PRESSURE SUPPORT VENTILATION DURING ISOFLURANE ANESTHESIA

We have studied the respiratory effects of 5 and 10 cmH(2)O pressure s upport ventilation during anaesthesia with 1.5% end-tidal concentratio n of isoflurane in nine healthy, spontaneously breathing, adult patien ts. Some of the patients demonstrated an irregular respiratory pattern with periods of apnoea and we therefore went on to study a further se ven patients with a continuous 500 s recording of airflow. Pressure su pport ventilation augmented mean (SD) tidal volume from 212 (56) ml to 350 (88) ml at 5 cmH(2)O and to 509 (108) ml at 10 cmH(2)O (n = 16, p < 0.05). Mean (SD) respiratory rate decreased from 26 (6) min(-1) to 22 (6) min(-1) at 5 cmH(2)O and 17(5) min(-1) at 10 cmH(2)O pressure s upport (n = 16, p < 0.05). Mean (SD) inspiratory work of breathing dec reased from 1.77 (0.70) J. min(-1) to 0.31 (0.36) J min(-1) at 10 cmH( 2)O pressure support ventilation (n = 9, p < 0.05). Analysis of the re spiratory rhythm in the second group ow seven patients revealed on osc illating respiratory pattern in four patients at 5 cmH(2)O and six of the seven patients at 10 cmH(2)O pressure support ventilation. The met abolic advantage of the decreased work of breathing during pressure su pport ventilation during and anaesthesia is unlikely to balance the di sadvantage of an oscillating respiratory rhythm.