Why does airway pressure rise sometimes near the end of inflation during pressure support?


Sir: Patient-ventilator non-synchrony is common [1–6] and may increase morbidity [4]. Some possible reasons for its adverse effects include increased need for sedation, sleep disruption, respiratory muscle injury and errors in assessing weanability (see [7] for review). Extension of the ventilator’s inflation phase much beyond the end of inspiratory effort (delayed cycling-off) is a very common form of non-synchrony, particularly with pressure support (PSV) [2, 3, 5, 6; see 8–10 for mechanisms]. With delayed cycling-off, the lungs are being forcibly inflated when the patient wants them to deflate. When this occurs in alert individuals, it results in considerable discomfort. Often, expiratory muscles are activated in an effort to force cycling-off [11–13]. Airway pressure (PAW) frequently overshoots the set PSV level before the ventilator cycles off [13]. Given the reported expiratory muscle activation in association with delayed cycling-off [11, 12], some researchers interpret the PAW overshoot as indicative of expiratory recruitment in response to delayed cycling-off (i.e. patient is fighting the ventilator). Whereas expiratory muscle activation may result in pressure overshoot, an overshoot need not reflect expiratory muscle activation or even excessive delayed cycling-off. This is important to recognize, since concluding that the patient is fighting the ventilator, or that cycling-off delay is excessive when it is not, may lead to unnecessary sedation or to increasing the cycling-off flow threshold when this is not indicated. The latter situation may result in the equally undesirable premature termination of the inflation cycle. To understand the mechanism of PAW overshoot it is necessary to review how the ventilator controls PAW. When gas is flowing into the patient, PAW (i.e. upstream pressure) is higher than downstream pressure (alveolar pressure; PALV) by an amount corresponding to flow rate and respiratory resistance (RRS; Fig. 1): PAW = PALV + Flow× RRS . Eq. (1)


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