Comparing Cardiovascular Responses Between Upper Body Negative Pressure and Lower Body Negative Pressure

Abstract

PURPOSE: LBNP stimulation distal to the iliac crest pulls blood to the legs resulting in a drop in blood pressure. In healthy individuals this is followed by a compensatory baroreflex-mediated increase in sympathetic nervous system activity resulting in global vasoconstriction and increased heart rate in order to recover blood pressure. LBNP is a relatively simple, inexpensive, and efficient method to reliably reproduce baroreflex responses and is therefore often employed as a research tool. Previous research utilizing LBNP is limited to the lower body, thus it is unknown as to whether responses to negative pressure vary based on the part of the body exposed to the stimulus. The aim of this investigation was to compare the cardiovascular responses to negative pressure when applied to the upper and lower limb. METHODS: Healthy participants performed a series of protocols to compare the cardiovascular responses to upper body negative pressure (UBNP) and LBNP. A volumetric edema gauge was first used to quantify the volume of the whole arm (distal to the acromion process) and then match an equal volume of participant’s leg to determine the depth of the leg. In a random order participants then placed their whole left arm (WA), whole leg (WL) (distal to the pubic tubercle) or a portion of the left leg that is volume-matched to the arm (VML) into the NP chamber. Once the limb was sealed in the chamber the subjects remained at rest for 5 minutes to collect baseline data. Following baseline each limb was subject to 5 minutes of -60 mmHg negative pressure. Heart rate (HR), mean arterial pressure (MAP) and brachial artery blood flow (BBF) in the control arm were measured continuously throughout each protocol. RESULTS: Relative to baseline, BBF during the WA, WL, and VML conditions decreased during the negative pressure by 21.2 ml/min, 41.8 ml/min, and 9.9 ml/min, respectively. HR during the WA and WL conditions decreased by 2.2 bpm and 1.2 bpm, respectively, however HR increased during the negative pressure during the VML condition by 2.0 bpm. During the negative pressure MAP increased during the WA condition by 2.4 mmHg and decreased during the WL and VML conditions by 0.9 mmHg and 3.0 mmHg, respectively. CONCLUSIONS: These data indicate that the cardiovascular responses to UBNP and LBNP are different and it is not simply due to a function of limb volume. Proximity of negative pressure relative to the baroreceptors may also have an impact.