Sympathetic neural and hemodynamic responses to head-up tilt during isoosmotic and hyperosmotic hypovolemia

Alexander M. Posch, Adam J. Luippold, Katherine M. Mitchell, Karleigh E. Bradbury, Robert W. Kenefick, Samuel N. Cheuvront, Nisha Charkoudian
2017 Journal of Neurophysiology  
Posch AM, Luippold AJ, Mitchell KM, Bradbury KE, Kenefick RW, Cheuvront SN, Charkoudian N. Sympathetic neural and hemodynamic responses to head-up tilt during isoosmotic and hyperosmotic hypovolemia. We hypothesized that muscle sympathetic nerve activity (MSNA) during head-up tilt (HUT) would be augmented during exercise-induced (hyperosmotic) dehydration but not isoosmotic dehydration via an oral diuretic. We studied 26 young healthy subjects (7 female, 19 male) divided into three groups:
more » ... rated (EUH, n ϭ 7), previously exercised in 40°C while maintaining hydration; dehydrated (DEH, n ϭ 10), previously exercised in 40°C during which 3% of body weight was lost via sweat loss; and diuretic (DIUR, n ϭ 9), a group that did not exercise but lost~3% of body weight via diuresis (furosemide, 80 mg by mouth). We measured MSNA, heart rate (HR), and blood pressure (BP) during supine rest and 30°and 45°HUT. Plasma volume (PV) decreased similarly in DEH (Ϫ8.5 Ϯ 3.3%) and DIUR (Ϫ11.4 Ϯ 5.7%) (P Ͼ 0.05). Plasma osmolality was similar between DIUR and EUH (288 Ϯ 4 vs. 284 Ϯ 5 mmol/kg, respectively) but was significantly higher in DEH (299 Ϯ 5 mmol/kg) (P Ͻ 0.05). Mixedmodel ANOVA was used with repeated measures on position (HUT) and between-group analysis on condition. HR and MSNA increased in all subjects during HUT (main effect of position; P Ͻ 0.05). There was also a significant main effect of group, such that MSNA and HR were higher in DEH compared with DIUR (P Ͻ 0.05). Changes in HR with HUT were larger in both hypovolemic groups compared with EUH (P Ͻ 0.05). The differential HUT response "strategies" in each group suggest a greater role for hypovolemia per se in controlling HR responses during dehydration, and a stronger role for osmolality in control of SNA.
doi:10.1152/jn.00403.2017 pmid:28747468 fatcat:wde5p4kdqjhblj5th5uezikcoa