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S27 Venous Bicarbonate As A Clinical Tool For Identifying Obesity Hypoventilation Syndrome In The Sleep Clinic
  1. B Prudon,
  2. SD West
  1. Newcastle Regional Sleep Service, Newcastle Upon Tyne, UK


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.

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