Measurement of cardiac output and pulmonary transit time for assessment of pulmonary vascular resistance in domestic piglets

The degree of elevated pulmonary vascular resistance (PVR) is a crucial clinical parameter. Cardiac output (CO) and pulmonary transit time (PTT) can be ascertained by a number of radiological methods. A close functional relationship between CO, PTT and PVR would facilitate non-invasive PVR measurements. One-hundred and fifty-one measurements were made in six piglets. Pressures in the pulmonary and systemic circulation were measured invasively. Cardic output was determined by the use of a Doppler flow probe around the truncus pulmonalis. Temperature sensors were placed in the pulmonary truncus and left atrium. Elevated PVR was produced by repeated air embolism. After injection of ice-cold saline, the time span between the minimal temperature in the truncus pulmonalis and the left atrium was taken as PTT. The CO and PTT were inserted into a new formula derived from the Hagen-Poiseuille law for the calculation of the PVR model. Numerical constants of the formula were calculated by the robust method of minimization. The PVR values, as calculated from invasively measured mean pulmonary artery pressure, mean left atrial pressure and CO, served as reference. In the six piglets, the PVR model and PVR reference showed a strong linear correlation with r = 0.923. The Bland-Altman plot revealed a standard deviation of -0.64/+0.67 Wood units. Cardiac output, PTT and PVR showed a close functional relationship. With a correction for blood viscosity and body size, this relationship could be used for non-invasive clinical measurements of PVR.