Blood gas changes during spinal and epidural analgesia

Conduction analgesia (extradural or subarachnoid block) combined with light general anaesthesia offers some advantages over general anaesthesia for certain kinds of surgery and in particular major vaginal surgeryl.Blood loss is reduced and the operating conditions are good?The adequacy of spontaneous ventilation during spinal analgesia without general anaesthesia, has been assessed by Moir3, Askrog et al. 4, de Jongs and Ward et aI.6 who found no serious disturbance in ventilatory function or arterial oxygenation.During spontaneous ventilation under halothane anaesthesia, however, there is an increase in the alveolar-arterial oxygen tension differences PA^, -Pao1)7-9 which is likely to be due to increased pulmonary shunting.A reduced cardiac output is unlikely to contribute to this increased PA^, -PaO, since the mixed venous oxygen content levels are usually within normal limits 10.Conduction analgesia involving the sympathetic outflow and without the use of pressor amines, generally results in hypotension and a fall in cardiac output67 1 1.If conduction analgesia is used in combination with halothane anaesthesia these haemodynamic changes, together with the increased P k , -Pa,, associated with general anaesthesia might result in serious impairment of pulmonary gas exchange.The purpose of this study was to investigate pulmonary gas exchange and arterial oxygenation before and after conduction analgesia in patients anaesthetised with halothane and breathing spontaneously.