The Pharmacologic Treatment of Hypotension cont.
Rationale for Pharmacologic Treatment of Hypotension
Rationale for Treating Hypotension
Increase Cardiac Output
- Increase blood volume
- Stimulate contractility
Increase Systemic Vascular Resistance
- Vasoconstrictor drugs
The rationale for treating hypotension is very clear, however, the desired outcome (restoration of arterial pressure and organ perfusion) is not always obtained. Arterial blood pressure can be elevated by increasing blood volume, which increases venous pressure and then cardiac output through the Frank-Starling mechanism. Therefore, in hypotensive states caused by volume depletion, intravenous fluids or whole blood are given to restore circulating volume and enhance cardiac output and arterial pressure. Drugs are also given that stimulate the heart to increase cardiac output by increasing heart rate and stroke volume. Many of these same drugs, as well as vasoconstrictor drugs that are specific for blood vessels, constrict the systemic vasculature and thereby increase systemic vascular resistance. Drugs that increase arterial pressure by cardiac stimulation and vasoconstriction are called pressor agents. Such drugs are usually effective in increasing arterial pressure unless the heart and vasculature becomes refractory to such interventions as can occur in the late stages of cardiogenic and circulatory shock.
Postural hypotension caused by autonomic neuropathy is difficult to treat pharmacologically because arterial pressure may be normal when lying down or sitting, but then falls upon standing because of impaired baroreceptor reflexes. There are, however, some non-drug interventions (e.g., breathing maneuvers, muscular contraction, limb compression) that may help patients in this situation. Other forms of postural hypotension may be due to volume depletion (e.g., heat exhaustion, overuse of a diuretic), or due to the autonomic blocking effects of alpha-blockers or beta-blockers. In these cases, restoration of blood volume, or reducing the dose of a diuretic or autonomic blocker may be sufficient to maintain normal arterial pressures when standing.