Overnight Transcutaneous Carbon Dioxide Monitoring in Eucapnic Patients with Obstructive Sleep Apnea Syndrome

We monitored increases in CO₂ levels during sleep by measuring transcutaneous pCO₂ (PtcCO₂) to determine its relationship with polysomnographic data in normocapnic patients with obstructive sleep apnea syndrome (OSAS).

Between October 2011 and December 2012, 139 patients underwent PtcCO₂ monitoring with polysomnography. All patients were evaluated with arterial blood gas (ABG) measurements and pulmonary function tests (PFTs). We excluded 13 patients with COPD and/or daytime hypercapnia and 29 patients whose PtcCO₂ records could not be evaluated.

The patients' mean age was 46.8±10.3 years. Fifty-nine patients (60.8%) were male, and 38 (39.2%) patients were female. The mean overnight PtcCO₂ was ≤45 mm Hg in 84 (86.6%) patients and >45 mm Hg in 13 (13.4%) patients. In the group with PtcCO₂>45 mm Hg, 10 patients had an apnea-hypopnea index (AHI) >15, and 3 patients had an AHI<15, without a statistically significant difference (p=0.078). The mean apnea and apnea/interapnea periods were similar. The mean PtcCO₂ values correlated with time spent when the SpO₂ was <90% (r=0.220, p<0.031). When we grouped the patients by AHI, 60 (61.8%) patients had an AHI>15 (moderate to severe OSAS), and 37 (37.2%) had an AHI<15 (mild OSAS). Of the former group, 16.7% had a mean PtcCO₂ >45 mm Hg, whereas this ratio was 8.1% in the latter group. The difference was not statistically significant (p=0.359). In the group with an AHI>15, the highest PtcCO₂ levels were significantly higher (p<0.05).

We conclude that seemingly eucapnic OSAS patients may experience hypercapnia when sleeping, and PtcCO₂ monitoring may be useful in the early diagnosis of hypercapnia.