Simulation of late inspiratory rise in airway pressure during pressure support ventilation

BACKGROUND: Late inspiratory rise in airway pressure (LIRAP, P_aw/ΔT) caused by inspiratory muscle relaxation or expiratory muscle contraction is frequently seen during pressure support ventilation (PSV), although the modulating factors are unknown. METHODS: We investigated the effects of respiratory mechanics (normal, obstructive, restrictive, or mixed), inspiratory effort (−2, −8, or −15 cm H₂O), flow cycle criteria (5–40% peak inspiratory flow), and duration of inspiratory muscle relaxation (0.18–0.3 s) on LIRAP during PSV using a lung simulator and 4 types of ventilators. RESULTS: LIRAP occurred with all lung models when inspiratory effort was medium to high and duration of inspiratory muscle relaxation was short. The normal lung model was associated with the fastest LIRAP, whereas the obstructive lung model was associated with the slowest. Unless lung mechanics were normal or mixed, LIRAP was unlikely to occur when inspiratory effort was low. Different ventilators were also associated with differences in LIRAP speed. Except for within the restrictive lung model, changes in flow cycle level did not abolish LIRAP if inspiratory effort was medium to high. Increased duration of inspiratory relaxation also led to the elimination of LIRAP. Simulation of expiratory muscle contraction revealed that LIRAP occurred only when expiratory muscle contraction occurred sometime after the beginning of inspiration. CONCLUSIONS: Our simulation study reveals that both respiratory resistance and compliance may affect LIRAP. Except for under restrictive lung conditions, LIRAP is unlikely to be abolished by simply lowering flow cycle criteria when inspiratory effort is strong and relaxation time is rapid. LIRAP may be caused by expiratory muscle contraction when it occurs during inspiration.