Acute kidney injury (AKI) is one of the most common and clinically significant complications following cardiac surgery performed with cardiopulmonary bypass (CPB). Even small increases in serum creatinine after cardiac surgery have been associated with increased morbidity, mortality, and healthcare costs. Early identification of renal hypoperfusion during surgery could allow clinicians to intervene before permanent kidney damage occurs. The article titled “A Prospective Observational Study on Role of Intraoperative Renal & Muscle Oxygen Saturation on Post-Operative Serum Creatinine Level in Patients Undergoing Cardiac Surgery on Cardiopulmonary Bypass” investigates the predictive value of tissue oxygenation monitoring for identifying patients at risk of post-operative renal dysfunction.
The researchers explored the use of near-infrared spectroscopy (NIRS), a non-invasive monitoring technique that measures regional tissue oxygen saturation in real time. NIRS has become increasingly popular in cardiac anesthesia and perfusion because it provides continuous information about tissue perfusion and oxygen delivery at the microcirculatory level. In this study, NIRS sensors were used to monitor renal regional oxygen saturation (SrO₂) and thenar muscle oxygen saturation (SmO₂) during cardiac surgery.
The study was conducted as a prospective observational trial at a tertiary care hospital over an 18-month period. A total of 55 adult patients between the ages of 18 and 60 years who were scheduled for elective open-heart surgery requiring cardiopulmonary bypass were enrolled. Patients with pre-existing renal impairment or anatomical factors preventing accurate NIRS measurements were excluded. Before surgery, all patients underwent ultrasound evaluation to confirm that the depth from the skin surface to the kidney was less than 4 cm, ensuring reliable NIRS measurements.
Baseline oxygen saturation values for both renal and muscle tissues were recorded prior to induction of anesthesia. NIRS sensors were positioned on the patient’s left flank to measure renal oxygen saturation and on the thenar eminence (thumb muscle) to measure muscle oxygen saturation. These values were continuously monitored throughout the surgical procedure.
The investigators focused on analyzing relative decreases from baseline oxygen saturation levels rather than absolute values. Specifically, they calculated the area under the curve (AUC) representing both the magnitude and duration of desaturation below defined thresholds. For renal oxygen saturation, a decrease greater than 20% from baseline was evaluated as a potential predictor of post-operative renal dysfunction. For muscle oxygen saturation, decreases of 10%, 15%, and greater than 20% from baseline were analyzed.
Post-operative kidney function was assessed using serum creatinine measurements at 24, 48, and 72 hours after surgery. Even small increases in serum creatinine were considered clinically relevant because previous research has demonstrated that minimal changes in creatinine after cardiac surgery may significantly impact patient outcomes.
The study results revealed several important findings. Among the 55 patients included in the analysis, 24 patients (43.6%) experienced a small post-operative increase in serum creatinine. Statistical analysis using receiver-operating characteristic (ROC) curves demonstrated that renal oxygen saturation was a strong predictor of these increases. A greater than 20% decrease in SrO₂ from baseline showed excellent predictive ability, with an area under the ROC curve (AUROC) of 0.921, indicating high diagnostic accuracy.
Muscle oxygen saturation also demonstrated predictive value, although slightly lower than renal measurements. A 15% decrease in SmO₂ from baseline was found to be the optimal threshold, with an AUROC of 0.843, outperforming the predictive value of a 20% decrease. This suggests that even moderate decreases in peripheral muscle oxygenation may signal systemic hypoperfusion and impending renal dysfunction.
A key objective of the study was to determine whether peripheral muscle oxygenation could serve as a surrogate marker for renal oxygenation. Renal NIRS monitoring can sometimes be technically challenging due to factors such as variable tissue depth, limited surgical access to the flank region, and potential sensor contamination during surgery. Monitoring the thenar muscle, however, is significantly easier because it is superficial and easily accessible.
The researchers performed Spearman correlation analysis to examine the relationship between renal and muscle oxygen saturation changes. The results showed a strong positive correlation between decreases in SmO₂ and decreases in SrO₂. Specifically, a 15% decrease in muscle oxygen saturation had a correlation coefficient of 0.593, while a 20% decrease showed an even stronger correlation of 0.606 with renal oxygen desaturation. These findings suggest that peripheral muscle oxygen saturation monitoring could serve as a practical surrogate for renal perfusion monitoring in clinical settings.
The study findings align with previous research indicating that intraoperative tissue oxygenation monitoring can help predict postoperative kidney injury. For example, earlier studies have shown that prolonged renal oxygen desaturation during cardiac surgery is associated with increased risk of AKI. Additionally, other investigations have demonstrated that muscle oxygen saturation monitoring can reflect systemic perfusion changes during major surgery.
Despite the promising findings, the authors acknowledged several limitations. The sample size was relatively small, and the study was conducted at a single center. Long-term follow-up was not performed to evaluate whether intraoperative desaturation events had lasting renal consequences. Furthermore, the study did not explore interventions aimed at correcting tissue oxygen desaturation in real time.
In conclusion, this study provides important evidence supporting the use of near-infrared spectroscopy monitoring during cardiac surgery. Both renal and thenar muscle oxygen saturation measurements were found to be reliable predictors of postoperative increases in serum creatinine. The strong correlation between renal and muscle oxygenation suggests that thenar muscle NIRS monitoring could serve as a practical and accessible surrogate for renal perfusion monitoring, particularly when renal sensor placement is difficult.
These findings highlight the potential of real-time tissue oxygenation monitoring to improve perioperative management, identify patients at risk of acute kidney injury, and guide interventions to optimize perfusion during cardiopulmonary bypass.
