Biologically variable ventilation prevents deterioration of gas exchange during prolonged anaesthesia

We have studied the time course of changes in gas exchange and respiratory mechanics using two different modes of ventilation during 7 h of isoflurane anaesthesia in pigs. One group received conventional control mode ventilat ion (CV). The other group received biologically variable ventilation (BW) w hich simulates the breath-to-breath variation in ventilatory frequency cn t hat characterizes normal spontaneous ventilation. After baseline measuremen ts with CV, animals were allocated randomly to either CV or BW (Fl(O2) 1.0 with 1.5% end-tidal isoflurane). With BW, there were 376 changes in f and t idal volume (V-T) over 25.1 min. Ventilation was continued over the next 7 h and blood gases and respiratory mechanics were measured every 60 min. The modulation file used to control the ventilator for BW used an inverse powe r law frequency distribution (1/f(a) with a = 2.3 +/- 0.3). After 7 h, at a similar delivered minute ventilation, significantly greater Pa-O2 (mean 72 .3 (SD 4.0) vs 63.5 (6.5) kPa) and respiratory system compliance (1.08 (0.0 8) vs 0.92 (0.16) mi cm H2O-1 kg(-1)) and lower Pa-CO2 (6.5 (0.7) vs 8.7 (1 .5) kPa) and shunt fraction (7.2 (2.7)% vs 12.3 (6.2)%) were seen with BW, with no significant difference in peak airway pressure (16.3 (1.2) vs 15.3 (3.7) cm H2O). A deterioration in gas exchange and respiratory mechanics wa s seen with conventional control mode ventilation but not with BW in this e xperimental model of prolonged anaesthesia.