Anti-hypertensives And Dosages: Vasodilators, Angiotensin Antagonists And Calcium Channel Blockers
Vasodilators And Angiotensin Antagonists
Since no single drug is entirely satisfactory for all cases and hypertension is multifactorial in origin, several groups of drugs have been introduced from time to time. These are diuretics, inhibitors of sympathetic responses, vasodilators, angiotensin-antagonists, converting enzyme inhibitors and calcium channel blocking agents.
Hydralazine: This drug causes predominantly arteriolar dilatation. It is effective in a dose of 20-50 mg thrice daily. This drug increases the renal, coronary and hepatic blood flow. Adverse side effects include angina (induced by reflex tachycardia caused by this drug) and lupus erythematosus, which regresses on withdrawing the drug.
Prazosin: This is a powerful vasodilator, acting both on the arterial and venous systems when given in a dose of 2-7 mg/day. Therapy should be started with a small dose since sudden hypotension may occur at times.
Sodium Nitroprusside: This is an effective vasodilator which rapidly lowers blood pressure and therefore, it is used in hypertensive emergencies. It is given as an intravenous drip, 50mg being added to 500 ml 5% glucose and infused at the rate of 1-8 ug/kg/min. The response being monitored continuously. The drug is withdrawn when the emergency is controlled. Adverse side effects include severe hypotension, nausea, psychotic behaviour and muscle spasm.
Diazoxide: This is a potent vasodilator with short-action and therefore, it is used in hypertensive emergencies. When given in a dose of 100-300 mg intravenously, it lowers the blood pressure rapidly.
Minoxidil: This is effective when given orally in doses up to 20-40 mg daily. It is effective in patients with severe hypertension associated with renal impairment. Side effects may be troublesome, but uncommon. These include hypertrichosis (increased hair growth0, coarsening of facial appearance, salt and water retention and tachycardia. Minoxidil is always given along with diuretics.
Saralasin is a synthetic drug which acts by competitive inhibition of angiotensin II. It is effective when given intravenously. The drug is under evaluation.
Converting Enzyme Inhibitors And Calcium Channel Blockers
Captopril, teprotide and enalapril are drugs which lower blood pressure by inhibiting the enzyme which converts angiotensin I to angiotensin II.
Captopril: It is effective in oral doses of 150-400 mg daily. It is particularly effective in high rennin hypertension. Captopril lowers peripheral resistance without altering the cardiac output and heart rate. It is effective in hypertension resistant to other drugs. Side effects include fever, rashes, pruritis, distaste in the mouth, orthostatic hypotension, nephropathy, bronchospasm, hyperkalemia and leucopenia.
Calcium Channel blocking drugs
These drugs act on the cell membrane selectively to block the access of calcium to the contractile mechanisms and thereby exert negative inotropic action. They increase myocardial oxygen supply, prevent coronary artery spasm, increase coronary blood flow and reduce peripheral vascular resistance to lower the mean arterial blood pressure. Nifedipine is a widely used drug belonging to this class. In a dose of 10-20 mg given thrice daily it lowers blood pressure rapidly, without any neurological side effects such as depression. This drug is increasingly employed in the management of hypertensive emergencies and for long-term control of high blood pressure.
© 2014 Funom Theophilus Makama