CONTROL OF BREATHING IN OBSTRUCTIVE SLEEP-APNEA AND IN PATIENTS WITH THE OVERLAP SYNDROME

In some patients obstructive sleep apnoea (OSA) may co-exist with chronic obstructive pulmonary disease (COPD) and respiratory failure; the so-called ''overlap syndrome''. Obstructive, hypercapnic patients have both blunted ventilatory and mouth occlusion pressure responses during CO2 stimulation. The purpose of this study was to compare the pattern of breathing and CO2 response between OSA patients and those with the overlap syndrome. Twenty obese men with OSA and normal lung function (Group A), 11 obese men with overlap syndrome (Group B) and 13 healthy nonobese subjects (Group C) were examined, Lung function tests, breathing pattern, mouth occlusion pressure (P0.2) at rest, and respiratory responses during CO2 rebreathing were investigated, Diagnosis of OSA was established by standard polysomnography. There were no statistical differences between Groups A and B in apnoea(+) hypopnoea index (62 vs 54), mean arterial oxygen saturation (Sao(2)) during sleep (85 vs 84%) and in body mass index (BMI) 34.3 vs 36.3 kg . m(-2). Minute ventilation, mean inspiratory flow and P0.2 at rest were increased in both groups of patients in comparison to controls. During CO2 rebreathing, group A had normal ventilatory and P0.2 responses, similar to controls, (2.7+/-1.1 vs 2.1+/-0.4 l . min(-1) . mmHg(-1) and 0.7 +/- 0.3 vs 0.71+/-0.25 cmH(2)O . mmHg(-1), respectively), However, Group B had significantly decreased ventilatory and P0.2 responses to CO2 (0.71+/-0.23 l . min(-1) . mmHg(-1) and 0.34+/-0.17 cmH(2)O . mmHg(-1), respectively). This comparison showed that patients with OSA had normal CO2 response when awake, whereas those with overlap syndrome had diminished CO2 response when awake. It seems that co-existence of COPD with hypercapnic respiratory failure is the main cause of decreased CO2 response in the overlap syndrome.