Clinical significance of coronary flow reserve: effect of papaverine and exercise

Assessment of endothelial (dys)function in atrial fibrillation

Atrial fibrillation (AF) is associated with an increased risk of mortality and morbidity from stroke and thromboembolism. Endothelial damage or dysfunction may contribute to this increased risk of thromboembolism via the mediation of a prothrombotic or hypercoagulable state. However, the precise pathophysiological mechanism(s) relating endothelial (dys)function to AF and thromboembolism are yet to be fully elucidated. This review article aims to provide a comprehensive overview of endothelial (dys)function and AF, as well as the merits and limitations of the different methods used to assess endothelial function in AF.

Beta-blockers and coronary flow reserve: the importance of a vasodilatory action

Coronary flow reserve (CFR) is the maximal increase in coronary blood flow (CBF) above its resting level for a given perfusion pressure when coronary vasculature is maximally dilated. Normally, hyperaemic CBF reaches values at least 2- to 3-fold greater than resting CBF. Reduction of CFR is mainly due to epicardial coronary artery stenosis or to coronary microvascular dysfunction. CFR can be determined by several techniques that measure CBF itself (e.g. positron emission tomography) or CBF velocities (Doppler methods) from which coronary flow velocity reserve is calculated. Hyperaemic coronary vasodilation can be obtained by pharmacological agents (e.g. adenosine and dipyridamole), but also by the cold pressure test. Long-term antihypertensive treatment induces significant improvement of CFR, which is parallel to the regression of left ventricular (LV) hypertrophy. First- and second-generation beta-adrenergic receptor antagonists (beta-blockers) have shown contradictory influences on CFR. This can be explained by the interaction of the effects on CBF at rest, generally reduced by these drugs, and after hyperaemia, when minimal coronary resistance appears to be either increased or reduced. Third-generation beta-blockers (e.g. carvedilol and nebivolol), which have vasodilating capacity, improve hyperaemic CBF. This occurs as a result of a reduction in minimal resistance, which can be attributed to alpha-adrenergic blockade and/or to a nitric oxide-mediated effect. This improvement is clearly beneficial in patients with coronary artery disease and indicates an improved coronary microvascular function. Changes of CFR due to vasodilating beta-blockers improve microvascular angina pectoris or silent ischaemia in patients without epicardial artery stenosis, and are also helpful in predicting the response or the further improvement of LV function to treatment.

Does the beta-blocker nebivolol increase coronary flow reserve?

INTRODUCTION

Nebivolol, a highly selective beta1-adrenergic receptor-blocker, increases basal and stimulated endothelial nitric oxide (NO)-release. It is unknown, whether coronary perfusion is improved by the increase in NO availability. Therefore, we sought to evaluate the effect of nebivolol on coronary flow reserve (CFR) and collateral flow.

METHODS

Doppler-flow wire derived coronary flow velocity measurements were obtained in ten controls and eight patients with coronary artery disease (CAD) at rest and after intracoronary nebivolol. CFR was defined as maximal flow during adenosine-induced hyperemia divided by resting flow. In the CAD group, collateral flow was determined after dilatation of a flow-limiting coronary stenosis. Collateral flow index (CFI) was defined as the ratio of flow velocity during balloon inflation divided by resting flow.

RESULTS

CFR at rest was 3.0+/-0.6 in controls and 2.1+/-0.4 in CAD patients. After intracoronary doses of 0.1, 0.25, and 0.5 mg nebivolol, CFR increased to 3.4+/-0.7, 3.9+/-0.9, and 4.0+/-0.1 (p<0.01) in controls, and to 2.3+/-0.7, 2.6+/-0.9, and 2.6+/-0.5 (p<0.05) in CAD patients. CFI decreased significantly with intracoronary nebivolol and correlated to changes in heart rate (r=0.75, p<0.001) and rate-pressure product (r=0.59, p=0.001).

DISCUSSION

Intracoronary nebivolol is associated with a significant increase in CFR due to reduction in resting flow (controls), or due to an increase in maximal coronary flow (CAD patients). CFI decreased with nebivolol parallel to the reduction in myocardial oxygen consumption.

Do beta-adrenergic blocking agents increase coronary flow reserve?

BACKGROUND

Beta-adrenergic blocking agents are the cornerstone in the treatment of coronary artery disease (CAD). The exact pathophysiologic mechanism is not clear but depends largely on the oxygen-sparing effect of the drug. Thus, the effect of metoprolol on coronary flow reserve and coronary flow velocity reserve (CFVR) was determined in patients with CAD.

METHODS

Coronary blood flow velocity was measured with the Doppler flow wire in 23 patients (age: 56 +/- 10) undergoing percutaneous transluminal coronary angioplasty for therapeutic reasons. Measurements were carried out at rest, after 1-min vessel occlusion (postischemic CFVR) as well as after intracoronary adenosine (pharmacologic CFVR) before and after 5 mg intravenous metoprolol. In a subgroup (n = 15), absolute flow was measured from coronary flow velocity multiplied by coronary cross-sectional area.

RESULTS

Rate-pressure product decreased after metoprolol from 9.1 to 8.0 x 10(3) mm Hg/min (p < 0.001). Pharmacologic CFVR was 2.1 at rest and increased after metoprolol to 2.7 (p = 0.002). Likewise, postischemic CFVR increased from 2.6 to 3.3 (p < 0.001). Postischemic CFVR was significantly higher than pharmacologic CFVR before as well as after metoprolol. Coronary vascular resistance decreased after metoprolol from 3.4 +/- 2.0 to 2.3 +/- 0.7 mm Hg x s/cm (p < 0.02).

CONCLUSIONS

The following conclusions were drawn from this study. Metoprolol is associated with a significant increase in postischemic and pharmacologic CFVR. However, postischemic CFVR is significantly higher than pharmacologic CFVR. The increase in CFVR by metoprolol can be explained by a reduction in vascular resistance. The increase in CFVR (= increased supply) and the reduction in oxygen consumption (= decreased demand) after metoprolol explain the beneficial effect of this beta-blocker in patients with CAD.

Reversal of abnormal coronary vasomotion by calcium antagonists in patients with hypercholesterolemia

BACKGROUND

It has been shown that exercise-induced coronary vasodilation of angiographically normal coronary vessels is reduced in hypercholesterolemic patients. The purpose of this study was to evaluate the effect of calcium channel blockers on coronary vasomotion of angiographically smooth coronary arteries in hypercholesterolemic patients.

METHODS AND RESULTS

A total of 57 patients were included in the present analysis. Vasomotion of angiographically normal coronary arteries was evaluated in 37 control subjects (group 1) without and 20 patients (group 2) with calcium blocker administration before physical exercise. Both groups were subdivided into subgroup A (normal cholesterol values: < or = 5.5 mmol/L or 212 mg%) and subgroup B (elevated cholesterol values: >5.5 mmol/L or 212 mg%). Coronary luminal area at rest and during exercise was assessed by biplane quantitative coronary angiography. The normal vessels showed a significant increase in coronary luminal area during exercise in subgroup A (n=13) with normal cholesterol values (31%; P<.05) but not in subgroup B (n=24; 13%; P=NS). In contrast, all patients in group 2 showed similar vasodilation during exercise, namely, 22% (P<.05) in subgroups A (n=8) and B (n=12) (P<.05). Independent of the actual cholesterol level, the stenotic lesions showed coronary vasoconstriction during exercise in group 1 but vasodilation in group 2 after pretreatment with calcium antagonists.

CONCLUSIONS

Coronary vasomotor response to exercise is inversely related to actual serum cholesterol level in angiographically normal vessels. Administration of calcium antagonists normalizes exercise-induced vasodilation and thus eliminates cholesterol-induced abnormal vasomotion, probably by a direct effect on the smooth muscles of the vasculature.

Abnormal coronary vasomotion in hypertension: role of coronary artery disease

OBJECTIVES

This study sought to evaluate the effect of dynamic exercise on coronary vasomotion in hypertensive patients in the presence and absence of coronary artery disease.

BACKGROUND

Endothelial dysfunction with abnormal coronary vasodilation in response to acetylcholine has been reported in patients with arterial hypertension.

METHODS

Coronary artery dimensions of a normal and stenotic vessel segment were determined in 64 patients by biplane quantitative coronary arteriography at rest and during supine bicycle exercise. Patients were classified into two groups: 20 patients without evidence of coronary artery disease (10 normotensive, 10 hypertensive [group 1]) and 44 patients with coronary artery disease (26 normotensive, 18 hypertensive [group 2]). Both groups were comparable with regard to clinical characteristics, serum cholesterol levels, body mass index, exercise capacity and hemodynamic data.

RESULTS

Mean aortic pressure was significantly higher in hypertensive than normotensive patients. Exercise-induced vasodilation of the normal vessel segment was similar in normotensive and hypertensive patients without coronary artery disease (group 1), namely, +19% versus +20%. However, in hypertensive patients with coronary artery disease, exercise-induced vasodilation was significantly less in both normal and stenotic vessel segments than in normotensive subjects (+1% vs. +20% for normal [p < 0.003] and -20% vs. -5% for stenotic vessels [p < 0.025]). Administration of 1.6 mg of sublingual nitroglycerin at the end of exercise led to a normalization of the vasodilator response in normotensive as well as hypertensive patients. However, this response became progressively abnormal in group 2 when coronary artery disease was present.

CONCLUSIONS

In the absence of coronary artery disease, the vasomotor response to exercise is normal in both normotensive and hypertensive patients. However, in hypertensive patients with coronary artery disease, an abnormal response of the coronary vessels can be observed, with a reduced vasodilator response to exercise in normal arteries but an enhanced vasoconstrictor response in stenotic arteries. This behavior of the epicardial vessels during exercise suggests the occurrence of endothelial dysfunction (i.e., functional defect) that is not evident in the absence of coronary artery disease. Nitroglycerin reverses impaired coronary vasodilation, but this effect is blunted in the presence of coronary artery disease (i.e., structural defect).

Coronary vasomotion during dynamic exercise: influence of intravenous and intracoronary nicardipine

OBJECTIVES

Our aim was to evaluate the influence of a calcium channel blocking agent of the dihydropyridine group (nicardipine) on coronary vasomotion during dynamic exercise.

BACKGROUND

Coronary vasomotion plays an important role in the pathophysiology of myocardial ischemia.

METHODS

Twenty-nine patients with coronary artery disease were studied at rest and during bicycle exercise with the use of biplane quantitative coronary angiography. Twelve patients without pretreatment (group 1) served as control subjects. Seventeen patients (group 2) received nicardipine, either 0.2 mg by intracoronary injection (n = 9) or 2.5 mg intravenously (n = 8) before exercise.

RESULTS

In the control group there was exercise-induced vasoconstriction (-29%, p < 0.001) of the stenotic segment but coronary vasodilation (+22%, p < 0.05) of the normal vessel segment. In group 2, nicardipine induced coronary vasodilation of both the normal (+16%, p < 0.001) and the stenotic vessel segment (+35%). During subsequent exercise there was some additional vasodilation of normal (+4%, p = NS) and stenotic arteries (+5%, p = NS). There was no difference between either intracoronary or intravenous nicardipine with regard to vasodilation. Application of sublingual nitroglycerin was associated with significant vasodilation of the normal vessel segment in groups 1 (+18%, p < 0.05) and 2 (+15%, p < 0.001). The stenotic vessels showed a significant increase in percent cross-sectional area after nitroglycerin in groups 1 (+12%, p = NS) and 2 (+51%, p < 0.001). Exertional angina pectoris occurred less frequently in group 2 (18%) than in group 1 (67% [p < 0.005 vs. group 2]); group 2 also had a smaller increase in mean pulmonary artery pressure (+14 vs. +21 mm Hg, p < 0.05).

CONCLUSIONS

Exercise induces vasoconstriction of stenotic, but vasodilation of normal, coronary vessel segments. Intravenous and intracoronary nicardipine prevent vasoconstriction of stenotic coronary arteries during exercise and exert a significant anti-ischemic effect. The combination of two anti-ischemic drugs, nitroglycerin and nicardipine, has an additive effect on coronary vasomotion that is seen only in the stenotic vessel segment. Thus, the anti-ischemic action of nicardipine is mainly due to a primary effect on coronary vasomotor response rather than to secondary effects such as changes in loading conditions.