On which ventilator mode should a patient be kept for an apnea test?

Ventilator Mode for Apnea Testing

For apnea testing, the patient should be disconnected from mechanical ventilation and placed on a T-piece or self-inflating bag valve system connected to the endotracheal tube after proper preoxygenation. 1

Proper Technique for Apnea Testing

Prerequisites

• Normalize pH and PaCO2 via arterial blood gas analysis
• Maintain core temperature >35°C
• Normalize blood pressure appropriate for patient's age
• Correct any factors that could affect respiratory effort
• Preoxygenate with 100% oxygen for 5-10 minutes

Procedure

1. Discontinue mechanical ventilation after ensuring the patient is well oxygenated with normal PaCO2
2. Change to one of these options:
   • T-piece attached to the endotracheal tube (preferred)
   • Self-inflating bag valve system (e.g., Mapleson circuit) connected to the ETT
   • Tracheal insufflation of oxygen (use with caution due to risk of barotrauma and CO2 washout)

Monitoring During Test

• Continuously monitor heart rate, blood pressure, and oxygen saturation
• Observe for any spontaneous respiratory effort
• Obtain serial arterial blood gases to monitor PaCO2 rise
• Continue until PaCO2 ≥60 mmHg AND ≥20 mmHg above baseline

Why CPAP Is Not Recommended

While some facilities use CPAP for apnea testing, this approach has significant limitations:

• Many current ventilators automatically switch from CPAP mode to mandatory ventilation when apnea is detected 1
• Spontaneous ventilation has been falsely reported during CPAP despite reducing trigger sensitivity to minimum levels 1
• CPAP may interfere with accurate assessment of spontaneous respiratory effort

Alternative Approaches for High-Risk Patients

For patients with poor baseline oxygenation or hemodynamic instability where disconnection from the ventilator poses significant risks:

1. Modified apnea test may be considered where the patient remains connected to the ventilator 2:

   • Increase PEEP by 2 mbar
   • Switch to CPAP mode
   • Turn off apnea back-up mode
   • Set flow trigger high (e.g., 10 L/min)

2. If apnea testing cannot be safely completed, ancillary studies should be performed to assist with brain death determination 1, 3

Common Pitfalls to Avoid

• Inadequate preoxygenation: Ensure full 5-10 minutes of 100% oxygen before testing
• Premature termination: The maximum rate of PaCO2 increase usually occurs within 5 minutes, but test should continue until target PaCO2 is reached 1
• High gas flow rates with tracheal insufflation: May promote CO2 washout preventing adequate PaCO2 rise 1
• Failure to recognize hemodynamic instability: If oxygen saturations fall below 85% or hemodynamic instability occurs, immediately terminate the test and resume ventilatory support 1

Remember that the apnea test is considered positive (consistent with brain death) only when there is complete absence of respiratory effort while PaCO2 rises to ≥60 mmHg and ≥20 mmHg above baseline.