Hemodynamic Stability to Surface Warming and Cooling During Sustained and Continuous Simulated Hemorrhage in Humans

Abstract

One in 10 deaths worldwide is caused by traumatic injury, and 30% to 40% of those trauma-related deaths are due to hemorrhage. Currently, warming a bleeding victim is the standard of care due to the adverse effects of combined hemorrhage and hypothermia on survival. We tested the hypothesis that heating is detrimental to the maintenance of arterial pressure and cerebral perfusion during hemorrhage, while cooling is beneficial to victims who are otherwise normothermic. Twenty-one men (31 ± 9 y) were examined under two separate protocols designed to produce central hypovolemia similar to hemorrhage. Following 15 min of supine rest, 10 min of 30 mm Hg of lower body negative pressure (LBNP) was applied. On separate randomized days, subjects were then exposed to skin surface cooling (COOL), warming (WARM), or remained thermoneutral (NEUT), while LBNP continued. Subjects remained in these thermal conditions for either 40 min of 30 mm Hg LBNP (N = 9), or underwent a continuous LBNP ramp until hemodynamic decompensation (N = 12). Arterial blood pressure during LBNP was dependent on the thermal perturbation as blood pressure was greater during COOL (P >0.001) relative to NEUT and WARM for both protocols. Middle cerebral artery blood velocity decreased (P <0.001) from baseline throughout sustained and continuous LBNP, but the magnitude of reduction did not differ between thermal conditions. Contrary to our hypothesis, WARM did not reduce cerebral blood velocity or LBNP tolerance relative to COOL and NEUT in normothermic individuals. While COOL increased blood pressure, cerebral perfusion and time to presyncope were not different relative to NEUT or WARM during sustained or continuous LBNP. Warming an otherwise normothermic hemorrhaging victim is not detrimental to hemodynamic stability, nor is this stability improved with cooling.

Figure 1

Experimental schematic for Protocol 1 (panel A) and Protocol 2 (panel B). Subjects were supine within the lower-body negative pressure (LBNP) device under thermoneutral conditions but without LBNP (15 min Baseline). Subjects were then exposed to 10 min of LBNP (30 mmHg) while remaining thermoneutral. For Protocol 1: With LBNP continuing, subjects remained thermoneutral, underwent warming, or skin surface cooling (each on a different day and randomized) for 40 min. For Protocol 2: With LBNP continuing subjects were exposed to 5 min of one of the thermal manipulations, LBNP then continuously increased at 5 mmHg·min⁻¹ until presyncope while the applied thermal manipulation continued. Thermal and hemodynamic variables were continuously measured throughout both protocols.

Figure 2

Mean (± SD) skin temperature (top panel) and core temperature (bottom panel) for Protocol 2 during Baseline, lower body negative pressure (LBNP only), LBNP with the thermal perturbation (Thermal), 20%, 40%, 60% and 80% of LBNP ramp, and Presyncope; squares-COOL, circles- NEUT, triangles- WARM; ^{^}, greater than Baseline and LBNP only for the WARM condition; ^#, less than Baseline and LBNP only for the COOL condition; ^*, greater than Baseline and LBNP only for all thermal conditions.

Figure 3

Mean (± SD) heart rate for Protocol 2 during Baseline, lower body negative pressure (LBNP only), LBNP with the thermal perturbation (Thermal), 20%, 40%, 60% and 80% of LBNP ramp, and Presyncope; squares- COOL, circles- NEUT, triangles- WARM; ^*, greater than Baseline for all thermal conditions.

Figure 4

Mean (± SD) arterial pressure for Protocol 2 during Baseline, lower body negative pressure (LBNP only), LBNP with the thermal perturbation (Thermal), 20%, 40%, 60% and 80% of LBNP ramp, and Presyncope; squares- COOL, circles- NEUT, triangles- WARM; ^{^}, greater than Baseline for the COOL condition; ^#, less than Baseline for the WARM condition; ^*, less than Baseline and LBNP ramp.

Figure 5

Mean (± SD) cerebral blood flow velocity for Protocol 2 during Baseline, lower body negative pressure (LBNP only), LBNP with the thermal perturbation (Thermal), 20%, 40%, 60% and 80% of LBNP ramp, and Presyncope; squares- COOL, circles- NEUT, triangles- WARM; ^*, less than Baseline for all thermal conditions.

Figure 6

Mean (± SD) compensatory reserve index Protocol 2 during Baseline, lower body negative pressure (LBNP only), LBNP with the thermal perturbation (Thermal), 20%, 40%, 60% and 80% of LBNP ramp, and Presyncope; squares- COOL, circles- NEUT, triangles- WARM; ^*, less than Baseline for all thermal conditions.

Figure 7

Proportion of subjects remaining during lower body negative pressure ramp for COOL (dotted line), NEUT (solid line), and WARM (dashed line).