Hydralazine causes reflex tachycardia when used alone for hypertension. This reflex is mediated primarily by:

• A Direct stimulation of cardiac beta-1 receptors by hydralazine
• B Hydralazine-induced release of adrenaline from the adrenal medulla
• C Baroreceptor-mediated sympathetic activation and renin release secondary to arteriolar dilation ✓
• D Blockade of vagal tone to the sinoatrial node

Explanation

Hydralazine selectively dilates arterioles, reducing peripheral resistance and blood pressure. The sudden fall in arterial pressure is detected by baroreceptors (carotid sinus, aortic arch), triggering a reflex increase in sympathetic nervous system activity. This raises heart rate via beta-1 stimulation, increases renin secretion (activating the RAAS), and causes sodium retention — all of which attenuate the antihypertensive effect. Combination with a beta-blocker and diuretic overcomes these compensatory responses.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

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