Conclusions-It appears feasible to close interatrial communications and atrial septal defects up to 26 mm stretched diameter safely with the Amplatzer septal occluder. Short term results confirm an early high occlusion rate with no major complications. Careful selection of cases based on the echocardiographic morphology of the ASD and accurate assessment of their stretched diameter is of utmost importance. Further experience with the larger devices and longer term results are required before a firm conclusion regarding its use can be made. (Heart 1999;82:300-306)

Background-Pulmonary vascular resistance (PVR) is the current standard for evaluating reactivity in children with pulmonary arterial hypertension (PAH). However, PVR measures only the mean component of right ventricular afterload and neglects pulsatile effects. We recently developed and validated an method to measure pulmonary vascular input impedance, which revealed excellent correlation between the zero-harmonic impedance value and PVR, and suggested a correlation between higher harmonic impedance values and pulmonary vascular stiffness (PVS). Here we show that input impedance can be measured routinely and easily in the catheterization laboratory, that impedance provides PVR and PVS from a single measurement, and that impedance is a better predictor of disease outcomes compared to PVR.