Tissue oximetry is a non-invasive monitoring technique that provides a localized measurement of oxygen saturation in peripheral tissues, often displayed as regional oxygen saturation (rSO2) or tissue oxygen saturation (StO2). This measurement reflects the balance between oxygen supply delivered to a specific tissue and the amount of oxygen that tissue is consuming. Unlike devices that measure systemic oxygenation, tissue oximetry focuses on a specific region, such as the brain, kidney, or muscle. Monitoring this regional oxygen balance offers an early and continuous indication of inadequate tissue perfusion, a state where oxygen supply cannot meet cellular metabolic demands. Clinicians use this technology to assess the microvascular health of organs or limbs in real-time.
How Near-Infrared Spectroscopy Measures Tissue Oxygen
The technology enabling tissue oximetry is Near-Infrared Spectroscopy (NIRS), a method that uses specific wavelengths of light to analyze tissue. NIRS works on the principle that near-infrared light can safely penetrate biological tissue like skin, bone, and fat. This light is then absorbed and scattered by chromophores within the tissue, primarily oxygenated and deoxygenated hemoglobin. Hemoglobin, the protein in red blood cells that carries oxygen, absorbs light differently depending on whether it is bound to oxygen.
By emitting light at two or more distinct wavelengths and measuring the amount of light that returns, the NIRS device can calculate the relative concentrations of oxyhemoglobin and deoxyhemoglobin. The final rSO2 or StO2 value is an estimate of the ratio of oxygenated hemoglobin to the total hemoglobin in the sampled tissue volume. The measurement is non-invasive, with a sensor placed directly on the skin, and typically assesses the microvasculature approximately 1 to 2.5 centimeters beneath the sensor.
The sampled blood volume contains a mix of blood from the arterioles, capillaries, and venules within the tissue bed. The resulting signal is heavily weighted toward the venous side, with estimates suggesting that 70% to 80% of the signal comes from the venous blood. This makes the rSO2 reading an indicator of the residual oxygen left in the blood after the tissue has extracted what it needs for metabolism.
Essential Clinical Applications
Tissue oximetry provides localized information valuable in clinical settings focused on organ protection. Monitoring cerebral oxygen saturation (cSO2 or rSO2) is a widespread application, particularly during complex procedures like cardiac surgery and carotid endarterectomy. Since these surgeries risk reduced blood flow to the brain, a drop in cerebral oxygenation can signal potential neurological complications.
In the neonatal intensive care unit, NIRS monitors vulnerable organs in infants, especially those with congenital heart disease. Splanchnic oximetry, which monitors the oxygenation of the gut, detects early signs of intestinal ischemia that can lead to necrotizing enterocolitis. Similarly, renal oximetry detects low oxygen levels in the kidneys, correlated with an increased risk of acute kidney injury after surgery.
The technology is also employed to assess peripheral tissue oxygenation, often measured over skeletal muscle in the thenar eminence of the hand or a lower limb. In trauma or vascular surgery, peripheral StO2 monitoring helps assess the adequacy of perfusion to a limb or region. A persistently low StO2 value in the muscles of a trauma patient has been associated with poor outcomes, including higher mortality rates and an increased risk of multi-organ failure.
Understanding the Readings
Tissue oximetry readings, such as StO2 or rSO2, are expressed as a percentage and represent the localized oxygen saturation within the microcirculation of the monitored tissue. While baseline values vary depending on the tissue and the individual, absolute values are less important than changes. Clinicians generally interpret significant downward trends—often a drop of 20% or more from the patient’s baseline—as an indication of tissue hypoxia or ischemia.
This monitoring provides different information compared to standard pulse oximetry, which measures systemic arterial oxygen saturation (SpO2). Pulse oximetry assesses the oxygen content of the blood in the pulsatile arteries, reflecting how well the lungs are oxygenating the blood. In contrast, tissue oximetry measures regional saturation in a mixed bed of arterial, capillary, and venous blood, providing insight into the oxygen delivery and utilization at the cellular level.
A patient may have a normal SpO2 reading, indicating sufficient oxygen in the main arteries, while the StO2 reading simultaneously drops, signaling that oxygen is not reaching or being exchanged effectively in a specific organ or limb. Because the tissue oximeter is measuring the oxygen remaining after cellular use, a falling reading often serves as an early warning sign of a potential problem with localized blood flow or oxygen extraction. In some scenarios, a decrease in NIRS values can precede a drop in SpO2 by 10 to 15 seconds, providing a brief but important window for intervention.