vasodilators

introduction

vasodilators are medications which relax the smooth muscle in blood vessels, and generally will result in lower blood pressure
those which primarily act on the venous side will reduce pre-load on the heart
those which act on the arteries will reduce after-load on the heart
vasodilators may be “direct-acting” - acting at the blood vessel smooth muscle itself, or, “indirect-acting” - acting on other mediating systems

nitrates including GTN
- mainly venous at low dose, then also arterial at higher doses
hydralazine
- thought to directly act on peripheral arterioles primarily
- slow acetylator phenotypes (50% of Caucasians and 20% of Asians) are at risk of toxicity with usual dosing
sodium nitroprusside
- potent, rapid direct-acting that greatly reduces arterial blood pressure
- relaxes both venous and arterial circulation
- must be given by iv infusion
- onset within minutes
calcium channel blockers
- mainly act on peripheral arterioles, reducing afterload and decreasing myocardial oxygen demand
- diltiazem tends to cause more coronary vasodilation than peripheral arterial vasodilation
- amlodipine and felodipine tend to cause more peripheral arterial vasodilation than coronary vasodilation
- nifedipine and verapamil tend to equally effect coronary and peripheral arteries
potassium channel activators
- eg. nicorandil, diazoxide, minoxidil
- act on ATP-sensitive potassium channels which are activated and results in antagonism of the action of ATP, preventing closure of the channel, resulting in hyperpolarisation and relaxation of smooth muscle
beta 2 adrenergic agonists - cause arteries to skeletal muscle to dilate