Cerebral Oxygenation Monitoring During Coronary Artery Bypass Grafting and Its Correlation with Hematocrit, Mean arterial pressure, and Partial pressure of Oxygen in Arterial Blood

Introduction: Optimal cerebral oxygenation is vital during coronary artery bypass grafting (CABG) to prevent neurological complications like cognitive decline and stroke. Non-invasive monitoring methods include near-infrared spectroscopy (NIRS), electroencephalography (EEG), and transcranial doppler (TCD). It offers real-time rSO2 assessment, detecting critical thresholds and reducing risks during cardiopulmonary bypass (CPB). Objective: This observational study aims to investigates cerebral oxygenation changes during CABG and correlations with hematocrit, mean arterial pressure (MAP), blood oxygen levels, CPB flows, and temperature. Methods: Seventy-two elective CABG patients underwent CPB with parameters including rSO2, hematocrit, MAP, PaO2, temperature, and pump flows assessed at specific time points: T1: Baseline pre-anesthesia; T2: Post-anesthesia induction (FiO2 100%); T3: Post-anesthesia induction (FiO2 50%); T4: CPB initiation; T5: CPB at 35°C; T6: CPB at 32°C; T7: CPB rewarming (36°C); T8: Post-CPB weaning (FiO2 100%); T9: Post-CPB weaning (FiO2 50%). Results: The mean baseline values for rSO2 were 72.14 for the right side and 71.90 for the left side. Upon initiating CPB at 35°C, a significant maximum reduction in rSO2 of 10.5% was observed, which remained below baseline during the hypothermia phase. The rSO2 values began to increase during the rewarming phase, nearly reaching baseline levels after CPB. A post hoc analysis indicated that changes in rSO2 were correlated with variations in hematocrit (correlation coefficient = 0.518), MAP (correlation coefficient = 0.399), and PaO2 (correlation coefficient = 0.001). Conclusion: This study explored the fluctuations in rSO2 during CABG with CPB and examined its correlations with hematocrit, MAP, PaO2, CPB flows, and temperature. The findings highlight significant correlations among these variables, providing insights into factors influencing cerebral oxygenation during cardiac surgery.

INTRODUCTION

Optimal cerebral oxygenation is crucial for maintaining brain function and preventing cerebral injury. The observation of cerebral oxygenation during Coronary artery bypass grafting (CABG) surgery has gained increasing attention in recent years as a means of identifying and mitigating potential cerebral oxygen imbalances. Despite progress, cerebral complications during and after CABG with Cardiopulmonary bypass (CPB) pose risks due to the brain's vulnerability to ischemic events, leading to cognitive decline, stroke, and mortality (¹). A significant correlation exists between regional cerebral oxygen desaturation and neurological complications after cardiac surgery (²). Monitoring regional cerebral oxygen saturation (rSO₂) has predictive value in decreasing the incidence of early postoperative cognitive decline (³). Therefore, maintaining optimal cerebral oxygenation during CABG is crucial to alleviate these risks.

Numerous non-invasive cerebral oxygenation monitoring techniques exist, including near-infrared spectroscopy (NIRS), Electroencephalography (EEG), and transcranial Doppler (TCD) ultrasound. EEG has proven useful for early detection of imbalances between cortical tissue oxygen supply and demand (⁴). However, EEG monitors are not highly specific for ischemic injuries, as such imbalances are not always caused by blood flow variations (⁴). Other factors, such as non-convulsive seizures or prior sub-clinical traumatic cortical injuries, can also influence the readings (⁵). TCD complements EEG by assessing blood flow velocity in cerebral arteries, aiding in emboli detection (⁴). However, maintaining a stable probe position during surgery is challenging, as it involves securing the probe in place with a sterile sleeve using a band strapped around the patient's head (⁴). NIRS emerges as a superior alternative since near-infrared light can easily penetrate the skull, it enables real-time assessment of rSO2 using sensors placed on the patient’s forehead (⁶). By monitoring both hemispheres, this technology can distinguish between global and unilateral causes of hypoperfusion, such as changes in head position or unilateral vessel occlusion. Additionally, it does not rely on pulsatile blood flow, making it particularly advantageous during cardiopulmonary bypass procedures (⁴). In addition to being noninvasive, NIRS has a response time of 10.9 seconds (⁷) about to changes in CBF. NIRS is established as a comprehensive and effective neuromonitoring modality in cardiovascular surgeries, surpassing the limitations associated with EEG and TCD.

Interrupted blood circulation during CABG makes the brain vulnerable to ischemic incidents, manifesting as temporary impairments or severe conditions like as stroke. An rSO2 level below 45% or a decrease of 25% from individual baseline values is considered a critical threshold, indicating a higher risk of adverse neurological outcomes (⁶).

Numerous unresolved questions remain about the effects of hemodilution, hypothermia, and PaO2 on cerebral oxygenation. This study aimed to investigate alterations in cerebral oxygenation in patients undergoing CABG with CPB and examine potential correlations between cerebral oximetry measurements and factors such as hematocrit levels, mean arterial pressure, arterial oxygen levels, CPB flows, and temperature.

METHODS

This observational study was conducted at Swai Man Singh Medical College, Jaipur from September to December 2023, following approval from the office of the ethics committee, S.M.S. Medical College and attached hospitals, Jaipur (No. 199/MC/EC/2023 Dated 05^thApril 2023) and registration with the Clinical Trials Registry India (CTRI/2023/07/055738 Dated 26^thJuly 2023). The study included adult patients scheduled for CABG with CPB under the care of a single surgeon, who met the inclusion criteria. Patients were excluded if they required emergency surgery, underwent off-pump CABG, had evidence of carotid disease, had a history of cerebrovascular accident or syncope, suffered from liver or kidney disease, acute coronary syndrome, or severe uncontrolled hypertension (MAP > 150 mmHg).

A total of 72 patients were selected for the study by purposive sampling. Upon arrival in the operating theatre, the patient’s fasting status, written informed consent, and pre-anesthetic assessment results were verified. Standard routine monitors, including Non-invasive blood pressure (NIBP), oxygen saturation (SpO2) probe, and Electrocardiogram (ECG), were applied, and baseline parameters (heart rate, systolic blood pressure, diastolic blood pressure, mean arterial pressure, and oxygen saturation) were recorded. Femoral arterial cannulation for invasive blood pressure monitoring and right internal jugular vein cannulation were performed under local anesthesia.

Two NIRS sensors (ForeSight Elite large sensor, model FSESL) were positioned bilaterally on the forehead, and rSO2 was constantly measured with the ForeSight Tissue Oximeter Monitor (Edwards Life Sciences). Measurements of rSO2R, rSO2L, hematocrit, MAP, and PaO2 were obtained at T1 [Baseline before anesthesia induction (FiO2 21%)]. Preoxygenation was conducted using 100% oxygen for a duration of 3 to 5 minutes. General anesthesia was induced with intravenous midazolam (0.05 mg/kg), fentanyl (3-5 mcg/kg), and etomidate (3-5 mg/kg). Endotracheal intubation was facilitated with intravenous rocuronium bromide (1 mg/kg), and the correct position of the tube was confirmed by 5-point auscultation and EtCO2 measurements.

Monitoring of end-tidal gases and nasopharyngeal temperature commenced post-intubation. Mechanical ventilation was configured with a tidal volume of 8 - 10 mL/kg and a 50:50 mixture of air and oxygen to achieve a PaCO2 of 35–40 mmHg. During the pre-CPB period, anesthesia was maintained with sevoflurane (1 MAC), supplemented with fentanyl (2-5 mcg/kg) and midazolam (0.05 mg/kg). Anesthesia management was maintained consistently during and after CPB according to institutional protocols, incorporating additional boluses of fentanyl, midazolam, and vecuronium bromide. Blood sugar levels were maintained below 180 mg/dL during the surgery in all patients.

Measurements of rSO2R, rSO2L, hematocrit, MAP, PaO₂, and CPB flows were recorded at

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