Association between ventilatory response to hypercapnia and obstructive sleep apnea-hypopnea index in asymptomatic subjects

The majority of awake ventilatory control studies have shown normal or decreased ventilatory response to hypercapnia (HCVR) in obstructive sleep apnea-hypopnea syndrome (OSAHS) patients. These findings are contrary to experimental studies suggesting increased loop gain and greater breathing instability in OSAHS patients. We have investigated the relationship between central chemoreflex sensitivity tested by HCVR and obstructive sleep apnea/ hypopnea index (OSAHI) in asymptomatic subjects. Twenty normal volunteers (10 men and 10 women) from the general population without physical complaints including sleep-related symptoms were included. The subjects were studied for awake ventilatory responses to hypoxia (HVR) and hypercapnia. Overnight polysomnography (PSG) was performed in two consecutive nights with the first night used as acclimatization. The subjects have an average body mass index ( BMI) of 27 +/- 5 SD kg/ m(2), ages of 35 +/- 9 SD years and Epworth sleepiness scale of 2.1 +/- 1.8 SD. A positive linear relationship was found between HCVR and logarithmically transformed OSAHI ( r= 0.67, p= 0.001). BMI and age were not significantly correlated to HCVR or Log OSAHI. No relationship was found between HVR and Log OSAHI ( r=0.25, p=0.29). Percentage oxygen saturation nadir during sleep was found to significantly correlate to both daytime HCVR (r=-0.60, p=0.005) and Log OSAHI (r=-0.65, p=0.002) and tended to correlate to HVR (r=-0.41, p= 0.07). Arousal index during sleep was not associated with either HCVR (p=0.93) or HVR (p=0.26). In conclusion, heightened central chemosensitivity was positively related to OSAHI in asymptomatic volunteers. We believe these findings are in keeping with the evolving theory of loop gain being a significant factor for respiratory control instability and obstructive apnea genesis. The mechanism can be applied to asymptomatic subjects with even minimal sleep-disordered breathing.