Haematocrit and urine volume response to acute intravenous saline and oral water loading in normal volunteers

Abstract

Introduction: Haematocrit (Hct) is widely recognized as a marker of haemodilution. Acute oral water (OW) and intravenous saline (IVS) have variable effects on blood volume. We aimed to test the utility of repeated measures of Hct in mapping this response.Methods: 30 normal volunteers (16 male; age 48.7 ± 8.6, range 48–63; body mass index 24.7 ± 2.9 Kg/m2, range 19.1–31.9) were given in random order, no intervention (C), OW (1.5 L over 20 minutes) or IVS (1.5 L over 20 minutes) on 3 separate days. Hct was measured at 0, 0.5, 1, 2, 4, 6 and 8 h, and urine volume was measured at 4 and 8 h post intervention.Results: Baseline Hct was similar before all treatments [C 40.8 (3.5); OW 41.5 (3.4); IVS 41.9 (4.3); p=0.083] and at 8h [C 39.4 (3.0); OW 39.5 (2.9); IVS 38.7 (3.3); p= 0.118]. On C, there was a significant decrease in Hct between 0 and 0.5 h [41.0 (3.4) versus 38.9 (2.8); p<0.0001]. The fall in Hct was significant and greater for IVS compared to OW, but not between C and either IVS or OW (Figure 1). Urine output rose significantly following both OW and IVS compared to C, with larger increase following OW (Figure 2).Conclusion: The small relative rise in Hct following OW compared to C suggesting a transient reduction in blood volume is in keeping with previous studies. Urine volume response following OW is in contrast to that seen with Hct changes. The mediators of this differential response are unclear.Funding: Haemostasis Thrombosis and Vascular Biology Unit charitable funds.