Hydralazine
The vasodilation produced by hydralazine (Apresoline) depends in part on the presence of an intact blood vessel endothelium. This implies that hydralazine causes the release of nitric oxide, which acts on the vascular smooth muscle to cause relaxation. In addition, hydralazine may produce vasodilation by activating K+ channels.
Pharmacological Actions Hydralazine
Hydralazine produces widespread but apparently not uniform vasodilation; that is, vascular resistance is decreased more in cerebral, coronary, renal, and splanchnic beds than in skeletal muscle and skin. Renal blood flow and ultimately glomerular filtration rate may be slightly increased after acute treatment with hydralazine. However, after several days of therapy, the renal blood flow is usually no different from that before drug use.
In therapeutic doses, hydralazine produces little effect on nonvascular smooth muscle or on the heart. Itspharmacological actions are largely confined to vascular smooth muscle and occur predominantly on the arterial side of the circulation; venous capacitance is much less affected. Because cardiovascular reflexes and venous capacitance are not affected by hydralazine, postural hypotension is not a clinical concern. Hydralazine treatment does, however, result in an increase in cardiac output.This action is brought about by the combined effects of a reflex increase in sympathetic stimulation of the heart, an increase in plasma renin, and salt and water retention.These effects limit the hypotensive usefulness of hydralazine to such an extent that it is rarely used alone.
Clinical Uses of Hydralazine
Hydralazine is generally reserved for moderately hypertensive ambulatory patients whose blood pressure is not well controlled either by diuretics or by drugs that
interfere with the sympathetic nervous system. It is almost always administered in combination with a diuretic (to prevent Na+ retention) and a B-blocker, such as propranolol (to attenuate the effects of reflex cardiac stimulation and hyperreninemia). The triple combination of a diuretic, Beta-blocker, and hydralazine constitutes a unique hemodynamic approach to the treatment of hypertension, since three of the chief determinants of blood pressure are affected: cardiac output (beta-blocker plasma volume (diuretic), and peripheral vascular resistance (hydralazine).
Although hydralazine is available for intravenous administration and has been used in the past for hypertensive emergencies, it is not generally employed for this purpose. The onset of action after intravenous injection is relatively slow, and its actions are somewhat unpredictable in comparison with those of several other
vasodilators.
DOSAGE AND ADMINISTRATION
Initiate therapy in gradually increasing dosages; adjust according to individual response. Start with 10 mg four times daily for the first 2-4 days, increase to 25 mg four times daily for the balance of the first week. For the second and subsequent weeks, increase dosage to 50 mg four times daily. For maintenance, adjust dosage to the lowest effective levels.
The incidence of toxic reactions, particularly the L.E. cell syndrome, is high in the group of patients receiving large doses of Apresoline.
In a few resistant patients, up to 300 mg of Apresoline daily may be required for a significant antihypertensive effect. In such cases, a lower dosage of Apresoline combined with a thiazide and/or reserpine or a beta blocker may be considered. However, when combining therapy, individual titration is essential to ensure the lowest possible therapeutic dose of each drug.
The vasodilation produced by hydralazine (Apresoline) depends in part on the presence of an intact blood vessel endothelium. This implies that hydralazine causes the release of nitric oxide, which acts on the vascular smooth muscle to cause relaxation. In addition, hydralazine may produce vasodilation by activating K+ channels.
Pharmacological Actions Hydralazine
Hydralazine produces widespread but apparently not uniform vasodilation; that is, vascular resistance is decreased more in cerebral, coronary, renal, and splanchnic beds than in skeletal muscle and skin. Renal blood flow and ultimately glomerular filtration rate may be slightly increased after acute treatment with hydralazine. However, after several days of therapy, the renal blood flow is usually no different from that before drug use.
In therapeutic doses, hydralazine produces little effect on nonvascular smooth muscle or on the heart. Itspharmacological actions are largely confined to vascular smooth muscle and occur predominantly on the arterial side of the circulation; venous capacitance is much less affected. Because cardiovascular reflexes and venous capacitance are not affected by hydralazine, postural hypotension is not a clinical concern. Hydralazine treatment does, however, result in an increase in cardiac output.This action is brought about by the combined effects of a reflex increase in sympathetic stimulation of the heart, an increase in plasma renin, and salt and water retention.These effects limit the hypotensive usefulness of hydralazine to such an extent that it is rarely used alone.
Clinical Uses of Hydralazine
Hydralazine is generally reserved for moderately hypertensive ambulatory patients whose blood pressure is not well controlled either by diuretics or by drugs that
interfere with the sympathetic nervous system. It is almost always administered in combination with a diuretic (to prevent Na+ retention) and a B-blocker, such as propranolol (to attenuate the effects of reflex cardiac stimulation and hyperreninemia). The triple combination of a diuretic, Beta-blocker, and hydralazine constitutes a unique hemodynamic approach to the treatment of hypertension, since three of the chief determinants of blood pressure are affected: cardiac output (beta-blocker plasma volume (diuretic), and peripheral vascular resistance (hydralazine).
Although hydralazine is available for intravenous administration and has been used in the past for hypertensive emergencies, it is not generally employed for this purpose. The onset of action after intravenous injection is relatively slow, and its actions are somewhat unpredictable in comparison with those of several other
vasodilators.
DOSAGE AND ADMINISTRATION
Initiate therapy in gradually increasing dosages; adjust according to individual response. Start with 10 mg four times daily for the first 2-4 days, increase to 25 mg four times daily for the balance of the first week. For the second and subsequent weeks, increase dosage to 50 mg four times daily. For maintenance, adjust dosage to the lowest effective levels.
The incidence of toxic reactions, particularly the L.E. cell syndrome, is high in the group of patients receiving large doses of Apresoline.
In a few resistant patients, up to 300 mg of Apresoline daily may be required for a significant antihypertensive effect. In such cases, a lower dosage of Apresoline combined with a thiazide and/or reserpine or a beta blocker may be considered. However, when combining therapy, individual titration is essential to ensure the lowest possible therapeutic dose of each drug.
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Unknown - Friday, 29 April 2011
