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

OBJECTIVES: We monitored increases in CO2 levels during sleep by measuring transcutaneous pCO(2) (PtcCO(2)) to determine its relationship with polysomnographic data in normocapnic patients with obstructive sleep apnea syndrome (OSAS). MATERIAL AND METHODS: Between October 2011 and December 2012, 139 patients underwent PtcCO(2) 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 PtcCO(2) records could not be evaluated. RESULTS: 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 PtcCO(2) was <= 45 mm Hg in 84 (86.6%) patients and >45 mm Hg in 13 (13.4%) patients. In the group with PtcCO(2) >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 PtcCO(2) values correlated with time spent when the SpO(2) 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 PtcCO(2) >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 PtcCO(2) levels were significantly higher (p<0.05). CONCLUSION: We conclude that seemingly eucapnic OSAS patients may experience hypercapnia when sleeping, and PtcCO(2) monitoring may be useful in the early diagnosis of hypercapnia.