Sympatholytics centrally acting

Centrally acting sympatholytics (e.g., clonidine) and calcium channel antagonists (e.g., diltiazem) may be useful for symptom control when contraindications to /3-blockade exist. 

Antihypertensive drugs can be divided into eight classes based on the mechanism of action diuretics, )3-adrenoblockers, centrally acting sympatholytics, peripherally acting sympatholytics, calcium channel blockers, myotropic hypotensive drugs, angiotensin-con-verting enzyme inhibitors, and calcium channel activators. 

Diuretic with beta-blocker, added calcium blocker, ACE inhibitor or alpha blocker, or centrally acting sympatholytic... 

Centrally Acting Sympatholytics (adrenergic blockers) clonidine HC1 (Catapres), methyldopa (Aldomet)... 

Fig. 29.1. Calcium second messenger mechanism of vascular smooth muscle contraction and relaxation and sites of action of the peripheral and centrally acting sympatholytics and vasodilators. (From Yeh DC, Michel T. Pharmacology of Vascular Tone. In Golan DE, Tashjian A, Armstrong E, et al., eds. Principles of Pharmacology The Pathophysiologic Basis of Drug Therapy. Baltimore Lippincott Williams Wilkins, 2004 317-330 with permission.)...

Overall, although the frequency of troublesome adverse effects produced by guanfacine is similar to that produced by clonidine and the other centrally acting sympatholytics, their incidence and severity are lower with guanfacine. Unlike clonidine, abrupt discontinuation of guanfacine rarely results in rebound hypertension. When a withdrawal syndrome has occurred, its onset was slower and its symptoms less severe than the syndrome observed with clonidine. 

If blood pressure does not decrease, the patient is given a diuretic with a beta-blocker or a second drug is added such as a calcium chaimel blocker, ACE inhibitor, alpha blocker, or centrally acting sympatholytic. 

Sympatholytic syndrome. Blood pressure and pulse rate are both decreased (peripheral alpha blockers may cause hypotension with reflex tachycardia). The pupils are small, often of pinpoint size. Peristalsis is often decreased. (Examples centrally acting alpha-2 agonists [clonidine and methyidopa], opiates, and phenothiazines.)... 

There are two types of sympatholytics those with a direct and an indirect mechanism of action. The direct sympatholytics act via the inhibition of adrenoceptors by competing as antagonists with the transmitters for the binding site. The indirect acting drugs, the so-called antisympathotonics, interfere with the central regulation and/or the peripheral mechanisms of sympathetic transmitter release. 

C. Trimethaphan is a ganglionic blocking agent that will lower blood pressure very rapidly. Hydralazine is a vasodilator hydrochlorothiazide and spironolactone are diuretics and methyldopa is a sympatholytic acting in the central nervous system. All of these drugs are used clinically as antihypertensive agents. None work as rapidly as trimethaphan. Clinically, however, either nitroprusside or clonidine is used much more commonly than trimethaphan in this situation. 

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