P Leung, A Jubran and MJ Tobin
Division of Pulmonary and Critical Care Medicine, Edward Hines Jr., Veterans Administration Hospital, Loyola University of Chicago Stritch School of Medicine, Illinois 60141, USA.

In 11 ventilator-dependent patients, we undertook a head-to-head comparison of patient-ventilator interaction during four ventilator modes: assist-control ventilation (ACV), intermittent mandatory ventilation (IMV), pressure support (PS), and a combination of IMV and PS. Progressive increases in IMV rate and PS level each decreased inspiratory pressure-time product (PTP) (p < 0.0001). These reductions in PTP were greater with PS than with IMV at lower but proportional levels of maximal assistance (p < 0.005). When PS 10 cm H2O was added to a given level of IMV, greater reductions in PTP were achieved not only during intervening (PS) breaths (p < 0.001), but also during mandatory (volume-assisted) breaths (p < 0.0005); this additional unloading during mandatory breaths was proportional to the decrease in respiratory drive (dP/dt) during intervening breaths (r = 0.67, p < 0.0001). Maximal unloading occurred with ACV, achieving more than a fivefold decrease in PTP compared with unassisted breathing. Decreases in PTP were confined to the post-trigger phase, and PTP of the post- trigger phase correlated with dP/dt (r = 0.78, p < 0.0001). Effort during the trigger phase remained constant despite marked changes in drive and intrinsic positive end-expiratory pressure (PEEPi). Ineffective triggering occurred with all modes, and wasted PTP increased with increasing levels of assistance as a result of the accompanying decrease in drive and increase in volume. Breaths preceding nontriggering efforts had shorter respiratory cycle times (p < 0.0005) and expiratory times (p < 0.0001) and higher PEEPi (p < 0.0001), indicating that neural-mechanical asynchrony resulted from inspiratory activity commencing prematurely before elastic recoil pressure had fallen to a level that could be overcome by a patient's muscular effort. Thus, increases in the level of ventilator assistance produced progressive decreases in inspiratory muscle effort and dyspnea,which were accompanied by increases in the rate of ineffective triggering.

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