Lower-body negative pressure restores leg bone microvascular flow to supine levels during head-down tilt.
Skeletal unloading and cephalic fluid shifts in microgravity may alter the bone microvascular flow and may be associated with the 1-2% bone loss per month during spaceflight. The purpose of this study was to determine if lower-body negative pressure (LBNP) can prevent microgravity-induced alterations of tibial microvascular flow. Head-down tilt (HDT) simulates the cephalad fluid shift and microvascular flow responses that may occur in microgravity. We hypothesized that LBNP prevents HDT-induced increases in tibial microvascular flow. Tibial bone microvascular flow, oxygenation, and calf circumference were measured during 5 min sitting, 5 min supine, 5 min 15° HDT, and 10 min 15° HDT with 25 mmHg LBNP using photoplethysmography (PPG), near-infrared spectroscopy (NIRS), and strain-gauge plethysmography (SGP). Measurements were made simultaneously. Tibial microvascular flow increased by 36% with 5 min 15° HDT [2.2 ± 1.1 V; repeated-measures ANOVA (RMANOVA) P < 0.0001] from supine (1.4 ± 0.8 V). After 10 min of LBNP in the 15° HDT position, tibial microvascular flow returned to supine levels (1.1 ± 0.5 V; RMANOVA P < 0.001). Tibial oxygenation did not change significantly during sitting, supine, HDT, or HDT with LBNP. However, calf circumference decreased with 5 min 15° HDT (-0.7 ± 0.4 V; RMANOVA P < 0.0001) from supine (-0.5 ± 0.4 V). However, with LBNP calf circumference returned to supine levels (-0.4 ± 0.1 V; RMANOVA P = 0.002). These data establish that simulated microgravity increases tibial microvascular flow and LBNP prevents these increases. The results suggest that LBNP may provide a suitable countermeasure to normalize the bone microvascular flow during spaceflight.
Department of Orthopedic Surgery, University of California, San Diego, California jsiamwala@ucsd.edu.
Jamila H
http://orcid.org/0000-0001-9482-9183
JH
Siamwala
Department of Orthopedic Surgery, University of California, San Diego, California.
Paul C
PC
Lee
Department of Orthopedic Surgery, University of California, San Diego, California.
Brandon R
BR
Macias
Department of Orthopedic Surgery, University of California, San Diego, California.
Alan R
AR
Hargens
D000328:Adult; D005260:Female; D018475:Head-Down Tilt / Q000502:physiology*; D006801:Humans; D007866:Leg / Q000502:physiology*; D008150:Lower Body Negative Pressure / Q000379:methods; D008297:Male; D055806:Microvessels / Q000502:physiology*; D010991:Plethysmography / Q000379:methods; D012039:Regional Blood Flow / Q000502:physiology*; D013026:Space Flight / Q000379:methods; D019265:Spectroscopy, Near-Infrared / Q000379:methods; D016683:Supine Position / Q000502:physiology; D013977:Tibia / Q000502:physiology*; D018474:Weightlessness Simulation / Q000379:methods; D055815:Young Adult
bone microvascular flow; head-down tilt; limb girth; oxygenation; photoplethysmography; spaceflight