Attenuation of circadian variation by combined antianginal therapy with suppression of morning and evening increases in transient myocardial ischemia

Circadian Variation of the Myocardial Ischemic Threshold in Patients with Ischemic Heart Disease

PURPOSE

Circadian variation of the heart rate (HR) at the myocardial ischemic threshold was compared with that of the HR at the sympathetic tone threshold in 16 cases of effort angina pectoris.

METHODS

The high (0.15-0.40 Hz, Hf) and low (0.03-0.15 Hz, Lf) frequency spectral components were extracted every twenty seconds, using frequency domain analysis of the RR interval recorded by a 24-hour Holter ECG with an accelerometer. HR-Hf data collected every five minutes were plotted as the X-Y coordinates, and a two-compartment analysis was performed. The HR at the autonomic tone threshold was then determined at the intersection point of the lines. A loop of the HR-ST segment was drawn during walking to determine the ischemic threshold at which heart rate ischemic ST segment depression began and circadian variation was observed.

RESULTS

The circadian rhythm at the sympathetic tone threshold and the HR at the ischemic threshold were high during the day and low at night. The HR at the ischemic threshold was 15-20% higher than that at the sympathetic tone threshold at night but only 9-13% higher during the day. Also, there was a period during which there was no significant difference between the HR at the two thresholds.

CONCLUSION

In comparing circadian variation between HR at the sympathetic tone threshold and HR at the ischemic threshold, there was a period during which there was no significant difference between the HRs. This might suggest a period in which a high incidence of cardiac ischemic attack is likely. This information could provide a feasible indicator for appropriate exercise training for patients with ischemic heart disease.

Chronobiology and chronotherapy of ischemic heart disease

The occurrence of the clinical manifestations of ischemic heart disease (IHD)--myocardial ischemia and angina pectoris, acute myocardial infarction, and sudden cardiac death--is unevenly distributed during the 24 h with greater than expected events during the initial hours of the daily activity span and in the late afternoon or early evening. Such temporal patterns result from circadian rhythms in pathophysiological mechanisms plus cyclic environmental stressors that trigger ischemic events. Both the pharmacokinetics (PK) and pharmacodynamics (PD) of many, though not all, anti-ischemic oral nitrate, calcium channel blocker, and beta-adrenoceptor antagonist medications have been shown to be influenced by the circadian time of their administration. The requirement for preventive and therapeutic interventions varies predictably during the 24 h, and thus therapeutic strategies should also be tailored accordingly to optimize outcomes. During the past decade, two first generation calcium channel blocker chronotherapies have been developed, trialed, and marketed in North America for the improved treatment of IHD. Nonetheless, there has been relatively little investigation of the administration-time (circadian rhythm) dependencies of the PK and PD of conventional anti-ischemic medications, and there has been little progress in the development of new generation IHD chronotherapies. Available epidemiologic, pharmacologic, and clinico-therapeutic evidence demonstrates how the chronobiologic approach to IHD can contribute new insight and opportunities to improve drug design and drug delivery to enhance therapeutic outcomes.

Effect of metoprolol in reducing myocardial ischemic threshold during exercise and during daily activity

In a study of 48 patients with coronary artery disease and evidence of ischemia during exercise and daily life, metoprolol reduced the threshold of myocardial ischemia in a dose-dependent manner. This effect of beta blockers is probably due to increased coronary tone.

Nitrates in silent ischemia

In recent years it has become clear that episodes of transient myocardial ischemia commonly occur in patients with coronary artery disease in the absence of chest pain or angina equivalent. These episodes of "silent myocardial ischemia" are particularly well documented during continuous ambulatory electrocardiographic monitoring in daily life. Evidence suggests that these episodes represent true ischemia, and appear to be a marker of unfavorable outcome. While the pathophysiology is not completely understood, it appears as though the mechanisms of angina and silent ischemia are the same. Both forms of ischemia respond to conventional antianginal medication. While long-acting nitrates are effective in reducing or preventing myocardial ischemia, because of their propensity to cause tolerance they should be used intermittently and in association with either beta-blockers or calcium antagonists. Nitrates are safe and comparatively inexpensive, and will continue to play an important role in the treatment and prevention of angina. However, in the light of current knowledge, there is no specific indication for the treatment of silent ischemia by nitrates.

Effect of beta-blockade on heart rate variability in patients with coronary artery disease

OBJECTIVES

This study assessed the effects of beta-blockade on heart rate variability in patients with coronary artery disease and determined whether the effects of metoprolol in a controlled-release formulation and atenolol differ with regard to electrocardiographic measures of cardiac autonomic control.

BACKGROUND

Low heart rate variability is common in coronary artery disease and is associated with increased mortality. Beta-adrenergic blocking drugs may increase heart rate variability in healthy subjects, but there is limited knowledge of whether they are able to modify heart rate variability in patients with uncomplicated coronary artery disease.

METHODS

In a randomly allocated, double-blind crossover study with three 2-week treatment periods, 200 mg of controlled-release metoprolol once a day, 100 mg of atenolol once a day or placebo once a day were administered in 18 male patients with stable coronary artery disease. The 24-h heart rate variability was measured in both the time and frequency domains.

RESULTS

Beta-blockade induced a significant increase in heart rate variability, but no significant differences were found between atenolol and metoprolol. The average 24-h high frequency power increased by 64% after atenolol and by 62% after metoprolol. The root-mean-square successive difference of normal RR intervals increased by 70% after atenolol and by 62% after metoprolol, and the standard deviations of RR intervals increased by 20% and 16%, respectively. Beta-blockade had no significant effects on the amplitude of the circadian rhythm of heart rate variability, although both metoprolol and atenolol blunted the abrupt decrease of high frequency power after arousal.

CONCLUSIONS

Beta-blockade by metoprolol and atenolol enhance the heart rate variability in patients with coronary artery disease. This may contribute to the protective effects of beta-blockade in ischemic heart disease.