The effect of hyperventilation on pulmonary surface activity.

The effect of IPPV with and without hyperventilation was studied in five groups of living anaesthetized guineapigs. The respiratory frequency used was in all cases 32 per min and the tidal volume was equal to either 15 per cent of the predicted vital capacity for a period of 15 min or to 75 per cent of the predicted vital capacity for a period of 2 breaths, 5 min, 10 min, or 15 min. Pressure/volume manoeuvres were done on the lungs of each animal at the end of the ventilation period and samples of lung tissue taken for measurement of the stability ratio of expressed lung bubbles. There was a small but non-significant fall in lung compliance and lung stability index following normal ventilation for 15 min. Hyperventilation reduced lung compliance, lung stability index, total lung volume and the bubble stability ratio to a degree which was related to the duration of ventilation. The opening pressure of the lungs and the pressure required to maximally inflate the lungs were increased following hyperventilation.