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 (pCO(2)) (pCO(2)v) with mathematically arterialised pCO(2) (pCO(2)ca) and to further evaluate whether pCO(2)ca follows change in arterial pCO(2) (pCO(2)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 pCO(2) between two consecutive samples (ΔpCO(2)) for pCO(2)v was compared with the change in calculated pCO(2)ca values. In the second data set taken from previously published work, we analysed 82 trend points (difference between consecutive samples) for change in pCO(2). There were pCO(2)v, pCO(2)a and pCO(2)ca values for each trend point. The primary outcome measures were the 95% limits of agreement (LOAs) between different sampling methods for ΔpCO(2). RESULTS: In the first data set, 46 patients had 203 VBG results giving 157 trend points for ΔpCO(2) analysis. The 95% LOAs for ΔpCO(2)ca and ΔpCO(2)v were −9.28 to 11.12 mm Hg. In the second data set, 95% LOAs for ΔpCO(2) were −9.46 to 9.48 mm Hg for ΔpCO(2)a and ΔpCO(2)v, −8.94 to 8.58 mm Hg for ΔpCO(2)ca and ΔpCO(2)v, and −4.54 to 4.91 mm Hg for ΔpCO(2)a and ΔpCO(2)ca. CONCLUSION: This study suggests that trending pCO(2)v is not an accurate way to trend pCO(2)a in patients with respiratory failure. ΔpCO(2)ca via vTAC trended differently to ΔpCO(2)v. Our data suggest pCO(2)ca more accurately trends pCO(2)a.