cardiac hypoxia by antagonizing the effects of catecholamine release and/or by reducing myocardial oxygen consumption, thus probable increasing the ability to withstand oxygen-want at altitude.