Brain death is a clinical diagnosis supported by imaging but confirmed by the bedside exam to assess for the irreversible loss of cortical and brainstem functions. The apnea test (AT) is a common method used to evaluate the definitive absence of a brainstem-mediated respiratory drive.
To ensure patient safety and accurate results, I want several prerequisites met before beginning the AT, including a body temperature > 36°C (many protocols are okay with > 32°C), adequate denitrogenation with 100% oxygen, normal or elevated arterial CO2 levels, normal blood pH, SBP > 90 mmHg, euvolemia, and ensuring the absence or reversal of any neuromuscular junction blockers. At that point, I’ll draw a baseline ABG (ideally, PaO2 > 200 mmHg and PaCO2 > 35 mmHg), disconnect the ventilator, pass an oxygen delivery cannula into the endotracheal/tracheostomy tube, and check an ABG every ~5 minutes.
The apnea test is considered positive for brainstem death when no breathing effort is observed at a PaCO2 of 60 mmHg or a 20 mmHg increase from the baseline. Conversely, if respiratory movements are detected, the test is classified as negative, indicating brainstem activity.
While the apnea test is generally safe, potential complications must be carefully monitored, such as low blood pressure, oxygen deprivation, acidosis, and cardiac arrhythmias. If any adverse events occur, the test should be immediately terminated and mechanical ventilation resumed.
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While on VV ecmo, do you automatically defer from performing the apnea test and go straight to a nuke med scan?
One can turn down the sweep flow rate and minute ventilation to achieve a desired CO2 (e.g., 60 mmHg) and look for signs of spontaneous efforts. VA-ECMO is more challenging because hemodynamics can often be affected by adjusting things like the pump speed, so one option is to bleed in CO2 into the sweep gas to maintain flow and oxygenation while driving up CO2.