Background
Studies using transthoracic thermodilution have demonstrated increased extravascular lung water (EVLW) measurements attributed to progression of edema and flooding during sepsis and acute lung injury. We hypothesize that redistribution of pulmonary blood flow can cause increased apparent EVLW secondary to increased perfusion of thermally silent tissue, not increased lung edema.
Methods
Anesthetized, mechanically ventilated canines were instrumented with PiCCO® (Pulsion Medical, Munich, Germany) catheters and underwent lung injury by repetitive saline lavage. Hemodynamic and respiratory physiologic data were recorded. After stabilized lung injury, endotoxin was administered to inactivate hypoxic pulmonary vasoconstriction. Computerized tomographic imaging was performed to quantify in vivo lung volume, total tissue (fluid) and air content, and regional distribution of blood flow.
Results
Lavage injury caused an increase in airway pressures and decreased arterial oxygen content with minimal hemodynamic effects. EVLW and shunt fraction increased after injury and then markedly following endotoxin administration. Computerized tomographic measurements quantified an endotoxin-induced increase in pulmonary blood flow to poorly aerated regions with no change in total lung tissue volume.
Conclusions
The abrupt increase in EVLW and shunt fraction after endotoxin administration is consistent with inactivation of hypoxic pulmonary vasoconstriction and increased perfusion to already flooded lung regions that were previously thermally silent. Computerized tomographic studies further demonstrate in vivo alterations in regional blood flow (but not lung water) and account for these alterations in shunt fraction and EVLW.
The effect of high-frequency oscillatory ventilation (HFOV) settings on the distribution of lung volume (V L ) with changes in mean airway pressure (Paw), frequency (f R ) and tidal volume (V T ) remains controversial. We used computer tomographic (CT) imaging to quantify the distribution of V L during HFOV compared to static continuous positive airway pressure (CPAP). In anesthetized, supine canines, CT imaging of the entire lung was performed during CPAP and HFOV at Paw of 5, 12.5 and 20 cmH 2 O, f R = 5, 10, 15 Hz. We found small, statistically significant decreases compared with CPAP in total and regional V L during HFOV that were greatest at lower f R and Paw. Apex and base sub-volumes underwent changes comparable to the lung overall. Increases in f R were accompanied by increases in Pa O2 . These finding provide additional insight into the impact of HFOV settings on the distribution of V L and suggest that there is low risk of occult regional over-distention during HFOV in normal lungs.
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