A Comparison of Protocols for Simulating Hemorrhage in Humans: Step vs. Ramp Lower Body Negative Pressure

Lower body negative pressure (LBNP) elicits central hypovolemia, and has been used to characterize the cardiovascular and cerebrovascular responses to simulated hemorrhage in humans. LBNP protocols traditionally employ a progressive stepwise reduction in pressure that is maintained for specific time periods. More recently, however, continuous ramp LBNP protocols have been utilized to simulate the continuous nature of most bleeding injuries. Purpose: The aim of this study was to compare tolerance and hemodynamic responses between a step LBNP protocol and a continuous ramp LBNP protocol until the onset of presyncope. Methods: Healthy human subjects (N=20; 8F, 12M) participated in two LBNP protocols to presyncope: 1) Step Protocol, where chamber pressure decreased every 5-min to -15, -30, -45, -60, -70, -80, -90 and -100 mmHg, and, 2) Ramp Protocol, where chamber pressure decreased 3 mmHg/min. Heart rate (HR), mean arterial pressure (MAP), stroke volume (SV), middle and posterior cerebral artery velocity (MCAv and PCAv), muscle and cerebral oxygen saturation (SmO2 and ScO2), and end-tidal CO2 (etCO2) were measured continuously. Time to presyncope, the cumulative stress index (CSI; summation of chamber pressuretime at each pressure), and hemodynamic responses were compared between the two protocols. Results: Time to presyncope (Step: 1611.8 ± 80.5 s vs. Ramp: 1675.4 ± 68.3 s; P=0.17), and the ensuing magnitude of central hypovolemia (%Δ SV, Step: -54.3 ± 2.5 % vs. Ramp: -51.9 ± 2.7 %; P=0.31) were similar between protocols, despite a higher CSI for the step protocol (Step: 946.5 ± 98.4 mmHgmin vs. Ramp: 836.7 ± 81.6 mmHg*min; P=0.06). While there were no differences at presyncope between protocols for the maximum change in HR, MCAv, or SmO2 (P≥0.21), the reduction in MAP was slightly less (Step: -17.1 ± 1.8 % vs. Ramp: -20.0 ± 1.4 %; P=0.02) and the reductions in PCAv, ScO2,and etCO2 (P≤0.08) were slightly greater for the step protocol compared to the ramp protocol. Conclusion: These results suggest that step and continuous ramp LBNP protocols elicit relatively similar tolerance times, reductions in central blood volume, and subsequent reflex hemodynamic responses, despite a greater cumulative stress in young healthy adults.