Mild dehydration modifies the cerebrovascular response to the cold pressor test

The cold pressor test (CPT) is widely used in clinical practice and physiological research. It is characterised by a robust autonomic response with associated increases in heart rate (HR), mean arterial pressure (MAP) and mean middle cerebral artery blood flow velocity (MCAvmean ). Hydration status is not commonly reported when conducting this test, yet blood viscosity alone can modulate MCAvmean , potentially modifying the MCAvmean response to the CPT. We investigated the effect of mild dehydration on the physiological response to the CPT in 10 healthy males (mean ± SD: age 28 ± 5y; body mass 83 ± 5 kg). All participants completed two CPTs, cold water (0 °C) immersion of both feet for 90 s, with the order of the euhydration and dehydration trials counter-balanced. Beat-to-beat MCAv, MAP, HR and breath-by-breath partial pressure of end-tidal CO2 (PET CO2 ) were measured continuously. Participants' pain perception was measured 1 min into the CPT using a visual analogue scale (no pain = 0; maximal pain = 10). Dehydration significantly elevated plasma osmolality, urine specific gravity and reduced body mass (all P < 0.01). MAP and HR responses were not different between treatments (both P > 0.05). After 90 s of immersion the change in MCAvmean from baseline was less in the dehydration compared to the euhydration trial (Δ0 ± 5 vs. 7 ± 7 cm s(-1) , P = 0.01), as was PET CO2 (Δ-3 ± 2 vs. 0 ± 3 mm Hg, P = 0.02). Dehydration was associated with greater relative pain sensation during the CPT (7.0 ± 1.3 vs 5.8 ± 1.8, P = 0.02). Our results demonstrate that mild dehydration can modify the cerebrovascular response to the CPT with dehydration increasing perceived pain, lowering PET CO2 and ultimately blunting the MCAvmean response. This article is protected by copyright. All rights reserved.