Does hydralazine affect heart rate more than metoprolol?

Hydralazine Increases Heart Rate While Metoprolol Decreases It

Hydralazine causes a reflex increase in heart rate through sympathetic activation, whereas metoprolol (a beta-blocker) decreases heart rate—these drugs have opposite effects on heart rate. 1, 2, 3

Mechanism of Hydralazine's Effect on Heart Rate

• Hydralazine causes reflex tachycardia as a compensatory mechanism to its arterial vasodilation, leading to increased sympathetic tone and elevated heart rate 1, 2
• The FDA label confirms that hydralazine's peripheral vasodilating effect results in increased heart rate, stroke volume, and cardiac output as direct consequences of decreased arterial blood pressure 2
• In clinical studies, hydralazine increased heart rate by 28% on average in patients with sinus bradycardia, primarily through decreased sinoatrial conduction time 4
• This chronotropic effect is mediated almost entirely by the autonomic nervous system—when autonomic blockade was achieved, hydralazine's heart rate effect was abolished 4

Mechanism of Metoprolol's Effect on Heart Rate

• Metoprolol decreases heart rate through beta-adrenergic blockade, slowing atrioventricular conduction and reducing sympathetic drive to the heart 3
• The FDA label specifically warns that concomitant use of metoprolol with digitalis glycosides increases the risk of bradycardia due to additive effects on slowing heart rate 3
• Beta-blockers like metoprolol are used specifically to counteract the reflex tachycardia caused by vasodilators like hydralazine 1

Clinical Implications and Combination Therapy

• The American College of Cardiology recommends using hydralazine with background beta-blocker therapy specifically because hydralazine increases sympathetic tone and causes reflex tachycardia 1
• When hydralazine is combined with metoprolol, the beta-blocker minimizes the reflex tachycardia that would otherwise occur 1
• The FDA drug interaction data confirms that concomitant administration of hydralazine may inhibit presystemic metabolism of metoprolol, leading to increased metoprolol concentrations and enhanced heart rate reduction 3
• In heart failure management, the combination of hydralazine/isosorbide dinitrate is always given with background beta-blocker therapy to prevent excessive tachycardia 5, 1

Quantitative Effects

• Hydralazine increased heart rate by approximately 17-28% in clinical studies of hypertensive patients 4, 6
• In one study, heart rate rose from 61.9 to 68.6 beats per minute after intravenous hydralazine administration 7
• Metoprolol's target dosing aims to achieve heart rate control, with typical reductions bringing heart rate into the 50-60 beats per minute range when combined with other agents like ivabradine 5

Critical Caveat

• Hydralazine should never be used as monotherapy in conditions where tachycardia could be harmful (such as myocardial infarction or heart failure), as the reflex tachycardia can provoke myocardial ischemia 1, 8
• The combination of these drugs requires careful monitoring, as the interaction can lead to unpredictable blood pressure responses and enhanced bradycardia if metoprolol levels increase 3