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

A pathophysiological compass to personalize antianginal drug treatment

Myocardial ischaemia results from coronary macrovascular or microvascular dysfunction compromising the supply of oxygen and nutrients to the myocardium. The underlying pathophysiological processes are manifold and encompass atherosclerosis of epicardial coronary arteries, vasospasm of large or small vessels and microvascular dysfunction - the clinical relevance of which is increasingly being appreciated. Myocardial ischaemia can have a broad spectrum of clinical manifestations, together denoted as chronic coronary syndromes. The most common antianginal medications relieve symptoms by eliciting coronary vasodilatation and modulating the determinants of myocardial oxygen consumption, that is, heart rate, myocardial wall stress and ventricular contractility. In addition, cardiac substrate metabolism can be altered to alleviate ischaemia by modulating the efficiency of myocardial oxygen use. Although a universal agreement exists on the prognostic importance of lifestyle interventions and event prevention with aspirin and statin therapy, the optimal antianginal treatment for patients with chronic coronary syndromes is less well defined. The 2019 guidelines of the ESC recommend a personalized approach, in which antianginal medications are tailored towards an individual patient's comorbidities and haemodynamic profile. Although no antianginal medication improves survival, their efficacy for reducing symptoms profoundly depends on the underlying mechanism of the angina. In this Review, we provide clinicians with a rationale for when to use which compound or combination of drugs on the basis of the pathophysiology of the angina and the mode of action of antianginal medications.

Myocardial ischemia: lack of coronary blood flow, myocardial oxygen supply-demand imbalance, or what?

This opinionated article reviews current concepts of myocardial ischemia. Specifically, the historical background is briefly presented. Then, the prevailing paradigm of myocardial oxygen-supply-demand imbalance is criticized since demand is a virtual parameter that cannot be measured and data on measurements of myocardial blood flow and contractile function rather support matching between flow and function. Finally, a concept of myocardial ischemia that focusses on the reduction of coronary blood flow to below 8-10 µl/g per beat with consequences for myocardial electrical, metabolic, contractile and morphological features is advocated.

Heart rate in the pathophysiology of coronary blood flow and myocardial ischaemia: benefit from selective bradycardic agents

Starting out from a brief description of the determinants of coronary blood flow (perfusion, pressure, extravascular compression, autoregulation, metabolic regulation, endothelium-mediated regulation and neurohumoral regulation) the present review highlights the overwhelming importance of metabolic regulation such that coronary blood flow is increased at increased heart rate under physiological circumstances and the overwhelming importance of extravascular compression such that coronary blood flow is decreased at increased heart rate through reduction of diastolic duration in the presence of severe coronary stenoses. The review goes on to characterize the role of heart rate in the redistribution of regional myocardial blood flow between a normal coronary vascular tree with preserved autoregulation and a poststenotic vasculature with exhausted coronary reserve. When flow is normalized by heart rate, there is a consistent close relationship of regional myocardial blood flow and contractile function for each single cardiac cycle no matter whether or not there is a coronary stenosis and what the actual blood flow is. beta-Blockade improves both flow and function along this relationship. When the heart rate reduction associated with beta-blockade is prevented by pacing, alpha-adrenergic coronary vasoconstriction is unmasked and both flow and function are deteriorated. Selective heart rate reduction, however, improves both flow and function without any residual negative effect such as unmasked alpha-adrenergic coronary vasoconstriction or negative inotropic action.

Vasopeptidase inhibition in a canine model of exercise-induced left ventricular dysfunction

1. The present study compared the acute efficacies of vasopeptidase inhibition with omapatrilat, nitroglycerin and angiotensin-converting enzyme (ACE) inhibition in exercise-induced myocardial dysfunction. Omapatrilat, a vasopeptidase inhibitor, inhibits both neutral endopeptidase and ACE. Whereas vasopeptidase inhibitors have demonstrated clinical efficacy in hypertension and heart failure, their effects in myocardial ischaemia remain unclear. 2. Omapatrilat (0.3 mg/kg) was compared with vehicle (saline), an ACE inhibitor (fosinoprilat; 0.44 mg/kg) and nitroglycerin (8.0 microg/kg per min), in an established canine model of exercise-induced myocardial dysfunction induced by progressive closure of an ameroid constrictor placed about the proximal circumflex coronary artery. Maximal treadmill exercise tests, terminated when heart rate failed to increase with increasing workload or failure to continue exercise, were performed in chronically instrumented dogs. 3. During exercise, omapatrilat and nitroglycerin similarly increased ischaemic wall thickening (P< or = 0.0001, ANOVA, 12 d.f.), whereas fosinoprilat and vehicle were without effect. Ischaemic zone ST changes were decreased with nitroglycerin (P = 0.0006, ANOVA, 12 d.f.) and tended to decrease with omapatrilat (P = 0.07, ANOVA, 12 d.f.). Peak exercise capacity was increased with nitroglycerin (9.7 +/- 1.1 vs 11.2 +/- 1.0 kcal, control vs 4 h, respectively; n = 6) and omapatrilat (9.7 +/- 0.8 vs 11.4 +/- 0.6 kcal, control vs 4 h, respectively; n = 6) and was unchanged with ACE inhibition (9.0 +/- 1.2 vs 9.5 +/- 1.1 kcal, control vs 4 h, respectively; n = 7). Omapatrilat differentially increased double product during exercise (P = 0.001, ANOVA, 12 d.f.) compared with other treatments. 4. During exercise-induced myocardial dysfunction, acute ACE inhibition did not attenuate ischaemic changes and failed to improve exercise capacity. Increased exercise capacity with omapatrilat was accompanied by a differential increase in double product, consistent with increased oxygen supply and demand. Improvements in ischaemic function were comparable between omapatrilat and nitroglycerin, suggesting that omapatrilat may represent a novel therapy in demand-induced ischaemia.

Heparin accelerates coronary collateral development in a porcine model of coronary artery occlusion

BACKGROUND

Coronary collaterals develop in response to an ischemic stimulus. However, collateral growth is not sufficient to result in the complete recovery of coronary reserves. Using a porcine model of gradual coronary artery occlusion, we investigated the effect of continuous heparin infusion on coronary collateral development.

METHODS AND RESULTS

We placed ameroid constrictors on the left circumflex coronary artery of 16 minipigs; the ameroid constrictors completely occluded the left circumflex coronary artery at 10 +/- 1 days. Half of the animals also were instrumented with subcutaneously placed osmotic pumps and catheters that delivered heparin (300 units/h) into the external jugular vein. At 2, 3, and 4 weeks, we assessed blood flow at rest and during vasodilation using radioactive microspheres. Our results indicate that the animals receiving heparin restored resting myocardial blood flow to normal levels at or before 2 weeks; in contrast, we did not see normal resting myocardial blood flow levels in the untreated-ameroid animals until 3 weeks. Under vasodilated conditions, untreated-ameroid animals experienced a severe loss of coronary reserves at 2 weeks. Although this improved with time, these animals still were significantly underperfused at 4 weeks. In contrast, in the heparin-treated animals, coronary reserves returned to near-normal levels between 3 and 4 weeks. In addition, infarct size was significantly smaller in the heparin-treated animals.

CONCLUSIONS

These experiments suggest that the administration of heparin in the early phases of gradual coronary occlusion accelerates the rate of return of normal blood flow under resting conditions, substantially increases the recovery of coronary reserve, and reduces the risk of infarction.

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.

Myocardial lactate release during ischemia in swine. Relation to regional blood flow

To determine the relation between regional myocardial blood flow, contractile function, and myocardial lactate release during mild-to-moderate regional myocardial ischemia, nine open-chest swine were instrumented for measurement of regional myocardial blood flow (microsphere method), contractile function (sonomicrometry), and hemodynamics. L-[1-14C]Lactate or L-[U-13C]lactate was infused intravenously using a primed continuous infusion technique to quantify regional myocardial lactate release. D-[U-13C]glucose or D-[6-14C]glucose was simultaneously infused to determine the contribution of exogenous glucose to lactate release. Graded coronary ischemia (two to three levels) was created in the left anterior descending coronary arterial distribution by mechanically constricting the artery in five animals or by decreasing flow through a cannulated left anterior descending artery in four animals. In all nine animals, subendocardial blood flow was 0.99 +/- 0.21 (ml/min)/g during control and 0.34 +/- 0.14 (ml/min)/g during the most severe grade of underperfusion (p less than 0.001) in the left anterior descending coronary arterial distribution. Regional myocardial lactate release was 0.15 +/- 0.09 and 1.19 +/- 0.75 mumols/ml, respectively (p less than 0.003). A highly significant inverse correlation was observed between subendocardial blood flow and myocardial lactate release during the graded reductions in blood flow (r = -0.71, p less than 0.001). Results from sonomicrometry showed a significant reduction in contractile ventricular function in the anterior wall during the graded reductions in blood flow. The regional arterial-venous glucose difference increased significantly with underperfusion in the left anterior descending coronary arterial distribution, from 0.14 +/- 0.15 to 0.56 +/- 0.37 mumols/ml (p less than 0.003). The contribution of exogenous glucose to lactate release also increased significantly; 0.04 +/- 0.03 mumols/ml of the lactate came from exogenous glucose during control compared with 0.64 +/- 0.59 mumols/ml during the most severe underperfusion (p less than 0.02). A significant positive correlation exists between lactate release and lactate from exogenous glucose during graded underperfusion (r = 0.96, p less than 0.001). In summary, these data demonstrate a close inverse relation between regional myocardial lactate release and regional subendocardial blood flow during graded ischemia.

Selective thromboxane A2 receptor blockade in experimental exercise-induced myocardial ischaemia in dogs

Thromboxane A2 receptor stimulation induces blood platelet aggregation and vasoconstriction, both potential causes of impaired perfusion of ischaemic myocardium. To study the potential role of thromboxane A2 receptor blockade in exercise-induced myocardial ischaemia and post-exercise myocardial dysfunction, nine conscious chronically instrumented dogs with single-vessel coronary artery stenosis (ameroid constrictor) were studied before, during and after steady-state treadmill runs which induced regional myocardial ischaemia. Three hours after a control run, the dogs were exercised again after the infusion of a selective thromboxane A2 receptor blocker: BM 13.177 (10 mg kg-1 i.v.). In the control run, systolic wall thickening (WTh, sonomicrometer) in the post-stenotic myocardium decreased from 22.1 +/- 9.1% at rest to 8.8 +/- 5.2% (mean +/- SD). Subendocardial blood flow (microspheres) in the ischaemic area decreased from 0.75 +/- 0.25 to 0.45 +/- 0.27 (ml min-1 g). The WTh in the ischaemic region remained depressed at 20 min after the run. BM 13.177 reduced peak left ventricular (+) dP/dt (micromanometer) and WTh in both control and post-stenotic myocardium at rest, during and after the run. WTh in the ischaemic area was reduced to approximately the same levels during running with BM 13.177 (not significantly different from control exercise) and remained depressed for at least 30 min after the run. Regional myocardial blood flow was not affected by BM 13.177. Thus, selective thromboxane A2 receptor blockade with BM 13.177 had a modest negative inotropic effect and did not improve regional function or blood flow in post-stenotic ischaemic subendocardium.

Consequences of regional inotropic stimulation of ischemic myocardium on regional myocardial blood flow and function in anesthetized swine

Determination of the effect of inotropic stimulation on regionally ischemic and hypokinetic myocardium is complicated when intravenous administration of the inotropic agent also causes stimulation of nonischemic adjacent and distant regions, thereby altering global ventricular hemodynamics. To obviate such events, 16 anesthetized swine were studied during regional inotropic stimulation by infusion of dobutamine hydrochloride (2.5 +/- 1 microgram/min) into the cannulated left anterior descending coronary artery. Coronary inflow was controlled by a pump in an extracorporeal circuit. Two groups of swine with different degrees of ischemia were studied. In the first group of animals (n = 8), reduction in coronary inflow to produce a fall in coronary artery pressure (CAP) from 114 +/- 7 mm Hg to 62 +/- 2 mm Hg caused a decrease in percent systolic wall thickening (%WTh) from 34.6 +/- 8.1% to 25.4 +/- 5.8% (p less than 0.005). In the second group of animals (n = 8), CAP was decreased to 46 +/- 5 mm Hg (control: 115 +/- 8 mm Hg) and % WTh decreased from 34.1 +/- 16.4% to 10.4 +/- 6.9% (p less than 0.001). Subendocardial blood flow was reduced from 1.41 +/- 0.38 ml/min/g to 0.65 +/- 0.13 ml/min/g (group 1, p less than 0.001) and from 1.08 +/- 0.22 ml/min/g to 0.24 +/- 0.08 ml/min/g (group 2, p less than 0.001). Regional infusion of dobutamine caused asynchronous ventricular contraction with early systolic augmentation in wall thickening followed by late systolic thinning. Therefore, during hypoperfusion regional myocardial function assessed by %WTh remained unchanged (26.2 +/- 5.8%, p = NS) in group 1 and decreased significantly to 1.6 +/- 5.1% (p less than 0.041) in group 2. Subendocardial blood flow decreased to 0.44 +/- 0.15 ml/min/g in group 1 (p less than 0.005) and to 0.15 +/- 0.07 ml/min/g in group 2 (p less than 0.012). To account for the augmented early systolic thickening that occurred during asynchronous contraction, a myocardial work index was developed in which the sum of the instantaneous left ventricular pressure-wall thickness product was calculated for estimation of regional myocardial work. Increases in this work index were apparent with the addition of dobutamine at both levels of hypoperfusion. This significant enhancement in regional myocardial function in group 2 caused a significant increase of 16% (p less than 0.009) in overall left ventricular power during ejection. Thus, regional inotropic stimulation with dobutamine caused enhancement of maximum work of the ischemic myocardium in the steady state despite a further decrease in subendocardial blood flow.

Beta-adrenoceptor antagonists plus nifedipine in the treatment of chronic stable angina pectoris

The antianginal effects of beta-adrenoceptor antagonists are achieved by a reduction in myocardial oxygen demand. This is a rational approach to treatment in patients whose angina is caused by a fixed stenosis. However, dynamic coronary vasospasm is an important factor in patients with chronic stable angina. Nifedipine increases myocardial oxygen supply by reducing coronary vascular tone and is a logical approach to treatment in these patients. For monotherapy of angina, nifedipine is less effective than the beta-adrenoceptor antagonists, but the combination has additive effects in reducing the frequency of anginal episodes and improving exercise tolerance. Plasma concentrations of nifedipine are closely related to clinical efficacy, and the variable first-pass metabolism of the drug leads to wide interindividual differences in peak concentrations and duration of action. Increasing the size of individual doses of nifedipine carries a risk of enhanced side effects due to high peak plasma concentrations. Optimal treatment may be more appropriately achieved in some patients by a slow release formulation, but with an increased frequency of administration.

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.

Coronary collateral reserve during exercise induced ischemia in swine

We determined coronary collateral vasodilator reserve during exercise-induced ischemia in 17 mini-swine. We induced coronary collateral development in the left circumflex bed by placing an ameroid occluder on that artery. Four weeks later we studied the animals at rest and during exercise (EX) eliciting heart rates (HR) of 240 and 265 beats/min. We measured myocardial blood flow with microspheres and myocardial function by wall thickness sonomicrometry gauges. At matched exercise HRs we treated the animals with nifedipine (10 micrograms/kg IV) (EXN 10), nifedipine (100 micrograms/kg IV), (EXN 100), and adenosine infusion (1.2 mg/min/kg) EXAD. EXN 10 did not significantly alter hemodynamics compared to EX but EXN 100 and EXAD both decreased blood pressure significantly (p less than 0.05). Ischemic endocardial/nonischemic endocardial flow ratios and collateral resistance served as indices of vasodilator reserve. In the ischemic zone exercise reduced vasodilator reserve to 24 +/- 3% in the endocardium and 64 +/- 7% in the epicardium. Neither EXN 10 nor EXAD improved exercise-induced ischemia measured either as flow or function. However EXN 100 improved function during exercise-induced ischemia without improving coronary collateral flow. We conclude there is no additional coronary flow reserve during exercise-induced ischemia in the collateral dependent bed of the pig a few days after occlusion that can be recruited. Large doses of nifedipine improve function by direct action on the myocardium or by reducing afterload. The lack of development and deep myocardial distribution of the coronary collateral vessels in the pig may be an important factor of why these nifedipine responses differ from those reported in species which have primarily large epicardial coronary collaterals.

Cardiac nuclear medicine, present status, future hopes as seen from the view of a cardiologist

The present techniques of nuclear medicine that are used in cardiology are critically evaluated. They are used mainly for the assessment of coronary circulation (measurement of regional flow and perfusion), on the determination of left ventricular muscle function (radionuclear ventriculography) and on the study of cardiac metabolism. The last-mentioned technique is unique insofar as (except for magnetic resonance imaging) there is no other method available to analyse myocardial metabolism noninvasively in living man. Of immense practical importance is the diagnosis and quantitation of the abnormal coronary flow and perfusion, as coronary artery disease is one of the major diseases of the Western world. Finally, the value of nuclear imaging techniques is compared with that of other nonnuclear methods. In present-day medicine, with its increasingly high costs, the value of nuclear techniques must be assessed very carefully.

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.

Mechanism of beneficial effect of beta-adrenergic blockade on exercise-induced myocardial ischemia in conscious dogs

We examined the importance of decreased heart rate in the beneficial effect of beta-adrenergic blockade on exercise-induced regional myocardial ischemia and contractile dysfunction in conscious dogs with single vessel coronary stenosis (ameroid constrictor). Studies were performed during control treadmill exercise, which produced regional myocardial ischemia (blood flow measured with microspheres) and wall dysfunction (measured using sonomicrometers). A second run was performed after the administration of atenolol (0.3-1.0 mg/kg i.v.), and the reduced heart rate caused by atenolol during early steady-state running was then prevented by atrial pacing during the latter portion of the run. Atenolol reduced the exercise heart rate from 217 +/- 25 beats per minute (SD, n = 9) to 166 +/- 15, and ischemic zone wall thickening during systole improved from 27 +/- 22% of the resting value in the control run to 50 +/- 25% of the resting value in the atenolol run (p less than 0.01). Atrial pacing then increased heart rate to 217 +/- 23 beats per minute, and regional wall thickening deteriorated to 15 +/- 25% of the resting value. Regional subendocardial blood flow in the ischemic zone during atrial pacing with atenolol was slightly less than that observed in the control run, in both ischemic and control zones, indicating no remaining beneficial effect of atenolol when heart rate reduction was eliminated. We conclude that the only significant mechanism for the improvement in exercise-induced ischemia and wall motion produced by atenolol is a reduction in the exercise heart rate.(ABSTRACT TRUNCATED AT 250 WORDS)

Elimination of exercise-induced regional myocardial dysfunction by a bradycardiac agent in dogs with chronic coronary stenosis

We have previously demonstrated that the beneficial effect of cardioselective beta-blockade on exercise-induced ischemia is due entirely to negative chronotropism. Therefore we studied the effect of a new bradycardiac agent (UL-FS 49) in 10 dogs with chronic coronary artery stenosis produced by an ameroid constrictor. Regional myocardial function (sonomicrometers, wall thickness) and blood flow (microspheres) were measured during a control treadmill exercise bout and an identical run 3 hr later after the administration of UL-FS 49 (1.0 mg/kg iv). In the control run, heart rate increased from 114 +/- 20 to 230 +/- 19 beats/min and systolic wall thickening (%WT) in the poststenotic myocardium decreased from 23.3 +/- 5.2% at rest to 9.3 +/- 5.0%, a 60% reduction. Subendocardial blood flow in the ischemic area decreased from 1.04 +/- 0.30 to 0.55 +/- 0.40 ml/min/g, blood flow per beat decreased from 9.1 X 10(-3) to 2.5 X 10(-3) ml/g, and mean transmural flow failed to increase (1.06 +/- 0.30 vs 1.08 +/- 0.39 ml/min/g). During exercise with UL-FS 49, heart rate increased from 89 +/- 10 to only 139 +/- 10 beats/min. End-diastolic left ventricular pressure was increased compared with that during the control run (35.7 +/- 3.0 vs 28.9 +/- 5.5 mm Hg) but left ventricular peak systolic pressure and dP/dt were unchanged. %WT in the ischemic zone did not change significantly during exercise with UL-FS 49 (23.3 +/- 7.9% at rest, 21.5 +/- 8.4% during the run), and in the nonischemic zone it increased to the same extent as during the control run.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuation of exercise-induced myocardial ischemia in dogs with recruitment of coronary vasodilator reserve by nifedipine

There is now evidence that under resting conditions coronary vasodilator reserve exists even in the presence of myocardial ischemia. Therefore, we tested the hypothesis that a vasodilator reserve may exist during exercise so that during exercise-induced ischemia a reduction in coronary constrictor tone can be produced that attenuates the decreases in regional myocardial blood flow and function distal to a severe coronary stenosis without changing the determinants of myocardial oxygen demand. Nine dogs were instrumented with an ameroid constrictor on the left circumflex coronary artery and were studied 2 to 3 weeks later. During a control treadmill run, heart rate increased from 119 +/- 20 to 225 +/- 20 beats/min and peak left ventricular pressure increased from 144 +/- 17 to 163 +/- 28 mm Hg. Poststenotic subendocardial blood flow (measured by a microsphere technique) fell from 1.19 +/- 0.36 to 0.51 +/- 0.30 ml/min X g and systolic wall thickening (by sonomicrometry) decreased from 24.3 +/- 5.8% to 6.0 +/- 6.1%. During an identical run after nifedipine (10 micrograms/kg iv), systemic hemodynamics were not significantly altered. However, subendocardial blood flow was increased to 0.85 +/- 0.51 ml/min X g (p less than .05) and systolic wall thickening to 11.4 +/- 7.8% (p less than .01). We conclude that in this study the amelioration of exercise-induced myocardial ischemia was due to the recruitment by nifedipine of coronary vasodilator reserve.

Ischemic myocardial dysfunction assessed by temporal Fourier transform of regional myocardial wall thickening

A Fourier analysis including the first 20 harmonics was performed on sonomicrometric measurements of regional myocardial wall thickness in eight conscious dogs under control conditions and at four levels of ischemia produced by a hydraulic occluder on the left circumflex coronary artery. Systolic wall thickening was reduced from 26.47 +/- 6.20% (S.D.) (control) to 22.05 +/- 5.73% (mild stenosis), 17.00 +/- 5.86% (moderate stenosis), 11.46 +/- 3.56% (severe stenosis), and 3.69 +/- 2.57% (30-second occlusion), values significantly different from each other (p less than 0.01). The amplitude of the first harmonic decreased stepwise from 1.35 +/- 0.31 to 1.08 +/- 0.29 mm, 0.90 +/- 0.27 mm, 0.69 +/- 0.24 mm, and 0.43 +/- 0.12 mm, all significantly different from each other (p less than 0.05). These amplitude values correlated to percent systolic wall thickening (r = 0.894, p = 0.001). A phase shift of the first harmonic from 137 +/- 11 to 139 +/- 14 degrees, 150 +/- 15 degrees (p less than 0.05 vs control), 161 +/- 21 degrees (p less than 0.01 vs control), and 191 +/- 21 degrees (p less than 0.01 vs control and severe stenosis) correlated with the increase in time from end diastole to the point of maximum wall excursion (r = 0.662, p less than 0.001). These data indicate that the extent of ischemic regional myocardial hypokinesis can be adequately described by the amplitude of the first harmonic, and that the asynchrony of ventricular contraction and relaxation can be detected from the phase of the first harmonic.