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Publication Date

11-6-2017

Keywords

apnea, brain death, health practice, lungs, ventilation

Abstract

Background: Ventilation encompasses both active and passive processes. Air is initially drawn into the lungs due to a negative intrathoracic pressure created using the respiratory muscles, most importantly the diaphragm. In contrast, expiration is the passive relaxation of the respiratory muscles. Oxygenation occurs when oxygen diffuses across the alveolar-capillary membrane. The ability to oxygenate without ventilation has been termed apneic diffusion oxygenation or apneic oxygenation. We believe it is crucial to keep alveoli open in order for adequate oxygenation to occur. This can be achieved with the aid of positive end-expiratory pressure (PEEP). We investigated this concept in patients who are brain-dead because they cannot ventilate. The stimulus to breathe originates from chemoreceptors in the brainstem. These cells respond to a decrease in pH by triggering the body to take a breath. A positive apnea test confirms that the patient has no functioning brainstem.

Purpose: Determine the rate of pO2 and pCO2 changes during different methods of the apnea test and identify variations in practice within Aurora Health Care.

Methods: Data were collected retrospectively on brain-dead patients older than 18 years. Data points pulled from Epic medical records included serial arterial blood gases (ABGs) that were completed during the apnea test and patient demographics. The rate of change in pCO2 and pO2 was evaluated using both Mann-Whitney and two-sample t-tests comparing a PEEP valve group to all other oxygenation methods.

Results: Eight variations of the test were performed, with median starting CO2 for the oxygenation and PEEP group of 43 and 44 mmHg, respectively (95% confidence interval: 26–53, P = 0.6771). Oxygenation group had a mean CO2 increase of 2.95 mmHg/minute, whereas the PEEP valve group increased at 4.60 mmHg/minute. No statistical significance was found (P = 0.0508). Neither was there significant difference between the rate of desaturation between the oxygenation and PEEP valve group (6.53 mmHg vs 2.60 mmHg, respectively; P = 0.5536).

Conclusion: We found no difference in the rate of CO2 increase comparing the oxygenation group to the PEEP valve group. This suggests that there is no significant component of CO2 washout in the lungs using the PEEP valve setup. A superior method of apneic oxygen was not able to be demonstrated with our results due to an insufficient sample size and practice variations. The most common method to perform the apnea test at our institutions is preoxygenation.