EFFECTS OF INSPIRATORY RESISTANCE ON CEREBRAL BLOOD VELOCITY DURING ORTHOSTASIS WITH DEHYDRATION

Purpose: 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. Methods: 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. Results: 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). Conclusions: 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 effect MCAv or the reporting of pre-syncopal symptoms.