Near-Infrared Spectroscopy Oximetry: Part Science, Part Art.

Unauthorized reproduction of this article is prohibited Pediatric Critical Care Medicine www.pccmjournal. org 89 Jobsis first reported in 1977 the relatively high degree of transparency of brain tissue in the near-infrared range, enabling the noninvasive assessment of tissue oxygenation using transillumination spectroscopy. By 1985, studies were published on the use of near-infrared spectroscopy (NIRS) to measure cerebral oxygenation in humans (1). In 1996, the first commercial cerebral oximetry device (INVOS 3100; Covidien Corporation, Boulder, CO) was approved for use in adults in the United States by the Food and Drug Administration (FDA). The technology was initially used to monitor cerebral oxygenation during cardiopulmonary bypass. In 2000, the device received FDA approval for use in children, and over the last several years NIRS oximetry has extended into the realm of pediatric cardiac critical care, and additional NIRS oximeters have been developed. In this issue of Pediatric Critical Care Medicine, Hoskote et al (2) present the results of a survey among pediatric cardiac centers in Europe that evaluated the prevalence and utilization of NIRS oximetry. They found that upward of 70% of centers use NIRS oximetry following high-risk neonatal cardiac surgery. The results of this study are similar to those found by Rossi et al (3) published in 2012, which also reported that upward of 90% of centers in the United States routinely use cerebral NIRS oximetry following repair of complex neonatal cardiac disease. In addition to its widespread use, the results of this survey highlight the “challenges” of NIRS oximetry, namely the wide variation in the interpretation and clinical application of the data within and among centers. Hoskote et al (2) call into question the widespread perhaps precocious deployment of this technology, considering the divergent practices surrounding its use, its high cost, and the fact that there have been no studies to evaluate the impact of this monitor on outcomes in children. What is consistent about the use of NIRS oximetry is how inconsistent providers are in their “use” of this technology. That being said, neither these “challenges” nor the lack of evidence demonstrating a beneficial impact of NIRS oximetry on outcomes should discourage or dissuade clinicians from using NIRS oximetry. These findings are the rule rather than the exception in medicine, particularly in the care of critically ill children and do not necessarily indicate futility. Studies such as those focusing on the role of the pulmonary arterial catheter in critical illness, for example, have found similar results, primarily wide variations in the understanding and interpretation of the data as well as divergent opinions on the indications for its use (4, 5). And the number of prospective randomized controlled trials in pediatrics and pediatric cardiac critical care are few and far between. The optimal use of NIRS oximetry depends on a clear understanding of cerebral and circulatory physiology, as well as its indications and limitations. In brief, NIRS cerebral oximetry, as with pulse oximetry, relies on the principle that the oxygen saturation of hemoglobin may be approximated by the differential absorption of two or more wavelengths of light (6). In contrast to pulse oximetry, NIRS oximetry evaluates the nonpulsatile signal where the majority of blood (upward of 75%) is venous and the oxygen saturation is used as a surrogate for venous saturations, providing an assessment of the relationship between oxygen supply and demand for the interrogated tissue (6). Cerebral oximetry correlates strongly with and tracks changes in jugular and superior vena cava saturations (6–8) and thus is used as an indicator of the adequacy of regional (cerebral) and systemic tissue oxygenation. However, NIRS oximeters rely on proprietary algorithms that are sensitive to biologic variation among humans to derive the oxygen saturation, and thus, there may be considerable variation between measured and calculated values. Pulse oximetry also relies on an algorithm to derive oxygen saturations, and the difference between measured and calculated values may also be appreciable, particularly in the setting of hypoxemia. Therefore, NIRS oximetry is useful as a trend monitor. However, if the value can be corroborated by an assessment of jugular, superior vena cava, or right atrial saturations (in the absence of cardiac shunting), then the absolute number may be used (7, 8). Cerebral metabolic autoregulation dictates that the oxygen extraction ratio (O 2 ER) remains normal until cerebral blood flow becomes limited (9), and while the critical O 2 ER appears to be at or somewhat greater than 60% (defined by the onset of anaerobic metabolism), it is unclear for how long the O 2 ER must exceed critical for hypoxic tissue injury to occur (10, 11). The critical systemic O 2 ER appears to be in a similar range (when serum lactate levels begin to rise or oxygen consumption begins to fall) (12–14). These physiologic underpinnings are responsible for establishing normal O 2 ERs for cyanotic and noncyanotic patients; identify what is an appreciable change in the O 2 ER; and establish severity of illness, clinical trajectory, and responses to interventions. Copyright © 2016 by the Society of Critical Care Medicine and the World Federation of Pediatric Intensive and Critical Care Societies DOI: 10.1097/PCC.0000000000000565 Ronald A. Bronicki, MD, FCCM, FACC Critical Care Medicine and Cardiology Baylor College of Medicine Texas Children’s Hospital Houston, TX