Blood Pressure Medicines Information
The term "hypertension" literally means an abnormally raised arterial blood pressure. There are many conditions which elevate arterial pressure, including primary renal disease, pheochromocytoma, hyperthyroidism, hyperaldosteronism and coarctation of aorta, leading to secondary hypertension. In about 80 to 85 per cent of patients of hypertension, no specific cause is evident, and such a condition is labeled as primary or essential hypertension.
Most antihypertensive drugs can effectively reduce mildly elevated blood pressure, but their use is associated with many side effects. Thus the decision whether to use a drug to control borderline or mild hypertension is made on the basis of the benefit: risk ratio.
Antihypertensive Drugs Classification
Drugs influence arterial blood pressure at four effector sites- arterioles (resistance vessels); veins (capacitance vessels); heart; and the kidneys—by several different mechanisms. They can be classified according to their site or mode of action as follows:I. Diuretics
(i) Thiazides and related agents (hydrochlorothiazide, chlorthalidone etc.)
(ii) Loop diuretics (frusemide, bumetanide, ethacrynic acid)
(iii) Potassium-sparing diuretics (spironolactone, triamterene, amiloride)
II Sympatholytic Drugs
(i) Centrally acting agents (methyldopa, clonidine).
(ii) Ganglion blocking agents (trimethaphan).
(iii) Adrenergic neuron blocking agents (guanethidine, guanadrel, reserpine).
(iv) Beta-adrenoceptor blockers ( metoprolol, atenolol etc.)
(v) Alpha-adrenoceptor blockers (prazosin, terazosin, doxazosin, phenoxybenzamine).
(vi) Alpha+beta blockers (labetalol, carvediol).
(i) Arterial (hydralazine, minoxidil, diazoxide)
(ii) Arterial and venous (nitroprusside)
IV. Calcium Channel Blockers
Verapamil, nifedipine, nicardipine, nitrendipine.
. Angiotensin Converting Enzyme Inhibitors
VI. Angiotensin II Receptor Blockers
VII. Potassium Channel Opener
Diuretics Antihypertensive Drugs
Diuretics are still used as initial drugs of choice for mild hypertension in the majority of cases. They can be used alone, or in combination with other classes of antihypertensive drugs.
Methyldopa (alpha-methyldopa) is recommended for the treatment of most types of hypertension. Its potency lies between that of guanethidine and reserpine. Methyldopa is a structural analogue of dihydroxyphenylalamine and differs from it only by the presence of a methyl group on the a-carbon of the side chain.
Trimethaphan is a short-acting ganglion blocking agent. The blocking action is exerted at the postsynaptic cholinergic receptor in autonomic ganglia. This competitive antagonism is short-lived.
Guanethidine is a potent drug for the relief of severe hypertension, and has relatively lower incidence of side effects.
After propranolol many beta-blockers have been introduced. All beta-blockers share the common property of being competitive inhibitors of catecholamines.
Antihypertensinve Alpha-Adrenoceptor Blockers
Prazosin acts by a competitive postsynaptic alphs1-adrenoceptor blockade. It does not block the presynaptic inhibitory effects, which allow reflex action to overcome the postural hypotension usual with alpha-blockade.
Doxazosin is a water-soluble quinazoline analogue to prazosin with selective alpha1-adrenoceptor inhibiting action. It has no direct action on the vascular smooth muscle.
Terazosin is a long-acting selective alpha1-adrenoceptor blocker. It reduces TPR, and has little effect on the heart rate, Terazosin may be used alone or with a diuretic in patients with mild to moderate hypertension.
Phenoxybenzamine is an orally effective, long-acting non-selective (alpha and alpha) alpha-adrenoceptor blocking agent.
Labetalol has an alpha blocking action, with a non-specific beta and beta-adrenoceptor blocking action. On oral administration the ratio of alpha to beta blockade is about 1:3.
Hydralazine (1-hydrazinopthalazine hydrochloride) is a potent direct relaxant of vascular smooth muscle, but often produces considerable reflex cardiac stimulation. This drug reduces both systolic and diastolic BP.
Minoxidil dilates arteriolar resistance vessels with minimal effects on venous capacitance vessels. It reduces calcium uptake through the cell membrane, thereby reducing arteriolar tone and lowering peripheral vascular resistance.
It is a non-diuretic thiazide. On prolonged use it leads to diabetes mellitus, although no permanent in nature. Hence, it is not used as a routine hypotensive.
Sodium nitroprusside is a short-term hypotensive agent. It reduces TPR by a direct action on blood vessels and may be used in the treatment of hypertensive crisis.
Antihypertensive Calcium Channel Blockers
Verapamil is a calcium blocker used in the chronic management of angina pectoris; and in the control of supraventricular arrhythmias. Verapamil is also useful in the control of mild to moderate hypertension.
Angiotensin converting enzyme inhibitors are orally effective antihypertensive, used either alone or in combination with other antihypertensive agents for treating mild to moderate hypertension. Captopril in higher doses can be used for controlling severe and resistant forms of hypertension. Enalparil is a prodrug, converted by de-esterification to a converting enzyme inhibitor.
Antihypertensive Angiotensin II Receptor Blockers
The older agent, saralasin is an Angiotensin II analogue, and a competitive inhibitor of Angiotensin II at both its receptors (AT1 and AT2 sites). It blocks the pressor and aldosterone releasing effects of Angiotensin II, and lowers BP in high rennin states like renal artery stenosis and ischemia.
Losartan is a recently introduced blocker of the AT1 receptor. It has no effect on bradykinin metabolism, and therefore a moiré selective blocker of Angiotensin effects than the ACE inhibitors.
Antihypertensive Potassium Channel Opener
Pinacidil is a potent vasodilator, and is the first antihypertensive drug classified as a potassium channel opener, although it is likely that minoxidil and diazoxide also lower BP by this mechanism.
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