Mechanism of immediate hemodynamic effects of chlorothiazide

The place of diuretics in the treatment of hypertension: a historical review of classical experience over 30 years

Thiazide diuretics have been the mainstay of antihypertensive therapy for over 30 years. Their precise mechanism of antihypertensive action is still incompletely understood. They reduce arterial pressure initially through a fall in plasma volume and cardiac output. However, with chronic administration cardiac output tends to return toward pretreatment levels, suggesting that the long-term pressure reduction is mediated through a reduction in vascular resistance. Although multiple lines of evidence suggest that salt and water loss is an essential part of the mechanism, at least in some cases an indirect vasodilator effect may play a role as well. The antihypertensive efficacy of diuretics is proven; they are at least as effective as other classes of antihypertensive drugs. They have been shown to protect against stroke, but not against mortality from myocardial infarction. There is some concern about the metabolic side effects, such as hypokalemia, hyperglycemia, and hyperlipidemia. In order to minimize these side effects the lowest effective dose should be used. Diuretics are likely to remain first-line antihypertensive agents, but they should be considered as one of several possible choices for the initial therapy among other classes, such as beta-blockers, ACE inhibitors, or calcium entry blockers.

Hemodynamic actions of diuretic agents

The blood pressure lowering effect of diuretics in hypertensive disease is well documented. The hemodynamic actions underlying this antihypertensive effect differ in the early phase from the effects observed during long-term treatment. This article first describes new experiments on the hemodynamic effects of three diuretics, viz. hydrochlorothiazide, chlorthalidon and furosemide in conscious, chronically instrumented spontaneously hypertensive rats. This animal model was chosen in view of its good applicability for the study of the hemodynamics of antihypertensive drug action. Results show a marked early fall in stroke volume and cardiac index with all three diuretics. Mean arterial blood pressure was decreased only after a delay of several hours, because of an early large increase in total peripheral resistance. This article furthermore discusses possible mechanisms involved in the long-term return of peripheral resistance close to or below pre-treatment values. On the basis of data in the literature it is concluded that adaptation of baroreceptor reflexes, auto-regulatory responses of the peripheral vascular resistance and enhanced production of endogeneous vasodilator substances play an increasingly important role during the long-term hemodynamic actions of diuretic agents.

The rational use of diuretics in the treatment of arterial hypertension

The development of modern pharmacologic diuretic agents has revolutionized the therapy of arterial hypertension. The diuretics currently available are easily administered orally, are effective in the presence of alkalosis or acidosis, are non-toxic and have a low incidence of side effects which are readily circumvented or treated. Loop diuretics such as furosemide have the capacity to be effective in patients with diminished renal function or clinical situations that have a powerful stimulus to sodium retention. In clinical circumstances when renal potassium loss is to be prevented such as in patients receiving digitalis, the addition of a potassium-sparing diuretic to either a thiazide or furosemide will achieve the clinical goal of providing an effective diuresis while inhibiting potassium excretion. The mechanism of antihypertensive activity of the diuretic agents appears to be the reduction of extracellular fluid volumes and plasma volumes. Hence, the clinical dictum that to be effective as an antihypertensive agent, diuretics should be administered in diuretic doses. Besides being the cornerstone of initial antihypertensive therapy, diuretics also play an important role in antihypertensive therapy of patients with moderate to severe hypertension who are receiving potent antihypertensive drugs of the vasodilator or sympatholytic class of compounds. Indeed, one of the most important steps in the successful therapy of these patients receiving multiple drugs, is the re-assessment of the diuretic agent. The sodium retention and consequent fluid volume expansion associated with the administration of these potent antihypertensive agents may often cause these patients to develop apparent drug resistance since the thiazide diuretics are not potent enough to counteract the powerful stimulus to sodium retention caused by these antihypertensive agents. The re-evaluation of the diuretic agent at this point will usually necessitate the substitution of furosemide for thiazide or the doubling of the dose of the present loop diuretic. A working knowledge of the physiology of urine formation and the sites of action of currently available diuretic agents will enable the clinician to tailor the diuretic agent to the clinical circumstances of an individual patient and allow the clinician to rationally select a diuretic for the treatment of arterial hypertension.