Background and objective:: Diagnosis of brain death usually requires the absence of spontaneous respiratory movements during the apnoea test and an arterial carbon dioxide partial pressure above 60 mmHg. On the other hand, although capnography (end‐tidal CO2) is currently monitored in intensive care unit patients, it has not been evaluated during the apnoea test in brain‐dead patients. Therefore, the aim of this prospective study was first to investigate the usefulness of capnography monitoring, and secondly to evaluate the variation of the carbon dioxide partial pressure‐end‐tidal CO2 gradient during the apnoea test in clinically brain‐dead patients.
Methods:: After local Ethics Committee approval, 60 clinically brain‐dead patients were investigated. End‐tidal CO2 was continuously recorded before, during and after the apnoea test. Arterial blood gases were sampled immediately before and after the apnoea test for calculation of the carbon dioxide partial pressure‐end‐tidal CO2 gradient.
Results:: The apnoea test was clinically positive in 58 patients, whereas end‐tidal CO2 was equal to 0 during the apnoea. During the 20‐min apnoea test, carbon dioxide partial pressure increased from 40 ± 7 to 97 ± 19 mmHg (P < 0.001) with a rate of 2.8 ± 0.9 mmHg min−1, end‐tidal CO2 increased from 31 ± 6 to 68 ± 17 mmHg (P < 0.001) and carbon dioxide partial pressure‐end‐tidal CO2 gradient increased from 9 ± 4 to 29 ± 10 mmHg (P < 0.001). In two patients, the apnoea test was clinically negative because of the occurrence of spontaneous respiratory movements, whereas capnography showed contemporaneously significant increases in end‐tidal CO2.
Conclusions:: End‐tidal CO2 should be systematically monitored and recorded, at least for medico‐legal considerations, during the apnoea test in brain‐dead patients. The high variability in the carbon dioxide partial pressure‐end‐tidal CO2 gradient increase precludes any extrapolation of the carbon dioxide partial pressure from the end‐tidal CO2 at the end of the apnoea test.