AIM To investigate the microvascular(skeletal muscle tissue oxygenation; SmO_2) response to transfusion in patients undergoing elective complex spine surgery.METHODS After IRB approval and written informed consent, 20 patients aged 18 to 85 years of age undergoing > 3level anterior and posterior spine fusion surgery were enrolled in the study. Patients were followed throughout the operative procedure, and for 12 h postoperatively. In addition to standard American Society of Anesthesiologists monitors, invasive measurements including central venous pressure, continual analysis of stroke volume(SV), cardiac output(CO), cardiac index(CI), and stroke volume variability(SVV) was performed. To measure skeletal muscle oxygen saturation(SmO_2) during the study period, a non-invasive adhesive skin sensor based on Near Infrared Spectroscopy was placed over the deltoid muscle for continuous recording of optical spectra. All administration of fluids and blood products followed standard procedures at the Hospital for Special Surgery, without deviation from usual standards of care at the discretion of the Attending Anesthesiologist based on individual patient comorbidities, hemodynamic status, and laboratory data. Time stamps were collected for administration of colloids and blood products, to allow for analysis of SmO_2 immediately before, during, and after administration of these fluids, and to allow for analysis of hemodynamic data around the same time points. Hemodynamic and oxygenation variables were collected continuously throughout the surgery, including heart rate, blood pressure, mean arterial pressure, SV, CO, CI, SVV, and SmO_2. Bivariate analyses were conducted to examine the potential associations between the outcome of interest, SmO_2, and each hemodynamic parameter measured using Pearson’s correlation coeffi-cient, both for the overall cohort and within-patients individually. The association between receipt of packed red blood cells and SmO_2 was performed by running an interrupted time series model, with SmO_2 as our outcome, controlling for the amount of time spent in surgery before and after receipt of PRBC and for the inherent correlation between observations. Our model was fit using PROC AUTOREG in SAS version 9.2. All other analyses were also conducted in SAS version 9.2(SAS Institute Inc., Cary, NC, United States).RESULTS Pearson correlation coefficients varied widely between SmO_2 and each hemodynamic parameter examined. The strongest positive correlations existed between ScvO_2(P = 0.41) and SV(P = 0.31) and SmO_2; the strongest negative correlations were seen between albumin(P =-0.43) and cell saver(P =-0.37) and SmO_2. Correlations for other laboratory parameters studied were weak and only based on a few observations. In the final model we found a small, but significant increase in SmO_2 at the time of PRBC administration by 1.29 units(P = 0.0002). SmO_2 values did not change over time prior to PRBC administration(P = 0.6658) but following PRBC administration, SmO_2 values declined significantly by 0.015 units(P < 0.0001).CONCLUSION Intra-operative measurement of SmO_2 during large volume, yet controlled hemorrhage, does not show a statistically significant correlation with either invasivehemodynamic, or laboratory parameters in patients undergoing elective complex spine surgery. AIM To investigate the microvascular (skeletal muscle tissue oxygenation; SmO_2) response to transfusion in patients undergoing elective complex spine surgery. METHODS After IRB approval and written informed consent, 20 patients aged 18 to 85 years of age undergoing> 3level anterior and posterior spine fusion surgery were enrolled in the study. Patients were followed throughout the operative procedure, and for 12 h postoperatively. In addition to standard American Society of Anesthesiologists monitors, invasive measurements including central venous pressure, continual analysis of stroke volume (SV), cardiac output ( CO), cardiac index (CI), and stroke volume variability (SVV) was performed. To measure skeletal muscle oxygen saturation (SmO_2) during the study period, a non-invasive adhesive skin sensor based on Near Infrared Spectroscopy was placed over the deltoid muscle for continuous recording of optical spectra. All administration of fluids and blood products followed standard procedures at the Hospital for Special Surgery, without deviation from usual standards of care at the discretion of the Attending Anesthesiologist based on individual patient comorbidities, hemodynamic status, and laboratory data. Time stamps were collected for administration of colloids and blood products, to allow for analysis of SmO_2 immediately before, during, and after administration of these fluids, and to allow for analysis of hemodynamic data around the same time points. Hemodynamic and oxygenation variables were collected throughout the surgery, including heart rate, blood pressure, mean arterial pressure, SV, CO, CI, SVV, and SmO_2. Bivariate analyzes were conducted to examine the potential associations between the outcome of interest, SmO_2, and each hemodynamic parameter measured using Pearson’s correlation coeffi- cient, both for the overall cohort and within-patients individually The association between receipt of packed red blood cells and SmO_2 was performed by running an interrupted time series model, with SmO_2 as our outcome, controlling for the amount of time spent in surgery before and after receipt of PRBC and for the inherent correlation between observations. Our model was fit using PROC AUTOREG in SAS version 9.2. All other Analyzes were also conducted in SAS version 9.2 (SAS Institute Inc., Cary, NC, United States) .RESULTS Pearson correlation coefficients varied widely between SmO_2 and each hemodynamic parameter examined. The strongest positive correlations existed between ScvO_2 (P = 0.41) and SV (P = 0.31) and SmO_2; the strongest negative correlations were seen between albumin (P = -0.43) and cell saver (P = -0.37) and SmO_2. Correlations for other laboratory parameters studied were weak and only based on a few observations. In the final model we found a small, but significant increase in SmO_2 at the time of PRBC administration by 1.29 units (P = 0.0002). SmO_2 values ​​did not change over time prior to PRBC administration (P = 0.6658) but foll due PRBC administration, SmO_2 values ​​significantly significantly by 0.015 units (P <0.0001) .CONCLUSION Intra-operative measurement of SmO_2 during large volume, yet controlled hemorrhage, does not show a said statistically significant with either invasive hemodynamic, or laboratory parameters in patients undergoing elective complex spine surgery.