Determinants of Wake Pco2 and Increases in Wake Pco2 over Time in Patients with Obstructive Sleep Apnea.

RATIONALE

The progression from obesity and obstructive sleep apnea to obesity with hypoventilation and daytime hypercapnia may relate to features of sleep-disordered breathing events that affect loading and unloading of carbon dioxide.

OBJECTIVES

To determine whether the wake Pco2 increases over time in untreated obstructive sleep apnea, and whether that increase is explained by changes in sleep-disordered breathing event duration, interevent duration, or postevent ventilation amplitude.

METHODS

We selected 14 adults who had two polysomnographic studies more than 1 year apart because of untreated or suboptimally treated moderate to severe obstructive sleep apnea. Demographic and polysomnographic data were reviewed for both sets of studies, including the evening wake end-tidal CO2, the ratio of mean event to mean interevent duration (subsuming apneas and hypopneas), and the ratio of mean post- to preevent breath amplitude.

MEASUREMENT AND MAIN RESULTS

The mean (SD) wake end-tidal Pco2 increased between studies from 35.9 (4.2) to 39.5 (3.9) mm Hg (P < 0.005). The wake end-tidal CO2 correlated inversely with the post- to pre-event breath amplitude and positively with the ratio of mean event to mean interevent duration and with body mass index. However, those three variables were not significantly changed between the two studies. The wake end-tidal CO2 did not correlate with the apnea-hypopnea index or age. There was a significant increase in bicarbonate level between studies (median, 24.0-26.5 mmol/L; P = 0.01).

CONCLUSIONS

In our study cohort, wake end-tidal CO2 correlated with body mass index and features of sleep apnea that influence the balance of loading and unloading of CO2. However, those features remained fixed over time, even as the wake Pco2 and bicarbonate levels increased with untreated sleep apnea.