Erector Spinae Oxygen Saturation and AKI After OPCAB

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.