17.2 Apnea Testing and Ancillary Studies (02.I)

Apnea testing is a procedure, not a blood gas

Quick Answer: After a valid brainstem examination, preoxygenate, document a near-normal baseline arterial blood gas, then observe off effective ventilation. The 2023 AAN/AAP/CNS/SCCM apnea target is no respirations, arterial pH <7.30, and PaCO2 ≥60 mm Hg and ≥20 mm Hg above the pre-test baseline. That is and, not the 2010 adult or. Abort for any breath, hypoxemia, or hypotension. Ancillary cerebral blood-flow tests complete an examination that cannot be finished. They do not replace missing confounder exclusion. EEG electrocerebral silence is not an accepted 2023 ancillary test.

Independent OpenExamPrep teaching here covers apnea testing and ancillary studies from the same BD/DNC topic in the ABPN Content Specifications. It is not an AAN or SCCM procedure manual and not a substitute for your hospital’s apnea checklist.

Apnea testing asks whether the medulla still generates a breath when PaCO2 is a strong stimulus and pH is acid. It is the last clinical pillar after coma and brainstem areflexia. It is also the step that destablizes donors and non-donors alike if you skip preoxygenation or ignore a crashing blood pressure.

Prerequisites unique to the apnea test

Do not disconnect the ventilator until:

• The neurologic examination (this round) is already consistent with BD/DNC.
• There is no hypoxemia, hypotension, or hypovolemia going into the test.
• Core temperature and neuromuscular blockade gates from the prior section still hold.
• A baseline arterial blood gas (ABG) shows pH about 7.35–7.45 and PaCO2 about 35–45 mm Hg, or, in a known chronic retainer, PaCO2 at that patient’s baseline.
• After 100% oxygen for at least about 10 minutes, PaO2 is typically >200 mm Hg (teaching summaries of 2023 use that preoxygenation target).

Place an arterial line if you do not already have one; you will draw gases during a test you may need to abort in seconds. Tell the family what will happen. Have vasopressors mixed. Suction the airway before you start so a mucus plug is not your “first breath.”

How the test is done

Preoxygenate on FiO2 1.0. Normalize the minute ventilation so the baseline PaCO2 is in the normal band (or the chronic baseline). Disconnect from the ventilator and supply oxygen by a catheter in the endotracheal tube at about 4–6 L/min, or use an apneic oxygenation method that preserves functional residual capacity (for example CPAP through a flow-inflating bag) if prior attempts desaturated. Watch the chest and the end-tidal CO2 tracing for any respiratory effort. Draw ABGs at intervals (often around 8–10 minutes in a stable adult, earlier if the patient is labile). Children may reach the PaCO2 target faster because of a smaller CO2 store — watch the patient, not a stopwatch copied from an adult protocol.

Positive test (consistent with BD/DNC) in a patient without chronic hypercarbia, or whose chronic baseline is unknown (see extra rule below):

1. No respirations, and
2. pH <7.30, and
3. PaCO2 ≥60 mm Hg and ≥20 mm Hg above the pre-apnea-test baseline.

The 2010 adult AAN update used PaCO2 ≥60 mm Hg or ≥20 mm Hg above baseline. That or is a 2023 trap. A PaCO2 of 61 with a rise of only 8 mm Hg from a baseline of 53 does not meet 2023 criteria even if pH drifted to 7.29. A rise of 22 mm Hg to a PaCO2 of 52 also fails because the 60 mm Hg floor was missed.

Chronic carbon dioxide retention

COPD, obesity hypoventilation, and chronic neuromuscular hypoventilation are the usual retainers. Guessing a baseline of 45 in a lifelong retainer is how you falsely call apnea.

When to abort — and what to do next

Stop immediately and draw an ABG if:

• The patient takes one or more spontaneous respirations — the patient does not meet BD/DNC; do not “finish the numbers anyway.”
• Hypoxemia develops (teaching summaries abort around saturation <85%).
• Hemodynamic instability develops (teaching summaries abort around SBP <100 mm Hg or MAP <75 mm Hg, or new unstable arrhythmia).

If the abort happened before pH and PaCO2 criteria were reached:

• After hypoxemia: repeat later with a method that holds FRC (CPAP / flow-inflating bag), repeat when the lungs are healthier, or perform an ancillary test.
• After hypotension: augment blood pressure and repeat when stable, or perform an ancillary test.

An aborted test is not a failed brainstem. It is an incomplete procedure. On venoarterial ECMO, 2023 requires pH and PaCO2 criteria from both a right radial (or other pre-oxygenator arterial) sample and a post-oxygenator sample, which is technically demanding because sweep gas removes CO2. If those paired gases cannot be achieved safely, that is an ancillary-testing problem, not a reason to ignore the circuit.

Pediatric apnea nuance

Each of the two pediatric examinations includes its own apnea test. You do not perform one apnea test and recycle it 12 hours later. The same abort rules apply. Ancillary testing does not shorten the 12-hour gap between the two examinations.

Ancillary studies: only to finish what the bedside cannot finish

The 2023 rule is narrow. Clinicians should perform ancillary testing only when the BD/DNC neurologic examination or the apnea test cannot be completed or the findings cannot be interpreted adequately. Typical legitimate indications:

• Facial trauma, packed canals, or cervical instability that blocks both oculocephalic and caloric testing.
• Unstable gas exchange or shock that makes apnea testing unsafe even after a careful retry.
• Metabolic derangements that cannot be adequately corrected, even though the examination and apnea test otherwise look consistent with BD/DNC.
• Inability to interpret a component (for example severe orbital trauma that destroys the pupils and corneas).

Ancillary testing is not indicated solely because of an open fontanelle, skull fracture, craniectomy, or CSF diversion device. It is not a way to skip lingering sedation, residual neuromuscular blockade, or the 24-hour post-rewarming wait. It is not a way to shorten the pediatric 12-hour interval. It is not a substitute for an incomplete reversible-confounder workup. If the pentobarbital level is still therapeutic, fix the pharmacology; do not order a scan to launder the examination.

All elements of the examination that can be assessed must still be assessed, and those findings must be consistent with BD/DNC. A present corneal reflex plus a “pretty” angiogram is not death.

Which tests 2023 accepts — and which it dropped

Accepted ancillary tests are tests of cerebral perfusion:

Not accepted as ancillary tests in the 2023 document:

• EEG electrocerebral silence — historically used in 2010/2011 pathways; 2023 does not use EEG, auditory evoked potentials, or somatosensory evoked potentials to complete BD/DNC because they do not establish absent brainstem function or absent cerebral blood flow.
• CT angiography — clinicians should not use CTA as an ancillary BD/DNC test.
• MR angiography is not a 2023 ancillary standard for this purpose.

If an older stem still offers “electrocerebral silence on EEG” as the way to finish an incomplete examination, the 2023 answer is a blood-flow study, not a 30-minute EEG.

Documentation and the legal time of death

Write the prerequisites, the temperature and blood-pressure values, the TOF, each reflex, the apnea method, the baseline and final ABGs, abort events, and any ancillary result. The time of death is the time the last required criterion is fulfilled — typically the clock time of the ABG that meets 2023 apnea targets, or the time an accepted ancillary test is completed when apnea could not be finished — not the time the ventilator is later removed. In children, death is declared only after the second complete examination and apnea test (and ancillary test, if one was required) are done. Notify the organ-procurement organization according to hospital policy; donor management is a later chapter. Families may observe the examination; observation is communication, not a second medical vote.

Worked apnea numbers

Baseline PaCO2 40 mm Hg, pH 7.40. At 10 minutes there are no breaths, pH 7.18, PaCO2 64 mm Hg. That meets 2023 (no breaths, pH <7.30, PaCO2 ≥60 and 24 above baseline).

Baseline PaCO2 58 mm Hg in a known retainer whose chronic PaCO2 is 58. At 10 minutes, no breaths, pH 7.28, PaCO2 72 mm Hg. Rise is only 14 mm Hg — does not meet the ≥20 rise above the chronic baseline even though PaCO2 is ≥60 and pH is <7.30. Continue the test if safe, or abort and use an ancillary study if the patient is unstable.

Suspected COPD, baseline unknown. No breaths, pH 7.22, PaCO2 70 mm Hg from a pre-test PaCO2 of 42. Gases meet the numeric target, but 2023 still requires an ancillary test because the chronic baseline was unknown.

Exam traps

Using the 2010 or rule (PaCO2 60 or a rise of 20). Declaring death after an aborted hypoxic apnea test without a repeat or an ancillary study. Ordering EEG silence to finish BD/DNC in 2026. Using TCD as the pediatric ancillary test. Ordering CTA “because angiography is angiography.” Using a nuclear study to skip pentobarbital or the rewarming clock. Forgetting that unknown-baseline chronic retainers need both qualifying gases and an ancillary study.