Lung water estimation using an external sensing catheter

A method for measuring pulmonary oedema (ETVL) is described using sodium ions as the intravascular indicator and heat as the extravascular indicator. The technique offers the advantage of virtually unlimited repeatability, relatively uncomplicated instrumentation and no blood loss. There is also the potential for differentiating between pulmonary oedema and pulmonary congestion and determining the efficacy of therapy. A sensing catheter to detect blood conductivity and temperature changes is required. Ideally the sensing catheter should be situated at a point just distal to the aortic valve. However, a commercially manufactured detecting catheter we had built proved troublesome to insert because of its relatively large diameter. Therefore, an external sensing catheter has been constructed to measure ETVL and the values obtained have been compared with those from an internal catheter. There was no difference between the internal and external catheter QT, but ETVL was consistently overestimated by 2.46 +/- 0.26 ml/kg. However, this value can be related to the internal value and can be used for clinical studies.