Hydralazine Increases Heart Rate
Hydralazine typically increases heart rate as a reflex response to its vasodilatory effects. This is clearly documented in the FDA drug label, which states that the peripheral vasodilating effect of hydralazine results in "decreased arterial blood pressure (diastolic more than systolic); decreased peripheral vascular resistance; and an increased heart rate, stroke volume, and cardiac output" 1.
Mechanism of Heart Rate Increase
- Hydralazine is a direct arterial vasodilator that relaxes vascular smooth muscle, leading to decreased peripheral vascular resistance 1
- The drop in blood pressure triggers a baroreceptor-mediated reflex sympathetic response, resulting in increased heart rate 2
- This reflex tachycardia is a compensatory mechanism to maintain cardiac output in the face of decreased peripheral resistance 1, 2
Clinical Evidence
- Research studies have consistently documented the heart rate-increasing effects of hydralazine:
- A clinical pharmacokinetics study specifically noted that hydralazine produces "reflex baroreceptor-mediated responses of tachycardia and increased ejection velocity" 2
- A study examining vasodilating drugs confirmed that hydralazine causes "increases in heart rate, cardiac output, cardiopulmonary blood volume" 3
- Hydralazine has even been used therapeutically to increase heart rate in patients with symptomatic sinus bradycardia 4
Clinical Considerations
- The chronotropic effect of hydralazine may be less pronounced when combined with medications that blunt sympathetic response, such as beta-blockers 5
- In hypertensive emergencies, the FDA-approved guidelines note that hydralazine's unpredictability of response and prolonged duration of action make it less desirable as a first-line agent for acute treatment 6
- The heart rate increase is part of why hydralazine is often combined with isosorbide dinitrate in heart failure patients - providing complementary effects with hydralazine causing arterial vasodilation and isosorbide dinitrate causing venous vasodilation 5
Magnitude and Timing of Heart Rate Effect
- Heart rate typically increases by approximately 11 beats per minute following hydralazine administration 7
- The onset of the heart rate effect correlates with the blood pressure reduction, which begins within 10-30 minutes of administration and lasts for 2-4 hours 6
Cautions
- The reflex tachycardia may be problematic in patients with coronary artery disease or certain arrhythmias 8
- In patients with pulmonary hypertension, the reflex increase in heart rate (11 beats per minute) combined with systemic hypotension can lead to adverse outcomes 7
- Patients with heart failure may show an attenuated tachycardic response compared to hypertensive patients without heart failure 2
In summary, hydralazine reliably increases heart rate through reflex sympathetic activation in response to its vasodilatory effects, though this response may vary based on patient characteristics and concurrent medications.