ADD-ONS
Apnea test for brain death diagnosis
Created 11/04/2023, last revision 19/09/2023
Why the apnea test is performed
- brain death (BD) is defined as the irreversible cessation of all brain functions (including those of the brainstem)
- the apnea test is a mandatory examination for determining brain death, serving as an essential indicator of definitive loss of brainstem function if positive
- used to assess the integrity of the brainstem respiratory centers and the absence of respiratory drive in response to increased PaCO2 levels
- the apnea test must be conducted with caution and only in appropriate clinical settings by qualified personnel due to the potential risks and complications associated with the procedure
Prerequisites
- body temperature ≥ 32°C (some authors recommend at least 36°C)
- SBP ≥ 90 mmHg
- preoxygenate the patient with 100% O2 for 10 minutes and adjust ventilation volume so that PaCO2 reaches 36-45 mmHg (5.3kPa) before test initiation
- required PaO2 levels are often not clearly defined, but hypoxia must be avoided
- oxygen saturation should not fall < 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 recommended
- test should not be performed when the subject is under the influence of drugs that could paralyze respiratory muscles (such as relaxants)
Procedure
Disconnection of the patient from the respirator
- disconnect ventilator and connect 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 blood, even in absence of respiratory movements
- monitor O2 saturation (should be maintained > 80-90%)
- repeat blood gas testing, aiming for a pCO2 > 60 mm Hg (7.9kPa)
- at this point, respiratory center should be stimulated and respiratory movements should be triggered
- absence of respiratory movements indicates brain death
Apnea test assessment
- absence of respiratory movements when hypercapnia is achieved (PaCO2 of 60 mmHg or increase ≥ 20 mmHg from baseline) correlates with brainstem death (apnea test is positive)
- respiratory movements or cough rule out brain death (test is negative); patient should be reconnected to ventilator
- if the test is stopped because of hemodynamic instability or other adverse events, further testing is required and the ventilator should be reconnected
Complications
- potential complications (risk increases with inadequate prerequisites) :
- severe hypotension (12-24%)
- barotrauma
- excessive hypercarbia and hypoxemia
- acidosis (68%)
- arrhythmias, incl. ventricular fibrillation or asystole (1%)
Alternative tests
- 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 preclude the use of the test
