Acute effects of ventilator settings on respiratory motor output in patients with acute lung injury

During assisted mechanical ventilation, changes in ventilator settings may acutely affect the respiratory motor output via the mechanoreceptor reflex feedback system, thus interfering with patient management. This feedback system in mechanically ventilated patients with parenchymal lung injury remains largely unexplored. To investigate this, the early response of respiratory motor output to varying ventilator settings was determined in 13 sedated patients with acute lung injury.

During assist/control and pressure support (PS) ventilation changes in (1) tidal volume (V(T)) at fixed inspiratory flow (V'(I)), (2) V'(I) at fixed V(T) and (3) PS level were employed and the response of respiratory motor output was followed for two breaths after the change. Respiratory motor output was assessed by total pressure generated by the respiratory muscles (Pmus), computed from esophageal pressure (Pes).

Neural expiratory time increased with increasing V(T) and PS, while it remained constant with V'I changes. Neural inspiratory time (T(I)n) increased with decreasing V'(I) and PS, but was not affected by V(T) changes. None of the changes in ventilator settings influenced significantly the rate of rise of Pmus, used as an index of respiratory drive. The changes in respiratory timing resulted in significant changes in breathing frequency, which increased with decreasing V(T) and PS and increasing V'(I). The time integral of Pmus, an index of respiratory effort, increased with increasing T(I)n. These acute responses were not related to the severity of deterioration of respiratory system mechanics.

We conclude that alterations in commonly used ventilator settings induce acute changes in respiratory timing, without affecting the respiratory drive. These changes, probably mediated via mechanoreceptor reflex feedback, are dependent on the type of the alteration in the ventilator settings.