Apnea test for brain death diagnosis

David Goldemund M.D.
Updated on 02/02/2024, published on 11/04/2023

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


  • 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)


Disconnection of the patient from the respirator

  • disconnect the ventilator and connect the 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 the 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, the 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); 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


  • 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

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Apnea test for brain death diagnosis