THE EFFECT OF MEAN AIRWAY PRESSURE (Paw) ON DEAD SPACE/TIDAL VOLUME RATIO(VD/VT) DURING HIGH-FREQUENCY OSCILLATORY VENTILATION (HFOV). † 1138

Introduction: During HFOV, lung inflation plays a critical role in decreasing ventilation-perfusion mismatch. While inadequate Paw can increase Vd/Vt by promoting alveolar atelectasis, excessive Paw can increase Vd/Vt by decreasing lung perfusion.

Objective: To evaluate the effect of changing Paw on Vd/Vt during HFOV.

Methods: Five healthy New Zealand rabbits were ventilated with HFOV under ketamine anesthesia. End tidal CO₂ (P_ETCO₂) was measured in the trachea using a Beckman CO₂ gas analyzer. Each animal was randomly ventilated at Paw of 3, 5, 7 and 9 cm H₂O pressure. Blood pressure, P_ETCO₂, arterial and venous blood gases were measured at each Paw. After collecting 2 sets of data at each Paw, the lungs were lavaged with 20cc/kg of normal saline to create lung injury. Following lung injury, measurements were repeated at Paw of 10, 13, 16, 19. Vd/Vt was estimated using the following equation: Vd/Vt(%) = 100*(PaCO₂ - P_ETCO2)/ PaCO₂. (P_ETCO₂ = mixed expired CO₂). In animals with acute lung injury, we noted that the PvCO₂ was significantly higher than PaCO2 and we also calculated a“venous” Vd/Vt using PvCO₂. We assessed changes using ANOVA.

Results: Changes in Paw were associated with significant changes in Vd/Vt. (table) In normal animals, changes in Paw were not associated with changes in mean blood pressure, central venous pressure, AaDO_2, or arterial pH. In animals with acute lung injury, increases in the Paw were associated with decreased mean blood pressure and lower pH.

Table 1

Conclusions: These data demonstrate the feasibility of Vd/Vt measurements during HFOV. Increases in Paw resulted in increased Vd/Vt without a concomitant change in AaDO₂. Vd/Vt measurements may be helpful for evaluating the effect of Paw on lung volume and pulmonary blood flow during HFOV.