To the Editor:
We thank Mr Chatburn for his positive comments on our review.1 Chatburn highlights our previous finding2,3 that asynchronies, in particular, ineffective efforts during expiration, were slightly more frequent in pressure support ventilation (PSV) compared with volume control continuous mandatory ventilation throughout mechanical ventilation. Chatburn rightly points out that, in theory, PSV should result in better synchrony than volume control continuous mandatory ventilation because PSV delivers only spontaneous breaths, so patients should retain substantial control of the breath. We fully agree with Chatburn's 3 recommendations for selecting the best mode and for adjusting settings to patients' needs. Recently, Tobin4 pointed out that “in critical care practice, no area demands greater understanding of physiological principles than ventilator management,” and this is particularly true for PSV.
The apparent simplicity of PSV makes it an attractive mode. It is often the mode of choice when the patient's level of consciousness improves and starts to trigger the ventilator.5 In this period, it is crucial to ensure optimal trigger sensitivity, appropriate post-trigger inflation (paying special attention to pressure rise time and the duration of the patient's inspiratory effort), and cycling (ie, the transition from inspiration to expiration).4,6,7 If the switch over from inspiration to expiration is not in phase with the patient's neural expiration, then expiratory muscles can be activated while the ventilator is still inflating the lungs. Moreover, over-assistance during PSV is common and often results in hyperinflation, which can cause respiratory muscular dysfunction or even atrophy, which favors the occurrence of ineffective efforts during expiration, which causes difficulties during sleep and weaning, and results in a prolonging of mechanical ventilation.4,6,7 Importantly, clusters of ineffective efforts during expiration can increase intensive care and hospital mortality.8
PSV is not a simple mode; as with any other mode, it requires continuous bedside adjustments to ensure that underrecognized forms of patient-ventilator asynchrony do not go undetected and inadequately treated. All patients on mechanical ventilation are susceptible to asynchronies, in all modes, from intubation to extubation.2 The effects of poor patient-ventilator interaction on the outcome could be significant9 and warrant urgent investigations.
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