An Evaluation of Transcutaneous Carbon Dioxide Partial Pressure Monitoring during Apnea Testing in Brain-dead Patients

Background: Diagnosis of brain death usually requires an arterial carbon dioxide partial pressure (PaCO2) of 60 mmHg during the apnea test, but the increase in PaCO2 is unpredictable. The authors evaluated whether transcutaneous carbon dioxide partial pressure (PtcCO2) monitoring during apnea test can predict that a PaCO2 of 60 mmHg has been reached. Methods: The authors compared PtcCO2 measured with a transcutaneous ear sensor (V-Sign Sensor, Sentec Digital Monitoring System; SENTEC-AG, Therwil, Switzerland) and PaCO2 obtained from arterial blood gas measurements in 32 clinically brain-dead patients. Results: In the first 20 patients, the mean PaCO2–PtcCO2 gradient was 0.7 3.6 mmHg at baseline and 8.7 7.1 mmHg after 20 min of apnea. Using receiver operating characteristic curve analysis (area under the curve: 0.983 0.013), the best threshold value of PtcCO2 to predict that a PaCO2 of 60 mmHg had been reached was 60 mmHg (positive predictive value: 1.00 [0.93– 1.00]). In the following 12 patients investigated with use of this PtcCO2 target value of 60 mmHg, the mean duration of the apnea test (11 4 vs. 20 0 min; P < 0.001), hypercapnia (74.0 4.9 vs. 98.3 20.0 mmHg; P < 0.001), acidosis (pH: 7.18 0.06 vs. 7.11 0.08; P < 0.001), and decrease in arterial oxygen partial pressure ( 47 44 vs. 95 89; P < 0.05) at the end of the test were reduced as compared with the 20-min apnea test group. Conclusion: During the apnea test in brain-dead patients, a PtcCO2 of 60 mmHg accurately predicts that a PaCO2 of 60 mmHg has been reached. This may allow a reduction in the duration of the apnea test and consecutively limit occurrence of complications.