Pulse Transit Time (PTT) is the sum of Pre-Ejection Period (PEP), the time interval between the onset of ventricular depolarization and the ventricular ejection, and Vascular Transit Time (VTT), the time it takes for the pulse wave to travel from the aortic valve to the peripheral arteries (Obrist et al. 1979). PEP variations are known to correlate with reductions in central blood volume induced by head-up tilt (Chan et al., 2007b, 2008). The same authors also demonstrated that PTT variations follow closely PEP variations and therefore central blood volume variations (Chan et al., 2007b). Following central blood volume reductions induced by head-up tilting ventricular diastolic filling time increases involving an increase in PEP and PTT. Chan et al. (Chan et al., 2007b) concluded that PTT could have been used to assess early central hypovolemia and suggested that joint analysis of PTT and RR intervals could help in predicting the extent of blood volume loss. The investigators hypothesized that sympathetic drive associated with trauma would act on cardiac contractility through beta activity thus shortening PTT without reducing RR interval to the same extent in healthy hearts. We also hypothesized that progressive hypovolemia would lead to a rising of PTT (augmented diastolic filling time) and a RR interval shortening (relative tachycardia). In this study the investigators propose and index based on the beat-to-beat PTT/RR ratio to assess central hypovolemia in traumatic patients enrolled by our Helicopter Emergency Medical System (HEMS) in a prehospital setting.