A mini volume loading test for indication of preoperative dehydration in surgical patients

Background and objective: Previously, a mini volume loading test (mVLT) detected signs of dehydration in healthy volunteers after an overnight fast. Our objective was to investigate whether mVLT could indicate preoperative dehydration in patients after an overnight fast.
Materials and methods: The mVLT was performed in 36 elective primary total knee arthroplasty patients. Each subject received three fluid challenges before anesthesia induction. These consisted of 5 mL/kg boluses of Ringer's acetate infused over 3–5 min and followed by a 5-min period without fluids. Invasive (arterial, venous) and noninvasive (capillary) measurements of hemoglobin concentration were performed before and after each fluid challenge, as well as after a 20-min period without fluids which followed the last bolus. Arterial, venous and capillary plasma dilutions were calculated in every data point. Dilution values were used to calculate the plasma dilution efficacy of each fluid challenge.
Results: Venous dilution was higher than capillary after the first fluid challenge (P = 0.030), but lower than capillary after 20 min period following the last bolus (P = 0.009). Arterial dilution was lower than capillary (P = 0.005) after 20 min following the last bolus. Venocapillary and arterio-capillary plasma dilution efficacy differences decreased (P = 0.004 and P = 0.033, respectively) from positive to negative during mVLT. These are signs of re-hydration from pre-existing dehydration according to a transcapillary reflux model.
Conclusions: Signs of dehydration were observed during mVLT in patients after pre-operative overnight fast. A revised transcapillary reflux model was proposed to explain the results.