Antihypertensive drugs direct vasodilators

Minoxidil is a peripheral vasodilator that directly relaxes vascular smooth musculature, thus, lowering systolic and diastolic pressure. Its action is linked to the activation of calcium channels. Open calcium channels cause hyperpolarization of smooth muscle cells, which in turn, reduces the flow of calcium ions into the cell, which is necessary for supporting vascular tonicity. However, when taking minoxidil, tachycardia, elevated renin secretion, and water and sodium ion retention all appear simultaneously with hypotension. Because of potentially serious side effects, it is used only for severe hypertension that does not respond to treatment with other drugs, and absolutely in combination with two other antihypertensive drugs. A synonym of this drug is loniten. 

Systemic heat Large whirlpool Hubbard tank Decreased muscle/joint stiffness in large areas of the body Opioid and nonopioid analgesics skeletal muscle relaxants Severe hypotension may occur if systemic hot whirlpool is administered to patients taking peripheral vasodilators and some antihypertensive drugs (eg., alpha-1 antagonists, nitrates, direct-acting vasodilators, calcium channel blockers)... 

Direct vasodilators frequently produce baroreflex-induced tachycardia, but rarely orthostatic hypotension. They are usually prescribed with a beta blocker or a centrally acting antihypertensive to minimize the reflex increase in heart rate and cardiac output. It should be noted that another member of the directly acting class of antihypertensives is minoxidil. This potent, long-acting drug has gained considerable notoriety for its use as a topical hair-restorer. Oral use can result in hirsutism (unwanted hair growth over the face as well as other parts of the body). 

Careful monitoring of the heart and hemodynamic status should be performed. Hypertension and symptoms of CNS stimulation usually resolve spontaneously with only supportive measures. Antiarrhythmic and antihypertensive agents may be necessary in severe exposures. If treatment of hypertension is necessary, a direct vasodilator such as nitroprusside or nifedipine should be utilized. Treat agitation and seizures as necessary with benzodiazepines. Management of concurrently ingested drugs should be appropriate to the agent involved. 

These drugs are direct vasodilators (see Chapter 13, Direct Vasodilators, for a complete discussion of direct vasodilators). They are primarily used as second line drugs in a stepwise antihypertensive regimen. Their use results in deceased arterial tone, which results in a decrease in peripheral resistance and decreased systemic pressure. 

The hemodynamic effects of diazoxide are similar to those of hydralazine and minoxidil. It produces direct relaxation of arteriolar smooth muscle with little effect on capacitance beds. Since it does not impair cardiovascular reflexes, orthostasis is not a problem. Its administration is, however, associated with a reflex increase in cardiac output that partially counters its antihypertensive effects. Propranolol and other -blockers potentiate the vasodilating properties of the drug. Diazoxide has no direct action on the heart. Although renal blood flow and glomerular filtration may fall transiently, they generally return to predrug levels within an hour. 

Antihypertensive Vasodilators - A common characteristic of hypertensive patients, regardless of the etiology of their disease, is abnormally high peripheral vascular resistance. Drugs that act directly on peripheral vasculature to decrease resistance are, therefore, logical agents for the treatment of hypertension. Unfortunately, sympathetic reflex actions leading to cardiac stimulation, hyperreninemia and fluid retention limit the hypotensive action of vasodilators and they are used mainly in combination with 6-blockers and diuretics for the treatment of more severe hypertension. A vasodilator that may not cause undesirable reflex... 

Diazoxide is a parenteral, rapid, and direct-acting vasodilating antihypertensive used in hypertensive emergencies. An IV injection can drop blood pressure by as much as 80 mmHg in 5 minutes. Unlike sodium nitroprusside, however, venous dilation is not part of its mechanism. Chemically it is a benzothiadiazide without the sulfamoyl function at the 7 position (see diuretics). In fact, diazoxide is not a diuretic. Chronic use of diazoxide reflexly increases renin release, which actually counteracts the antihypertensive effect of the drug by expanding the volume of circulating fluid. 

Hydralazine (apresoline) causes direct relaxation of arteriolar smooth muscle, possibly secondary to a fall in intracellular Ca concentrations. The drug does not dilate epicardial coronary arteries or relax venous smooth muscle. Hydralazine-induced vasodilation is associated with powerful stimulation of the sympathetic nervous system, likely due to baroreceptor-mediated reflexes, which results in increased heart rate and contractility, increased plasma renin activity, and fluid retention all of these effects counteract the antihypertensive effect of hydralazine. Although most of the sympathetic activity is due to a baroreceptor-mediated reflex, hydralazine may stimulate NE release from sympathetic nerve terminals and augment myocardial contractility directly. Most of hydralazine s effects are confined to the cardiovascular system the decrease in blood pressure after administration is associated with a selective decrease in vascular resistance in the coronary, cerebral, and renal circulations, with a smaller effect in skin and muscle. Because of preferential dilation of arterioles, postural hypotension is not common, and hydralazine lowers blood pressure equally in the supine and upright positions. 

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