Hydralazine Does Not Cause Bradycardia—It Causes Reflex Tachycardia
Hydralazine is associated with reflex tachycardia, not bradycardia, as a direct consequence of its vasodilatory mechanism. The drug causes arterial vasodilation leading to blood pressure reduction, which triggers a compensatory baroreceptor-mediated increase in heart rate 1.
Mechanism of Heart Rate Effect
The characteristic cardiovascular response to hydralazine includes:
- Reflex tachycardia occurs as the primary heart rate effect due to arterial vasodilation and subsequent blood pressure reduction 1
- The baroreceptor reflex detects the fall in blood pressure and activates sympathetic outflow, increasing heart rate 2, 3
- This positive chronotropic effect is mediated primarily through the autonomic nervous system, specifically by decreasing sinoatrial conduction time 3
- The heart rate increase begins within 10-30 minutes of administration and lasts 2-4 hours 4
Clinical Management Strategy
Because of the reflex tachycardia, hydralazine should be used with a beta blocker and diuretic:
- The ACC/AHA guidelines explicitly state that direct vasodilators like hydralazine are "associated with sodium and water retention and reflex tachycardia; use with a diuretic and beta blocker" 1
- Beta blockers blunt the sympathetic response and reduce the magnitude of reflex tachycardia 5, 4
- Diuretics counteract the sodium and water retention that accompanies vasodilation 1
Paradoxical Bradycardia: An Exceptional Circumstance
Bradycardia with hydralazine is extremely rare and occurs only under specific experimental conditions:
- In sino-aortic deafferented rats (with baroreceptor reflex surgically removed), hydralazine produced bradycardia instead of tachycardia through activation of vagal cardiodepressant reflexes 6
- One small study from 1981 deliberately used hydralazine to treat symptomatic sinus bradycardia, achieving heart rate increases in approximately 60% of patients 7
- These findings represent experimental or therapeutic manipulation, not typical clinical adverse effects 7, 6
Clinical Implications
In routine clinical practice:
- Expect tachycardia, not bradycardia, when prescribing hydralazine 1
- The reflex tachycardia can provoke myocardial ischemia in patients with coronary artery disease, occurring in approximately 23% of heart failure patients with ischemic cardiomyopathy 8
- In heart failure, hydralazine is typically combined with isosorbide dinitrate (providing complementary arterial and venous vasodilation), which may modify the heart rate response 5, 4
- The chronotropic effect is less pronounced when beta blockers are co-administered 5, 4
Common Pitfall to Avoid
Do not confuse hydralazine's reflex tachycardia with bradycardia-inducing antihypertensives such as beta blockers, non-dihydropyridine calcium channel blockers, or central alpha-2 agonists, which have entirely different mechanisms 1.