How do you tell a brain-dead patient from a living one?

Question

One theme that's been explored somewhat in popular culture is brain death, and the interesting dynamics of what separates dead from alive.

In chemistry, death is defined as the loss of the ability to control entropy within an organism. The parts that are supposed to be wet become dry, the parts that are supposed to be dry become wet, the parts that are supposed to be at a specific temperature... well, you get the idea.

In medicine, heart rate, respiration rate, blood pressure, EKG, and oxygen saturation are often used to detect death of the body, but none of these measures adequately address brain death in cases like cervical spinal cord injury. Rarely, there will be a patient with a fully functioning brain but loss of function otherwise, a patient who, while perhaps not destined for a particularly high quality of life, is otherwise a fine candidate for a heart or lung transplant, depending on the level and type of injury.

In such cases, where the only reliably functioning or even injured but recoverable organ is the brain, how are patients with otherwise weak or undetectable vital signs, kept on life support, distinguished from those who are well and truly dead?

P.S. For lack of better tags, I used the diagnostics and symptoms tags. I'm not quite sure that I've ever heard spoken the term "symptoms of death", but it does serve its purpose rather well.

P.S. #2 At the suggestion of Carey Gregory, a little clarification about my use of the term brain death:

I use the term brain death in the more general sense to mean death of the brain independent of the rest of the organism. The question being asked here is, if you had a body in front of you with, for example, a cervical spinal cord injury effectively severing the head from anything below it, but the body kept on ECMO to maintain oxygenation, how could you tell a living brain from a dead brain? How could you differentiate a patient who, given massive resources, has a chance, from one who has none? This especially applies in cases of coma or similar

Answer 1

Once the decision to proceed with the brain death determination has been made, three conditions must be present: coma, the absence of brainstem reflexes, and apnea.

Coma should be evaluated by ensuring a lack of responsiveness to noxious stimuli; no eye or motor reflex should be present in response to stimuli. Additionally, the cause of coma should be identified by neuroimaging, history, and physical examination or laboratory testing.

The following brainstem reflexes should be tested in the physical examination of a patient deemed for brain death evaluation. They all must be absent for a patient to be diagnosed as brain dead:

• The pupillary reflex to light–must be fixed at a mid-position; usually, around 4 mm and must not respond to light.
• Oculocephalic reflex – performed by rapidly turning the head.
• Oculovestibular reflex (cold caloric test)–absence when the head is held at 30 degrees and cold water instilled in the ear canal.
• Corneal reflex–stimulate with a swab.
• Gag reflex–stimulate the posterior pharynx.
• Cough reflex–stimulate with endotracheal suctioning.
• The facial movement to noxious stimuli–apply noxious pressure to the supraorbital ridge, there should be no facial muscle responses.

Apnea testing is performed by the following procedure as recommended by the AAN:

• Preoxygenate for at least 10 minutes with 100% fraction of inspired oxygen (FiO2) to a partial pressure of oxygen, arterial (PaO2) greater than 200 mmHg.
• Reduce ventilator frequency to 10 breaths per minute. Reduce positive end-expiratory pressure to 5 cm H2O.
• If the peripheral capillary oxygen saturation (SPO2) remains greater than 95%, obtain baseline blood gas.
• Disconnect the patient from the ventilator, preserve oxygenation with oxygen delivered through insufflation tubing given at 100% FiO2 at 6 L/min near the level of the carina through the endotracheal tube.
• Look for respiratory movements for 8-10 minutes. If no respiratory drive is observed, repeat blood gas at approximately 8 minutes.
• If no respiratory movements are observed, and partial pressure of carbon dioxide (PCO2) is greater than 60 mmHg, the apnea test result is positive.

If the above list is completed, and coma, the absence of brainstem reflexes and a positive apnea test are present, the diagnosis of brain death can be made. This procedure is validated and supported by several professional organizations.

^{Starr R, Tadi P, Pfleghaar N. Brain Death. [Updated 2020 Jul 8]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2020 Jan-.}

Sometimes, 2 independent physicians are required to make the diagnosis, sometimes 1 physician suffices.

There are a few other conditions that have to be met to make the diagnosis^ibid.

• body temperature > 36 Degree Celsius
• systolic pressure > 100 mmHg (if it is lower, they raise it with vasoconstrictors)
• negative drug screen
• 5 times the half-life of neuromuscular blocking agents must have passed if they were administered