Absence of a Respiratory Drive.
Absence of a breathing drive is tested with a CO2 challenge. Documentation of an increase in PaCO2 above normal levels is typical practice. It requires preparation before the test.
eucapnia (PaCO2 35–45 mm Hg)
absence of hypoxia
no prior evidence of CO2 retention (i.e., chronic obstructive pulmonary disease, severe obesity).
Adjust vasopressors to a systolic blood pressure _100 mm Hg.
Preoxygenate for at least 10 minutes with 100% oxygen to a PaO2 _200 mm Hg.
Reduce ventilation frequency to 10 breaths per minute to eucapnia.
Reduce positive end-expiratory pressure (PEEP) to 5 cm H2O (oxygen desaturation with decreasing PEEP may suggest difficulty with apnea testing).
If pulse oximetry oxygen saturation remains_95%, obtain a baseline blood gas (PaO2, PaCO2, pH, bicarbonate, base excess).
Disconnect the patient from the ventilator.
Preserve oxygenation (e.g., place an insufflations catheter through the endotracheal tube and close to the level of the carina and deliver 100% O2 at 6 L/min).
Look closely for respiratory movements for 8–10 minutes. Respiration is defined as abdominal or chest excursions and may include a brief gasp.
Abort if systolic blood pressure decreases to _90 mm Hg.
Abort if oxygen saturation measured by pulse oximetry is _85% for _30 seconds. Retry procedure with T-piece, CPAP 10 cm H2O, and 100% O2 12 L/min.
If no respiratory drive is observed, repeat blood gas (PaO2, PaCO2, pH, bicarbonate, base excess) after approximately 8 minutes.
If respiratory movements are absent and arterial PCO2 is _60 mm Hg (or 20 mm Hg increase in arterial PCO2 over a baseline normal arterial PCO2), the apnea test result is positive(i.e., spports the clinical diagnosis of brain death).
If the test is inconclusive but the patient is hemodynamically stable during the procedure, it may be repeated for a longer period of time (10–15 minutes) after the patient is again adequately pre-oxygenated.