Trending peripheral venous PCO₂ in patients with respiratory failure using mathematically arterialised venous blood...

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) (pCO₂v) with mathematically arterialised pCO₂ (pCO₂ca) and to further evaluate whether pCO₂ca follows change in arterial pCO₂ (pCO₂a) more accurately.

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 pCO₂v was compared with the change in calculated pCO₂ca 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 pCO₂v, pCO₂a and pCO₂ca values for each trend point. The primary outcome measures were the 95% limits of agreement (LOAs) between different sampling methods for ΔpCO₂.

In the first data set, 46 patients had 203 VBG results giving 157 trend points for ΔpCO₂ analysis. The 95% LOAs for ΔpCO₂ca and ΔpCO₂v 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 ΔpCO₂a and ΔpCO₂v, -8.94 to 8.58 mm Hg for ΔpCO₂ca and ΔpCO₂v, and -4.54 to 4.91 mm Hg for ΔpCO₂a and ΔpCO₂ca.

This study suggests that trending pCO₂v is not an accurate way to trend pCO₂a in patients with respiratory failure. ΔpCO₂ca via vTAC trended differently to ΔpCO₂v. Our data suggest pCO₂ca more accurately trends pCO₂a.