Calculation of Blood Loss in Cardiac Surgery: How Should We Monitor?

Abstract

Introduction Perioperative blood loss in cardiac surgery is still a significant problem [1]. There is no gold standard method to assess actual blood loss [2]. Traditional methods are based on volumetric measurement, which can be inaccurate as they can be under- or overestimated [3]. It is important to understand how the same amount of blood loss affects hemodynamics and postoperative complications in patients with different body mass [4]. Moreover, the fluid draining from the chest tube can be either haemorrhagic, serous or a combination of both [5]. We hypothesized and calculated blood loss using the Hb/kg index, which is based on hemoglobin loss per kilogram and proved to be a more accurate method of calculating actual blood loss. Aim To develop an empirical formula and a novel approach to calculate actual blood loss in terms of hemoglobin loss per kilogram (Hb/kg index). Methods In a prospective observational study, a total of 195 patients who underwent cardiac surgery with cardiopulmonary bypass were included over a period of 13 months (October 2023- November 2024). To calculate the Hb/kg index, hemoglobin loss via chest tube drainage, intraoperative hemoglobin loss, transfusion of packed red blood cells and patient weight were taken into account. An analysis of 86 predictors of the Hb/kg index was performed. Correlation coefficients Pearson r and Spearman rho between the predictors and the Hb/kg index were computed. Predictors that showed statistically significant Spearman correlations were selected for further analysis in Robust Linear Regression model. Results The robust linear regression model was fitted using Huber’s T norm estimator in Ordinary Least Squares (OLS). The determination coefficient R-squared was equal 0.983 and indicated that 98% of the variance in the Hb/kg index was explained by the predictors. The adjusted R-squared was equal 0.981. The linear regression model was statistically significant, F(27,167) = 366.3, p < 0.001. The linear regression analysis revealed several significant predictors of the Hb/kg index. Body mass index (-0.0303, p < 0.001) showed a significant negative relationship, indicating that low body mass index leads to an increase in the Hb/kg index. Predictors such as hemoglobin mass before surgery (0.0430, p < 0.001), hemoglobin loss via chest tube (0.0110, p < 0.001) and intraoperative hemoglobin loss (0.0033, p < 0.001) were found to be highly significant and positively correlated. Conversely, transfusion of hemoglobin mass (packed red blood cells) showed significant negative effects (-0.0136, p < 0.001). Conclusions This method provides a more reliable and clinically relevant tool for calculation actual blood loss, providing a more precise and individualized evaluation. This approach could be a robust research tool, although further studies are needed to demonstrate its reliability.