To the Editor.
—Recent reports1-3 on the hypertensive response to inversion provide hemodynamic data that may have clinical utility.This rise in pressure apparently occurs without change in cardiac output (CO). Prompted by personal observations,4 my pilot study of 20 young athletes showed no significant change in CO (P<.05) during ten minutes of 90° head-down tilt, as measured by a nuclear stethoscope (Cardiac Probe). To my knowledge, there are no published reports of CO at this degree of tilt, but Gazenko et al5 used invasive monitoring with subjects at a 30° head-down tilt and found no significant change in CO over a one-hour period.If CO is constant, that simplifies Poiseuille's law, which states that pressure is proportional to the product of flow and resistance. That is, mean arterial pressure (MAP) equals flow (CO) times total peripheral resistance (TPR), or TPR equals MAP divided by CO.