Introduction COPD exacerbations are a common cause of emergency hospital admission in the UK, with an estimated 94,000 per year. Identifying hypercapnic respiratory failure is crucial. Guidelines recommend obtaining arterial blood samples but these are more difficult to obtain than venous samples. Furthermore, administration of local anaesthetic prior to arterial sampling is seldom used. We assessed whether blood gas values derived from venous samples could replace arterial at initial assessment.

Methods Patients treated for a COPD exacerbation had paired arterial and venous samples taken. Bland Altman analyses were performed to assess agreement between arterial and venous pH, PCO₂ and HCO₃ ^-, and between SpO₂ and SaO₂. The number of attempts and pain scores for each sample were measured.

Results 234 patients had paired arterial and venous samples. There was good agreement between arterial and venous measures of pH and HCO₃ ^- (mean difference 0.03 and -0.04, limits of agreement -0.54 to 0.11, and -2.90 to 2.82), and between SaO₂ and SpO₂ (in patients with a SpO₂ of greater than 80%).

We calculated the sensitivity and specificity of a VBG pH and HCO₃ ^- to correctly identify an arterial pH of ≥7.35, and an arterial HCO₃ ^- of ≥21, as well as a SpO₂ to identify a SaO₂ of ≥ 92%. A venous pH of 7.34, a venous HCO₃ ^- of 21.45 and a SpO₂ of 91.5 would have correctly classified 87% (95% CI 82% to 91%), 97% (95% CI 93% to 98%), and 71% (95% CI 65% to 77%), of patients respectively. 96% of patients with an ABG pH of ≥7.35 also had a VBG pH of ≥7.35.

Arterial sampling took more attempts and was more painful than venous (mean pain score 4 (IQR 2–5) and 1 (IQR 0–2), p < 0.001).

Conclusion Arterial sampling is more difficult and painful than venous sampling. There is good agreement between pH and HCO₃^- values derived from venous and arterial blood, and between pulse oximetry and arterial blood gas oxygen saturations. This could allow the initial assessment of COPD exacerbations to be based on venous blood gas analysis and pulse oximetry, simplifying the care pathway and improving the patient experience.