Ventilator-Delivered Continuous Positive Airway Pressure for Apnea Test in the Diagnosis of Brain Death in Patient With Extremely Poor Baseline Lung Function—Case Report

Prospective Evaluation of a Modified Apnea Test in Brain Death Candidates that Does Not Require Disconnection from the Ventilator

BACKGROUND

The apnea test (AT) is an important component in the determination of brain death/death by neurologic criteria (BD/DNC) and often entails disconnecting the patient from the ventilator followed by tracheal oxygen insufflation to ensure adequate oxygenation. To rate the test as positive, most international guidelines state that a lack of spontaneous breathing must be demonstrated when the arterial partial pressure of carbon dioxide (PaCO2) ≥ 60 mm Hg. However, the loss of positive end-expiratory pressure that is associated with disconnection from the ventilator may cause rapid desaturation. This, in turn, can lead to cardiopulmonary instability (especially in patients with pulmonary impairment and diseases such as acute respiratory distress syndrome), putting patients at increased risk. Therefore, this prospective study aimed to investigate whether a modified version of the AT (mAT), in which the patient remains connected to the ventilator, is a safer yet still valid alternative.

METHODS

The mAT was performed in all 140 BD/DNC candidates registered between January 2019 and December 2022: after 10 min of preoxygenation, (1) positive end-expiratory pressure was increased by 2 mbar (1.5 mm Hg), (2) ventilation mode was switched to continuous positive airway pressure, and (3) apnea back-up mode was turned off (flow trigger 10 L/min). The mAT was considered positive when spontaneous breathing did not occur upon PaCO2 increase to ≥ 60 mm Hg (baseline 35-45 mm Hg). Clinical complications during/after mAT were documented.

RESULTS

The mAT was possible in 139/140 patients and had a median duration of 15 min (interquartile range 13-19 min). Severe complications were not evident. In 51 patients, the post-mAT arterial partial pressure of oxygen (PaO2) was lower than the pre-mAT PaO2, whereas it was the same or higher in 88 cases. In patients with pulmonary impairment, apneic oxygenation during the mAT improved PaO2. In 123 cases, there was a transient drop in blood pressure at the end of or after the mAT, whereas in 12 cases, the mean arterial pressure dropped below 60 mm Hg.

CONCLUSIONS

The mAT is a safe and protective means of identifying patients who no longer have an intact central respiratory drive, which is a critical factor in the diagnosis of BD/DNC. Clinical trial registration DRKS, DRKS00017803, retrospectively registered 23.11.2020, https://drks.de/search/de/trial/DRKS00017803.

Proposed safe apnea test using positive end-expiratory pressure valve and short-term blood gas analysis: Observational study

The apnea test is an essential examination for the determination of brain death; however, hypotension, hypoxemia, and other complications during the apnea test can affect the stability of brain-dead patients, as well as organ function for recipients. Therefore, it is necessary to establish standard guidelines for apnea testing.The modified apnea test (MAT) comprises delivery of 100% oxygen through the endotracheal tube connected to manual resuscitator (Ambu bag) with the positive end-expiratory pressure (PEEP) valve after disconnection of the mechanical ventilator for maintenance of PEEP. Forty-nine instances of the conventional apnea test (CAT) were performed in 25 brain-dead patients; 77 instances of the MAT were performed in 39 brain-dead patients.The mean duration of the apnea test was 3.5 ± 1.4 minutes in the CAT group and 3.0 ± 1.2 minutes in the MAT group. There were no significant changes in PaCO2, PaO2, or pH between the CAT and MAT groups (P = .341, .593, and .503, respectively). In overweight patients (body mass index ≥ 23 kg/m), MAT prevented dramatic reductions in PaO2 and SaO2 (P < .05 for both). In the patients who had hypoxic brain injury due to hanging, differences in PaO2 and SaO2 in the MAT group were significantly smaller than in the CAT group (P < .05).Although MAT, which was invented to maintain PEEP, was not efficient for all brain-dead patients, it could be helpful in selected patient groups, such as overweight patients or those who had hypoxic injury due to hanging. And clinicians should consider short-term apnea test to avoid unnecessarily prolonged hypoxemia.

Prevention of Hypoxemia During Apnea Testing: A Comparison of Oxygen Insufflation And Continuous Positive Airway Pressure

BACKGROUND

Apnea testing is an essential step in the clinical diagnosis of brain death. Current international guidelines recommend placement of an oxygen (O₂) insufflation catheter into the endotracheal tube to prevent hypoxemia, but use of a continuous positive airway pressure (CPAP) valve may be more effective at limiting arterial partial pressure of O₂ (PO₂) reduction.

METHODS

We performed a multicenter study assessing consecutive apnea tests in 14 intensive care units (ICUs) in two cities utilizing differing protocols. In one city, O₂ catheters are placed and arterial blood gases (ABGs) performed at intervals determined by the attending physician. In the other city, a resuscitation bag with CPAP valve is attached to the endotracheal tube, and ABGs performed every 3-5 min. We assessed arterial PO₂, partial pressure of carbon dioxide (PCO₂), pH, and blood pressure at the beginning and termination of each apnea test.

RESULTS

Thirty-six apnea tests were performed using an O₂ catheter and 50 with a CPAP valve. One test per group was aborted because of physiological instability. There were no significant differences in the degree of PO₂ reduction (-59 vs. -32 mmHg, p = 0.72), rate of PCO₂ rise (3.2 vs. 3.9 mmHg per min, p = 0.22), or pH decline (-0.02 vs. -0.03 per min, p = 0.06). Performance of ABGs at regular intervals was associated with shorter test duration (10 vs. 7 min, p < 0.0001), smaller PCO₂ rise (30 vs. 26 mmHg, p = 0.0007), and less pH reduction (-0.20 vs. -0.17, p = 0.0012). Lower pH at completion of the apnea test was associated with greater blood pressure decline (p = 0.006).

CONCLUSION

Both methods of O₂ supplementation are associated with similar changes in arterial PO₂ and PCO₂. Performance of ABGs at regular intervals shortens apnea test duration and may avoid excessive pH reduction and consequent hemodynamic effects.