Cardiopulmonary effects of positive pressure ventilation during acute lung injury

To assess the gas exchange and hemodynamic effects of pressure-limited ventilation (PLV) strategies in acute lung injury (ALI). We hypothesized that in ALI, the reduction of plateau airway pressure (Paw) would be associated with less alveolar overdistention and thus have better hemodynamic and gas exchange characteristics than larger tidal volume (Vr) ventilation.

Laboratory.

Prospective time-controlled sequential animal study.

Right atrial, pulmonary artery, left atrial, arterial, lateral pleural (Ppl), and pericardial (Ppc) pressures, Paw, ventricular stroke volume, mean expired CO2, and arterial and mixed venous oxygen contents. Airway resistance and static lung compliance were also measured.

Intermittent positive pressure ventilation (IPPV) given before (control) and after induction of ALI by oleic acid infusion (0.1 mL/kg). IPPV at FIO2 of 1, VT of 12 mL/kg, and frequency adjusted to maintain normocarbia. ALI PLV was given during ALI and defined as that VT which gave a similar plateau Paw to that of control IPPV. High-frequency jet ventilation (HFJV) and ALI HFJV were also given and defined as frequency within 10% of heart rate and mean Paw similar to that during control IPPV.

After ALI, static lung compliance, PaO2, and pH decreased, whereas airway resistance and PaCO2 increased. For a constant lung volume, Ppl and Ppc were not different between control and ALI. Both absolute dead space (VD) and intrapulmonary shunt fraction increased after ALI, but absolute VD was lower with ALI PLV and ALI HFJV when compared with ALI IPPV. Ventilation did not alter hemodynamics during ALI.

Changes in lung volume determine Ppc and Ppl. PLV strategies do not alter hemodynamics but result in less of an increase in VD/VT than would be predicted from the obligatory decrease in VT.