REVIEW ARTICLENear infrared spectroscopy for evaluation of the trauma patient: a technology review☆
Introduction
Shock is a complex entity, traditionally defined as a state in which the metabolic demands of the tissue are not matched by sufficient delivery of oxygen and other metabolic substrates. In the setting of trauma, this most commonly occurs as the direct result of hemorrhage. Although it is now realized that inflammatory and immune related events occur shortly after the initial insult and may contribute to later development of multisystem organ failure and death, these secondary events are believed to be closely linked to the severity of the initial insult (degree of oxygen debt).1, 2, 3, 4, 5 It is, thus, generally agreed that recognizing inadequate oxygen delivery at the earliest possible time and preventing its progression is a sound strategy to prevent the occurrence of these horrific secondary sequelea. Unfortunately, traditional clinical signs of tissue perfusion that have been used over the last century (capillary refill, mental status, heart rate, pulse pressure, systemic blood pressure and even urine output) are now recognized as being limited in their ability to act as sensitive indicators of tissue perfusion.6, 7, 8, 9 Therefore, detecting compensated shock (which is the most common presentation of shock) continues to remain a tremendous challenge. Even after beginning resuscitation from a state of uncompensated shock, mounting evidence exists that supports ongoing inadequate tissue perfusion despite normalization of these historic indices.6, 10, 11
Over the last two decades, an intense search for more sensitive monitoring technology and methods to serve as resuscitation end-points has and continues to take place.12, 13 Perhaps a misnomer, an ideal endpoint to resuscitation of shock states should also serve as an ideal marker of the presence of an altered state of oxygen delivery at its earliest stages, even before clinical signs of such a state are evident. Conversely, the chosen monitoring modality or endpoint should be capable of ensuring that the patient is fully resuscitated and can be maintained once the endpoint is normalized.
Figure 1 demonstrates this principle using a simplified whole-body biphasic oxygen delivery–consumption relationship. As can be noted, oxygen consumption (VO2) can remain constant over wide ranges of oxygen delivery (DO2). This is possible because tissues are capable of increasing the ratio of extracted oxygen (OER) efficiently. This will be reflected by decreasing venous oxygen saturation (SvO2) from the tissues. However, when DO2 reaches a critical threshold, tissue extraction of oxygen cannot be increased further to meet tissue demands. It is at this point that VO2 becomes directly dependent on DO2 (DO2crit) and cells begin to convert mainly to anaerobic metabolism as manifested by significant increases in metabolic byproducts, and other cellular entities reflective of this state such as lactate, NADH, and reduced cytochrome oxidase. Although Figure 1 represents the whole-body DO2–VO2 relationship, each individual organ has its own biphasic relationship with differing points of DO2crit, which can vary significantly based on the severity of the insult, the system's metabolic activity, vascular responsiveness to a myriad of mediators, and type of insult.
To date, a combination of global and regional measures have been reported.12, 13 Regional indicators of perfusion are now being given greater emphasis since it has been known for some time that regional perfusion changes can occur significantly earlier than traditional global indices if hemorrhage is submassive.12, 14 Examples of technologies which may take advantage of regional changes, and which may help identify these states, include transcutaneous pO2 and pCO2, subcutaneous and interstitial pH, pCO2, and pO2 measurements, gastric and sublingual tonometry, and near-infrared absorption spectroscopy (NIRS).12
The technique of NIRS is exciting because potentially it may allow for noninvasively derived information concerning all of the major components of oxygen transport ranging from bulk transport of oxygen to its cellular utilization at the level of the mitochondria. This paper will review the principles of NIRS and its application to the trauma patient to assess tissue oxygenation. Its use for monitoring traumatic brain injury is excluded since we believe its use in this setting deserves a dedicated review.
Section snippets
Principles of NIRS
NIRS research for measuring tissue properties is not new. The technique was pioneered by Millikan who first developed a dual wavelength oximeter for muscle and Jobsis who was the first to note the differential spectral absorption of hemoglobin and the mitochondrial enzyme cytochrome oxidase or cytochrome a,a3 (CtOx) in vivo with NIR transillumination.15, 16 Chance actually introduced and reported the concept of differential spectroscopy in 1951 which provides the basis for modern in vivo
Equipment and algorithms
The basic equipment components required for an NIRS monitoring system include: (1) light sources capable of generating multiple wavelengths or a single light source with the necessary filters to produce the required wavelengths in the NIR region (or spectrophotometers to record the intensity of reflected light as a function of wavelength), (2) fiberoptic bundles to transmit light from the source to the probe and from the probe to a computer, (3) tissue probe containing an optodes(s) for the NIR
Signal derivation, challenges, and basis for use in the trauma patient
Figure 2 demonstrates the relative absorbances of the Hb, HbO2, and CtOx. Of note is that oxyhemoglobin and oxymyoglobin have basically identical extinction coefficient as do deoxyhemoglobin and deoxymyoglobin. The absorbance spectrum for CtOx actually represents the oxidized minus reduced form of the enzyme (discussed in depth below). It is also important to note that, although the extinction coefficient of CtOx is almost twice that of hemoglobin, its overall contribution to absorption is
Laboratory and clinical evaluation
In terms of absolute use, the majority of clinical NIRS work has been performed in an attempt to monitor the state of cerebral oxygenation in neonatal critical care and in adults during operative interventions such as carotid endarterectomy, subarrachnoid hemorrhage surgery, or cardiopulmonary bypass.18, 69 NIRS also enjoys popularity as a research tool in the evaluation and understanding of exercise induced changes in skeletal muscle oxygen transport.46, 57
The aggressive application of NIRS in
Summary
The present review provides the foundation to understand and evaluate the potential value and limitations of NIRS as a tool in the assessment of victims of trauma. Despite continuing technical controversies concerning signal derivation, accuracy, precision, and quantitative ability, NIRS clearly demonstrates promise in being able to monitor the balance of oxygen delivery and consumption at the end-organ level in several trauma-related settings, including traumatic shock, compartment syndrome,
Acknowledgements
Supported in part by the U.S. Office of Naval Research (Grants N00014-02-10344, N00014-02-1-0642, and N00014-03-1-0253) and in part by Grants from the U.S. Army Medical Research and Materiel Command (Grants: USAMRMC W81K00-04-0998 and DAMD17-03-1-0172).
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A Spanish translated version of the summary of this article appears as Appendix in the online version at 10.1016/j.resuscitation.2005.06.022.