Key result
Lower body negative pressure substantially reduced aortic wave reflection compared to baseline (1% vs -15% and -12%; P<0.01), whereas passive head-up tilt did not significantly decrease it.
Why the study?
Does venous pooling or body position manipulation influence aortic wave reflection during orthostatic challenges in young healthy adults?
Does venous pooling or body position manipulation influence aortic wave reflection during orthostatic challenges in young healthy adults?
Lower body negative pressure, but not passive head-up tilt, substantially reduces aortic wave reflection, suggesting changes in body position alone do not explain differences in AIx during orthostatic conditions.
Answers are grounded in this paper and cite their sources.
LBNP-associated reduction in aortic wave reflection is hypothesis-generating; should not change practice pending prospective validation in hypertension.
BACKGROUND: Aortic wave reflection (augmentation index; AIx) decreases during orthostatic challenges despite increased peripheral resistance, which is thought to be due to venous pooling. The purpose of this study was to examine if the decrease in AIx during an orthostatic challenge is due to venous pooling alone or body position manipulation. METHODS: Twenty-three young, healthy adults (11F/12M) participated in 3 separate orthostatic challenges (5 minutes each); 60° head-up tilt (HUT), 60° HUT with bilateral rhythmic blood pressure (BP) cuff inflation on calves (75 mm Hg) to minimize venous pooling, and lower body negative pressure (LBNP; -30 mm Hg) for venous pooling independent of body position. High-fidelity radial artery pressure waveforms using applanation tonometry were recorded at minutes 2:30 and 5:00 during each condition. Aortic BP and wave reflection were analyzed from a synthesized aortic BP waveform. RESULTS: Compared to resting (baseline) measurements, AIx did not significantly decrease at minutes 2:30 or 5:00 of HUT conditions (HUT 0 ± 2% vs. -3 ± 3%, 0 ± 2%; HUT w/cuffs 0 ± 2% vs. -4 ± 2%, 0 ± 2%). Conversely, LBNP substantially reduced AIx at minutes 2:30 and 5:00 (1 ± 2% vs. -15 ± 2% and -12 ± 2%; P < 0.01). When standardized to heart rate (AIx@75), AIx@75 increased relative to baseline during HUT conditions (P < 0.05). CONCLUSIONS: In contrast to previous studies, AIx did not decrease during passive HUT, yet decreased substantially during LBNP. Despite being well matched for peripheral hemodynamics, it appears that LBNP elicits a greater effect on central hemodynamics, relative to passive HUT. Collectively, changes in body position alone do not explain differences in AIx during orthostatic conditions.
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Hughes et al. (2016) studied Healthy (n=23). Lower body negative pressure (LBNP) vs. Passive head-up tilt (HUT) and baseline was evaluated on Aortic wave reflection (augmentation index; AIx) (p=< 0.01). Lower body negative pressure substantially reduced aortic wave reflection compared to baseline (1% vs -15% and -12%; P<0.01), whereas passive head-up tilt did not significantly decrease it.
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