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

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.

Accuracy of the double indicator method for measurement of extravascular lung water depends on the type of acute lung injury*

“…This redistribution could prevent indicator diffusion and therefore edema detection. Our results are in agreement with previous studies that found an underestimation of EVLW by 30 -40% in similar OA models (23,24). Other studies in normal lungs or after indirect injury found either overestimation by the DI method or underestimation (6,11,(25)(26)(27)(28).…”

Transpulmonary dilution-derived extravascular lung water as a measure of lung edema

“…As alluded to above, some types of pulmonary edema, in animal studies [18,[39][40][41], are less well reflected by EVLW measurements than others, partly associated with redistribution of intrapulmonary blood flow. Cardiac output may also be too high for thermal equilibration with the extravascular distribution volume, and positive end-expiratory pressure ( …”

“…PEEP) may increase the distribution of the thermal indicator and increase EVLW, although this is controversial and opposite observations have been made, dependent on models, techniques and effects of PEEP on cardiac output and ventilator-associated lung injury[6,8,17,19,31,40,41]. Boldt et al[44] observed that altering cardiac output after cardiac surgery in humans did not affect the (old) thermal-dye EVLW (densitometer technique).…”

The PiCCO Monitor

“…91 In contrast, PEEP may induce a redistribution of pulmonary blood flow toward previously excluded areas and hence artificially ''increase'' EVLW by recruiting the lungs. 87,92 It is important to appreciate that in addition to potentially affecting measurement of EVLW by dilution method, PEEP may also have an effect on the real amount of EVLW; in case of elevated pulmonary capillary pressure due to LV dysfunction, the application of PEEP may decrease EVLW by decreasing pulmonary capillary pressure. 93,94 In contrast, PEEP may increase EVLW by increasing central venous pressure leading to reduced lymph flow from the lungs (and thus lymphatic congestion), and by increasing lung volume leading to vascular congestion and edema.…”