Volume-independent elastance: a useful parameter for open-lung positive end-expiratory pressure adjustment

A decremental positive end-expiratory pressure (PEEP) trial after full lung recruitment allows for the adjustment of the lowest PEEP that prevents end-expiratory collapse (open-lung PEEP). For a tidal volume (Vt) approaching zero, the PEEP of minimum respiratory system elastance (PEEP(minErs)) is theoretically equal to the pressure at the mathematical inflection point (MIP) of the pressure-volume curve, and seems to correspond to the open-lung PEEP in a decremental PEEP trial. Nevertheless, the PEEP(minErs) is dependent on Vt and decreases as Vt increases. To circumvent this dependency, we proposed the use of a second-order model in which the volume-independent elastance (E1) is used to set open-lung PEEP.

Pressure-volume curves and a recruitment maneuver followed by decremental PEEP trials, with a Vt of 6 and 12 mL/kg, were performed in 24 Wistar rats with acute lung injury induced by intraperitoneally injected (n = 8) or intratracheally instilled (n = 8) Escherichia coli lipopolysaccharide. In 8 control animals, the anterior chest wall was surgically removed after PEEP trials, and the protocol was repeated. Airway pressure (Paw) and flow (F) were continuously acquired and fitted by the linear single-compartment model (Paw = Rrs·F + Ers·V + PEEP, where Rrs is the resistance of the respiratory system, and V is volume) and the volume-dependent elastance model (Paw = Rrs·F + E1 + E2·V·V + PEEP, where E2·V is the volume-dependent elastance). From each model, PEEPs of minimum Ers and E1 (PEEP(minE1)) were identified and compared with each respective MIP. The accuracy of PEEPminE1 and PEEPminErs in estimating MIP was assessed by bias and precision plots. Comparisons among groups were performed with the unpaired t test whereas a paired t test was used between the control group before and after chest wall removal and within groups at different Vts. All P values were then corrected for multiple comparisons by the Bonferroni procedure.

In all experimental groups, PEEPminErs, but not PEEPminE1, tended to decrease as Vt increased. The difference between MIP and PEEPminE1 exhibited a lower bias compared with the difference between MIP and PEEPminErs (P < 0.001). The PEEPminE1 was always significantly higher than the PEEPminErs (7.7 vs 3.8, P < 0.001) and better approached MIP (7.7 vs 7.3 cm H2O with P = 0.04 at low Vt, and 7.8 vs 7.1 cm H2O with P < 0.001 at high Vt).

PEEPminE1 better identifies the open-lung PEEP independently of the adjusted Vt, and may be a practical, more individualized approach for PEEP titration.