Hydralazine Does Not Cause Bradycardia—It Typically Causes Reflex Tachycardia
Hydralazine causes an increase in heart rate (tachycardia), not a low heart rate (bradycardia), through reflex sympathetic activation in response to its arterial vasodilating effects. 1, 2
Mechanism of Heart Rate Increase
- Hydralazine is a direct arterial vasodilator that reduces peripheral vascular resistance with minimal effect on venous tone 1, 3
- The drop in blood pressure triggers baroreceptor-mediated reflex tachycardia as a compensatory mechanism 4, 5
- This sympathetic activation increases both cardiac sympathetic nerve activity and heart rate within minutes of administration 4
- The magnitude of tachycardia correlates with individual baroreceptor sensitivity, which varies among patients 5
Clinical Context and Modifying Factors
The chronotropic (heart rate increasing) effect may be blunted when hydralazine is combined with beta-blockers or other medications that suppress sympathetic response. 2
- In heart failure management, hydralazine is typically combined with isosorbide dinitrate rather than used as monotherapy 1, 2
- This combination provides complementary hemodynamic effects: hydralazine dilates arteries while isosorbide dinitrate dilates veins 1, 6, 3
- The American Heart Association notes that hydralazine may help prevent nitrate tolerance through potential antioxidative properties 6, 3
Rare Exception: Paradoxical Bradycardia
Bradycardia with hydralazine is extremely rare and occurs only under specific experimental conditions:
- Paradoxical bradycardia has been documented in sino-aortic deafferented animals (with arterial baroreceptors surgically removed), resulting from activation of vagal cardiodepressant reflexes originating in the heart 7
- This is not clinically relevant in humans with intact baroreceptor function 7
Therapeutic Use for Bradycardia
Interestingly, hydralazine has been used therapeutically to increase heart rate in patients with symptomatic sinus bradycardia, producing a 20% or greater increase in heart rate in approximately two-thirds of hypertensive patients and half of normotensive patients 8
Important Caveats
- The ACC/AHA guidelines note that hydralazine has an unpredictable BP response and prolonged duration of action (2-4 hours), making it less desirable as a first-line agent for acute hypertensive treatment 1
- When used for hypertensive emergencies, the onset of action is relatively slow (15 minutes) with difficulty in dose titration 1
- Common side effects include headache and gastrointestinal complaints, but bradycardia is not among them 1