Detecting Change in Left Ventricular Ejection Time During Head-Up Tilt-Induced Progressive Central Hypovolemia Using a Finger Photoplethysmographic Pulse Oximetry Wave Form

Abstract

Change in cardiac preload caused by mild hypovolemia can alter left ventricular ejection time (LVET) without noticeable change in blood pressure (BP). Previously our group has explored a novel method of LVET monitoring using a noninvasive finger photoplethysmographic pulse oximetry wave form. The current study investigated the ability of photoplethysmographic pulse oximetry wave form-derived LVET (LVETp) to identify progressive central hypovolemia induced by head-up tilt and evaluated the potential use of LVETp as an early noninvasive indicator of blood loss. Thirteen healthy subjects underwent graded head-up tilt from 0 degrees to 80 degrees. The response of LVETp to tilt was compared with that of interbeat heart interval (RR) and BP. Least-squares linear regression analysis was performed on an intrasubject basis between various physiologic variables and sine of the tilt angle (which is associated with the decrease in central blood volume). During graded tilt, LVETp had a very strong negative linear correlation with sine of the tilt angle, with correlation coefficients (r) ranging from -0.961 to -0.985. At a very mild hypovolemic state (10 degrees), there was a significant decrease in LVETp compared with baseline (0 degrees) but without a significant change in RR and BP. Gradient analysis showed that LVETp was sensitive to central volume loss at all volume states (0 degrees-80 degrees), whereas RR was only responsive at mild-to-moderate and moderate hypovolemic states (20 degrees-80 degrees) but not mild hypovolemic state (0 degrees-20 degrees). LVETp has a strong association with the change in central blood volume and may be a sensitive early marker of nonhypotensive progressive central hypovolemia. Joint interpretation of LVETp and RR trends may help to characterize the extent of blood volume loss.