A comparison of the end-tidal-CO2 documented by capnometry and the arterial pCO2 in emergency patients

Satisfactory artificial ventilation is defined as sufficient oxygenation and normo- or slight arterial hypocarbia. Monitoring end tidal CO2 values with non-invasive capnometry is a routine procedure in anaesthesia, emergency medicine and intensive care. In anaesthesia the ventilation volume is adjusted to the capnometric end tidal CO2 (ETCO2), taking into account a normal variation from the pACO2 of 3-8 mmHg. We evaluated the usefulness and practicability of using ETCO2 for correctly adjusting ventilation parameters in prehospital emergency care, by comparing arterial pCO2 and ETCO2 of 27 intubated and ventilated patients. We used the side-stream capnometry module of the Defigard 2000 (Bruker, ChemoMedica Austria) and a portable blood gas analyzer (OPTI 1, AVL Graz, Austria). Evaluation of the group of patients as a whole showed that there was no correlation whatsoever between the end expiratory and arterial CO2. Dividing the patients into three subgroups (1, During CPR; II, respiratory disturbances of pulmonary and cardiac origin; III, extrapulmonary respiratory disturbances), we found that only patients without primary cardiorespiratory damage showed a slight, but not statistically significant, correlation. This can be explained by the fact that almost any degree of cardiorespiratory failure causes changes of the ventilation-perfusion ratio, impairing pulmonary CO2 elimination. We conclude, that the ventilation of emergency patients can only be correctly adjusted according to values derived from an arterial blood gas analysis and ETCO2 measurements cannot be absolutely relied upon for accuracy except, perhaps, in patients without primary cardiorespiratory dysfunction.