Respiratory flow characteristics during isoflurane/nitrous oxide anaesthesia

Ventilatory characteristics during isoflurane anaesthesia and spontaneous breathing were studied in ten adults during surgery. After premedication with diazepam and induction with thiopental sodium and suxamethonium, 1.2% isoflurane in a 50% mixture of nitrous oxide in oxygen was introduced via a non-rebreathing circuit. Respiratory flow was measured by means of a pneumotachograph, arterial gases were sampled and carbon dioxide elimination and dead-space to tidal volume ratios (VD/VT) calculated. The time axis of one respiratory cycle was divided into 20 equidistant sections and the flow at the end of each section was expressed as a percentage of the maximum flow rate during inspiration and expiration, respectively. In this manner, a relative respiratory flow pattern was constructed. The total ventilation was 5.8 +/- 0.5 1.min-1 (mean +/- s.d.) with a tidal volume of 191 +/- 45 ml and a respiratory rate of 31 +/- 6.min-1. The PaCO2 was 7.2 +/- 0.6 kPa, the carbon dioxide elimination 151 +/- 38 ml (STPD) and VD/VT 0.53 +/- 0.12. The respiratory flow pattern showed early peak flows during both inspiration and expiration. The expiratory flow rate after 90% of the total respiratory cycle time was on average 43% of the maximum expiratory flow rate. The conclusion was that isoflurane seems to have a similar respiratory flow pattern to halothane. The significant hypercarbia that resulted is still acceptable but a lighter anaesthetic level is recommended for routine surgery.