An assessment of the potential use of transthoracic impedance measurement for the non-invasive monitoring of lung injury following cardiac surgery

Aim. Lung injury commonly occurs following cardiac surgery. More severe forms are associated with a high mortality. In order to diagnose it and monitor its progress with treatment, measurement of extravascular lung water is necessary. This is usually measured using either a double-indicator dilution or single-indicator technique. Both require intravascular lines and expensive consumables which make them unsuitable for the routine monitoring of cardiac surgical patients. It has been suggested that the measurement of bioelectrical impedance could provide a non-invasive alternative which would allow routine measurement in these patients. Methods. Extravascular lung water (EVLW) and index (EVLWI) were measured in 23 patients undergoing elective coronary artery bypass graft surgery at 3 time points before and after surgery using the PiCCO system. At the same times the Transthoracic Impedance ratio (TTIr) was also measured using the Bodystat Dual Scan 2005. Results. The corresponding measurements of TTIr were compared statistically with both EVLW and EVLWI. The resulting correlation coefficients were very low (<0.1). Conclusion. Transthoracic Impedance does not appear to reflect extravascular lung water and is therefore unlikely to be useful in the routine monitoring of cardiac surgical patients.