Acute kidney injury (AKI) is a frequent and serious complication after cardiac surgery, including off-pump coronary artery bypass (OPCAB). Because AKI is diagnosed on the basis of serum creatinine, which rises 1 to 2 days after the actual renal insult, early detection and timely intervention remain difficult. Near-infrared spectroscopy (NIRS) allows non-invasive and continuous measurement of regional tissue oxygen saturation, but the effective penetration depth of commercially available devices is only approximately 2 to 2.5 cm. In most adults the renal cortex lies deeper than this, so the signal obtained from a flank sensor may originate largely from the abdominal wall musculature rather than from renal parenchyma. This limitation may explain the inconsistent association between renal regional oxygen saturation (renal rSO2) and AKI reported so far.
The erector spinae muscle at the level of the renal hilum lies within 2 cm of the skin in most patients and is supplied by the lumbar arteries and by vessels adjacent to the renal hilum. Erector spinae muscle regional oxygen saturation (ESrSO2) may therefore provide a technically more reliable NIRS signal that reflects perfusion of a vascular territory close to that of the kidney, particularly during the transient low cardiac output state induced by mechanical displacement of the heart for coronary anastomosis during OPCAB.
This single-center prospective observational cohort study will enroll 138 adult patients scheduled for elective OPCAB at Severance Hospital, Yonsei University Health System, Seoul, Republic of Korea. ESrSO2 and renal rSO2 will be measured continuously with an INVOS oximeter from before anesthetic induction until the end of surgery, in addition to cerebral rSO2, which is part of standard care at the participating institution. The ESrSO2 and renal rSO2 channels will be physically masked on the monitor display during surgery, no alarms will be set for these two channels, and their values will not be used for any intraoperative clinical decision. The recorded data will be extracted after surgery using the INVOS Analytics Tool. Apart from placement of the additional NIRS sensors and a brief pre-induction ultrasound measurement of tissue depth, no study-specific procedure, laboratory test, or imaging study will be performed, and all anesthetic, surgical, and postoperative care will follow the standard institutional protocol.
The primary objective is to identify which ESrSO2-derived variable best predicts postoperative AKI, defined by the Kidney Disease: Improving Global Outcomes (KDIGO) criteria within 7 days after surgery. The candidate variables are the baseline value, the intraoperative nadir, the area under the threshold (AUT), and the duration under the threshold below absolute values of 60%, 55%, and 50% and below 80% of the baseline value, calculated separately for the period before cardiac displacement and for the cardiac displacement period. Secondary objectives are to assess whether the selected ESrSO2 variable provides independent and incremental predictive information beyond a pre-specified baseline risk model, to compare the predictive performance of ESrSO2 with that of cerebral rSO2 and renal rSO2, and to evaluate the prediction of severe AKI (KDIGO stage 2 to 3) and persistent AKI (lasting 48 hours or longer).
If ESrSO2 proves to be a useful early marker of AKI, it could allow real-time, non-invasive identification of patients at high risk during surgery and provide a basis for future trials of renal protective strategies.
Adults aged 19 years or older scheduled for elective off-pump coronary artery bypass (OPCAB) surgery.
Able to receive a full explanation of the purpose and procedures of the study and to provide written informed consent voluntarily.
Depth from the skin to the erector spinae muscle at the level of the left renal hilum, measured by ultrasonography in the operating room before anesthetic induction, of less than 2.0 cm, so that stable near-infrared spectroscopy measurement is considered feasible. Consenting patients who do not meet this criterion are not enrolled and are classified as screen failures.
Emergency surgery.
Concomitant surgery performed by another department.
Preoperative or postoperative cardiogenic shock, or need for mechanical circulatory support.
Estimated glomerular filtration rate below 30 mL/min/1.73 m2.
Consecutive adult patients (19 years of age or older) scheduled for elective off-pump coronary artery bypass surgery at Severance Hospital, a tertiary university hospital in Seoul, Republic of Korea, who provide written informed consent. After consent, ultrasonography performed in the operating room before anesthetic induction measures the depth from the skin to the erector spinae muscle at the level of the left renal hilum; only patients in whom this depth is less than 2.0 cm, so that a reliable near-infrared spectroscopy signal can be obtained, are finally enrolled. Patients requiring emergency or concomitant surgery, those with severe renal impairment (estimated glomerular filtration rate below 30 mL/min/1.73 m2), previous kidney transplantation, left renal disease or a solitary kidney, a body mass index above 30 kg/m2, cardiogenic shock or mechanical circulatory support, or active bleeding or infection are not eligible.
Cohort
Prospective
Near-infrared spectroscopy monitoring of erector spinae muscle and renal regional oxygen saturation
DeviceNon-invasive near-infrared spectroscopy monitoring performed for research purposes only. Before anesthetic induction, ultrasonography is used to measure the depth from the skin to the erector spinae muscle and to the renal cortex at the level of the left renal hilum. In eligible participants, disposable NIRS sensors are applied over the left erector spinae muscle and over the ipsilateral renal cortex, and regional oxygen saturation is recorded continuously from before induction until the end of surgery. The erector spinae and renal channels are masked on the display and no alarms are set, so the values are not used for clinical decision-making; data are extracted after surgery with the INVOS Analytics Tool. Cerebral regional oxygen saturation is monitored as part of standard institutional care. No blood sampling, imaging, or other test is performed in addition to routine care, and no cost is charged to participants.
Predictive performance (area under the receiver operating characteristic curve, AUROC) of intraoperative erector spinae muscle regional oxygen saturation (ESrSO2) variables for postoperative acute kidney injury
Receiver operating characteristic curves are constructed for each candidate ESrSO2 variable and the AUROC values are calculated and compared in order to identify the variable with the highest predictive performance for acute kidney injury. Candidate variables are: (1) baseline ESrSO2 measured before anesthetic induction on room air; (2) intraoperative nadir ESrSO2; (3) area under the threshold (AUT), defined as the integral of the magnitude and duration of desaturation below a given threshold, for absolute values below 60%, 55%, and 50% and for values below 80% of baseline; and (4) duration under the same thresholds. AUT and duration under the threshold are calculated separately for the period before cardiac displacement and for the cardiac displacement period, which is defined as the interval from the deep pericardial stitch to heart return. All values are computed with the INVOS Analytics Tool Software. Acute kidney injury is defined according to the KDIGO criteria as an increase in
Independent and incremental predictive value of the selected ESrSO2 variable for acute kidney injury after adjustment for pre-specified established risk factors
The ESrSO2 variable selected in the primary analysis is added as an incremental term to a pre-specified baseline risk model consisting of age, preoperative renal function (baseline estimated glomerular filtration rate or chronic kidney disease), diabetes mellitus, and hypoalbuminemia. These four covariates are fixed on the basis of clinical importance and previous OPCAB-AKI literature and are combined into a single linear predictor rather than being re-tested in this cohort; the only formally tested term is the ESrSO2 variable. Firth penalized logistic regression with bootstrap internal validation is used because of the limited number of expected events per variable. Improvement in discrimination is assessed by the difference in AUROC using the DeLong test, and clinical usefulness is explored by decision curve analysis (net benefit). This analysis is explicitly exploratory and is not intended to derive or validate a definitive prediction model.
Comparison of the predictive performance (AUROC) for acute kidney injury among ESrSO2, cerebral rSO2, and renal rSO2
Receiver operating characteristic curves are constructed separately for ESrSO2, cerebral rSO2, and renal rSO2 variables, and the AUROC values are calculated and compared with the DeLong test.
Predictive performance of the selected ESrSO2 variable for severe acute kidney injury (KDIGO stage 2 to 3)
Severe acute kidney injury is defined as KDIGO stage 2 or 3 within 7 days after surgery, and predictive performance is assessed with the same methods as for the primary outcome.
Predictive performance of the selected ESrSO2 variable for persistent acute kidney injury (lasting 48 hours or longer)
Persistent acute kidney injury is defined as acute kidney injury lasting 48 hours or longer, and predictive performance is assessed with the same methods as for the primary outcome.
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