Introduction The need for early detection of Obesity Hypoventilation Syndrome (OHS) is clear because delay in the diagnosis and treatment is associated with significant morbidity and mortality.

Objective To determine the prevalence of obesity hypoventilation syndrome among obese patients with suspected sleep apnoea and determine the validity of previously reported predictors of OHS such as serum bicarbonate level.

Methods A retrospective analysis of prospectively collected sleep clinic data on 525 consecutive obese patients referred to sleep clinic from January 2009 to January 2011 to a university hospital was performed. Subjects with suspected sleep disordered breathing were evaluated according to our clinical protocol and capillary blood gases were measured in obese (BMI> 30) subjects.

Results 525 consecutive patients (mean age 51.44±12.7, 65.71% males, mean BMI 34.59±8.1) were evaluated. A total of 344 (65.52%) were obese (mean age 52.29±12.4, 63.66% males) of which 128 (37.2%) were morbidly obese (BMI>40 kg/m²). Daytime hypercapnia (paCO₂>6 kPa) was detected in 20.63% (71/344) obese and 22.1% (61/275) obstructive sleep apnoea (OSA) patients. Univariate analysis of potential predictors of OHS showed significant correlations between paCO₂ and BMI, FEV1, FVC, AHI, mean nocturnal SpO₂, minimum nocturnal SpO₂, sleep time spent with SpO₂ <90%, paO₂ and serum HCO₃. Following stepwise multiple regression, paO₂ and HCO₃ were found to be independent predictors of OHS explaining 27.7% of paCO2 variance (p<0.0001).

On logistic regression analysis, serum HCO₃ cut-off of >27 mmol was found to have 85% sensitivity and 90% specificity for diagnosis of OHS.

Conclusion We confirmed high prevalence of OHS in obese patients with OSA (22.1%) that would be possible to diagnose by measuring serum HCO3 levels, thereby eliminating the need for arterial blood gas sampling.