The position of the lower inflection point depends on volume history

ARDS is a difficult to treat disease and is associated with a high mortality. For lung protective ventilation strategies the interaction of ventilator settings and respiratory mechanics is essential. In the past the static PV curve was used to determine ventilator settings in ARDS patients. Atelectasis was assumed to occur at pressures below the lower inflection point (LIP). This study was performed to investigate the influence of ventilatory patterns with different tidal volumes (Vt) on the shape of the static PV curve.

After induction of anesthesia and tracheotomy, 14 surfactant-depleted piglets were ventilated at ZEEP with three different Vt (8, 12, 16 ml/kg) in a randomised order. For data acquisition a BICORE CP100 monitor (Bicore Monitoring Systems, Irvine, CA, USA) was used. The protocol consisted of a static maneuver at the end of each ventilator setting. In addition a baseline measurement (12 ml/kg) was performed before saline lavage. At the end of each setting a recruitment maneuver was performed before Vt change. After determining the LIPs their corresponding pressure values were compared using ANOVA and the Fisher PLSD post-hoc test.

Figure 1 shows the pressure belonging to the LIP (mean ± SD) prelavage and postlavage with different Vt. In healthy lungs the LIP is located at lower pressures than after surfactant depletion. Postlavage the pressure at LIP increases significantly with increasing Vt.

The analysis of static respiratory mechanics shows interdependence between the ventilator settings prior to the static maneuver and the LIP. This could be interpreted as an effect of volume history on the relative position of the LIP. As we observed this effect despite standardisation of the maneuver itself the value of the current interpretation of static measurements has to be questioned.