Clinical Question

In patients with an acute exacerbation of chronic obstructive pulmonary disease (COPD), is a venous blood sample as useful as an arterial sample?

Bottom Line

There is very good agreement between arterial and venous measurements of pH and bicarbonate, and fairly good agreement at higher levels between arterial and peripheral measures of oxygen saturation. These authors suggest an algorithm for patients with acute exacerbation of COPD that includes an arterial blood gas analysis only if the patient's initial pulse oximetry is less than 80% or if the venous pH is less than 7.35, which would obviate the need for two-thirds of arterial blood gasses. A more conservative approach would also include an arterial blood gas analysis for patients with oxygen saturation between 80% and 84%, where there was also some misclassification. (Level of Evidence = 1b)

Synopsis

Arterial blood draws are painful. These British researchers asked, sensibly, whether we could get the same information from a venous blood sample. Specifically, can it identify patients with hypercapnia and respiratory acidosis who are at risk of respiratory failure and worse outcomes? This study included 234 patients (mean age of 71 years) hospitalized for an acute exacerbation of COPD. Patients had arterial and venous samples drawn, and the pain of each procedure was measured using a 10-point visual analog scale. The authors then evaluated the agreement between arterial and venous samples for pH, bicarbonate, and CO₂, and between arterial oxygen saturation and oxygen saturation by pulse oximetry.

Overall, agreement was very good between arterial and venous measures of pH (mean difference = 0.03; 95% confidence interval [CI], 0.02 to 0.04) and bicarbonate (mean difference = −0.04 mEq per L; 95% CI, −0.22 to 0.15). The venous CO₂ consistently overestimated pCO₂ (mean difference of arterial minus venous = −0.75 kPa; 95% CI, −0.89 to −0.61; −5.6 torr; 95% CI, −6.7 to −4.6). Agreement regarding oxygen saturation was good for patients with a pulse oximetry greater than 80%, but not if it was lower. Overall, the venous pH was 90% sensitive and 96% specific compared with arterial pH, correctly classifying 87% of patients. The median pain score was significantly higher for arterial samples (a score of 4 vs. 1 on a scale of 10; P < .001), and arterial samples were more likely to require a second stick.

Study design: Cross-sectional

Funding source: Government

Setting: Inpatient (any location)

Reference: McKeeverTMHearsonGHousleyGet alUsing venous blood gas analysis in the assessment of COPD exacerbations: a prospective cohort study. Thorax2016;71(3):210–215.