What is an apnea test Arterial Blood Gas (ABG)?

Apnea Test ABG: Critical Component in Brain Death Determination

An apnea test arterial blood gas (ABG) is a mandatory examination for brain death determination that documents the absence of respiratory drive despite an adequate PaCO2 stimulus, requiring a PaCO2 ≥60 mmHg and ≥20 mmHg above baseline with no respiratory effort observed. 1

Purpose and Clinical Significance

• The apnea test is essential for diagnosing brain death as it confirms definitive loss of brainstem function by demonstrating the absence of respiratory drive despite a strong physiological stimulus to breathe 2
• It is one of the core components of brain death determination, along with clinical examination confirming coma, absence of brainstem reflexes, and flaccid tone 1

Prerequisites Before Performing the Test

• Normalization of pH and PaCO2 measured by arterial blood gas analysis 1
• Core temperature maintained above 35°C 1
• Normalization of blood pressure appropriate for patient's age 1
• Correction of any factors that could affect respiratory effort 1
• Adequate preoxygenation with 100% oxygen for 5-10 minutes before initiating the test 1

Technique for Performing the Apnea Test

1. Preparation phase:

   • Obtain baseline ABG to document initial PaCO2 and pH 1
   • Preoxygenate with 100% oxygen for 5-10 minutes 1
   • Discontinue mechanical ventilation once the patient is well oxygenated and has a normal PaCO2 1

2. Oxygen delivery methods during test:

   • Change to a T-piece attached to the endotracheal tube (ETT) 1
   • Alternatively, use a self-inflating bag valve system (Mapleson circuit) connected to the ETT 1
   • Tracheal insufflation of oxygen via catheter inserted through ETT is another option, though caution is needed to prevent barotrauma 1

3. Monitoring during the test:

   • Continuously monitor heart rate, blood pressure, and oxygen saturation 1
   • Observe for any spontaneous respiratory effort 1
   • Obtain serial ABGs to monitor the rise in PaCO2 1

4. Interpretation of results:

   • The test is consistent with brain death if no respiratory effort is observed from the initiation of the test until PaCO2 ≥60 mmHg and ≥20 mmHg above baseline 1
   • Any evidence of respiratory effort is inconsistent with brain death, and the test should be terminated 1

Safety Considerations and Potential Complications

• Hypoxemia (oxygen saturation <85%) 1
• Hemodynamic instability or hypotension 3
• Cardiac arrhythmias 2
• Pneumothorax 2
• Excessive hypercarbia and acidosis 2

Special Circumstances

• If the apnea test cannot be completed safely due to hemodynamic instability, desaturation below 85%, or inability to reach target PaCO2, the patient should be placed back on ventilator support 1
• In such cases, another attempt may be performed later or ancillary studies (EEG, cerebral blood flow studies) should be pursued to assist with brain death determination 1
• For patients with chronic respiratory disease, the PaCO2 should still increase to ≥20 mmHg above their baseline level 1
• For patients on extracorporeal membrane oxygenation (ECMO), modifications to the technique are required, such as decreasing the sweep gas flow to allow CO2 accumulation 4

Common Pitfalls and Caveats

• The rate of PaCO2 increase is variable and unpredictable, ranging from 0.5 to 10.5 mmHg/min 5
• The rate of PaCO2 increase tends to decline throughout the duration of the test, which can lead to unnecessarily prolonged testing 5
• High gas flow rates with tracheal insufflation may promote CO2 washout, preventing adequate PaCO2 rise 1
• Continuous positive airway pressure (CPAP) ventilation during testing may falsely report spontaneous ventilation 1
• For overweight patients or those with hypoxic brain injury due to hanging, maintaining positive end-expiratory pressure (PEEP) during the test may prevent dramatic reductions in PaO2 6

Documentation Requirements

• Complete documentation of the apnea test is required, including baseline ABG values, duration of the test, serial ABG measurements, and absence of respiratory effort 1
• The test must be performed twice as part of the complete brain death determination protocol, with each examination separated by an observation period 1