The thermal-dye method of lung water measurement is reliable at a low cardiac output

Bioimpedance: a novel method for the determination of extravascular lung water

Extravascular lung water (EVLW) can be measured using the double indicator dilution technique (DD). However, because this method is highly invasive and complicated, its clinical used has been limited. In theory, changes in thoracic conductivity, or bioimpedance (BI), can reflect changes in EVLW. However, past studies were unable to directly quantitate changes in EVLW since the contribution of dynamic variables such as ventricular volumes, hematocrit (HCT), and EVLW to this impedance signal could not be discerned. Recent studies have shown that a thermodilution pulmonary artery catheter mounted with a fast response thermistor accurately measures right ventricular end-diastolic volume (RVEDV). With changes in the RVEDV and HCT known, the contribution of EVLW to the bioimpedance signal may be isolated and used to more directly measure changes in EVLW. This hypothesis was tested by creating acute sepsis in seven pigs by infusion of Pseudomonas aeruginosa. Changes in EVLW from baseline were measured using DD at 30 min and at 1, 2, and 4 hr and compared with the change in EVLW computed from a mathematical model comprising the measured changes in BI, RVEDV, and HCT at the same time points. Changes in EVLW using DD and BI were significantly correlated over the length of the study (r = 0.85, P less than 0.01). In early sepsis (30 min), BI overestimated EVLW when compared with DD (P less than 0.05). However, at 1, 2, and 4 hr there was no significant difference between the two methods. In conclusion, the use of bioimpedance and a volumetric catheter may provide a relatively simple and reliable method for continuously monitoring changes in EVLW in the intensive care setting.

Noninvasive estimation of extravascular lung water using bioimpedance

Currently, accurate measurements of extravascular lung water (EVLW) are obtained using the double dye dilution technique (DD). However, this method is invasive and complicated and has limited its clinical use. The purpose of this study was to develop a noninvasive method for determining changes in EVLW using bioimpedance (BI) and compare these measurements with DD in a model of acute pulmonary injury. Nine adult dogs were anesthetized and instrumented with a pulmonary artery and an EVLW arterial catheter with EVLW-DD and cardiac output calculated from the dye and thermal dilution curves. Eight external electrodes were placed on the thorax and impedance changes were measured by computer. Changes in EVLW-BI were computed as a function of the thoracic volume and impedance. Cardiac output (CO) from BI was obtained by electronically differentiating the bioimpedance signal. EVLW and CO were measured using DD and BI at 0, 15, 30, and 60 min following injection of oleic acid (0.1 cc/kg). Changes in EVLW were correlated over all time periods using DD and BI (r = 0.70, P less than 0.05); however, several impedance EVLW estimates varied greatly from double dilution methods. There was no difference in computed EVLW using DD and BI at 15, 30, and 60 min following oleic acid infusion (P greater than 0.35). CO was significantly correlated using DD and BI (r = 0.81, P less than 0.05). In summary, bioimpedance may hold promise as a noninvasive and continuous means for estimation of EVLW in the critical care setting.