Smooth muscle relaxants act as Vasodilators- These drugs act directly and dilate the blood vessels. Due to this, there occurs fall in blood pressure. Their action is independent of the innervations of the vascular smooth muscle and is not mediated by adrenergic, cholinergic or histaminergic receptors.
Hydralazine, indaparnide and xipamide are effective orally whereas sodium nitroprusside is administered by intravenous route. Important side effects of these vasodilator are headache, flushing, nasal congestion, tachycardia and palpitation. On prolonged administration in high doses, hydralazine produces reversible lupus-like syndrome.
Hydralazine: It is a synthetic drug acting as vasodilator. It is used in a dose of 100—200mg per day. It acts directly on arteriolar smooth muscle and not on veins. It is used to treat moderate to severe hypertension and is also used in chronic congestive cardiac failure.
It is given in combination with a diuretic (to prevent sodium and water retention) or a beta blocker (to prevent tachycardia and increased renin secretion due to reflex sympathetic stimulation).
Indapamide and xipamide: They are related to chlorthalidone diuretic. They are given in single sub-diuretic doses in mild to moderate hypertension. They should be avoided in pregnancy, severe renal or hepatic insufficiency and in combination with diuretics.
Sodium nitroprusside: It is a very potent vasodilator. It acts directly on arterioles and venules. The onset of action is instantaneous and the effect lasts for 5—10 mm. So it is given by continuous i.v. infusion at a rate of 0.001 mg/kg/mm for the treatment of hypertensive emergencies.
Action, effect and main uses of various types of vasodilators
• Vasodilators act
- To increase local tissue blood flow
- To reduce arterial pressure
- To reduce central venous pressure
• Net effect is reduction of:
- Cardiac preload (reduced filling pressure)
- Cardiac after-load (reduced vascular resistance)
- Cardiac work
• Main uses are: a
- Hypertension (e.g. ACM and ct1 antagonists)
- Angina pectoris (e.g. calcium channel blockers)
– Cardiac failure (e.g. ACM)