Correlation of arterial PCO2 and PETCO2 in prehospital controlled ventilation.

INTRODUCTION: This study was carried out to estimate the relationship between arterial PCO2 (PaCO2) and end-tidal carbon dioxide (PETCO2) during prehospital controlled ventilation and also to evaluate variation of the gradient between PCO2 and PETCO2 during prehospital transport.

METHODS: Measurements of PETCO2 from capnography values and PaCO2 from arterial blood gases were registered at the beginning (T(0)) and at the end (T(end)) of out-of-hospital management. For all patients requiring invasive ventilation, the gradient between PCO2 and PETCO2 was calculated for T(0) and T(end), the PaCO2-PETCO2 variation between T(end) and T(0) was also calculated.

RESULTS: One hundred patients were included in this study (mean age, 58.4 +/- 16.4 years; 57 were male). There was no variation of the mean gradient (DeltaPaCO2-PETCO2 ) during transport (8.64 +/- 13.5 mm Hg at T(0) and 7.26 +/- 12.94 mm Hg at T(end)). Thirty-six percent of patients (n = 36) had a gradient above +10 mm Hg, and for 6% of patients (n = 4) the gradient was lower than -10 mm Hg. The PaCO2-PETCO2 gradient was not significantly different according to the pathology, but was significantly higher in hypercapnic patients compared with hypocapnic or normocapnic patients. In patients with severe head injury, the capnia was normalized in 80% of patients at the end of the transport according to the last blood gas result. In this subgroup the DeltaPaCO2-PETCO2 (T(end) - T(0)) gradient was stable between T(0) and T(end) except in 20% of the patients for whom the DeltaPaCO2-PETCO2 was lower than -10 mm Hg. Fifty-four percent of critical care physicians had modified the respiratory setting after the first arterial blood gas results.

CONCLUSIONS: The PaCO2 cannot be estimated by the PETCO2 in the prehospital setting. There is wide variation in the gradient between PCO2 and PETCO2 depending on patient condition, and over time, the relationship does not remain constant and thus cannot be useful in prehospital ventilation management.