Rib cage contribution to resting and carbon dioxide stimulated ventilation during 1 MAC isoflurane anaesthesia.

Using respiratory inductive plethysmography, we have measured rib cage and abdominal motion during isoflurane anaesthesia in 16 healthy day-surgery patients. Anaesthesia was induced with propofol and maintained with 1 MAC isoflurane in air-oxygen via a laryngeal mask. Measurements were taken during both resting ventilation and hyperpnoea induced by rebreathing carbon dioxide. For resting ventilation, the rib cage contributed a mean (SD) of 33 (15)% of the total ventilation whilst awake, and 39 (12)% during anaesthesia (ns). With increasing end-tidal carbon dioxide whilst awake, the subjects showed a mean increase in the percentage rib cage contribution of 7.1 (12.5)%/kPa of carbon dioxide. With isoflurane anaesthesia, there was significant depression of this rib cage recruitment with the mean contribution decreasing by 3.6 (7.4)% kPa-1 (P less than 0.05). These results indicate that 1 MAC of isoflurane does not selectively depress rib cage motion, except during carbon dioxide stimulated hyperpnoea.