Respiratory responses to chemical stimulation in patients with obstructive sleep apnoea

Chemical control of breathing in obstructive sleep apnoea (OSA) patients has been studied by many authors. The results of previous studies, especially those dealing with hypoxic drive, are discordant. Respiratory responses were studied during hypercapnic and hypoxic stimulation in a group of 37 normocapnic patients with OSA during wakefulness. The diagnosis of OSA was established by standard polysomnography. These patients had increased apnoea/hypopnoea index (AHI; 51 +/- 22 (mean +/- SD)), obesity (body mass index (BMI) 32.4 +/- 5.6 kg.m-2) and normal lung function tests. The control group consisted of 14 healthy obese subjects (BMI 31.2 +/- 3.3 kg.m-2). Respiratory responses (ventilatory and mouth occlusion pressure (P0.1)) during hypercapnic and hypoxic rebreathing tests were measured with the use of computerized equipment. Respiratory responses during hypercapnic stimulation were similar to those in the control group (change in (delta) minute ventilation (V'E)/delta carbon dioxide tension (PCO2) 23.5 +/- 14.8 versus 22.3 +/- 10.0 L.min-1.kPa-1, delta P0.1/delta PCO2 4.6 +/- 3.6 versus 4.2 +/- 2.6 cmH2O.kPa-1). During isocapnic hypoxic stimulation in OSA patients the mean ventilatory response was higher than in the control group (delta V'E/delta arterial oxygen saturation (Sa,O2) 2.6 +/- 1.7 versus 1.7 +/- 0.7 L.min-1.%-1) but this difference was not statistically significant. Nevertheless, it was found that 13 (35%) OSA patients had increased ventilatory responses. The mean P0.1 response in OSA patients was higher but did not differ significantly from those in the control group (delta P0.1/delta Sa,O2) 0.43 +/- 0.38 versus 0.35 +/- 0.12 cmH2O.%-1). The results demonstrated that respiratory responses to chemical stimulation in awake normocapnic patients with obstructive sleep apnoea were in the normal range, similar to those in control obese subjects. During hypoxic stimulation some of them had increased ventilatory (35%) and mouth occlusion pressure (16%) responses.