Effect of PEEP and type of injury on thermal-dye estimation of pulmonary edema

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Abstract
We investigated the effect of positive end-expiratory pressure (PEEP) on the extravascular thermal volume of the lung (ETV) determined by the thermal-dye technique in three canine models of pulmonary edema created by injection of alpha-naphthylthiourea (ANTU) or oleic acid (OA) into the pulmonary circulation or intrabronchial instillation of hydrochloric acid (HCl). ETV was determined before, during, and after ventilation with 14 cmH2O PEEP, and final ETV was compared with the extravascular lung mass (ELM) determined postmortem. Final ETV correctly estimated ELM in 12 animals with ANTU injury, ETV/ELM = 1.04 +/- 0.13, but underestimated after HCl injury (n = 5), ETV/ELM = 0.61 +/- 0.23, and OA injury (n = 6), ETV/ELM = 0.73 +/- 0.19. Whereas PEEP had no consistent effect on extravascular thermal volume in ANTU edema, there was a reversible increase in ETV during PEEP in animals with HCl or OA injury and underestimation of ELM. The increase in ETV during PEEP averaged 9.3 +/- 3.8 ml/kg (62 +/- 42%) over the mean of the pre- and post-PEEP values after HCl injury (P less than 0.01) and 6.7 +/- 4.4 ml/kg (47 +/- 35%) after OA injury (P less than 0.02). There was an inverse correlation between the change in ETV during PEEP and the ETV/ELM ratio for animals with HCl and OA injury (r = -0.94). We conclude that PEEP produces a reversible increase in ETV in some models of lung injury by allowing for distribution of thermal indicator through a larger fraction of the lung water and that this response may be useful to detect underestimation when gravimetric measurements are not available.