Volume recruitment and oxygenation in pulmonary edema: A comparison between HFOV and CMV

Purpose: In acute lung injury, edema floods alveoli decreasing mean lung volume (MLV) and increasing pulmonary venous admixture (Qva/Qt). We reasoned that a ventilatory strategy that uses large tidal volumes (VT) might recruit volume differently than a strategy that uses very small VT (high-frequency oscillatory ventilation, HFOV) which may require an inflation maneuver to total lung capacity (TLC) for full recruitment. Materials and Methods: We studied six dogs with pulmonary edema induced by oleic acid injury and compared HFOV with conventional mechanical ventilation (CMV). Increasing mean airway opening pressure (Pao) from 6 to 14 cm H₂O raised MLV from 932 ± 162 to 1550 ± 210 mL and from 872 ± 145 to 1242 ± 192 mL during CMV and HFOV, respectively, whereas Qva/Qt decreased from 24.1 ± 8.5 to 9.3 ± 4.3% and from 42.2 ± 6.8 to 30.4 ± 9.3%. We repeated our measurements at a Pao of 14 cm H₂O after an inflation maneuver to TLC. Results: Intlation to TLC recruited additional lung volume and decreased Qva/Qt further only during HFOV. After an inflation to TLC, we observed a rapid isobaric volume loss from the deflation limb of the pressure-volume curve during both CMV and HFOV. Conclusions: We conclude that after oleic acid injury in dogs pressure-volume hysteresis has two components: a recruitable portion associated with gas exchange improvement and a nonrecruitable portion. At the level of PEEP used in this study (8.5 cm H₂O), full lung recruitment during HFOV required inflation to TLC, whereas during CMV it was accomplished by the relatively large VT.