Background: Bicarbonate (either venous or calculated from arterial blood gas analysis) above the upper limit of normal is used as an indicator of sleep-related hypoventilation in patients with diseases frequently associated with hypoventilation, such as obesity or neuromuscular diseases, to allow timely planning of sleep studies and evaluation of treatment for sleep-related breathing disorder.

Purpose: To evaluate the diagnostic value of bicarbonate from daytime arterial blood gas analysis for predicting sleep-related hypoventilation.

Methods: ROC analysis was performed to investigate the diagnostic value of different bicarbonate thresholds for detecting sleep-related hypoventilation.

Results: In 534 patients (mean±SD age 51.6±15.5 years, BMI 33.4±10.0 kg/m2, 61% with BMI ? 30 kg/m2 and 24.5% with BMI ? 40 kg/m2, 12.2% with neuromuscular disease) who underwent timely awake arterial blood gas analysis and polygraphy with transcutaneous capnometry, the sensitivity of an HCO3- of <26.5, 27.5, and 28.5 mmol/l was 71, 87, and 93%, respectively, to predict a mean nocturnal tcpCO2 of ? 6.7 kPa. The area under the ROC curve for bicarbonate and median nocturnal tcpCO2 ? 6.7 (286 yes, 248 no) was 0.69 (95%CI 0.65-0.74), p<0.0001. Interestingly, the sensitivity of these thresholds was lower in obesity with a BMI ? 40 kg/m2 than in obesity with a BMI ? 30 kg/m2 or the entire study population.

Conclusions: The sensitivity of elevated bicarbonate from arterial blood gas analysis using common cut-off values such as 27 mmol/l is modest and must be interpreted in the context of other assessments.