The Apnea Test: Rationale, Confounders, and Criticism

Abstract

The apnea test is recommended for the diagnosis of brain death. There are several reasons this test should be reconsidered. Confounding factors for performing the test are vaguely and poorly specified. The following 2 confounders are usually present and not considered: potentially reversible high cervical spinal cord injury and central endocrine failure of adrenal and thyroid axes. There are case reports of breathing at a higher partial pressure of arterial carbon dioxide threshold and cases of recovery of breathing after brain death is diagnosed. The test is dangerous for an injured brain in the setting of high intracranial pressure. It can convert viable penumbral brain tissue to irreversibly nonfunctioning tissue via a transient increase in intracranial pressure and no-reflow phenomena. Hyperoxia during the apnea test can further suppress the function of medullary respiratory rhythm centers. Finally, the philosophical rationale for the need to show lack of spontaneous breathing in brain death is lacking.

Clinical and ethical perspectives on brain death

“…However, high cervical spinal cord injury and endocrine dysfunction can also invalidate the apnea test, and these confounds are not as often considered in the diagnosis, though they can be common in patients with suspected brain death. 34 Perhaps more concerning, hypercarbia (which is deliberately induced in the apnea test) is known to increase ICP and therefore can worsen ischemic injury to the brain, potentially contributing to herniation, and thus can be dangerous to a recently injured …”

“…[34][35][36] "Indeed", Joffe et al wrote, "it is reasonable to suggest that the apnea test itself can result in failing the apnea test, creating a self-fulfilling prophecy". 34 Finally, although eupneic inspiratory efforts are stimulated by hypercarbia in the setting of normal physiology, in the context of the specific pathophysiology of a cascade of rostral-caudal dysfunction in which the lower medulla will be the final anatomic area to be compromised, the appropriate stimulus would be hypoxia (not hypercarbia), because hypoxia stimulates the lower medulla-mediated gasping reflex. 34 Some concerns regarding the safety of the apnea test have been discussed in the literature.…”

“…34 Finally, although eupneic inspiratory efforts are stimulated by hypercarbia in the setting of normal physiology, in the context of the specific pathophysiology of a cascade of rostral-caudal dysfunction in which the lower medulla will be the final anatomic area to be compromised, the appropriate stimulus would be hypoxia (not hypercarbia), because hypoxia stimulates the lower medulla-mediated gasping reflex. 34 Some concerns regarding the safety of the apnea test have been discussed in the literature. [37][38][39][40][41] For example, in a review of a large series of brain death determinations at a single institution, Wijdicks et al wrote "The apnea test is a fixture in brain death determination, but there is a perceived concern about the safety of the procedure".…”

Ewolucja próby bezdechu w rozpoznawaniu śmierci mózgu

“…Additionally, these criteria listed the adverse changes in the patient's general condition that indicate the need to disrupt the test: cardiac arrhythmia, hypotension with SBP ≤ 90 mm Hg, and significant desaturation (SpO2 < 85%) [7]. The need to perform AT has been questioned (in paediatric literature), and some authors have proposed to limit the test to only one probe [8,9].…”

A Brain Death Dilemma: Apnea Testing While on High-Frequency Oscillatory Ventilation

“…The apnea test has specifically been criticized for its nonspecificity, variability in its application, and lack of solid philosophical rationale. 6 In fact, some clinicians have abandoned the current apnea test guidelines. The authors of a recent retrospective chart review reported that in Canadian PICUs 18% of patients who were declared "brain dead" did not undergo an apnea test.…”