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COLLECTIVE REVIEWS

ear-Infrared Spectroscopy:otential Clinical Benefits in Surgery

tephen M Cohn, MD, FACS

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Freedom is the oxygen of the soul.Moshe Dayan

xygen is essential to our survival. Nevertheless, as cli-icians, we still struggle to assess the adequacy of oxygenelivery to key organ beds.1 The state of organ perfusionould ideally be assessed by a monitor of tissue oxygen-tion. An important contribution to the search for such

monitoring method is near-infrared spectroscopy,hich can assess tissue oxygen sufficiency. This reviewill discuss the current state of our knowledge aboutear-infrared spectroscopy and its potential place in theurgical arena.

VOLUTION OF OXYGEN TRANSPORTn our absolute need for oxygen, we are no different fromhe unicellular organisms from which we evolved.2

ingle-celled eukaryotes obtain their oxygen throughimple diffusion; the rapidity of this process is directlyroportional to the differences in partial pressure of theas and the area of the membrane and is inversely pro-ortional to the distance the gas must travel. Approxi-ately 600 million years ago, single-celled organisms

volved into multicellular forms, the earliest of whichere small, segmented, and highly adapted to meet theirxygen needs by simple diffusion. But as body plansecame more complex, the time-distance constraints ofiffusion had to be overcome, so the cardiovascular sys-em evolved to provide bulk flow to the various tissues ofhe body (Fig. 1). The body plan of most multicellularrganisms can be reduced to a simple scheme with fourrucial steps in the chain of oxygen transport: bulk flowrom the environment to a highly vascularized surface,hether skin, gills, or lung; diffusion into the blood;

ompeting Interests Declared: None.

eceived December 8, 2006; Revised January 31, 2007; Accepted February 6,007.rom the Department of Surgery, University of Texas Health Science Center,an Antonio, TX.orrespondence address: Stephen M Cohn, MD, FACS, Department of Sur-ery, University of Texas Health Science Center, 7703 Floyd Curl Dr, Mail

oode 7840, San Antonio, TX 78229-3900.

3222007 by the American College of Surgeons

ublished by Elsevier Inc.

ulk flow to the various tissues of the body; and diffu-ion into the mitochondrial sink of every cell. Thischeme does not defy the laws of physics; oxygen trans-ort ultimately depends on simple diffusion at the levelf the lung-blood interface and the blood-tissuenterface.

Because oxygen is poorly soluble in water and plasma,ost multicellular organisms with a cardiovascular sys-

em have evolved a respiratory pigment that serves toind oxygen for transport in the blood. In invertebrates,he respiratory pigment (usually hemocyanin) circulatesreely in solution. In vertebrates, the respiratory pigmentalways hemoglobin) is packaged in red blood cells,here it is protected from the oxidative stress of the

nvironment and where oxygen binding may be finelyuned according to allosteric and cooperative interac-ions. The red blood cells of fish, amphibians, reptiles,nd birds are nucleated; the anucleate red blood cell isnique to mammals. There are several possible evolu-ionary explanations for the loss of the red blood cellucleus. First, exclusion of the nucleus potentially pro-ides more room in the cell for hemoglobin. Second, annucleate red cell should weigh less and be more deform-ble. Finally, perhaps the most compelling explanation ishat a red blood cell without a nucleus is a cell withoutitochondria and oxidative phosphorylation. So the

nucleate red blood cell avoids the conflict of interesthat would be associated with being both a consumernd a deliverer of oxygen.

ISTORY OF OXYGEN AND OXIMETRYhe history of oxygen and its measurement are beauti-

ully described in two publications by Astrup and Sev-ringhaus3 and Severinghaus.4

iscoverers of oxygenir was poorly understood until the late 18th century,hen Carl Wilhelm Scheele, a Swedish pharmacist (in772), and Joseph Priestley, an English minister andmateur chemist (in 1774), independently discovered

xygen. Both men performed mouse survival experi-

ISSN 1072-7515/07/$32.00doi:10.1016/j.jamcollsurg.2007.02.024

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323Vol. 205, No. 2, August 2007 Cohn Benefits of Near-Infrared Spectroscopy in Surgery

ents using gas formed by heating red calx of mer-ury, now referred to as mercuric oxide. Priestley, atne time, lived next to a brewery and became fasci-ated by the immense quantities of gas that were gen-rated in the brewing vats, where a candle could noturn. He discovered nine gases and elucidated therinciples involved in refrigeration, soda water, andxygen generation by photosynthesis. Scheele identi-ied seven elements and many compounds. It wasavoisier, perhaps France’s greatest chemist, who, in779, first coined the term oxygen (oxygène); the terms taken from the Greek words for “acid” and “toorm,” because Lavoisier believed that this elementas present in all acids.

he origins of oximetryhe first person to monitor human blood oxygenationas German physiologist Karl von Vierordt in 1874. In

he two previous decades, Bunsen and Kirckhoff hadnvented the spectroscope, and George Gabriel Stokesad discovered that oxygen absorption led to changes inhe color of hemoglobin. These innovations enabled vonierordt to perform experiments demonstrating that the

mount of red light transmitted through the hand de-reased when the hand was made ischemic. These pio-eering studies were essentially ignored until the 1930s,hen German physician-scientist Karl Matthes firstsed variable transmission of red and infrared lighthrough the human ear to assess oxygenation. Americanesearcher Glen Milliken coined the term oximeter in942. He developed the first small portable ear oximeter,device that was used for many years in pulmonary andhysiology laboratories. Pulse oximetry was described in972 by Japanese bioengineer Takuo Aoyagi; the tech-ique was later modified by American anesthesiologistilliam New for use in anesthesia and critical care. To-

ay, the oximeter is no longer an experimental physio-ogic laboratory tool; it has evolved to become an inte-

Abbreviations and Acronyms

CPP � cerebral perfusion pressureMODS � multiorgan dysfunction syndromeNIRS � near-infrared spectroscopyStO2 � local tissue oxygen saturation

ral component of clinical care. i

EAR-INFRAREDPECTROSCOPY TECHNOLOGYhe physical and mathematic basis for near-infrared

pectroscopy (NIRS) is provided by the Beer-Lambertaw, which states that a portion of the light transmittedhrough a solution containing a colored compound isbsorbed by the compound, with the result that the in-ensity of the emerging light is reduced (Fig. 2). Lightcatters in tissues; in the microcirculation, it is absorbedifferently by oxygenated and deoxygenated hemoglo-in (Fig. 3). Our eyes see this difference when bright redxygenated blood changes to deep bluish and blackisheoxygenated blood. NIRS measures the amount of

ight returned to the sensor, producing a ratio of oxygen-ted hemoglobin to total hemoglobin (expressed as aercentage).Interestingly, because biologic materials are transpar-

nt to light in the near-infrared region of the light spec-rum, transmission of photons through organs is possi-le. This characteristic was noted by Jobsis,5 whoerformed pioneering work with NIRS as a noninvasiveethod of investigating living tissues. He observed that

eoxygenated hemoglobin exhibits a weak absorptioneak at 760 nm, and oxygenated hemoglobin does not;n the basis of these findings, he was the first to suggesthat NIRS might have value as a monitor of oxygenufficiency. Mancini and colleagues6 performed detailedxperiments involving human volunteers to validate theunctions of the NIRS device. Measuring skeletal musclehanges during forearm exercise. They found that ab-orption in the near-infrared spectrum (760 to 800 nm)as highly correlated with venous oxygen saturation,as minimally affected by skin blood flow, was alteredy changes in limb perfusion, and was related primarilyo absorption of light by deoxygenated hemoglobin, notyoglobin.How accurately does NIRS measure oxygen satura-

ion? In a comparison study, inline measurements wereaken in a closed, circulating blood loop with an NIRSevice (InSpectra Model 325, Hutchinson Technology),hile blood was simultaneously drawn for measurementith a CO-oximeter, the laboratory gold standard foreasuring hemoglobin oxygen saturation in circulating

lood. The researchers took 384 paired readings at oxy-en saturation levels ranging from 5% to 95%. The re-ults obtained by the two methods were very highly pos-

tively correlated (r2, 0.992),7 as shown in Figure 4. But

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324 Cohn Benefits of Near-Infrared Spectroscopy in Surgery J Am Coll Surg

IRS may be less accurate when used to assess individ-als with dark skin pigmentation.8

Unlike pulse oximetry, which uses fewer and differentavelengths of light, requires a pulsatile flow, and targetsnly the small additional arterial blood volume pro-uced at the measurement site during systole,9 NIRSssesses primarily the hemoglobin saturation of venouslood, which, along with capillary blood, composes ap-roximately 90% of the blood volume in tissues. Be-ause of these differences, NIRS is believed to reflect thexygen saturation of hemoglobin in the postextractionompartment of any particular tissue.

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Figure 2. Beer-Lambert Law: a portion of thecolored compound is absorbed by the com

emerging light is reduced.

In summary, NIRS technology facilitates continuous,oninvasive, and accurate monitoring of tissue oxygenaturation and can interrogate tissues by using lightaves transmitted through biologic materials (including

kin and bone).

EAR-INFRARED SPECTROSCOPY IN SHOCKND RESUSCITATIONnimal worky initial interest in NIRS was stimulated by the work

f Peter Rhee and colleagues,10 who studied 23 rabbitseach weighing 2 to 3 kg) with prototype NIRS probes

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325Vol. 205, No. 2, August 2007 Cohn Benefits of Near-Infrared Spectroscopy in Surgery

Biospectrometer-MB Oximeter, Hutchinson Technol-gy) placed on muscles of the stomach, liver, kidney, andeg. Animals were subjected to hemorrhage until cardiacutput decreased to 40% of baseline; output was main-ained at this level for 30 minutes, and the animals werehen resuscitated until cardiac output returned to base-ine and remained there for 1 hour. NIRS was used tonterrogate tissues for hemoglobin oxygen saturationuring hemorrhagic shock. In this early investigation,

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igure 3. Light absorption of hemoglobin (Hb) related to its state ofxygenation. In the near-infrared range, absorption of light is differ-nt in deoxygenated hemoglobin when compared with oxygenatedemoglobin. These differences can be measured and used to dif-erentiate the adequacy of hemoglobin oxygenation in blood.

Figure 4. Correlation of near-infrared spectroration in blood with CO-oximeter data. In a clat hemoglobin oxygen saturation levels ranginwere highly correlated with CO-oximeter measLD, Seifert RP, et al. Noninvasive method fortissue using wide gap second derivative nea

034017 with permission). StO2, local tissue oxyge

IRS appeared to detect the changes in cardiac output,ut this early NIRS prototype could be calibrated only ifhe animals were put to death.

My colleagues and I then embarked on a series ofnimal experiments evaluating a prototype NIRS deviceBiospectrometer-MB Oximeter) whose optical probeas placed on the end of a nasogastric tube. We per-

ormed NIRS in a porcine liver-injury model usingontrolled hemorrhage,11 uncontrolled hemorrhage,12

bdominal compartment syndrome,13 and a variety ofhock and resuscitation strategies. We found that theIRS device rapidly detected changes in blood flow

hat were reflected in systemic perfusion (pulmonaryrtery-derived SvO2) or regional perfusion (superioresenteric artery flow). In similar experiments with

igs, Beilman and colleagues14 found that oxygen de-ivery was highly correlated with NIRS-measured he-

oglobin oxygen saturation when either leg muscler2, 0.94) or the stomach wall was evaluated (r2,.91). In subsequent experiments, our group foundhat NIRS measurements of muscle tissue reflectedoth the adequacy of resuscitation and the magnitudef injury (Fig. 5).15 In addition, when NIRS measure-ents were used as a sole end point of resuscitation,ortality rates in our LD50 (median lethal dose)odel were decreased.

y (NIRS)-measured hemoglobin oxygen satu-circulating blood loop, 384 paired readings

m 5% to 95% obtained with an NIRS device,ents (r2, 0.992). (From: Myers DE, Andersonsuring local hemoglobin oxygen saturation inared spectroscopy. J Biomed Opt 2005;10:

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Chaisson and colleagues16 studied the use of a closed-oop resuscitation method in a sheep aortotomy modelf uncontrolled hemorrhage. Tissue oxygen saturationas monitored with the Somanetics INVOS 4100 de-ice placed on the flank muscle. Interestingly, whenuscle NIRS measurements rather than cardiac outputere used as the target end point of resuscitation, onlyalf of the resuscitation volume was required for fullestoration of perfusion.

Studies of other types of oxygen monitors have alsoemonstrated that tissue oxygen monitoring reflects theagnitude and duration of shock. Tonometric measure-ents of subcutaneous oxygen tension during hemor-

hage and resuscitation have been shown to be a func-ion of blood flow.17 A number of investigators haveccomplished percutaneous monitoring of tissue oxygenension with fiberoptic sensors and have shown that ox-gen tension measured in muscle or visceral organsimics perfusion changes in a variety of different animal

hock models.18-20

linical workairns and coworkers21 found a strong association be-

ween diminished NIRS measurements (using an earlyutchinson Technology prototype) of hemoglobin oxy-

en saturation and oxidized cytochrome aa3 in trauma

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igure 5. Mean arterial pressure (MAP) data compared with legear-infrared spectroscopy (NIRS) data in a porcine shock model.nimals underwent hemorrhage followed by resuscitation with nouid (No res), lactated Ringer’s (LR), or Hextend (Hex) (Hospira)olutions. NIRS-measured muscle tissue oxygen saturation wasimilar in magnitude and duration to blood pressure changes founduring hemorrhage and resuscitation. (From: Crookes BA, Cohn SM,urton EA, et al. Noninvasive muscle oxygenation to guide fluidesuscitation after traumatic shock. Surgery 2004;135:662–670,ith permission.)

atients during 12 hours of shock resuscitation, and the w

ubsequent development of organ dysfunction. McKin-ey and colleagues22 closely monitored eight severely in-ured trauma patients during resuscitation for 24 hoursfter admission. Deltoid muscle NIRS readingsBiospectrometer-MB Oximeter) were correlated withystemic oxygen delivery during the entire monitoringeriod (r, 0.95).The results of these studies indicated that NIRS tech-

ology might have promise in assessing shock and resus-itation, but little information was available about theormal distribution of hemoglobin oxygen saturation inealthy tissues. A better understanding of the distribu-ion of values in healthy persons was essential if we wereo determine threshold criteria for diagnosing hypoper-usion. To resolve this issue, we measured the normalange of thenar muscle local tissue oxygenation (StO2)n human volunteers. Readings were taken by using annSpectra device with a nonsterile polyethylene coverOptoshield, Hutchinson Technology) on intact skin.n analysis of the data collected from 707 ambulatoryolunteers showed a mean thenar StO2 of 87% � 6%.23

frequency distribution of thenar StO2 measurementss shown in Figure 6.

We then performed a prospective, nonrandomized,bservational study to determine the relationship be-ween the severity of traumatic shock and StO2 measure-ents. Thenar muscle NIRS measurements were made

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igure 6. Near-infrared spectroscopy (NIRS) data from normal vol-nteers. In 707 subjects, the range of thenar muscle oxygen satu-ation values was noted. A target value, two standard deviationselow the mean, was postulated as a reasonable goal for resusci-ation. (From: Crookes BA, Cohn SM, Bloch S, et al. Can near-nfrared spectroscopy identify the severity of shock in trauma pa-ients? J Trauma 2005;58:806–816, with permission.)

hile patients were in the resuscitation area of a Level I

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327Vol. 205, No. 2, August 2007 Cohn Benefits of Near-Infrared Spectroscopy in Surgery

rauma center. Traumatologists blinded to the NIRSeasurements were asked to place patients into one of

our groups based on the severity of shock: no shockn � 98), mild shock (n � 19), moderate shockn � 14), and severe shock (n � 14). The lowest thenartO2 measurement was 83% � 10% for the no shocknd the mild shock groups, 80% � 12% for the mod-rate shock group, and 45% � 26% for the severe shockroup. The thenar StO2 measurements of the severehock group were considerably different from those ofach of the other shock groups (all p � 0.05). We con-luded that NIRS could continuously and noninvasivelyonitor tissue oxygen saturation in muscle and was

robably an indicator of the severity of shock.Ikossi and colleagues24 monitored 28 well-resuscitated

ritically ill patients simultaneously with an intramuscu-ar Licox oxygen probe (CC1.G2 Oxygen Catheter Mi-ro Probe, Integra NeuroSciences) and a transcutaneousIRS probe (InSpectra Model 325) placed in the del-

oid region approximately 10 hours after hospital admis-ion. Early low values obtained by either device appearedo predict the risk of infectious complications or multi-rgan dysfunction syndrome (MODS).

We recently completed a multicenter study evaluatingow well StO2 measurements predicted the outcomes of83 patients with high-risk trauma who were in shockhen they arrived at one of seven Level I trauma centersarticipating in the study. The primary outcomes in thisrospective study were MODS (which occurred in 50 ofhe patients) and death (which occurred in 55 of the

able 1. Outcomes Results in Trauma Patients Related toystolic Blood Pressure, or Base Deficit

Min StO2

atients with bad/goodoutcomes, n 96/284

utoff 75ensitivity, % (95% CI) 84 (78, 90)pecificity, % (95% CI) 39 (34, 44)PV, % (95% CI) 32 (27, 37)PV, % (95% CI) 88 (83, 93)UC 0.70ad outcomesMean � SD (95% CI) 54 � 24 (49, 58)

ood outcomesMean � SD (95% CI) 68 � 17 (66, 70)

UC, area under the receiver operating characteristic curve; NPV, negative prFrom: Cohn SM, Nathans AB, Moore FA, et al. Tissue oxygen saturation preTrauma 2007;62:44–54, with permission.)

atients). StO2 was measured on arrival and for the next t

4 hours; other known predictors of hypoperfusion andlinical outcomes were also measured. Clinicians werelinded to StO2 measurements. A minimum StO2 mea-urement of 75%, a minimum systolic blood pressure of0 mmHg, and a maximum base deficit of 6 mEq/Lere similarly effective in predicting which patientsould experience bad outcomes (MODS or death) asemonstrated in Table 1. These measures had a sensitiv-

ty of about 80%; specificity was about 32%; positiveredictive value was about 30%; and negative predictivealue was approximately 80%. We concluded that NIRSeasurements of muscle tissue oxygen saturation per-

orm similarly to base deficit levels or systolic bloodressure in detecting poor perfusion and predictingODS or death after severe torso trauma; the advantage

f NIRS is that it allows continuous and noninvasiveeasurement.25

We can get along on only 20% of our lung capacity, butthat dragging sort of existence is a poor substitute for thevitality we enjoy when the twin bellows of our lungs aretaking in great drafts of oxygen.

Gene Tunney

EAR-INFRARED SPECTROSCOPY INXTREMITY MONITORINGompartment syndromeiagnosing compartment syndrome can be very diffi-

ult after trauma or vascular procedures to the extremity.elays in diagnosis can lead to catastrophic complica-

t Hour Measurements of Local Tissue Oxygen Saturation,

ximum base deficit Minimum systolic blood pressure

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77 (69, 84) 79 (72, 86)32 (27, 38) 32 (27, 36)31 (25, 36) 28 (24, 33)78 (71, 85) 82 (76, 88)0.65 0.62

11 � 6 (10, 13) 78 � 23 (74, 83)

8 � 5 (8, 9) 87 � 21 (84, 89)

ve value; PPV, positive predictive value; StO2, local tissue oxygen saturation.the development of organ dysfunction during traumatic shock resuscitation.

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328 Cohn Benefits of Near-Infrared Spectroscopy in Surgery J Am Coll Surg

equire amputation. Arbabi and coworkers26 and Garrnd associates27 placed NIRS (Biospectrometer-MBximeter) probes on the leg muscles in a pig model of

ompartment syndrome. Hypotension and hypoxia hadminimal effect on tissue oxygen measurements, but

nduction of compartment syndrome caused a dramaticecrease in StO2 measurements (from 82% � 4% to6% � 12%); after fasciotomy, these measurements im-roved to near baseline values. Subsequently, we used anIRS device (Biospectrometer-MB Oximeter) to assess

issue oxygen saturation in the legs of nine patients withn obvious diagnosis of compartment syndrome, as de-ermined by clinical examination and compartmentressures (64 � 17 mmHg). The tissue oxygen satura-ion in these limbs was only 56% � 27%; after fas-iotomy, this measurement improved to 82% � 16%Fig. 7). We concluded that NIRS may complementlinical examination in diagnosing compartmentyndrome.28

ecrotizing fasciitisike compartment syndrome, necrotizing soft-tissue in-

ection can be difficult to diagnose in its early stages andan result in profound consequences if the diagnosis iselayed. Investigators in Taipei29 used NIRS (RunmanW2000, NIM, Inc) to diagnose lower-extremity soft-

issue infections. Of the 234 patients assessed, 215 hadoutine cellulitis and 19 had necrotizing fasciitis (ac-ording to the criteria established by the Centers forisease Control and Prevention). For these 19 patients,IRS measurements were substantially lower (52% �

8%) than were reference measurements obtained fromhe contralateral extremity, from the biceps muscle, orrom patients with cellulitis (eg, the NIRS measurementf hemoglobin oxygen saturation for the biceps muscleas 86% � 12%). The NIRS device had a sensitivity of00% and a specificity of 97% for predicting necrotiz-ng fasciitis when a threshold value of 70% was used.his appears to be the only report in the English medical

iterature of a study that has used the NIRS device as aethod of diagnosing soft-tissue infection; the results

re quite interesting.

eripheral vascular diseaselogical use for a tissue oxygen monitor would be in

ssessing peripheral blood flow to predict the outcomesf vascular procedures. This technology might provide

aluable assessments of the severity of preoperative hy- l

operfusion and the improvements in tissue oxygen-tion after procedures aimed at restoring blood flow ton ischemic extremity. Svendsen and colleagues30 usedIRS (INVOS 3100, Somanetics) to evaluate lower-

xtremity muscle oxygenation during urinary tract sur-ery with the patient in the lithotomy position. On ele-ation of the leg, perfusion pressures decreasedramatically, with a corresponding reduction in NIRS-easured oxygen saturation of the gastrocnemius mus-

le (68% to 58%, p � 0.05). Eiberg and colleagues31

laced an NIRS device (Somanetics IVOS 3100) on theorsum of the foot of 14 patients undergoing infrain-uinal bypass. After proximal clamping, tissue oxygen-tion decreased from baseline by a mean of 61 U (rangeto 94 U). After bypass grafting, tissue oxygenation

igure 7. Near-infrared spectroscopy (NIRS) data from patients withompartment syndrome (CS), (A) before and (B) after fasciotomy.refasciotomy, patients with compartment syndrome had substan-ially lower tissue oxygen saturation values than uninjured limbs.fter fasciotomy, there was typically a dramatic improvement inuscle oxygenation. (From: Giannotti G, Cohn SM, Brown M, et al.tility of near-infrared spectroscopy in the diagnosis of lowerxtremity compartment syndrome. J Trauma 2000;46:396–401,ith permission.)

evels increased to a mean of 28 U (range 10 to 81 U)

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329Vol. 205, No. 2, August 2007 Cohn Benefits of Near-Infrared Spectroscopy in Surgery

ver baseline. These authors concluded that NIRS wasppropriate for perioperative monitoring of peripheralascular procedures.

Comerota and associates32 investigated the ability ofIRS (InSpectra Model 325) to measure tissue oxygen-

tion before and after exercise in patients with claudica-ion and in normal volunteers. Baseline measurementsere similar in both groups, but the NIRS-measuredxygen saturation levels of patients with claudicationere profoundly lower and required more time for re-

overy to baseline values. Additional studies will be re-uired to determine whether NIRS should be used rou-inely in the perioperative assessment of patients witheripheral vascular disease.

ree flapsnother seemingly ideal function for NIRS technologyould be in postoperative evaluation of tenuous microvas-

ular grafts.These free flaps are prone to arterial and venouscclusion because their placement requires tiny vascularnastomoses. Irwin and colleagues33 suggested that NIRSay play a role in monitoring these flaps. They used theIRO 500 NIRS device (Hamamatsu Photonics KK) toeasure oxygen saturation in the hind limb muscles of 10

abbits. The device rapidly detected arterial occlusions asrops in tissue oxygen saturation; venous occlusion aloneid not result in as great a change.The NIRS device’s rapidetection of changes in the vascular perfusion of humanuscle is well described.34,35 But there are limited data

bout the clinical use of NIRS to monitor patients under-oing microvascular grafting. Scheufler and coauthors36

sed an NIRS device (Multiscan OS 10, NIOS-Medicalechnologies) to assess 11 patients undergoing a pedicleram flap for breast reconstruction.They assessed flap tissuet a depth of 4 mm and found that tissue oxygenationlummeted and NIR-derived hemoglobin values rose dra-atically in the immediate postoperative period and nor-alized late postoperatively. Holzle and colleagues37 foundIR (O2C, Oxygen-to-see, LEA-Medizintechnik GmbH)easuring tissue at a depth of 2 to 4 mm to be a good

dentifier of impending tissue loss, before clinical indica-ors. In 61 patients undergoing radial forearm flaps beforeicrosurgical reconstruction of the head and neck, 9 pa-

ients (15%) had vascular disturbances detected, and pro-ound decreases in tissue oxygenation along with increasesn venous hemoglobin concentration were noted in flaps

ost because of venous congestion. Alterations were noted 4 d

ours before clinical signs of flap compromise. These datauggest a major role for NIR as a monitor of free flaps.

What oxygen is to the lungs, such is hope to the meaningof life.

Emil Brunner

EAR-INFRARED SPECTROSCOPYONITORING OF THE BRAINost clinicians still regard the brain as a “black box,”

nd our ability to assess brain function in comatose pa-ients is still limited. NIRS technology may offer aeans of monitoring the brain in these patients who

ave complex conditions. Because light in near-infraredavelengths can penetrate skull and brain, transcranialonitoring is feasible with the NIRS device. Wyatt and

olleagues38,39 were among the first to suggest that NIRSechnology can be used to measure cerebral blood vol-me and assess pathophysiologic brain damage in new-orn infants. An NIRS device (Somanetics IVOX 3100)as been compared with the LICOX oxygen sensor inssessing comatose critically ill patients with subarach-oid hemorrhage (n � 3) or severe brain injury (n � 9).easonable coherence of results was demonstrated when

hese monitors were placed over the frontal lobe in therea demonstrated by CT to have the greatest degree ofathologic changes.40 In another study, the SomaneticsVOS 4100 system was used to assess four patients afterevere brain injury. NIRS values � 75% were associatedith normal cerebral perfusion pressure in 96% of in-

tances. When NIRS measurements were � 55%, cere-ral perfusion pressure was dangerously low 68% of theime.41

NIRS has been used to evaluate cerebral oxygenationuring carotid surgery42,43 and during cardiac surgery,oth on-bypass44-46 and off pump.47 In a study of carotidndarterectomy, Kuroda and colleagues42 found thatIRS measurements (OM-100 or OM-110 device, Shi-adzu Co) identified severe cerebral hypoxia during

lamping of the carotid artery. Despite concerns that theccuracy of intracranial monitoring may be negativelyffected by extracranial tissues, the sensitivity and spec-ficity of NIRS in reflecting cerebral tissue oxygenationave been extremely good.43 Recent analyses of data ob-ained during coronary bypass procedures demonstratedhat the NIR device (Hamamatsu Photonics KK) mea-ured notable differences in brain tissue oxygenation

uring conventional extracorporeal circulation and dur-

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ng off-pump surgery.48 It is unclear whether the infor-ation derived from the NIRS device provides an ade-

uate sample of the brain so that areas of hypoperfusionan be detected. In addition, we require more investigationso determine whether monitoring brain tissue oxygenations clinically relevant and can translate into therapeutic ac-ions that will improve neurologic outcomes.

EAR-INFRARED SPECTROSCOPYONITORING IN NEONATEShe size of premature neonates and their high frequencyf associated critical illnesses make monitoring particu-arly important. Schulz and colleagues49 used NIRSechnology for transcutaneous measurement of liver ox-genation. They assessed 100 critically ill neonates with40-mm Hamamatsu (Photonics) probe placed over thepper abdomen (liver). NIRS values were correlatedith mixed venous oxygen saturation levels (r2, 0.72).he correlation was even higher when the central venousressure monitor was located in the right atrium and theatient had neither shunts nor sepsis (r2, 0.88). Theossibility of transcutaneous measurement of oxygenaturation in the tissue of abdominal organs is quiteuturistic and fascinating. But a monitor is helpful onlyf it leads to clinically relevant changes in management.

recent large multicenter trial assessed the use of fetalulse oximetry during the obstetric delivery of the in-ants of 5,341 women.50 Availability of oximetry dataid not affect the rates of cesarean delivery or the out-omes of the infants.

EAR-INFRARED SPECTROSCOPYONITORING IN HEART FAILUREhe ability of NIRS measurements of thigh muscle ox-genation to distinguish patients with congestive heartailure from normal volunteers during exercise was dem-nstrated nearly 20 years ago.51 Recently, Soller and col-eagues52 performed an elegant study in which 18 pa-ients undergoing coronary artery bypass grafting wereonitored with an NIRS device placed on the hypothenar

minence and with a reference fiberoptic oxygen probelaced into the abductor digiti minimi muscle. NIRS mea-urement of hemoglobin oxygen saturation was comparedith invasive oxygen tension; both were recorded during

he perioperative and intraoperative periods and were rea-onably well correlated (r2, 0.66). The authors concludedhat NIRS muscle monitoring was sensitive to changes in

issue perfusion during cardiopulmonary bypass surgery.

gain, additional clinical work is needed to assess the clin-cal impact of NIRS in cardiac patients.

Money is like Oxygen. . .– Not enough is NOT compatible with life.– But too much is toxic,not to you, but to your children. . .

Stanley Rosenbaum, MD

In summary, published studies have demonstrated theenefits of near-infrared spectroscopy in monitoring pa-ients with various conditions, such as shock and resus-itation or extremity disorders (compartment syn-rome, necrotizing fasciitis, peripheral vascular disease,nd microsurgical grafts). Other reports have describednnovative uses of NIRS for monitoring neonates andatients with brain injury or cardiac failure. Future ap-lications for NIRS will depend on the results of addi-ional clinical experience and on the therapeutic valueerived from our gaining a better understanding of thetate of tissue oxygenation. From my perspective, theutlook for near-infrared spectroscopy is “rosy.”

cknowledgment: I would like to thank Flo Witt, Michellerice, and Catherine Hornsby for their assistance in the prep-ration and editorial review of this paper.

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