CARDIOPULMONARY EFFECTS OF POSITIVE PRESSURE VENTILATION DURING ACUTELUNG INJURY

Study objectives: To assess the gas exchange and hemodynamic effects o f pressure-limited ventilation (PLV) strategies in acute lung injury ( ALI). We hypothesized that in ALI, the reduction of plateau airway pre ssure (Paw) would be associated with less alveolar overdistention and thus have better hemodynamic and gas exchange characteristics than lar ger tidal volume (VT) ventilation. Setting: Laboratory. Design: Prospe ctive time-controlled sequential animal study. Measurements: Right atr ial, pulmonary artery, left atrial, arterial, lateral pleural (Ppl), a nd pericardial (Ppc) pressures, Paw, ventricular stroke volume, mean e xpired CO2, and arterial and mixed venous oxygen contents. Airway resi stance and static lung compliance were also measured. Interventions: i ntermittent positive pressure ventilation (IPPV) given before (control ) and after induction of ALI by oleic acid infusion (0.1 mL/kg). IPPV at FIo(2) of 1, VT of 12 mL/kg, and frequency adjusted to maintain nor mocarbia. ALI PLV was given during ALI and defined as that VT which ga ve a similar plateau Paw to that of control IPPV. High-frequency jet v entilation (HFJV) and ALI HFJV were also given and defined as frequenc y within 10% of heart rate and mean Paw similar to that during control IPPV. Results: After ALI, static lung compliance, PaO2, and pH decrea sed, whereas airway resistance and PaCO2 increased. For a constant lun g 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 co mpared with ALI IPPV. Ventilation did not alter hemodynamics during AL I. Conclusions: Changes in lung volume determine Ppc and Ppl. PLV stra tegies do not alter hemodynamics but result in less of an increase in VDNT than would be predicted from the obligatory decrease in VT.