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Apnea test for brain death diagnosis

Created 11/04/2023, last revision 29/04/2023

Why the apnea test is performed

brain death (BD) is defined as the irreversible cessation of all brain function (including brainstem function)
the apnea test is a mandatory examination for determining brain death, as it is an essential sign of definitive loss of brainstem function if positive
- it is used to assess the integrity of the brainstem respiratory centers, and the absence of respiratory drive in response to increased PaCO₂
- the apnea test must be performed with caution and only in appropriate clinical settings by qualified personnel due to the potential risks and complications associated with the procedure

body temperature ≥ 32°C (some authors recommend at least 36°C)
SBP ≥ 90 mmHg
the patient is preoxygenated with 100% O₂ for 10 minutes and adjust ventilation volume so that PaCO₂ reaches 36-45 mmHg (5.3kPa) before starting the test
required PaO₂ levels are often not clearly defined, but hypoxia must be avoided
- oxygen saturation should not fall below 80-85%
- hypoxemia can cause cardiac arrhythmias or hypotension
blood pH should be normal or in the low basic range prior to testing
euvolemia or a positive fluid balance during the previous 6 h is also recommended
the test should not be performed when the subject is under the influence of drugs that may paralyze the respiratory muscles, i.e., relaxants

disconnect the ventilator and connect a T-tube with 100% O₂ for apneic oxygenation ( 4-10 liters of O₂ per minute), usually for 8 minutes
- this should provide adequate alveolar ventilation and oxygen transport to the blood, even in the absence of respiratory movements
monitor O₂ saturation (should be kept > 80-90%)
repeat blood gas testing, aiming for a pCO₂ > 60 mm Hg (7.9kPa)
- at this point, the respiratory center should be stimulated, and respiratory movements should be triggered
- the absence of respiratory movements indicates brain death

the absence of respiratory movements when hypercapnia is achieved (PaCO₂ of 60 mmHg or increase ≥ 20 mmHg from the baseline) correlates with brainstem death (apnea test is positive)
respiratory movements or cough rule out brain death (test is negative), and the patient should be reconnected to the ventilator
if the test is stopped because of hemodynamic instability or other adverse events, further testing is required, and the ventilator should be reconnected

if the apnea test cannot be performed or completed, alternative tests must be used (CTA, TCD/TCCD, EP, atropine test, etc.) → brain death diagnosis
reasons for test failure or inapplicability:
- significant hemodynamic instability
- poor baseline PaO₂ despite adequate preoxygenation
- inability to achieve target PaCO₂ levels
- extensive thoracic trauma or significant lung disorders interfering with ventilation
- other conditions that may prevent the use of the test