We read with interest the recent case report ‘Brainstem death and ventilator trigger settings’ (Willatts & Drummond. Anaesthesia 2000; 55: 676–7). We regularly face the same technical problem in setting ventilators in mechanically-ventilated patients (Puritan Bennett 7200 ventilator) with a left ventricular assist device (LVAD) in our cardiothoracic intensive care unit. The LVAD is implanted in patients with end-stage cardiac failure. In the LVAD system, a diaphragm pump is fixed intraperitoneally and is connected to left ventricular cavity and aorta beyond the aortic valve with preclotted, woven Dacron graft [1]. The pump maintains the cardiac output for the patient and it creates enough negative intrathoracic pressure to trigger the ventilator for synchronised intermittent mandatory ventilation (SIMV) or pressure support ventilation if the set sensitivity is at its most sensitive. The usual pressure below PEEP trigger has been found to be the cause of ventilator auto cycling, even at relatively insensitive levels of 4 or 5 cmH2O; this with the patient apnoeic on disconnection of the circuit. Obviously, this will add to the work of breathing and make weaning from IPPV difficult when the time comes.

We have found flow triggering to be the answer in these circumstances using the flow-by function on the Puritan Bennett 7200. An average base flow of 20 l.min−1 and trigger setting of 3 l.min−1 are generally suitable. We have found this demonstration of apnoea on disconnection of the ventilator circuit plus the elimination of ‘auto-cycling’ with ‘flow-by’ has reassured all intensive care staff of the true state of the patient regarding spontaneous inspiratory effort.


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