Noninvasive input impedance of the human systemic circulation.

Noninvasive Input Impedance of the Human Systemic Circulation To the Editor: The study of noninvasive aortic input impedance by Segers et al1 is a very important step in understanding age-related changes in humans and an advance on previous limited invasive studies conducted decades ago.2,3 One reason that these lapsed was the difficulty in accurately measuring aortic pulsatile flow, attributable to turbulence in the left ventricular outflow tract (LVOT) and proximal aorta. Subsequent emphasis was placed on pressure waveforms2,3 after introduction of high fidelity manometers2 and tonometers3 for noninvasive studies. Segers et al present baseline data for a longitudinal study; long-term information is eagerly awaited. We offer the following explanations for major findings.