Abstract / Summary
Peak expiratory flow (PEF) is the maximal expiratory airflow reached at the beginning of expiration. During controlled mechanical ventilation, it is determined by the driving pressure and expiratory airway resistance. Any change in PEF might reflect a change in driving pressure. Our aim was to verify whether PEF measured in static conditions (after end-inspiratory pauses of different durations) is different from that measured under dynamic conditions (no pause) and to discuss possible implications of a positive finding. In this prospective observational study we enrolled 145 adult patients, expected to require ventilatory support for > 24 h, ventilated either with a Drager Evita V800 or a Maquet Servo-U ventilator. We measured PEF following breaths without any pause (PEF dyn ) and following a 5-second end-inspiratory pause (PEF stat_5 ). In 34 patients PEF following a 0.5-second end-inspiratory pause was also measured (PEF stat_0.5 ). PEF stat_5 was always smaller than PEF dyn . The difference was 4 (3–6) L/min or 12 (9–16)% in the Drager group and 7 (5–9) L/min or 14 (11–16%) in the Maquet group. In the subgroup of 34 patients PEF stat_0.5 was always smaller than PEF dyn . The difference was 2 (1–2) L/min or 5 (3–6)% in the Drager group and 7 (6–8) L/min or 12 (10–20)% in the Maquet group. During volume-controlled constant-flow ventilation, PEF after an end-inspiratory pause (static conditions) appears smaller than before the pause (dynamic conditions). This may reflect a difference in driving pressure in the two conditions, but other reasons cannot be excluded.