Diagnostic predictors of obesity-hypoventilation syndrome in patients suspected of having sleep disordered breathing.

Introduction: Obesity-hypoventilation syndrome (OHS) is associated with significant morbidity and mortality and requires measurement of arterial pCO(2) for diagnosis. Objective: To determine diagnostic predictors of OHS among obese patients with suspected obstructive sleep apnea/hypopnea syndrome (OSAHS). Methods: Retrospective analysis of data on 525 sleep clinic patients (mean age 51.4 +/- 12.7 years; 65.7% males; mean BMI 34.5 +/- 8.1). All patients had sleep studies, and arterialized capillary blood gases (CBG) were measured in obese subjects (BMI > 30 kg/m(2)). Results: Of 525 patients, 65.5% were obese, 37.2% were morbidly obese (BMI > 40 kg/m(2)); 52.3% had confirmed OSAHS. Hypercapnia (pCO(2) > 6 kPa or 45 mm Hg) was present in 20.6% obese and 22.1% OSAHS patients. Analysis of OHS predictors showed significant correlations between pCO(2) and BMI, FEV1, FVC, AHI, mean and minimum nocturnal SpO(2), sleep time with SpO(2) < 90%, pO(2), and calculated HCO3 from the CBG. PO 2 and HCO3 were independent predictors of OHS, explaining 27.7% of pCO(2) variance (p < 0.0001). A calculated HCO3 cutoff > 27 mmol/L had 85.7% sensitivity and 89.5% specificity for diagnosis of OHS, with 68.1% positive and 95.9% negative predictive value. Conclusion: We confirmed a high prevalence of OHS in obese OSAHS patients (22.1%) and high calculated HCO3 level (> 27 mmol/L) to be a sensitive and specific predictor for the diagnosis of OHS.