Prevention and treatment of hypotension during central neural block.

Hypotension during central neural block may occur by three main mechanisms: decrease in venous return (in turn influenced by posture, bleeding and inferior vena cava compression), vasodilatation and decreased cardiac output. It is also important to recognize that, occasionally, other factors play a part. Bladder distension during central nerve block has been shown to produce hypotension inappropriate to the level of block [48, 62] and vagal overactivity may contribute in the unsedated patient. Preventive measures to reduce the likelihood of hypotension include correction of hypovolaemia, restriction of the upper level of block, use of a slight head-down tilt to maintain venous return and judicious use of sedation, especially in anxious patients. In the obstetric patient, the single most important factor in eliminating hypotension is the use of full left-lateral tilt. Mechanical methods to improve venous return by compressing the legs are not particularly helpful. Volume loading does not guarantee maintenance of arterial pressure and excessive fluid may be harmful in patients with bladder neck obstruction or at risk of pulmonary oedema. The administration of up to 1 litre before surgery may be particularly advisable if significant blood loss is expected (no matter what the anaesthetic technique), but colloid solutions do not have clear proven benefit over crystalloid. The prevention or treatment of hypotension induced by central block by administration of large volumes of fluid is a more contentious subject, although the practice is widespread. Review of the literature has shown that many studies have been poorly designed and the results have often been contradictory, even in such basic principles as the incidence of hypotension in control groups.(ABSTRACT TRUNCATED AT 250 WORDS)