The Effect of Hetastarch on Specific Gravity and Osmolality of Urine in the Dog
William R. Pritchard Veterinary Medical Teaching Hospital, University of California
Davis, CA, USA
Urine osmolality (UOSM) is a measure of the number of solutes within a given weight of urine, and signifies renal concentrating ability. Urine specific gravity (USG) is used clinically to estimate UOSM. Although USG has been shown to have a linear correlation with UOSM in dogs, the relationship is altered when there are significant numbers of high molecular weight (MW) molecules in the urine. The purpose of this investigation was to evaluate the effect of hetastarch (HES), with an average MW 600 and degree of substitution 0.7, on USG as compared to UOSM. Our hypothesis was that USG would no longer predict UOSM in dogs given intravenous HES.
Eight healthy, employee-owned dogs were included in the treatment group. Four of these dogs also served as the control group. USG and UOSM were measured every 30 minutes from t=0 minutes to t=360 minutes. Dogs were administered 20mL/kg of either NaCl 0.9% (control group, n=4) or HES (treatment group, n=8) IV over one hour starting at t=90 minutes. Washout period between experimental protocols for the control group was a minimum of 4 weeks. Two-way repeated measures analysis of variance was used to assess changes over time and differences between control and treatment groups. Time-specific group differences were evaluated using Student's t-test with a sequentially rejective method of multiple comparison adjustment.
There was a decrease in UOSM in both groups starting at t=120 minutes and continuing for the study duration, and there was no significant difference in UOSM between treatment and control groups across all time points. There was an appropriate decrease in USG from t=120 minutes for the control group. In the treatment group, USG increased significantly at t=120 minutes (p=0.0006), t=150 minutes (p=0.0002) and t=180 minutes (p=0.0044). The most remarkable increase in USG occurred at t=150 minutes with a mean USG of 1.070 ± 0.021 (range 1.038-1.104).
Given the results of this study, USG does not correlate with UOSM, and therefore does not reflect renal concentrating ability, after a one-hour infusion of 20mL/kg of HES. Interpretation of an increased USG after this therapy as an indicator of concentrated urine, rather than as an effect of hetastarch molecules within the urine, may lead to unfavorable treatment decisions.