Ventilatory responses after major surgery and high dependency care

Disturbed breathing during sleep, with episodic upper airway obstruction, is frequent after major surgery. Ventilatory responses to hypercapnia and hypoxia during episodes of airway obstruction are difficult to investigate because the usual measure, that of ventilation, has been attenuated by the obstruction. We simulated the blood gas stimulus associated with obstruction to allow investigation of the responses.

To assess ventilatory responses, we studied 19 patients, mean age 59 (19-79), first at discharge from high dependency care after major abdominal surgery and then at surgical review, ~6 weeks later. Exhaled gas was analysed and inspired gas adjusted to simulate changes that would occur during airway obstruction. Changes in ventilation were measured over the following 45-70 s. Studies were done from air breathing if possible, and also from an increased inspired oxygen concentration.

During simulated obstruction, hypercapnia developed similarly in all the test conditions. Arterial oxygen saturation decreased significantly more rapidly when the test was started from air breathing. The mean ventilatory response was 5.8 litre min(-2) starting from air breathing and 4.5 litre min(-2) with oxygen breathing. The values 6 weeks later were 5.9 and 4.3 litre min(-2), respectively (P=0.05, analysis of variance). There was no statistical difference between the responses starting from air and those on oxygen.

After major surgery, ventilatory responses to hypercapnia and hypoxaemia associated with airway obstruction are small and do not improve after 6 weeks. With air breathing, arterial oxygen desaturation during simulated rebreathing is substantial.