Brain death: a clinical overview

Overview

This review by Spears, Mian and Greer, published open access in the Journal of Intensive Care in 2022, describes the philosophy, history and clinical determination of brain death, also called death by neurologic criteria (BD/DNC). It covers the bedside examination, apnea testing and ancillary tests, and gives special attention to ECMO, targeted temperature management and children.

Concepts

The idea of brain death arose when resuscitation and ventilation made it possible to support the body without brain function: the 'coma dépassé' of 1959, the 1968 Harvard criteria and the 1980 Uniform Determination of Death Act. The most accepted formulation is whole-brain death, used by the US and most countries with national protocols and by the World Brain Death Project. The UK uses a brainstem formulation, and the 'higher brain' concept is not accepted because such patients can still breathe.

Prerequisites and clinical examination

A clear, irreversible cause of catastrophic brain injury must be established with consistent imaging, and confounders must be excluded: drugs (wait at least five half-lives, longer with liver or kidney injury), metabolic disturbance, hypothermia and hypotension. A conservative adult requirement is systolic pressure above 100 mmHg and temperature above 36 °C. After cardiac arrest, with or without temperature management, a wait of at least 24 hours is advised. The examination confirms coma, loss of the pupillary, corneal, oculocephalic, oculovestibular, gag and cough reflexes, and absence of motor responses other than spinal reflexes. If a test cannot be done, for example because of severe facial trauma, ancillary testing is needed.

Apnea testing

After pre-oxygenation the patient is disconnected from the ventilator with oxygen delivered at the carina, and an arterial blood gas is taken after 8 to 10 minutes. The test is positive if PaCO2 reaches at least 60 mmHg (and 20 mmHg above baseline in chronic CO2 retainers). Hypotension, hypoxaemia or arrhythmia lead to abortion in 1.6 to 4.8% of cases, and the test is usually done last. One to three examinations are required depending on the country.

Ancillary tests

Ancillary tests are used when the clinical examination cannot be completed or confounders cannot be excluded. Digital subtraction angiography is the gold standard and SPECT is a close alternative, while transcranial Doppler is easy at the bedside but depends on bone windows. CT angiography and MR angiography are not recommended because of problems such as stasis filling, and EEG is not recommended in adults unless local rules require it.

Special situations

On ECMO the sweep gas flow is turned down to 0 to 1 L/min for apnea testing, and gases from the distal artery and the oxygenator are taken together (pH below 7.3 and PaCO2 at least 60 mmHg). After therapeutic hypothermia, testing should start at least 24 hours after rewarming. In children, two examinations are usually recommended, the minimum age cited is 36 to 37 weeks of gestation, and SPECT is the preferred ancillary test. The authors call for global minimum standards and better training.