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Apnea test for brain death determination: an alternative approach

Introduction

Complications that may occur during the 'classical' apnea test include severe respiratoy acidosis causing hemodynamic instability, hypoxemia and an inadequate increase in CO2 requiring repeat testing. We present our experience administering carbon dioxide (CO2) during mechanical ventilation as a means of raising arterial CO2 (PaCO2).

Methods

An arterial blood gas and end-tidal CO2 (EtCO2) were measured at baseline and hemodynamic monitoring and pulse oximetry were monitored throughout. Using the formula: PaCO2 of 10 mmHg = pH of 0.8, it was predicted what EtCO2 was required to achieve a PaCO2 sufficient to cause a pH 7.20. A gas mixture of 3% CO2:97% O2 was then administered through the ventilator adjusting an IMV rate of 2–4 according to the rise in EtCO2. Once the predicted EtCO2 was reached, an blood gas was repeated. The PaCO2-EtCO2 gradient was also calculated pre and post testing. Respiratory movements were monitored by both the respiratory flow loops and by direct visualization by a physician.

Results

Sixteen patients aged 49 ± 15 years were studied. There were no incidences of hemodynamic instability or arterial desaturation during the studies. At the end of the apnea test, the predicted and measured EtCO2 were 52 ± 9 and 56 ± 10 torr, respectively,and the predicted and measured PaCO2 were 60 ± 10 and 67 ± 10 torr, respectively. All patients achieved an adequate arterial pH and there was no change in the PaCO2-EtCO2 gradient during the testing (P = 0.195, Student's t-test). (Table; mean ± SD).

Table

Conclusions

Advantages of this technique over the previous method include: 1. allows for continuous measurement of EtCO2 during the apnea test (EtCO2 is predictive of rises in PaCO2); 2. eliminates the likelihood of desaturation episodes; 3. better monitoring for respiratory effort than provided by visual inspection alone.

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Sharpe, M., Young, G. & Harris, C. Apnea test for brain death determination: an alternative approach. Crit Care 6, P53 (2002). https://doi.org/10.1186/cc1754

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Keywords

  • Pulse Oximetry
  • Brain Death
  • Hemodynamic Instability
  • Respiratory Movement
  • Hemodynamic Monitoring