Near-infrared spectroscopy helps to monitor tissue oxygenation of various organs during cardiopulmonary bypass, ECMO, or critical illness. The main advantage of NIRS is that it's noninvasive and the measurement takes place continuously. Visual demonstration of this technique is critical to point out the handling and pitfalls of this method, especially for abdominal applications.
Demonstrating the procedure will be Frank. He's a perfusionist in the children's hospital of the University of Erlangen. Begin by turning on the NIRS device and connecting the pre-amplifiers.
Enter the patient's data and select the proper probe according to the patient's weight and intended site of use. Connect the probe to the pre-amplifier. Make sure that the patient's skin is clean and dry for optimum adhesion.
Dry the skin with a swab, if necessary, being very careful, or omitting cleaning if the skin is vulnerable. After identifying the correct probe position, carefully bend the center of the probe toward the side of the white cover until it starts to come off, then gently peel off the cover without touching the probe sticky surface. Place the sensor on the skin from the center of the probe to the sides, making sure that the edges of the probe are firmly connected to the skin.
If the probe disconnects, wrong NIRS values will be obtained. To avoid skin lesions, do not place the probe on very immature or vulnerable skin. If the probe must be placed on vulnerable skin, use a layer of cellophane between the skin and the probe or leave the cover on.
If the cerebral position is selected, place the NIRS probe in the supraorbital region on the forehead below the hairline to obtain values from the frontal cortex. Do not place the probe above hair, the frontal sinus, the temporal muscle, nevi, the superior sagittal sinus, intracranial hemorrhages, or other anomalies. Placement of two probes allows for selective analysis of both hemispheres if the clinical setting requires this.
Neighboring probes emit and measure signals alternately to avoid interference. If placing the probe in a sematic position, select a position above the region of interest. Avoid fat deposits, hair, and bones.
Do not place the probe above nevi, hematoma, and injured skin. Keep in mind that the depth of the NIRS signal is approximately 2.5 centimeters. To place the probe on the kidney, first locate the kidney via a dorsal sagittal sonogram.
Make sure that the skin-to-organ distance does not exceed the maximum depth of the probe. When placing the probe on the intestines, simply place it in the region of interest, such as below the umbilicus or in the right or left lower quadrant. For liver placement, place the probe exactly above the liver and confirm its positioned by ultrasound, making sure that the liver tissue is at least as deep as the emitted light penetrates.
The probe can also be placed on the plantar portion of the foot. Measuring NIRS in the most distant part of the body gives information about peripheral perfusion during hypothermia, in patients with shock, or in any situation where pulse oximetry does not work, such as during cardiopulmonary bypass. Finally, the probe can be placed on a muscle of interest.
Set the baseline one to two minutes after placing the probe by pushing the corresponding button on the device which reflects the starting point of the measurement. The evolution of tissue perfusion in each monitored area can be observed and interpreted individually by relying on the change from the baseline value. If the device indicates bad recording quality or the values are implausible, confirm that all previous steps have been taken correctly.
Check all electrical plug contacts and the pre-amplifier. If necessary, replace the probe. If external light sources are affecting the sensor and they cannot be eliminated, cover the probe.
After ruling out technical problems, check the patient for clinical complications and adjust the treatment to optimize the following parameters. The measured regional tissue oxygenation value results from the ratio between oxygen supply and consumption. Metabolic characteristics lead to slightly different normal values depending on age and organ.
Except for the brain, scientifically evaluated reference values exist only for preterm infants and newborns. If the oxygen supply and demand are balanced at physiological values, tissue oxygenation is within normal range. Changes in either cause the regional tissue oxygenation value to fall or rise.
A typical curve revealing normal cerebral and renal near-infrared spectroscopy or NIRS values is displayed here. Clamping the descending aorta causes cerebral perfusion and the corresponding regional tissue oxygenation to rise. Another cause of increased cerebral blood flow and elevated cerebral regional tissue oxygenation is hyperdynamic shock in conjunction with high cardiac output.
When using two cerebral NIRS probes, values from the right and left sides should be similar. Dissonance between the right and left channel can be caused by the sensor's incomplete adhesion or indicate a complication. During some heart surgeries, the brain is perfused selectively via one carotid artery, making use of intracerebral collaterals to supply the opposite side.
Throughout this procedure, dissonance between the two cerebral NIRS channels can help diagnose a dysfunctional Circle of Willis. NIRS can also help discover a dislocated vena cava superior cannula during cardiopulmonary bypass, leading to venous stasis and lowered cerebral oxygen supply. The most important thing when administering NIRS is to choose the right position and place the probe thoroughly.
