Clinical significance of pulmonary arterial input impedance.

C Cl li in ni ic ca al l s si ig gn ni if fi ic ca an nc ce e o of f p pu ul lm mo on na ar ry y a ar rt te er ri ia al l i in np pu ut t i im mp pe ed da an nc ce e Pulmonary arterial input impedance excites the attention of few clinicians, but there are compelling reasons why they should be more interested in this subject. Physicians are well-versed in the intricacies of pulmonary vascular resistance, which is a measure of the opposition to the mean components of flow. Impedance is a measure of the opposition to the pulsatile components of flow. Right ventricular afterload is usually considered in terms of pulmonary vascular resistance. Yet, between one third and one half of the hydraulic power in the main pulmonary artery is contained in the pul-satile components of flow [1]. Therefore, measurement of arterial input impedance is needed to obtain a complete description of ventricular afterload [2]. The primary cause of cor pulmonale is an increase in the right ventricular afterload [3]. Therefore, impedance is likely to make a substantial contribution to this perennial problem. Most pulmonary vascular research has focused on the small pulmonary arteries, which appear to be the main site of resistance. Impedance is dependent on the mechanical properties and the geometry of the proxi-mal pulmonary arteries. The pulmonary arterial input impedance spectrum is dependent primarily on the first five orders from the main pulmonary artery in decreasing levels of importance [4]. The proximal pulmonary arteries are the first line of opposition to right ventric-ular output and have a dominant effect on the input impedance. The fact that the mean and the pulsatile components of flow are dependent on different portions of the pulmonary circulation suggests that they could be controlled separately, without much overlap. It seems likely that this may be an unexplored avenue that may open the possibility of developing new therapeutic interventions. Although the factors that affect pulmonary vascular resistance are many, resistance, unlike impedance, can be expressed as a single number. Impedance is a function of frequency and requires a graphical display of the spectrum to show its magnitude and the phase relation between pressure and flow over a range of frequencies that are contained in the measured pressure and flow waveforms. The magnitude of impedance …