Extravascular lung water volume measurement by a novel lithium-thermal indicator dilution method: comparison of three techniques to post-mortem gravimetry

Objective  To compare the lithium-thermal double indicator dilution (Li-thermal), indocyanine green-thermal double indicator dilution (ICG-thermal), single thermal indicator dilution (single-thermal) and gravimetric techniques of extravascular lung water volume (EVLW) measurement in porcine models of acute lung injury. Design  Two animal models designed to invoke a systemic inflammatory response. Setting  Laboratory study. Subjects  A total of 12 immature Deutsches Landschwein pigs. Interventions  Extravascular lung water volume was measured at four time points using Li-thermal, ICG-thermal and single-thermal techniques. Measurements were performed using existing technology according to manufacturer’s instructions. Post-mortem gravimetric EVLW measurements were performed by measuring wet and dry mass of lung tissue. Measurements were compared using the Bland–Altman method. Data are presented as mean (SD). Measurements and main results  Data were collected in 12 animals and comparison between all 4 techniques was possible in 10 animals. EVLW measured by gravimetry was 9.2 (±3.0)ml kg⁻¹. When compared to gravimetry, both Li-thermal and ICG-thermal techniques showed minimal bias but wide limits of agreement (LOA) [Li-thermal: bias −1.8 ml kg⁻¹ (LOA ± 13.1); ICG-thermal bias −1.0 ml kg⁻¹ (LOA ± 6.6)]. Comparison between the single-thermal and gravimetric methods identified both considerable bias and wide LOA [+8.5 ml kg⁻¹ (LOA ± 14.5)]. Conclusion  Clinically significant differences between EVLW measurements obtained with the gravimetric method and three in vivo indicator dilution techniques were identified. While none of the techniques could be considered ideal, the ICG-thermal method appeared more reliable than either the Li-thermal or single thermal techniques. Further research is required to determine whether the accuracy of the prototype Li-thermal technique can be improved. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.