Cerebral Oximetry (NIRS)

🔹 Introduction

Cerebral oximetry using Near-Infrared Spectroscopy (NIRS) is a non-invasive monitoring modality that provides continuous, real-time estimation of regional cerebral oxygen saturation (rSO₂). It is especially useful during surgeries with a high risk of cerebral hypoperfusion or desaturation, such as cardiac, carotid, and neurosurgical procedures.

🔹 Principle of Near-Infrared Spectroscopy (NIRS)

NIRS is based on the absorption characteristics of hemoglobin in the near-infrared light spectrum (700–1000 nm).

• Oxyhemoglobin (HbO₂) and deoxyhemoglobin (Hb) absorb light differently at specific wavelengths.
• Near-infrared light penetrates the scalp, skull, and superficial brain tissue (approximately 2.5–3.5 cm deep).
• The reflected light is analyzed to calculate the ratio of oxy- to deoxyhemoglobin, giving a measurement of regional cerebral oxygen saturation (rSO₂).

📌 Key Point: NIRS provides a venous-weighted saturation estimate (~70–75% venous, ~20% arterial, ~5% capillary), reflecting cerebral oxygen balance.

🔹 Technology Components

• Light Source: Emits near-infrared light at multiple wavelengths (typically 2–4).
• Detectors: Placed at varying distances (e.g., 3 cm and 4 cm from source) to differentiate superficial (scalp) and deep (cerebral) signals.
• Algorithm: Processes differential absorption data to output a numerical rSO₂ value.

🔹 Normal Values of rSO₂

• Normal baseline rSO₂ ranges from 55–75% in most adults.
• A decrease of >20% from baseline or absolute values <50% is considered clinically significant and may be associated with cerebral ischemia.

🔹 Applications of Cerebral Oximetry

🧠 1. Cardiac Surgery

• Monitoring during cardiopulmonary bypass (CPB), off-pump CABG, and aortic arch surgeries.
• Predicts neurological complications such as postoperative cognitive dysfunction (POCD) and stroke.
• Guides perfusion management and CPB flow adjustments.

🧠 2. Carotid Endarterectomy (CEA)

• Detects cerebral hypoperfusion during carotid clamping.
• May guide the need for shunt placement.

🧠 3. Neurosurgery

• Ensures adequate cerebral oxygenation during tumor resections, aneurysm surgeries, and awake craniotomies.

🧠 4. Orthopedic Surgery

• During procedures with a risk of embolism or hypotension (e.g., spine or hip surgeries in the sitting position).

🧠 5. Liver Transplantation

• Provides real-time monitoring during major hemodynamic shifts, especially during anhepatic and reperfusion phases.

🧠 6. Pediatrics and Neonates

• Particularly useful in congenital heart disease surgeries and extracorporeal membrane oxygenation (ECMO).
• Provides early detection of cerebral hypoxia before clinical signs manifest.

🔹 Advantages of Cerebral Oximetry

• Non-invasive and easy to apply
• Continuous and real-time monitoring
• Insensitive to movement artifacts
• Detects hemispheric asymmetry
• Useful in patients under deep anesthesia or sedation where neurological exams are not feasible

🔹 Limitations

🔹 Interpretation and Clinical Use

📌 Note: Always correlate with clinical context and other monitoring modalities (e.g., BIS, EEG, ICP, CPP).

🔹 Comparative Modalities

🔍 Suggested References

1. Miller’s Anesthesia, 9th Edition
2. British Journal of Anaesthesia (BJA) – Articles on cerebral monitoring
3. StatPearls – Cerebral Oximetry [NLM]
4. WFSA Resources – Cerebral monitoring in low-resource settings
5. Murkin JM, et al. Monitoring brain oxygen saturation improves outcome. Ann Thorac Surg. 2007.