Perioperative Changes in Blood Volume and Fluid Distribution in Total Hip Arthroplasty-A Protocol for a Prospective Single-Center Observational Study.

Abstract

BACKGROUND: Surgical stress response initiates a complex pathophysiological cascade that alters fluid distribution and intravascular volume. In elective total hip arthroplasty (THA), the perioperative blood loss is potentially substantial, a significant part of the loss being hidden caused by tissue extravasation, residual joint blood, and hemolysis. This far conventional methods to quantify blood loss rely on either visual estimation of bleeding or formulas based on hematological and/or anthropometric parameters, both of which are considered imprecise. Accurate assessment of perioperative blood volume changes may inform more individualized postoperative fluid therapy and improve postoperative outcomes. METHODS: The study design is a single-center prospective observational cohort study with enrollment of 25 patients undergoing elective THA in a standardized fast-track setting. Perioperative blood volume will be assessed using the carbon monoxide (CO)-rebreathing method, enabling quantification of total hemoglobin mass, red cell-, plasma-, and total blood volume. Bioimpedance spectroscopy will be used to evaluate fluid distribution between intra- and extracellular compartments, as well as total body water. Measurements will be performed preoperatively before surgery (baseline), at 6 h postoperatively, and at 24 h postoperatively when feasible. Demographic data, inflammatory markers, and data on perioperative fluid administration will also be collected. The primary outcome is change in total blood volume from baseline to 6 h postoperatively. Secondary outcomes include changes in hemoglobin mass, fluid compartment shifts, and their associations with postoperative orthostatic intolerance. DISCUSSION: By combining CO rebreathing and bioimpedance, our study applies accurate, minimally invasive methods not previously used in surgical patients to characterize blood loss and perioperative fluid shifts. Strengths include the standardized surgical setting, detailed protocol, and transparent statistical plan, whereas limitations include the observational design, minor sample size, and exclusion of patients with major intraoperative bleeding. CONCLUSION: The study will provide detailed information on the changes in perioperative blood volume and its association with orthostatic intolerance following THA. CO rebreathing may offer a precise alternative to conventional estimations of blood loss and contribute to the development of more individualized perioperative fluid management strategies in major orthopedic surgery.