Bicarbonate from arterial blood gas analysis as predictor of sleep-related hypoventilation: a diagnostic accuracy study.

Background: Bicarbonate 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.

Research question: The aim was to evaluate the diagnostic accuracy of bicarbonate from daytime arterial blood gas analysis for sleep-related hypoventilation measured via transcutaneous nocturnal capnometry.

Methods: In a diagnostic accuracy study, the sensitivity and specificity of bicarbonate concentration for detecting nocturnal hypoventilation were tested using a receiver operating curve (ROC) and logistic regression analysis in patients undergoing both an in-laboratory sleep study with transcutaneous capnometry and daytime arterial blood gas analysis.

Results: In 510 patients (mean±SD age 52±15 years, BMI 34±10 kg/m²), the area under the ROC curve for bicarbonate (HCO_3-) and median nocturnal tcpCO₂ ≥50 mm Hg (45% yes, 55% no) was 0.70 (95% CI 0.65 to 0.75). Sensitivity of HCO_3- of 26.5 mmol/L and 27.0 mmol/L was 61.0% and 54.1%, specificity 71.3% and 79.6%, and the likelihood ratio for sleep-related hypoventilation (median tcpCO₂ ≥50 mm Hg) 2.1 and 2.6. HCO_3- was significantly higher in those with (n=231, 27.6±3.0 mmol/L) versus without (n=279, 25.8±1.9 mmol/L) sleep-related hypoventilation (p<0.001). Increasing awake HCO_3- was associated with a higher likelihood of nocturnal hypoventilation (OR 1.4 (95% CI 1.3 to 1.5), p<0.001).

Conclusions: The sensitivity of elevated bicarbonate from awake arterial blood gas analysis using common cut-off values such as 27 mmol/L is only moderate, and bicarbonate as an integral part of the assessment of sleep hypoventilation (likelihood ratio) must be interpreted in the context of other assessments. A normal bicarbonate does not rule out sleep hypoventilation.