Accuracy and precision of three different methods to determine PCO2 (PaCO2 vs. PetCO2 vs. PtcCO2) during interhospital ground transport of critically ill and ventilated adults

Background: Interhospital transportation of critically ill and mechanically ventilated patients represents a common, yet difficult problem. Three different methods to determine PCO2 during transport are available: arterial blood gas analysis (PaCO2), end-tidal (PetCO2) and transcutaneous (PtcCO2) measurement. The aim of the present study is to analyze accuracy and precision of those different methods simultaneously in critically ill and ventilated adults during interhospital transport. Methods: Patients scheduled for interhospital transport were investigated after approval of the local ethics committee in the prospective study. PCO2 was determined five times in each patient during the transport simultaneously by (1) arterial blood gas analysis (PaCO2[Immediate Response Mobile Analyzer, IRMA]), (2) end-tidal (PetCO2), and (3) transcutaneous (PtcCO2) measurements. The results were compared with an in-hospital reference measurement performed by an ABL 625 blood gas analyzer (PaCO2[ABL625]). For statistical analysis the Bland-Altman method was used. A p<0.05 was considered statistically significant. Results: One hundred seventy data sets (PaCO2[IRMA], PaCO2[ABL625], PetCO2, PtcCO2) were obtained in 34 patients (61 years +/- 16 years old; 19 male patients, 15 female patients). The mean PaCO2(ABL625) was 43.2 min Hg +/- 8.8 min Hg ranging from 24.9 min Hg to 72.4 min Hg. Bland-Altman analysis revealed a bias and precision of -0.6 min Hg +/- 2.5 min Hg for the arterial blood gas analysis with the mobile IRMA device and -0.6 mm Hg +/- 7.5 min Hg for the transcutaneous measurement (p > 0.05). Bias and precision (-5.3 min Hg +/- 6.1 min Hg) of endexpiratory CO2-measurement differed significantly (p < 0.003) when compared with the reference. Conclusions: During interhospital transport PaCO2(IRMA) and PtcCO2 provide the best accuracy when compared with the reference measurement. Patients who either require a tight control of PCO2 or endured lengthy transportation could benefit greatly from the combination of expiratory calmography with mobile arterial blood gas analysis or the transcutaneous measurement of PCO2.