Transpulmonary dilution system identification for pulmonary blood volume measurements by contrast echocardiography

Pulmonary blood volume (PBV) is an important parameter to assess the condition of the lungs and the transpulmonary circulation. Unfortunately, its measurement is limited by the need for central catheterization. Contrast ultrasonography allows overcoming this problem. A small bolus of ultrasound contrast agent is injected in a peripheral vein and its passage through the right ventricle and left atrium is detected non-invasively by an ultrasound transducer. The PBV is then given by the product of the mean transit time (MTT) of the contrast between the two measurement sites times the cardiac output. The MTT is estimated by specific model interpolation of the measured dilution curves. In this paper we present a new method for PBV measurements based on a system identification approach. This method identifies the parameters of the model that represents the dilution system impulse response. No subsequent model interpolation is needed. The local density random walk model is adopted to represent the transpulmonary dilution system. Volume measurements show accurate in-vitro results with a correlation coefficient higher than 0.99. The clinical feasibility is confirmed by 70 measurements in patients. Beyond an accurate quantification of pulmonary blood volume, the proposed method also permits the characterization of the transpulmonary hemodynamics, possibly adding novel diagnostic value to the measurement.