ADD-ONS

Apnea test for brain death diagnosis

David Goldemund M.D.
Updated on 28/06/2024, published on 11/04/2023

Why the apnea test is performed

brain death (BD) is defined as the permanent cessation of all brain functions (including those of the brainstem)
the apnea test is a mandatory examination for determining brain death (BD)/death by neurologic criteria (DNC); it is a crucial indicator of the loss of brainstem function if the test is positive
- it assesses the integrity of the brainstem respiratory centers, detecting the presence/absence of respiratory drive in response to a hypercarbic and acidotic challenge
- 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 ≥ 36°C
SBP ≥ 90 mmHg
preoxygenate the patient with 100% O₂ for 10 minutes and adjust ventilation volume so that PaCO₂ reaches 35-45 mmHg (5.3kPa) and pH 7.35–7.45 before test initiation
- if a patient is known to have chronic hypercarbia, and the patient’s chronic baseline level is known, the PaCO₂ level before apnea testing should be at the patient’s chronic baseline level (AAN guidelines, 2023)
- if a patient is known/suspected to have chronic hypercarbia but the chronic baseline PaCO₂ level is unknown, the PaCO2 level before apnea testing should be at the patient’s estimated chronic baseline. However, in this circumstance, if apnea is present, clinicians should perform ancillary testing in addition
required PaO₂ levels are often not clearly defined, but hypoxia must be avoided
- AAN guidelines suggest a PaO2 level >200 mmHg
- oxygen saturation should not fall < 80-85%; hypoxemia can cause cardiac arrhythmias or hypotension
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)

disconnect the ventilator and connect the T-tube with 100% O₂ for apneic oxygenation ( 4-10 L/min), usually for 8-10 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%)
an ABG measurement should be performed after 8–10 minutes of apnea
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 BD/DNC

apnea test consistent with BD/DNC criteria: absence of respiratory movements when target hypercapnia and pH are achieved (see the table)
respiratory movements or cough are inconsistent with BD/DNC criteria (test is negative); the test should be aborted 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
- clinicians should repeat the apnea test when it can be safely completed or perform an ancillary test (AAN guidelines, 2023)
if the PaCO₂ and pH level criteria are not reached, and the patient did not experience hemodynamic instability/hypoxemia, clinicians should:
- continue the test beyond 10 minutes with ABG measurements checked at least every 2 minutes OR
- repeat apnea testing for a longer period after again preoxygenating to a PaO2 level >200 mm Hg and reestablishing normal PaCO₂ and pH levels

Patients who are known NOT TO HAVE chronic CO2 retention	no respiration arterial pH level is <7.30 PaCO2 ≥ 60 mm Hg and ≥ 20 mm Hg above the patient’s preapnea test baseline level
Patients who are KNOWN TO HAVE chronic CO2 retention, and the baseline PaCO2 level is KNOWN	no respiration arterial pH level is <7.30 PaCO2 level is ≥ 60 and ≥ 20 mm Hg above the patient’s known chronic elevated premorbid baseline level
Patients who are SUSPECTED TO HAVE chronic CO2 retention, but the baseline PaCO2 is UNKNOWN	no respirations occur arterial pH level <7.30 PaCO2 level is ≥ 60 and ≥ 20 mm Hg above the patient’s baseline (pretest) level, AND ancillary testing must be performed

if the patient experiences hemodynamic instability or hypoxemia at any point during the test
- SBP <100 mm Hg or MAP <75 mm Hg in adults despite titration of vasopressors, inotropes, and/or IV fluids
- progressive decrease in oxygen saturation < 85%
- a cardiac arrhythmia with hemodynamic instability
if cough or respiratory movements occur

if the apnea test cannot be performed or completed, ancillary tests must be used (DSA, TCD/TCCD) → 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