Introduction
The Pitt — Episode 3, apnea test scene:
"For the apnea test, we keep him on 100% oxygen, but the ventilator won't be giving him breaths." — Dr. Robby
"For how long?" — Family member
"Ten minutes. We monitor his carbon dioxide levels, and if they come back way too high — above 60 — it means his brain stem is not telling him to breathe." — Dr. Robby
This scene is one of the heaviest moments of Episode 3. Dr. Robby explains to Nick's family — an 18-year-old in fentanyl overdose — what will be done and what the result may mean. Nick's CO2 reached 82 mmHg without any spontaneous breathing. The diagnosis was confirmed.
The apnea test is a rigorous, standardized, and legally supported clinical procedure that forms part of the brain death diagnostic protocol. Understanding it — as a physician, patient, or family member — is essential for comprehending one of the most difficult decisions a medical team and a family can face together.
What is the Apnea Test?
The apnea test is a diagnostic procedure that evaluates the absence of spontaneous respiratory activity — the respiratory drive — mediated by the brainstem. It is one of the mandatory criteria for diagnosing brain death in both Brazil and the United States.
The physiological principle is simple and precise: the brainstem contains respiratory centers that respond to rising blood CO2. When CO2 rises above a critical threshold — generally 60 mmHg — the healthy brainstem fires the impulse to breathe. If this impulse does not occur even with very elevated CO2, it means the brainstem is not functioning.
The test is performed by temporarily disconnecting the mechanical ventilator while maintaining oxygen delivery by other means, allowing CO2 to rise naturally. The patient is observed for 8 to 10 minutes for any spontaneous respiratory movement — chest expansion, abdominal movement, or any attempt to breathe.
The complete absence of respiratory movements with CO2 above 60 mmHg — as with Nick, who reached 82 mmHg — is a positive result for brain death.
Causes and Clinical Context
The apnea test is indicated within the brain death diagnostic protocol, which is initiated when:
- The patient is in deep coma of known and irreversible cause
- Treatable causes of coma have been excluded — hypothermia, severe metabolic disturbances, active sedatives or neuromuscular blockers
- The neurological examination demonstrates absence of all brainstem reflexes: pupillary, oculocephalic, oculovestibular, corneal, cough, and gag
The most common causes leading to the brain death protocol include:
- Severe cerebral anoxia: as in Nick's case — cardiac arrest from fentanyl overdose with prolonged cerebral ischemia time.
- Severe traumatic brain injury with transtentorial cerebral herniation.
- Massive hemorrhagic stroke.
- Ruptured cerebral aneurysm with devastating subarachnoid hemorrhage.
In the episode, Dr. Robby was explicit about the expectation: CO2 of 82 mmHg without any spontaneous breathing was practically diagnostic even before the results of the confirmatory cerebral perfusion scintigraphy.
Signs and Symptoms
The patient who is a candidate for the apnea test presents a very specific clinical picture, already well-established before the test:
- Deep coma without response to stimuli — Glasgow 3
- Absent bilateral pupillary reflex — fixed and dilated pupils
- Absent oculocephalic reflex (doll's eye)
- Absent oculovestibular reflex (caloric testing)
- Absent corneal reflex
- Absent cough and gag reflexes
- Complete dependence on mechanical ventilator for breathing
All these criteria must be confirmed by two different physicians — one of them mandatorily a neurologist or neurosurgeon — in two separate evaluations separated by a time interval defined by institutional protocol and current legislation.
Diagnosis
The apnea test is part of a larger diagnostic protocol. The steps that precede it include:
Mandatory prerequisites: body temperature above 35°C, systolic pressure above 100 mmHg, absence of active sedatives or neuromuscular blockers, correction of severe metabolic disturbances.
Complete neurological exam: confirmation of absence of all brainstem reflexes listed above, by two qualified physicians.
Confirmatory complementary tests: in the U.S., electroencephalography (EEG), transcranial Doppler, or cerebral perfusion scintigraphy may be used. The scintigraphy, as performed for Nick, shows absence of radiopharmaceutical uptake by the cerebral parenchyma — confirming absence of cerebral blood flow.
The apnea test is the final step in this protocol. Its result is binary: the patient breathes spontaneously (negative result for brain death) or does not breathe (positive result for brain death).
The Procedure Step by Step
The apnea test follows a rigorous protocol to ensure safety and diagnostic validity:
- Confirm all prerequisites: temperature, blood pressure, absence of active sedation, and metabolic balance.
- Pre-oxygenate the patient with 100% FiO2 for 10 minutes on the mechanical ventilator to raise oxygen reserve and prevent desaturation during the test.
- Collect a baseline arterial blood gas to confirm initial PaCO2 and pH.
- Disconnect the ventilator — or configure it to deliver no active ventilation — while maintaining O2 supply via tracheal catheter at 6 L/min.
- Continuously observe the patient for 8 to 10 minutes for any spontaneous respiratory movement.
- Continuously monitor: SpO2, blood pressure, heart rate, and cardiac rhythm.
- Collect a new arterial blood gas at the end of the period to confirm the CO2 level reached.
- Immediately reconnect the ventilator after sample collection.
- Interpret the result: absence of respiratory movements with PaCO2 equal to or above 60 mmHg (or a rise of 20 mmHg above baseline) is a positive result for absent brainstem respiratory drive.
The test must be immediately stopped if: severe desaturation occurs (SpO2 below 85%), refractory hypotension develops, serious arrhythmia appears, or any significant hemodynamic instability occurs.
Prognosis and Complications
The apnea test itself has no prognosis — it is a diagnostic examination. Its positive result confirms brain death, which is irreversible and legally equivalent to the death of the individual.
Complications of the test include:
- Hypoxemia and desaturation: the most frequent. Prevented by adequate preoxygenation and continuous monitoring.
- Hypotension: from progressive hypercapnia and absence of positive-pressure ventilation.
- Cardiac arrhythmias: especially bradycardia and asystole from extreme hypercapnia.
- Pneumothorax: rarely, from pressure changes during ventilator disconnection.
When the test needs to be interrupted before completion, complementary tests — such as cerebral perfusion scintigraphy or transcranial Doppler — can be used as substitute confirmatory criteria.
Frequently Asked Questions
Is the apnea test the only criterion for confirming brain death?
No. In the United States, the determination of brain death requires: a known irreversible cause of coma, exclusion of confounding conditions, complete neurological examination demonstrating absence of all brainstem reflexes by qualified physicians, and the apnea test. Confirmatory tests such as EEG or cerebral blood flow studies may supplement but are not always required. All criteria must be concordant for the diagnosis to be established.
What happens to the patient after brain death is diagnosed?
After brain death confirmation, ventilatory support may be temporarily maintained — as with Nick — to preserve organs while the family is counseled and the transplant team evaluates donation viability. Withdrawal of support, as shown in the episode, is a medical and family decision carried out with respect and adequate palliative care.
Can the family refuse the apnea test?
The apnea test is part of a medical diagnostic protocol — not a therapeutic intervention that requires family consent to be performed. However, communication with the family must be clear, respectful, and continuous throughout the process. The decision about continuing or withdrawing support after diagnosis may involve the family depending on the cultural, legal, and ethical context of each case.
Why must CO2 reach 60 mmHg?
The 60 mmHg threshold was established because it represents the maximum stimulus for the brainstem respiratory centers. In healthy individuals, CO2 between 50 and 60 mmHg would already cause an intense sensation of suffocation and vigorous respiratory effort. If the brainstem does not respond to this extreme level of hypercapnia, it simply is not functioning.
Conclusion
The apnea test is one of the simplest procedures technically, but one of the most complex humanly. As portrayed with sensitivity in Episode 3 of The Pitt, it transforms a number — 82 mmHg of CO2 — into an irreversible certainty, and demands from the medical team the ability to communicate clearly and compassionately what that number means.
Explore more in our Medical Procedures category. Also read about anoxic brain injury, the mechanical ventilator, brain death in the ER, and fentanyl overdose cardiac arrest.
Disclaimer: This content is for educational purposes only and does not substitute professional medical evaluation, diagnosis, or treatment. In case of emergency, call 911 immediately.
