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 chro nic obstructive pulmonary disease (COPD) and respiratory failure; the so-called ''overlap syndrome''. Obstructive, hypercapnic patients have both blunted ventilatory and mouth occlusion pressure responses durin g CO2 stimulation. The purpose of this study was to compare the patter n of breathing and CO2 response between OSA patients and those with th e overlap syndrome. Twenty obese men with OSA and normal lung function (Group A), 11 obese men with overlap syndrome (Group B) and 13 health y nonobese subjects (Group C) were examined, Lung function tests, brea thing pattern, mouth occlusion pressure (P0.2) at rest, and respirator y responses during CO2 rebreathing were investigated, Diagnosis of OSA was established by standard polysomnography. There were no statistica l differences between Groups A and B in apnoea(+) hypopnoea index (62 vs 54), mean arterial oxygen saturation (Sao(2)) during sleep (85 vs 8 4%) and in body mass index (BMI) 34.3 vs 36.3 kg . m(-2). Minute venti lation, 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 sign ificantly 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), respective ly). This comparison showed that patients with OSA had normal CO2 resp onse when awake, whereas those with overlap syndrome had diminished CO 2 response when awake. It seems that co-existence of COPD with hyperca pnic respiratory failure is the main cause of decreased CO2 response i n the overlap syndrome.