How to Administer Near-Infrared Spectroscopy in Critically ill Neonates, Infants, and Children

The measured rSO₂ value results from the ratio between oxygen supply and consumption (Figure 1A); differing metabolic characteristics lead to slightly different normal values depending on age and organ (Table 2). Note that – except for the brain – scientifically evaluated reference values exist only for preterm infants and newborns^{26,27,28,29,30,31} and most of the protocol steps rely on manufacturers’ recommendations, personal experience, and expert opinion (Table 3). This is due to the fact the values depend on the device and sensors used and reveal high inter-individual variability^30,32. Critically low values and critical changes relative to the baseline originate from experience and expert opinion.

If the oxygen supply and demand are balanced at physiological values, tissue oxygenation is within normal range. Changes in either oxygen supply or consumption cause the rSO₂ value to fall or rise (Figure 1B,1C). A typical curve revealing normal cerebral and renal NIRS values is displayed in Figure 2 from the beginning until 14:25 pm.

In the following, we provide examples to show how changes in underlying physiologic conditions affect rSO₂. During heart surgery, doctors manipulate circulation in a controlled manner – therefore the effects on rSO₂ are easy to observe. For example, clamping the descending aorta causes cerebral perfusion and the corresponding rSO₂ to rise; perfusion of the lower body results in an rSO₂ decrease (Figure 2). Another – non-surgical – cause of increased cerebral blood flow and elevated cerebral rSO₂ is hyperdynamic shock in conjunction with high cardiac output (Figure 3).

In cold shock, a dropping renal rSO₂ together with stable cerebral rSO₂ can be the first sign; a decrease in both renal and cerebral rSO₂ can occur later in the course²³. Combined cerebral and renal NIRS can help identify early stages of shock in which cerebral perfusion is maintained at a normal level, but somatic perfusion is already impaired²³.

When using two cerebral NIRS probes, values from the right and left sides should be similar – dissonance between the right and left channel NIRS can be caused by the NIRS sensor’s incomplete adhesion (Figure 4, red star) or indicate a complication: During some heart surgeries, the brain is perfused selectively via one carotid artery, making use of intracerebral collaterals (the circle of Willis) to supply the opposite side. Throughout this procedure, dissonance between the two cerebral NIRS channels can help diagnose a dysfunctional circle of Willis (Figure 5).

Another example of a complication discovered by NIRS is a dislocated vena cava superior cannula during cardiopulmonary bypass leading to venous stasis and lowered cerebral oxygen supply (Figure 6). The use of NIRS can help to identify impaired cerebral perfusion that would otherwise remain undetected and result in severe brain damage.

Besides heart surgery and cardiac intensive care, rSO₂ measurements can also facilitate “standard” pediatric intensive care – complications and changes in therapy can be accompanied by changes in cerebral rSO₂ (Figure 7).

Figure 1: Balancing the ratio between oxygen supply and demand.
(A) Under physiologic conditions, oxygen supply and consumption are balanced, and regional tissue oxygenation is within normal range. (B) A decreasing cerebral rSO₂ results from either increased oxygen consumption or decreased oxygen supply. Reasons for low or decreasing cerebral NIRS values are illustrated in the figure. For example, fever increases cerebral oxygen consumption by 10-13% per 1 °C increase in body temperature. Cerebral spasms can increase oxygen consumption by up to 150-250%. (C) An increase in cerebral rSO₂ results from reduced oxygen consumption or increased oxygen supply. Reasons for high or rising cerebral NIRS values are provided in the figure. A cerebral rSO₂ above 80%, caused by high cerebral blood flow after the loss of cerebral vascular autoregulation, is also called "luxury perfusion". Please click here to view a larger version of this figure.

Figure 2: Evolution of cerebral and renal rSO₂ during clamp out of the descending aorta.
Initially, cerebral (blue) rSO₂ is lower than renal rSO₂ (yellow), as in physiological conditions. During clamp-out of the descending aorta, cerebral blood flow increases while the lower half of the body is undersupplied. Thus, cerebral rSO₂ rises and renal rSO₂ drops. The red area indicates that renal rSO₂ values are critically low because they decreased more than 25% below the baseline. After removing the aortic clamp and establishing reconstruction of the aorta and establishing normal circulation, both rSO₂ curves normalize. Please click here to view a larger version of this figure.

Figure 3: Hyperdynamic shock.
After arriving at the intensive care unit after cardiac surgery and changing respirator tubes, we experienced severe problems with mechanical ventilation (reaching only low tidal volumes at high ventilation pressures due to a defective filter). The patient developed hyperdynamic shock and respiratory acidosis with increased central venous saturation of 90% and increasing cerebral rSO₂ up to 92%. After changing the filter, fluid resuscitation, and vasopressor treatment, the patient stabilized quickly and cerebral rSO₂ normalized. Please click here to view a larger version of this figure.

Figure 4: Evolution of NIRS values during hypothermia and deep hypothermic cardiac arrest.
This figure illustrates how cerebral and renal NIRS values change under hypothermia, adjustment of cardiopulmonary bypass flow and in deep hypothermic cardiac arrest (arterial switch surgery in a patient with transposition of the great arteries and ventricular septal defect). The patient’s baselines rSO₂ values are 59% (left, yellow) and 64% (right, blue) for the brain and 32% (green) for the left kidney. The blood supply to the lower half of the body depends on the ductus arteriosus. Intraoperatively-induced hypothermia reduces oxygen consumption, which leads to rising NIRS values, especially in the kidney. With increasing NIRS values we reduced the flow rate of cardiopulmonary bypass. Due to falling NIRS values caused by an altered metabolic situation (e.g., due to insufficiently deep anesthesia), the flow was adjusted again. During deep hypothermic cardiac arrest, renal and cerebral rSO₂ fell to critically low values and rose again immediately after reestablishing physiological circulation. The red star with arrows shows two dips in the right cerebral NIRS curve due to incomplete probe adhesion. After gently remolding the sensor onto the skin, the values again run parallel to the left side’s. Please click here to view a larger version of this figure.

Figure 5: Dysfunctional circle of Willis during aortic arch surgery.
As soon as the brain is selectively perfused via the right carotid artery (red arrow), the rSO₂ measured on the left side (dark blue) decreases because the intracerebral collaterals via the circle of Willis are insufficient. After placing an additional cannula in the left carotid artery, sufficient perfusion of both hemispheres and thus normal NIRS values are achieved. Please click here to view a larger version of this figure.

Figure 6: Detection of upper vena cava obstruction caused by a dislocated cardiopulmonary bypass cannula.
Shortly after the start of cardiopulmonary bypass (for closure of an atrial septal defect), cerebral NIRS values dropped. Troubleshooting showed that the venous cardiopulmonary bypass cannula had become dislocated, leading to occlusion of the superior vena cava and obstructed cerebral venous drainage. This caused a cerebral undersupply of oxygen, which was only detected through the low rSO₂ value. After repositioning the superior vena cava cannula, venous flow was restored and NIRS values normalized. No. 6: start cardiopulmonary bypass; No. 36 aorta clamped; No. 11 end of ischemia. Please click here to view a larger version of this figure.

Figure 7: Changes in cerebral rSO₂ in a pediatric patient.
After near drowning, this patient was put on extracorporeal membrane oxygenation. Due to side differences in arterial blood gas analyses, we put a second cerebral NIRS sensor in place (yellow). The end of muscle relaxation (A), change of extracorporeal membrane oxygenation system (B), blood pressure fluctuations (A, C), and the effect of a hemothorax (C) are reflected by changes in the NIRS curves. Please click here to view a larger version of this figure.

Figure 8: Placing the NIRS probe over hair.
(A) This patient has a lot of hair on the forehead. (B) The NIRS probe was still put in place. (C) The device indicates that the signal intensity is suboptimal. (D) The NIRS curve values and the course of the curve follow the actions during the surgical procedure (reconstruction surgery in Ebstein’s anomaly). Please note that the absolute values cannot be interpreted, even if they seem normal. Please click here to view a larger version of this figure.

Table 1: NIRS probes by manufacturer and weight range.

Table 2: Typical rSO₂ values by organs and age group.

Table 3: Levels of evidence of the protocol steps.