Hypotension, subarachnoid block and the elderly patient.

This article reviews the current literature on the management of hypotension during subarachnoid block in the elderly. Hypotension results from blockade of the sympathetic nervous system, which causes decreases in both systemic vascular resistance and cardiac output. Abolition of normal cardiovascular reflexes is also important and may explain unexpected cardiac arrests during subarachnoid block. Untreated block in the elderly results in decreases in systolic arterial pressure, systemic vascular resistance and central venous pressure. Cardiac output appears not to decrease as has been previously reported and heart rate is affected by several different factors. Preload to the heart should be maintained during block by giving adequate intravenous fluids and 8 ml.kg-1 is satisfactory in most cases. Adequate preloading prevents decreases in cardiac output and unexpected cardiac arrests. In this respect, mild head down till is also beneficial. Ideally, intravenous fluid should be given as the block is developing. Excessive fluid administration serves no useful purpose and can cause fluid overload and urinary retention. If systolic arterial pressure decreases by more than 25%, or to below 90 mmHg, treatment with a vasopressor is indicated. The efficacy of ephedrine has recently been questioned, as it is a poor vasoconstrictor and inotrope in the elderly. The alpha-adrenoceptor agonists may prove a more logical choice, because they increase both peripheral resistance and preload. Metaraminol by infusion (< 10 ml.h-1 of 10 mg in 20 ml) has been used successfully, though hypertension can occur.