Validation of venous pCO2 to screen for arterial hypercarbia in patients with chronic obstructive airways disease.

To validate a previously derived venous pCO2 (pvCO2) cut-off for ruling out arterial hypercarbia in patients with chronic obstructive pulmonary disease (COPD), matched arterial and venous blood gas samples were taken from a convenience sample of patients who presented to the Emergency Department (ED) with COPD deemed by their treating doctor to require arterial blood gas (ABG) analysis as part of their care. The screening cut-off was defined as pvCO2 of > 45 mm Hg and arterial hypercarbia was defined as arterial pCO2 (paCO2) of > 50 mm Hg. Descriptive statistics were employed. Sensitivity, specificity and negative predictive value were calculated. There were 112 patients enrolled in the study, of whom 107 had complete data for analysis. Forty-three patients had arterial hypercarbia (range of 51 to 90 mm Hg, median 60 mm Hg). All cases of arterial hypercarbia were detected by the screening cut-off (sensitivity 100%; 43/43; 95% CI 91-100%; specificity 47%, 95% CI 35-59%). The negative predictive value of pvCO2 < 45 mm Hg was 100% (30/30, 95% CI 89-100%). Assuming the ABG was performed to assess hypercarbia, 29% of ABGs potentially could have been avoided if a venous screening test was employed. In conclusion, pvCO2 can be used as a screening test for arterial hypercarbia, and if employed, can potentially reduce the requirement for ABG sampling.