Answer:

The -blocker (metoprolol) should be continued not only up until surgery but also throughout the perioperative period. In patients with unstable angina, sudden withdrawal of -blocker may produce an exacerbation of symptoms and may precipitate an acute MI. The dose of -blocker does not need to be reduced before surgery for fear of bradycardia, hypotension, or difficulty in weaning from CPB. The half-life of oral metoprolol is 3 hours. It is metabolized in the liver.

Reductions in HR with a -blocker occur at lower serum levels than depression of myocardial contractility. Accordingly, as drug levels decrease after discontinuation of therapy, reductions in chronotropic response last longer than reductions in inotropy. This is an important concept in treating tachycardias in patients with significant ventricular dysfunction and CHF—a small dose is appropriate and indicated, whereas a large dose may suppress even further the inotropic state.

Numerous studies have confirmed improvements in cardiac function, exercise capacity, and long-term survival in patients with heart failure resulting from MI, hypertrophic cardiomyopathy, or idiopathic dilated cardiomyopathy with -blockers. -Blockers may also be of benefit in patients with diastolic dysfunction, secondary to hypertension.

Potential benefits of -adrenergic blockade in heart failure include decreased HR and normalization of -receptor function. A slower HR improves diastolic function by increasing the diastolic filling time and myocardial perfusion and by decreasing the myocardial oxygen consumption. -Adrenergic receptors are downregulated in heart failure, but their response is normalized by long-term -blockade.