Trending peripheral venous PCO 2 in patients with respiratory failure using mathematically arterialised venous blood gas samples.

BACKGROUND: Trending venous blood gases (VBGs) has been suggested as an alternative to arterial blood gases (ABGs) in patients with respiratory failure, but there are limits to its utility. The aim of this study was to compare the trending of venous carbon dioxide partial pressure (pCO2 ) (pCO2 v) with mathematically arterialised pCO2 (pCO2 ca) and to further evaluate whether pCO2 ca follows change in arterial pCO2 (pCO2 a) more accurately.

METHODS: We analysed two data sets. The first was a retrospective study of patients with respiratory failure admitted to the intensive care unit. Venous samples were mathematically arterialised using the vTAC method. The change in pCO2 between two consecutive samples (ΔpCO2 ) for pCO2 v was compared with the change in calculated pCO2 ca values. In the second data set taken from previously published work, we analysed 82 trend points (difference between consecutive samples) for change in pCO2 . There were pCO2 v, pCO2 a and pCO2 ca values for each trend point. The primary outcome measures were the 95% limits of agreement (LOAs) between different sampling methods for ΔpCO2 .

RESULTS: In the first data set, 46 patients had 203 VBG results giving 157 trend points for ΔpCO2 analysis. The 95% LOAs for ΔpCO2 ca and ΔpCO2 v were -9.28 to 11.12 mm Hg.In the second data set, 95% LOAs for ΔpCO2 were -9.46 to 9.48 mm Hg for ΔpCO2 a and ΔpCO2 v, -8.94 to 8.58 mm Hg for ΔpCO2 ca and ΔpCO2 v, and -4.54 to 4.91 mm Hg for ΔpCO2 a and ΔpCO2 ca.

CONCLUSION: This study suggests that trending pCO2 v is not an accurate way to trend pCO2 a in patients with respiratory failure. ΔpCO2 ca via vTAC trended differently to ΔpCO2 v. Our data suggest pCO2 ca more accurately trends pCO2 a.