Venous vs arterial blood gases in the assessment of sleep disordered breathing in an Irish bariatric population

Background: Arterial blood gas (ABG) analysis is an important component in assessing daytime hypoventilation in patients with suspected sleep disordered breathing (SDB). The diagnosis of obesity hypoventilation syndrome requires a body mass index (BMI) of >30kg/m2, and a raised daytime PaCO2 without another cause for hypoventilation. ABGs done in outpatients can be distressing for patients. Prior studies have explored the arteriovenous agreement for pH and pCO2 in an emergency setting1. We address this question for SDB in an Irish bariatric population. Methods: This was a prospective study of patients undergoing assessment for sleep disordered breathing, recruited through an ambulatory weight management programme. ABG and venous blood gas (VBG) were performed in immediate succession at outpatient assessment. Data collected included anthropometrics, domiciliary sleep study results and blood gas analysis. The primary outcome was arteriovenous comparison of pH, pCO2 and HCO3-. Results: Blood gas pairs from 27 patients were analysed. Average BMI was 51.94 +/- 8.92kg/m2. Median age was 49. 40.7% were male. The mean difference for pH (a-v) was 0.025 units (95% CI 0.018, 0.033). Mean difference for pCO2 was -0.73 kPa (95% CI -0.94, -0.52). Mean difference for HCO3- was -0.07 mmol/L (95% CI -0.22, 0.08). Conclusions: This data suggests that for obese patients with suspected SDB undergoing assessment for daytime hypoventilation, there is good arteriovenous agreement for pH and HCO3-, with less agreement for pCO2. References 1. A Kelly et al. Agreement between arterial and venous pH and pCO2 in patients undergoing non-invasive ventilation in the emergency department. Em Med Aus 2013