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 5min sitting, 5min supine, 5min of 15o HDT, 10min 15o HDT with 25mmHg LBNP using photoplethysmography (PPG), near infrared spectroscopy (NIRS) and strain gauge plethysmography (SGP). Measurements were made simultaneously. Tibial microvascular flow increased by 36% with 5min 15o HDT (2.2±1.1V; RMANOVA p<0.0001) from supine (1.4±0.8V). After 10min of LBNP in the 15o HDT position, tibial microvascular flow returned to supine levels (1.1±0.5V; RMANOVA p<0.001). Tibial oxygenation did not change significantly during sitting, supine, HDT, or HDT with LBNP. However, calf circumference decreased with 5min 15o HDT (-0.7±0.4V; RMANOVA p<0.0001) from supine (-0.5±0.4V). However, with LBNP calf circumference returned to supine levels (-0.4±0.1V; 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.
- Bone microvascular flow
- head down tilt
- Copyright © 2015, Journal of Applied Physiology