Timing of Apnea Testing During Brain Death Determination in Adults
The apnea test should be performed with each neurological examination during brain death determination, requiring two separate examinations separated by an observation period—though the specific observation period guidelines provided focus on pediatric populations rather than adults.
When to Perform the Apnea Test
Core Principle
The apnea test must be conducted as an integral component of each complete neurological examination used to determine brain death 1. This means:
- Two apnea tests are required (one with each neurological examination)
- Each test must be performed by the examining physician
- The tests are separated by the observation period between examinations
Prerequisites Before Testing
Before initiating any apnea test, the following conditions must be met 1:
- Core temperature > 35°C documented
- Normalization of pH and PaCO2 via arterial blood gas analysis
- Blood pressure normalization (mean arterial pressure > 60 mmHg in adults) 2
- Correction of factors affecting respiratory effort
- Preoxygenation with 100% oxygen for 5-10 minutes
Clinical Context and Safety Considerations
Apnea testing can be safely performed in hemodynamically unstable patients requiring vasopressors 3. In a large series of 228 brain death determinations, all patients were on one or more vasopressors, and 61% received vasopressin for diabetes insipidus. The apnea test was completed safely in 93% of cases with no major complications (cardiac arrest or pneumothorax) 3.
When NOT to Perform the Apnea Test
Absolute Contraindications 1, 2
The apnea test should not be attempted when:
- High cervical spine injury (invalidates the test)
- Severe baseline hypoxemia requiring high oxygen requirements
- Extreme ventilator settings that cannot be safely discontinued
- Hemodynamic instability at baseline that precludes safe testing
Risk Factors for Test Failure
Be particularly cautious in patients with 4, 5:
- Pre-test systolic blood pressure < 105 mmHg
- Pre-test pH < 7.33 (acidemia independently predicts failure)
- Elevated A-a gradient > 556 mmHg (indicates significant lung injury)
- Younger age (associated with higher failure rates)
- Polytrauma (significantly more common in aborted tests)
In 10% of patients, the apnea test cannot be completed due to hypoxemia or hypotension developing during the procedure 3, 5. When this occurs, ancillary studies must be performed to support the diagnosis 1, 2.
Technical Requirements for a Valid Test
Target PaCO2 Levels
The apnea test is positive (supporting brain death) when 1:
- PaCO2 ≥ 60 mmHg AND
- ≥ 20 mmHg increase above baseline PaCO2
- No spontaneous respiratory effort observed
This threshold is critical because patients with residual brainstem function will demonstrate spontaneous ventilatory effort at PaCO2 levels below 40 mmHg 6. The 60 mmHg threshold provides adequate stimulus to trigger any remaining medullary respiratory centers.
Test Duration
- Typical duration: 15 minutes 1
- PaCO2 rises at approximately 2.6 mmHg/minute in brain-dead patients 6
- Maximum rate of PaCO2 increase usually occurs within 5 minutes 1
Alternative Approach: Modified Apnea Test
Recent evidence supports a modified apnea test (mAT) that maintains ventilator connection 7:
- Increase PEEP by 2 mbar
- Switch to continuous positive airway pressure mode
- Turn off apnea back-up mode
- This approach is safer in patients with pulmonary impairment and avoids rapid desaturation
- Successfully completed in 99% of 140 patients with no severe complications 7
When Ancillary Testing is Required
If the apnea test cannot be performed or completed safely, ancillary investigation becomes mandatory 2:
- Cerebral CT angiography (increasingly preferred)
- EEG showing electrocerebral silence
- Radionuclide cerebral blood flow studies
Ancillary studies should also be considered to promote family understanding when there is uncertainty or non-acceptance of the diagnosis 2.
Critical Pitfalls to Avoid
- Do not proceed with testing if prerequisites are not met - this invalidates the results
- Do not continue the test if hypotension (MAP < 60 mmHg) or severe hypoxemia develops - abort and use ancillary studies instead
- Do not interpret irregular respiratory-like movements as spontaneous breathing - spinal reflexes can produce non-ventilatory movements even at PaCO2 > 60 mmHg 1
- Do not perform testing within 24-48 hours of cardiopulmonary resuscitation or severe acute brain injury - neurologic function may be unreliable 1
The apnea test remains a cornerstone of brain death determination but requires meticulous attention to safety parameters and physiologic prerequisites to ensure both validity and patient safety during this critical diagnostic procedure.