Introduction Obesity Hypoventilation Syndrome (OHS) is defined as sleep disordered breathing, obesity, and daytime hypercapnia, without another cause of ventilatory impairment. Literature suggests 10–25% of patients assessed for Obstructive Sleep Apnoea (OSA) have OHS, with significantly increased morbidity and mortality. Early identification may be beneficial. Studies suggest venous bicarbonate (vHCO₃ ^-) ≥27 mmol/l can be used to screen for OHS. We assessed the impact of incorporating this measurement into patient assessments.

Methods Obese out-patients referred for possible OSA had vHCO₃ ^- measured. Patients with a vHCO₃ ^- ≥27 mmol/l underwent arterial blood gas (ABG) analysis. Those with pCO₂ >6.2 kPa underwent further assessments to identify the cause of ventilatory impairment. None had been referred specifically for investigation of OHS. Patients had domiciliary or in-patient sleep studies as per standard practice.

Results There were 288 patients included: 65% males, mean (SD) age 50 years (range 21–79 years), BMI 39.2 kg/m² (7.8), Epworth Sleepiness Scale 13 (6), daytime SpO₂ on air 97% (2.1). Sleep study results showed the Apnoea-Hypopnea Index (AHI) to be ≥5 in 88%, and ≥30 in 49%. Mean vHCO₃ ^- was 26.2 mmol/l (2.7). vHCO₃ ^- correlated significantly (r = 0.3–0.4, p < 0.005) with daytime SpO₂, mean overnight SpO₂, time spent <80% and <90%, but not AHI or ODI.

vHCO₃ ^- was ≥27 mmol/l in 123 (43%), of whom 80 had an ABG measurement; mean pCO₂ 5.4 kPa (0.8), ten patients >6.2 kPa. Ventilatory impairment was due to OHS in four (5% of ABG cohort); there was additional lung or chest wall disease in the other six. Overall, 25 patients had a base excess ≥3. The vHCO₃ ^- range was 28–36 mmol/l in patients with OHS, with a BMI range of 38–53 kg/m².

Three additional outpatients with BMI >50 kg/m² were diagnosed with OHS on ABG without vHCO₃ ^- measurement. ­ In all seven OHS patients, CPAP was initiated. One was non-compliant, four improved and two required home non-invasive ventilation due to non-improvement in ABG.

Conclusions In this large cohort of patients assessed for OSA, 43% had a vHCO₃ ^- ≥27 mmol/l indicating possible OHS, but only 5% were actually diagnosed with OHS. In isolation this strategy to identify OHS seems inefficient. An increased vHCO₃ ^- in combination with sleep study data may be superior.