Effects of inspiratory resistance on cerebral blood velocity during orthostasis with dehydration

Abstract

Inspiratory resistance increases low frequency (LF) middle cerebral artery blood velocity (MCAv) oscillations and delays symptoms of pre‐syncope during lower body negative pressure, despite reductions in absolute MCAv. We tested the hypothesis that during central hypovolemia induced by combined dehydration and orthostasis (via head‐up tilt, HUT), inspiratory resistance would increase MCAv oscillations and attenuate pre‐syncopal symptoms. 10 subjects were tilted to 70° for 10‐min under euhydration (EUH) and dehydration (DEH; 40 mg of furosemide/fluid restriction) conditions, with and without inspiratory resistance (Impedance Threshold Device, ITD) in a randomized, cross‐over design. Arterial pressure (AP) and MCAv were measured continuously. DEH decreased blood (6.5±0.9%) and plasma (11.2±1.4%) volume (both P<0.01), but did not impact AP or MCAv (P≥0.25) at rest. HUT decreased AP and MCAv under both EUH and DEH (P≤0.016) with no difference between hydration conditions (P=0.498). ITD breathing did not attenuate the fall in AP or MCAv under either hydration condition (P≥0.123), and subjective pre‐syncopal symptoms were similar across all conditions (P=0.78). ITD breathing increased MCAv LF oscillations under both hydration conditions during HUT (P≤0.05). While ITD breathing increased MCAv LF oscillations during DEH and HUT, it did not attenuate the decrease in MCAv or the reporting of symptoms.