Ventilator self-cycling may falsely suggest patient effort during brain death determination

Brain death is suspected when a patient with a destructive neurologic brain injury on a ventilator fails to generate respirations and other brainstem reflexes are absent. An apnea test is mandated in brain-death evaluation. Apnea is concluded when no breathing effort is observed at PaCO₂ of 60 mm Hg or with a 20 mm Hg increase from normal baseline.¹ There are no reported cases of adult patients who were declared brain dead and later initiated respirations. Two cases from the U.K. (brain death and cardiac death) have been described where the ventilator readings were erroneous but remotely suggested patient effort.^2,3 We have recently come across several instances during brain-death determination when it appeared that patients falsely triggered the ventilator. We would like to call attention to this phenomenon of ventilator self-cycling.

Methods.

From January 2002 to February 2005, we performed apnea tests in 83 patients in our neurologic-neurosurgical intensive care unit for brain-death determination. All patients fulfilled the clinical criteria of brain death and apnea tests were performed using the American Academy of Neurology Practice Parameter guidelines.¹ In four patients (aneurysmal subarachnoid hemorrhage in three patients and traumatic brain injury in one patient), we noted occasional “triggering” of the ventilator. The apnea test proved positive and remained positive after repeating the test (no breathing of patient after disconnection of the ventilator at PaCO₂ of 60 mm Hg or more). This false triggering of the ventilator disappeared after the flow-by trigger mechanism (typically set at –2 cm H₂O) was changed to pressure trigger mechanism (typically set at 2.0 L/min) or when the trigger sensitivity was adjusted upward. In one patient on an intermittent mandatory ventilation (IMV) of 10 and peak end-respiratory pressure (PEEP) of 5 cm of water, the displayed frequency correlated linearly with the sensitivity (sensitivity of 3.0 L/min; frequency of 10 breaths/min, sensitivity of 2.0 L/min; frequency of 13 breaths/min, sensitivity of 1.5 L/min; frequency of 15 breaths/min; sensitivity of 0.2 L/min, frequency of 19 breaths/min). To confirm the highly sensitive trigger mechanism of current mechanical ventilators, we conducted a brief experiment. An ICU ventilator PB 840 (Puritan Bennett, Pleasanton, CA) connected to a test lung (Michigan Instruments, Grand Rapids, MI) was used for this simulation. Ventilator settings were assist-control mode: respiratory rate, 12 breaths/min; tidal volume, 500 mL; inspiratory flow rate, 30 L/min; positive end expiratory pressure of 5 cm/H₂O; and maximum trigger sensitivity threshold, 0.2 L/min. Test lung settings used a Rp20 resistor, which is equivalent to a 6.5 endotracheal tube with a compliance of 0.05 L/cm H₂O (50 mL/cm H₂O).

Apnea simulation was conducted by switching the ventilator to a spontaneous mode of respiration with a pressure support of 0 cm H₂O and a PEEP of 5 cm H₂O (CPAP 5 cm H₂O). Upon completion of this change, the ventilator began to self-cycle with a frequency of 28 breaths/min with a tidal volumes of 90 mL (see video clip). Self-cycling was eliminated by increasing the trigger flow sensitivity threshold to 2.0 L/min.

Discussion.

This simulation and patient examples demonstrate that ventilator settings below a certain trigger sensitivity threshold may cause the ventilator to self-cycle, simulating spontaneous respirations. Ventilator self-cycling may become apparent in clinical settings when factors other than the patient's inspiratory effort cause the trigger threshold value to be achieved. Common causes of ventilator self-cycling include leaks which could cause pressure changes, e.g., chest tube in polytraumatized patients; secretions or water in the ventilator circuit; or the cardiac cycle itself, which could change transpleural pressure and flow in a compliant lung. This phenomenon may be more common in new-generation ventilators that incorporate extremely sensitive flow trigger settings.^2–6 Ventilator self-cycling is a well-known phenomenon in the anesthesiology literature, although we believe it is not familiar to most neurologists involved in the declaration of brain death. If not recognized, it may suggest that patients with a major destructive brain injury can still “trigger the ventilator” even though all brainstem reflexes have disappeared. This may be interpreted as presence of respiratory function and could unnecessarily prolong the determination of brain death. In other instances, when the apnea tests result conflicts with the ventilator display of a possible spontaneous respiratory effort, it may lead to unnecessary confirmatory tests. We noted false triggering in 5% of apnea tests, but prospective studies could provide a more accurate prevalence and we suspect it is more common.