COMPARISON OF ASSISTED VENTILATOR MODES ON TRIGGERING, PATIENT EFFORT, AND DYSPNEA

In 11 ventilator-dependent patients, we undertook a head-to-head compa rison 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. Progress ive increases in IMV rate and PS level each decreased inspiratory pres sure-time product (PTP) (p < 0.0001). These reductions in PTP were gre ater 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 interv ening (PS) breaths (p < 0.001), but also during mandatory (volume-assi sted) 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 comp ared 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-e xpiratory pressure (PEEPi). Ineffective triggering occurred with all m odes, 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 cycl e times (p < 0.0005) and expiratory times (p < 0.0001) and higher PEEP i (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 dyspn ea,which were accompanied by increases in the rate of ineffective trig gering.