Hydration assessment using the cardiovascular response to standing.

The cardiovascular response to standing (sit-to-stand change in heart rate; SSΔHR) is commonly employed as a screening tool to detect hypohydration (body water deficit). No study has systematically evaluated SSΔHR cut points using different magnitudes or different types of controlled hypohydration. The objective of this study was to determine the diagnostic accuracy of the often proposed 20 b/min SSΔHR cut point using both hypertonic and isotonic models of hypohydration. Thirteen healthy young adults (8M, 5F) underwent three bouts of controlled hypohydration. The first bout used sweating to elicit large losses of body water (mass) (>3 % sweat). The second two bouts were matched to elicit 3 % body mass losses (3 % diuretic; 3 % sweat). A euhydration control trial (EUH) was paired with each hypohydration trial for a total of six trials. Heart rate was assessed after 3-min sitting and after 1-min standing during all trials. SSΔHR was compared among trials, and receiver operator characteristic curve analysis was used to determine diagnostic accuracy of the 20 b/min SSΔHR cut point. Volunteers lost 4.5 ± 1.1, 3.0 ± 0.6, and 3.2 ± 0.6 % body mass during >3 % sweat, 3 % diuretic, and 3 % sweat trials, respectively. SSΔHR (b/min) was 9 ± 8 (EUH), 20 ± 12 (>3 % sweat; P < 0.05 vs. EUH), 17 ± 7 (3 % diuretic; P < 0.05 vs. EUH), and 13 ± 11 (3 % sweat). The 20 beats/min cut point had high specificity (90 %) but low sensitivity (44 %) and overall diagnostic accuracy of 67 %. SSΔHR increased significantly in response to severe hypertonic hypohydration and moderate isotonic hypohydration, but not moderate hypertonic hypohydration. However, the 20 beats/min cut point afforded only marginal diagnostic accuracy.