Near-infrared Spectroscopy (NIRS) is a non-invasive monitoring technology that assesses the balance between oxygen supply and demand within a specific area of the body. It operates by measuring the oxygen saturation of hemoglobin directly in the tissue beneath the sensor. This provides a continuous, real-time window into how well an organ, such as the brain or kidney, is being perfused and oxygenated. Monitoring these localized oxygen levels is important in critical care settings, where early detection of a supply-demand mismatch guides treatment decisions.
How Near-Infrared Spectroscopy Works
The core of Near-Infrared Spectroscopy relies on the principle that biological tissue is relatively transparent to light in the near-infrared range, typically between 700 and 1100 nanometers. This specific wavelength range allows the light to penetrate several centimeters deep through skin, bone, and fat to reach the underlying blood vessels and tissue. The most important light-absorbing molecules, or chromophores, in this region are oxygenated hemoglobin ($OxyHb$) and deoxygenated hemoglobin ($DeoxyHb$).
The NIRS monitor employs a sensor pad, which contains a light source and at least one detector placed a set distance apart on the skin. The light source emits photons into the tissue, and as these photons travel, they are scattered and absorbed by the $OxyHb$ and $DeoxyHb$ molecules before a portion of them returns to the detector. Because $OxyHb$ and $DeoxyHb$ absorb light differently at various wavelengths, the monitor can distinguish between them.
The device measures the amount of light absorbed at two or more distinct wavelengths to calculate the relative concentrations of both forms of hemoglobin. This calculation uses a mathematical model, based on the Beer-Lambert law, to account for the light scattering properties of biological tissue. The resulting value, known as regional saturation of oxygen ($rSO_2$), represents a weighted average of the oxygen saturation in the entire microcirculation beneath the sensor, which includes blood from the arteries, capillaries, and veins.
This mechanism is distinct from pulse oximetry, which only measures the oxygen saturation in the arterial blood ($SaO_2$ or $SpO_2$) by detecting a pulsatile signal. NIRS measures the regional saturation, which is heavily weighted toward the venous side, with approximately 75% to 85% of the signal originating from venous blood. This focus on the venous side is significant because venous blood oxygen saturation reflects the amount of oxygen that the tissue has consumed, making $rSO_2$ a direct indicator of the local balance between oxygen delivery and oxygen consumption.
Critical Uses in Patient Monitoring
The ability to measure localized oxygen balance makes NIRS useful in patient care where systemic measurements may be insufficient. In the cardiac operating room, for example, cerebral NIRS monitoring is frequently used during complex procedures, particularly those involving circulatory arrest or cardiopulmonary bypass. The monitor provides an early warning system for potential cerebral ischemia, or poor blood flow to the brain, which can occur when blood pressure drops or when air or debris enters the circulation.
In the Neonatal Intensive Care Unit (NICU), NIRS monitors premature infants who are susceptible to injury from inadequate oxygen delivery to organs like the brain and kidneys. Clinicians monitor cerebral $rSO_2$ to help prevent brain injury and track somatic $rSO_2$ over the abdomen to assess perfusion to the gut and kidneys. A sudden drop in oxygen saturation in the abdominal region can be an early indicator of necrotizing enterocolitis, a severe inflammation of the intestine common in newborns.
NIRS is also used in critical care units and trauma settings to monitor patients in shock or low perfusion states. When a patient is experiencing shock, the body attempts to protect the brain and heart by diverting blood flow away from the extremities and the gut. Monitoring somatic $rSO_2$ provides an objective measure of the severity of this compensatory response. By tracking tissue oxygenation in these peripheral areas, clinicians can quickly determine if resuscitation efforts are effectively restoring blood flow beyond the central organs.
Why Clinicians Choose NIRS
Clinicians choose NIRS technology for patient monitoring due to features that address the limitations of other methods. The technology is non-invasive, requiring only a small sensor pad placed on the skin, which eliminates the risks and complications associated with invasive lines or probes. This ease of application makes it suitable for continuous, long-term monitoring in vulnerable patient populations, such as newborns or those undergoing lengthy surgical procedures.
The continuous nature of the monitoring provides a dynamic trend of tissue oxygenation rather than a single snapshot in time. A monitor that generates data every few seconds allows the team to observe the immediate effect of an intervention, such as a change in ventilation or the administration of a medication. This responsiveness helps guide precise therapy adjustments and titration of support.
NIRS offers localized information that systemic measurements often miss. While a pulse oximeter may indicate a healthy arterial oxygen saturation, the NIRS reading for the brain or a peripheral organ can reveal that the blood flow to that specific region is compromised. This data is beneficial in situations where continuous cerebral monitoring is desired, but the risks of inserting invasive brain oxygen probes are deemed too high.
Understanding the Data Output
The data point displayed on a NIRS monitor is the $rSO_2$ value, which is expressed as a percentage, representing the regional saturation of oxygen. This percentage reflects the proportion of oxygenated hemoglobin to total hemoglobin in the sampled tissue volume. For a healthy adult, cerebral $rSO_2$ values typically fall within a range of 55% to 75%, depending on the specific organ being monitored.
Clinicians focus more on the trend of the $rSO_2$ value than on any single absolute number. A sudden or sustained drop of 20% or more from a patient’s established baseline value is considered clinically significant, signaling an imbalance between oxygen supply and demand that requires intervention. The absolute $rSO_2$ number is often compared to the systemic arterial saturation ($SpO_2$) measured by a pulse oximeter to help assess oxygen delivery efficiency.
If the arterial saturation is high, but the regional $rSO_2$ value is low, it suggests that blood flow to the specific organ is restricted. Conversely, if both values are high, it indicates sufficient oxygen delivery to the tissue. However, the interpretation of the $rSO_2$ reading must also account for potential data quality issues, as the readings can be affected by external factors, including excessive patient movement or the presence of external light sources in the room.