Invasive methods of CO2 measurement have been shown to be impractical for sleep monitoring.¹ However, airway monitoring of End-tidal CO2 (ETCO2) provides a noninvasive method of monitoring CO2 retention in the sleeping subject.² In this study we examined the frequency of alveolar hypoventilation in the awake and asleep patient with obstructive sleep apnea syndrome.

Methods

We retrospectively selected 80 consecutive patients from our database based on the following criteria; no previous history of COPD, alveolar hypoventilation. or pulmonary disease; an RDI > 15 (based on overnight polysomnography), and no previous use of NCPAP. Mean age was 52, mean % IBW was 139, and mean RDI was 62. 68 of the patients were male and 12 were female. ETCO2, measured by nasal cannula using Biochem capnometer model 8800, was analyzed during wake, NREM,and REM in each patient We compared the wake ETCO2 to the NREM and REM ETCO2in each patient.

Results

The patients were divided into 2 groups, A and B. Group A presented ETCO2 > 45 mm Hg while awake and represented 32% (26) of the patients. 59%(36) of Group A demonstrated a rise in ETCO2 of >45 mm Hg in sleep. 15 of these demonstrated a rise only in REM sleep. The mean rise in ETCO2 for Group A from awake to sleep was 11 mm HG (see graph).

96% (25) of Group B demonstrated a rise in ETCO2 in sleep, with a mean ETCO2 of 48 mm Hg while awake rising to a mean TECO2 of 52 mm Hg(see graph). Simultaneous arterial CO2 samples were taken in 11 of the 80 patients while awake. The PaCO2 and ETCO2 values of these 11 patients presented a correlation coefficient of .95.

Conclusion

68% of our total population presented normal CO2 levels while awake. 32% presented above normal CO2 levels while awake. 71% of our total population demonstrated above normal CO2 levells while asleep. 40% of our total population presented normal CO2 levels while asleep that rose an average of 11mm Hg during sleep.

This data suggests that there is a high prevalence of alveolar hypoventilation in the awake and sleeping OSA patient. Alveolar hypoventilation can be detected by using noninvasive End-tidal CO2 monitoring. _________________________________________________

1 J.S. Clark, B.A. Votteri,et al., Noninvasive Assessment of Gases., Am.Rev.Res.Dis., Vol. 145(1):Jan,1992 220-232

2 T.Pace, et al., Laboratory Methods for Detecting Hypoventilation in Sleep., Sleep Res. Vol.18, 1989, p405