Patients
Institutional review board approval was waived as patients’ private information was not retrieved. We retrospectively reviewed the medical date of 765 patients undergoing posterior spinal fusion surgeries for adolescent idiopathic scoliosis from January 2014 to December 2018 in our hospital. All the surgeries were performed by highly experienced surgeons using a uniform approach. Inclusion criteria were patients (1) younger than 18 years and diagnosed adolescent idiopathic scoliosis; (2) underwent posterior spinal fusion surgery; (3) obtained stable perioperative fluid balance and hemodynamics. Exclusion criteria were patients (1) with coagulation disorders or perioperative infection; (2) with anticoagulant drugs; (3) with intraoperative blood loss greater than 2.5 L on account to larger bias with excessive blood loss[6]; (4) suffered cerebrospinal fluid leakage.
Date Extraction
Demographic information were collected from electronic medical record including age, sex, height, weight, body mass index (BMI), preoperative and postoperative hematocrit (Hct) and hemoglobin (Hb), American Society of Anesthesiologists (ASA) classification, preoperative Cobb angle, prothrombin time and activated partial thromboplatin time; Surgery information included operation time, pedicle screw number, bone mineral density, the number of surgical segments, whether undergoing osteotomy, the use of tranexamic acid, intraoperative blood loss, postoperative drainage volume, and allogeneic and autologous blood transfusion volume.
Blood Loss Management and Calculation of HBL
Intraoperative blood loss was recorded by the anesthesiologist, which was mainly comprised of the blood in the suction apparatus and in the soaked gauzes that were used during the entire operation. Postoperative blood loss was calculated though measuring the blood volume in the hemovac. Most of the hemovac was removed on the third postoperative day, if the drainage tube can not be removed when calculating the blood loss, we measured the blood in the drainage tube on the day when the blood was taken. The total visible blood loss was calculated as the sum of intraoperative blood loss and postoperative drainage. The blood loss during the operative procedure could be collected by blood salvage technique and reinfusion to the patient as autologous blood, which was decided by the anesthesiologist. Patients were given blood transfusion when hemoglobin level was below 70 g/dL, or below 80 g/dL with a significant symptom of anemia, such as increased heart rate hypotension. We calculated the patients blood volume (PBV) according to the formula described by Nadler et al. [7] PBV (L) = k1 × height (m)3 + k2 × weight (kg) + k3, (k1 = 0.3669, k2 = 0.03219, k3 = 0.6041 for male, and k1 = 0.3561, k2 = 0.03308, k3 = 0.1833 for female). The total red cell volume can be calculated through multiplying the PBV and the patients Hct together. Therefore, the reduction in Hct would reflect the change in red cell volume [8]. As the hemorrhage is continuing, the fluid persist transfusing to sustain the patients circulating volume. The linear formula proposed by Gross was found intimately following logarithmic formula [9]. According to Gross formula [9], total blood loss = PBV (Hctpre - Hctpost)/Hctave, where Hctpre is the initial preoperative Hct, Hctpost was Hct on the second or the third day postoperatively, and Hctave is the average of the Hctpre and the Hctpost. Hidden blood loss was calculated by subtracting the visible blood loss from the total blood loss according to the formula of Sehat et al. [10] The formula was
Hidden blood loss = total blood loss - visible blood loss;
When perioperative blood transfusion was performed, Hidden blood loss = total blood loss + allogeneic blood transfusion + autologus blood transfusion - visible blood loss.
Statistical Analysis
All date analyses was performed using IBM SPSS 22.0 software. Categorical variables are presented as frequencies and the chi-squared test was took to compare dichotomous variables, normally distributing continuous variables are presented as means ± SD deviations and independent sample t test was used to compare intergroup difference. The Pearson correlation (used for the normal date), Spearman correlation analysis (used for the non-normal date) and multivariate linear regression analysis were performed to identify risk factors of HBL, including 13 quantitative variables (age, ASA classification, BMI, preoperative Hct, preoperative Cobb angle, prothrombin time, activated partial thromboplatin time, bone mineral density, operation time, segment number, pedicle screw number, autologous blood transfusion, allogeneic blood transfusion, postoperative drainage) and qualitative variables (osteotomy, tranexamic acid and gender). In the qualitative variables, osteotomy and the tranexamic acid were set as “1”. Non-osteotomy and non-tranexamic acid were set as “0”. The binary logistic regression model was established to identify the the relationship between hidden blood loss and postoperative transfusion. Statistically significant continuous variables were converted to categorical variables through their cut-off points, which was determined by receiver operating characteristic curve (ROC). The level of statistical significance was set at P < 0.05.