Near-Infrared Spectroscopy : A Window on Cerebral Microcirculation and Metabolism
Keyword : Cerebral haemodynamics; Near infrared spectroscopy; Tissue oxygenation
Although light has been used as an indicator of hemoglobin oxygenation for almost half a century, the concept of using light in the near-infrared (NIR) range of the spectrum to monitor tissue oxygenation and hemodynamics was first described by Jobsis1 and his colleagues in 1977.2 Since then the technology has been applied to provide information of hemodynamic changes and metabolic status in intact tissues and body parts in animal research, adults and infants in various states of health. Of particular interest to neonatologist is the pioneer work of Brazy and her colleagues in applying the near-infrared (NIRS) method to obtain information on the intraneuronal availability of oxygen and redox state of intraneuronal enzyme cytochrome aa3 (Cyt aa3), that catalyzes the use of oxygen in the process of oxidative phosphorylation, in neonates with hypoxia, hypertensive or crying episodes, and during various procedures such as temperature taking, echocardiography, and gavage feeding.3-5 Concurrently and since then a large number of investigators have contributed many valuable information with the use of NIRS in neonatal medicine. This review is focused on summarizing some of the major applications of this technique in neonates.
Near-infrared spectroscopy method is based on the fact that light in the near-infrared (NIR) range (within 700-1000 nm) can pass through skin, bone and other tissues relatively easily and that there are characteristic absorption bands of oxygenated and deoxygenated hemoglobin (HbO2 and Hb-) and of Cyt aa3 in this range.6 Thus it is possible to separately assess hemodynamic changes and cytochrome reactions or their interactions in response to these changes. In addition, because of the relative transparency of biological tissue to NIR spectrum it is possible to detect photons that have passed through 8 cm or more of tissue thus rendering it feasible to transilluminate the preterm infant's head from one side to another. Although the principle of NIRS involves similar principle used in the pulse oximeter, NIR monitoring provides information on the actual delivery of oxygen to the tissues (more importantly to the brain) rather than in peripheral arterial blood. When the NIR beam passes through tissues, a decrease in signal intensity results from the absorbance of chromophores (HbO2 and Hb-) and oxidised copper moieties of Cyt aa3 in its path (Figure 1). Depending on the characteristics of the lasers used in different near-infrared spectroscopes the light spectrum will vary slightly. Generally the light spectrum of special interest is 750-920 nm. At the lower end of this spectrum Hb- has greater absorbance and at the higher end of the spectrum HbO2. The point of equal light absorbance is 810 nm and is termed the "Isobestic point". The sum of Hb- and HbO2 indicate tissue blood volume (By). Absorbance band for the copper moieties of cytochrome-c-oxidase (Cyt aa3), is 780-900 nm with maximum absorbance at 840 nm. Changes in optical absorption are measured in optical densities (arbitrary termed Vander Units by Jobsis) and can be converted into concentration changes (in mmol/L) of these chromophores by employing a modification of the Beer-Lambert principle.7,8 The NIRS principally provides 4 traces to indicate the tissue (regional) contents of HbO2, Hb-, B V, and redox state of Cyt aa3. These traces provide trends viz, they are linearly related to changes in concentration but not absolute concentration of monitored parameters. However, with additional data on simultaneous circulatory status (changes in arterial oxygen saturation (SaO2) ), tissue blood flow and blood volume can be obtained.9-11
The basic system of NIRS consist of four laser diodes and fiberoptic means of monitoring one area of the head, or other organs, using either a transillumination (placed on opposite side of the head as in premature infants or small animals) or reflectance mode (placed close together at acute angles to each other as in term infants or larger body parts such as muscle). The light emanating from the tissue traversed by the photons is captured by a second optode and is either carried by fiberoptics to the photomultiplier in NIRS console or detection takes placed in the detecting optode directly on the skin. The laser diodes are addressed for brief periods in rapid, sequential alternation. After demolution the individual wavelength signals are averaged over a larger fraction of a second. The four biological signals (HbO2, Hb-, BV, and Cyt aa3) are calculated by means of algorithms obtained by earlier experimentation in which only one of these parameters was varied at a time.12 More detailed description of the instrumentation and relevance of signals may be obtained from several publications.1,13-16
In recent years, attention has been focused on the quantification of NIRS measurements. This requires an estimation of optical pathlength through the tissues. Because of multiple scattering, this pathlength is considerably longer than the distance between the sites of entry and exit. Using the time of flight (TOF) of an ultrashort pulse of infrared light as it traverse the particular tissue of interest (head) Delpy and his colleagues were able to obtain the dimensions of the NIR path which they termed "Pathlength factor" (B). The value for B has been found to vary from an average of 3.9 ± 0.6 by van der Zee, et al17 to 4.4 ± 0.3 by Delpy, et al.8 This factor (B) can then be applied to the modification of the Beer-Lambert equation8 and used to calculate the quantification of the NIRS data. Quantification of NIRS data on the neonate's head is possible with the optical fibers positioned at different angles provided the distance of the optodes is greater than 2.5 cm.18 TOF analysis has also been applied to phase-shift techniques to the delay in moment of occurrence of peak photon flux described by Sevick et al19 as time-resolved spectroscopy. It is to be noted that measurement of optical pathlength of all infants to be studied is still not feasible, differences between the very low birth weight infants, premature infants and term infants have still to be properly defined. Further pathlength measurements using different optode geometrics should allow quantitative observations to be made in infants of different gestation ages and whether they are small for dates, appropriate or large for gestational age.
The ability of the NIRS to simultaneously provide information on the redox state of Cyt aa3 with hemodynamic changes in the microcirculation adds an important dimension to the usefulness of the technique. Cyt aa3 is the terminal member of the mitochondrial respiratory chain. Mitochondrial respiration is the predominant means by which cells utilizes oxygen. Cyt aa3 catalyses 90% of all O2 used in the body and more than 95% of the fraction used by the brain leading to the reduction of oxygen to water according to the redox reaction O2 + 4H+ + 4e- _____ 2H2O. Transport of reducing equivalents or electrons, derived mainly from the redox reactions in the tricarboxylic cycle (Krebs cycle), through the respiratory chain provides the free energy for conversion of ADP to ATP by the process of oxidative phosphorylation (Figure 2). Thus cytochrome c oxidase plays a key role in the mitochondrial reaction that couples these redox reactions to oxygen utilization to the production of high-energy phosphates (ATP) for cell maintenance, function and growth. Lack of oxygen or the introduction of inhibitors such as cyanide or carbon monoxide would interfere with proper respiratory-chain function and lead rapidly to cellular malfunction and injury.
Earlier studies were largely focused on the effect of hypoxia (or oxygen sufficiency) Table. The data obtained from these studies indicate that the Cyt aa3 optical tracings invariably tracks the HbO2 tracings; in other words when HbO2 increased the Cyt aa3 also became more oxidised and vise versa. Questions arose as to whether the optical tracings for Cyt aa3 were actually due to Cyt aa3 or that they represent "cross talk" between the two signals. Studies by Sylvia et al (1982)20, Piandadosi and Jobsis-VanderVliet FF (1984)21 in animals (Sprague-Dawley rats) replacing blood with fluorocarbon (Fluosil 43) showed that the Cyt aa3 optical tracings were independent of HbO2. Additional studies by Sylvia AL et al. (1985)22 of rats subjected to graded arterial hypoxia demonstrated that the NIRS signals of Cyt aa3 correlated with cerebral concentrations of pyruvate, lactate, phosphocreatine, ATP and ADP changes. Direct inhibition of Cyt aa3 by cyanide in bloodless rats was reported by Piantadosi CA and Sylvia AL.23 More recent studies by Wu et al,24 demonstrated that the entrance of bilirubin into neuronal cells is followed by increased reduction of Cyt aa3 independent of the Hb signals. Other studies by these investigators,25 using an animal model (rats) demonstrated that Cyt aa3 was also markedly reduced by potassium cyanide (KCN) and was associated with decreased glutathione in the brain independent of Hb signals. These studies validate the NIRS optically derived Cyt aa3 signals.
The ability of the NIRS to provide noninvasively, continuous, real-time information on cerebral hemodynamics and tissue oxygenation makes this a potentially useful instrument to monitor hypoxia and its effects on cerebral metabolism. Thus early studies with the NIRS were focused on characterization of oxygen insufficiency. In fact the Niros-scope made by Josis stands for near infrared oxygen sufficiency spectroscope. Thus the earliest applications of the NIRS in neonatal medicine by Brazy and her colleagues3-5 were to monitor pathophysiologic responses in cerebral HbO2, Hb-, BV and mitochondrial Cyt aa3 in infants who were at risk for hypoxia, due to diseased states (e.g. respiratory distress) or to handling or various procedures involved in the care of the infant. The findings are in agreement with our studies (using newborn piglets) that demonstrated that hypoxia caused reduction in HbO2, increased Hb-, and BV with concomitant increased in reduction of Cyt aa326 (Figure 3). Additionally with graded degrees of hypoxia, these changes became more marked (Figure 4). Studies by Sylvia, et al21 showed that the redox state of Cyt aa3 at different FiO2 was directly related to in vitro measured changes in cortical metabolites known to reflect energy production. Thus the redox state of cerebral Cyt aa3 would therefore reflect the state of cerebral energy metabolism.
Identification of the redox state of Cyt aa3 had been shown to be of particular usefulness in identifying, so called undedected (silent), episodes of cellular hypoxia as found in association with a large patent ductus arteriosus (PDA) during conditions of reduced cardiac output.4-5 These studies also showed that recovery of Cyt aa3 to baseline may not parallel the recovery of Hb oxygenation.6 The implications of this delay will need to be elucidated. It is quite possible that this delay in recovery may signal neuronal dysfunction or impending cellular injury. The effects of indomethacin used in the closure of the PDA were studied by several investigators.27-29 These investigators found that following indomethacin administration, cerebral blood flow, oxygen delivery, blood volume, and reactivity of blood volume to changes in arterial carbon dioxide tension fell sharply, with concomitant increased reduction of Cyt aa3. These changes in cerebral perfusion may therefore result in cerebral haemorrhage or cellular damage.
Another potential use of the NIRS had been applied to monitoring RDS during surfactant replacement therapy. Cerebral HbO2 fell transiently, while Hb- and BV increased during the drug administration and then recovered quickly.30-32 The significance of these fluctuations in hemodynamics is not quite clear at present. Studies by Greisen33 and his associates found that these changes were associated with marked depression in EEG activity lasting for 10 - 20 mm after surfactant administration. Whether these findings have a permanent effect are unknown.
The effects of positive and negative pressure ventilation on cerebral hemodynamics were recently studied in infants with RDS with the use of NIRS.34 Continuous negative extrathoracic pressure caused a median decrease in cerebral volume of 0.14 ml/100 ml brain compared with no respiratory support. HbO2 and Hb- both decreased, suggesting increased venous drainage as the main effect. Intermittent positive pressure ventilation (IPPV) also caused a median reduction in cerebral BV of 0.06 ml/100 ml brain. Hb- was found to increase while HbO2 decreased. The increase in Hb- suggests decreased venous drainage due to increase in intrathoracic pressure.35 This may lead to increase in venous blood within the cerebral circulation. HbO2 changes may be associated with fluctuations in CBV during the IPPV circle.36 Further, deLemos and Tomasovic37 showed transmission of positive airway pressure to the thorax increased as lung compliance improved with resolving RDS. Applications of the NIRS to monitor cerebral blood flow with various forms of ventilatory settings and ventilators for infants of differing sizes and diseases would be useful to arrive at parameters that would result in maximal effectiveness but decreased risks for cerebral hemorrhage.
The possibility of applying the NIRS to obtain insights into conditions apart from hypoxia (or oxygen sufficiency) has recently been reported by Wu and his colleagues in several studies aimed at characterizing the pathophysiological changes in cerebral hemodynamics and mitochondrial metabolism, as reflected by the redox state of Cyt aa3, in pathogenic Escherichia coli (E. coli) sepsis.38-40 Their studies found that following infusion of E. coli into the blood stream there was an initial hyperdynamic reaction with increases in cerebral BV, Hb- and Cyt aa3 which lasted from 1-2 hours. Subsequently, the BV and HbO2 decreased markedly while Hb- increased and Cyt aa3 became more reduced. These data suggest that the initial hyperdynamic reaction in cerebral microcirculation with increase in oxygen uptake leading to increased oxidation of Cyt aa3 (thereby cell metabolism). As sepsis continues the decrease in cerebral circulation lead to inadequate oxygen for the increased metabolism of brain cells resulting ultimately in neuronal injury and cell death. These studies also showed that pretreatment with dexamethasone as well as monoclonal antibody (specific anti 018) can modulate or prevent the changes in cerebral microcirculation and maintain the steady redox state of Cyt aa3.
Observations with the NIRS have recently been extended to the fetus during the intrapartum period.41 Special optical probes, designed by Peebles and his colleagues, that can be pass through the cervix and applied to the fetal scalp to provide continuous information on the cerebral hemodynamic changes during uterine contractions.42,43 Assessment of the usefulness of interventions during labor (e.g. oxygen or oxytocin) may also be obtained.44,45
Thus data from animal and clinical studies in research settings indicate that the near infrared spectroscopy has great potential as a noninvasive device for providing continuous, real-time information on cerebral hemodynamics, oxygenation, and tissue oxygen sufficiency. The technique can be applied to characterize the evolution of pathophysiologic responses during diseased states and the effectiveness of treatment modalities. Predictive value of the results obtained from such monitoring needs to be assessed. Advances in development for the quantification of the measured parameters will still need to be refined.
The author gratefully acknowledge the invaluable guidance and assistance by Drs. Frans F Jobsis, Avis L Sylvia, Kwan S Kim, Manual Durand, Won S Park, Rowena G Cayabya and Maria-Ellen S Mendoza in various studies cited in this paper.
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