A mini volume loading test (mVLT) using 2.5-mL kg⁻¹ boluses of crystalloid for indication of perioperative changes in hydration status

Background and objective: A mini volume loading test (mVLT) evaluating hemodilution during step-wise crystalloid infusion has established that the arterio-capillary plasma dilution difference is inversely correlated to the body hydration level of subjects. This observational study aimed to test whether this can be replicated in a perioperative setting using a 2.5-mL kg⁻¹ boluses.
Materials and methods: The mVLT was performed before induction of regional anesthesia and 24 h later. Step-wise infusion implied six mini fluid challenges. These consisted of 2.5-mL kg⁻¹ boluses of Ringer's acetate infused during 2–3 min and followed by 5-min periods with no fluids. Invasive (arterial) and noninvasive (capillary) measurements of hemoglobin were performed before and after each mini fluid challenge, as well as after a 20-min period without fluid following the last bolus. Hemoglobins were used to calculate the arteriocapillary plasma dilution difference which is used as an indication of changes in body hydration level. The 24-h fluid balance was calculated.
Results: Subjects were 69.5 (6.0) years old, their height was 1.62 m (1.56–1.65), weight was 87.0 kg (75.5–97.5) and body mass index (BMI) was 33.5 kg/m² (31.0–35.1). Preoperative arterio-capillary plasma dilution difference was significantly higher than postoperative (0.085 [0.012–0.141] vs. 0.006 [−0.059 to 0.101], P = 0.000). The perioperative 24-h fluid balance was 1976 mL (870–2545).
Conclusions: The mVLT using 2.5-mL kg⁻¹ boluses of crystalloid was able to detect the higher postoperative body hydration level in total knee arthroplasty patients.