HEAVy HAT-HEAlthy Volunteers Heart to Arm Time. Haemorrhage Simulation Protocol in Healthy Volunteers

In prehospital settings, hypovolemic shock diagnosis is based on Advanced Trauma Life Support (ATLS) shock classification. The most often used clinical signs are heart rate (HR), arterial blood pressure (BP), respiratory rate, neurologic status, diuresis, skin colour and temperature. However, some of these signs, such as hypotension and tachycardia, lack specificity and sensitivity and do not occur early enough. Even with an early preload reduction, blood pressure can remain constant due to compensatory mechanisms, such as vasoconstriction and positive chronotropism. Tachycardia occurs earlier, but has poor specificity and sensitivity. A retrospective analysis of 25,287 trauma patients showed that among 489 patients presenting with systolic BP \ 90 bpm), while 39% of patients with systolic BP \> 120 mmHg were tachycardic, probably resulting from other stimuli influencing heart rate, such as pain, fear, circulating hormones and endogenous enkephalins. Therefore, it could be very useful to have an index that identifies initial volume variation, when physiological regulatory mechanisms are still effectively maintaining normal BP. Pulse transit time (PTT) is the sum of pre-ejection period (PEP; the time interval between the onset of ventricular depolarization and ventricular ejection) and vascular transit time (VTT; the time it takes for the pulse wave to travel from the aortic valve to peripheral arteries). PEP and VTT variations depend on preload variation, and PTT increases with PEP, showing a linear correlation (R2 = 0.96). Chan et al. subjected 11 healthy volunteers to the head-up tilt test, and demonstrated that PEP increased and VTT decreased for increasing tilt angles from 0° to 80°, corresponding to light-moderate bleeding. They also observed early sympathetic activation, expressed by decreases of both RR interval (RR) and VTT, dampening the PTT increase, since PTT is influenced by both continuous PEP increase and prog