Use of near-infrared spectroscopy (NIRS) in cerebral tissue oxygenation monitoring in neonates

Near-infrared spectroscopy (NIRS) is a technology capable of non-invasive, continuous measuring of regional tissue oxygen saturation (StO2). StO2 represents a state of hemodynamic stability, which is influenced by many factors. Extensive research has been done in the field of measuring StO2 of various organs. The current clinical availability of several NIRS-based devices reflects an important development in prevention, detection and correction of discrepancy in oxygen delivery to the brain and vital organs. Managing cerebral ischemia remains a significant issue in the neonatal intensive care units (NICU). Cerebral tissue oxygenation (cStO2) and cerebral fractional tissue extraction (cFTOE) are reported in a large number of clinical studies. This review provides a summary of the concept of function, current variability of NIRS-based devices used in neonatology, clinical applications in continuous cStO2 monitoring, limitations, disadvantages, and the potential of current technology.


INTRODUCTION
Non-invasive measurement of cerebral tissue oxygenation using near-infrared spectroscopy (NIRS) is attracting increasing attention in neonatology 1 . NIRS provides a continuous assessment of regional cerebral oxygenation (cStO 2 ), and cerebral tissue oxygen extraction (cFTOE). Yet, it is not seen as a routine brain monitoring tool in neonatal intensive care units (NICU) (ref. 2,3 ). The aim of this review is to provide a summary of the relationship between cStO 2 and immediate postnatal adaptation, hypoxic-ischemic encephalopathy (HIE), patent ductus arteriosus (PDA), intraventricular hemorrhage (IVH), and respiratory distress syndrome (RDS), and to describe the clinical relevance of this method. Furthermore, the review summarizes the mode of function, current variability of NIRS-based devices used in neonatology, clinical applications of continuous monitoring of cerebral tissue oxygenation. It covers the limitations and disadvantages of current technology and ends with the potential of such technology if implemented in NICU environment.

History of NIRS-based technology
Transillumination, or the passage of the light through the body, has been studied since the early 1800s (ref. 4 ). Transillumination of the head, first described by Richard Bright in 1831, would be eventually recognized as the first light-based diagnostic technique to identify hydrocephalus 4 . In 1977, Jobsis reported that the tissue absorbs light (wavelength of 700-1,000 nm), and this fact can be used for vital tissue measurement in the diameter of 5-6 cm. He used a model of the myocardium and brain tissue 5 .
In 1985, Ferrari et al. published human cerebral oximetry studies using NIRS (ref. 6 ). Since the first report of pediatric application of NIRS for monitoring cerebral oxygenation in sick preterm infants in 1986, the possibilities of NIRS monitoring have significantly increased 7 . A major milestone in terms of availability of equipment capable of continuous monitoring was the year 1993. In May 1993, INVOS 3100® (Somanetics Corporation, Troy, Mich., USA) entered the market as the first commercial cerebral oximetry device approved by the Food and Drug Administration (FDA) (ref. 8 ). The discovery that functional activation of the human cerebral cortex can be explored by NIRS, has added a new dimension to the research. To obtain simultaneous multiple and localized information, a further major step forward was achieved by introducing NIR imaging and tomography 9 .

Physical principles of NIRS
Visible light (wavelength 450-700 nm) is strongly attenuated as a result of powerful absorption and scattering by the tissue constituens 10 . Near Infrared Spectroscopy (NIRS) is a type of vibrational spectroscopy that employs photon energy in the energy range of 2.65 x 10 -19 to 7.96 x 10 -20 J (corresponding the wavelength range of 750 to 2,500 nm). This energy range is higher than necessary to promote molecules only to their lowest excited vibrational states (through a fundamental vibrational transition). On the other hand, the energy range is lower than typical values necessary for electron excitation in molecules (except for some rare earth compounds). The overall objective of spectroscopy is to probe a sample in order to acquire qualitative and/or quantitative information coming from the interaction of near-infrared electromagnetic waves with its constituents 11 .
Near-infrared spectroscopy has been used as a tool to determine the redox state of light absorbing molecules. This technology is based on the Beer-Lambert Law, which states that light transmission through a solution with a dissolved solute decreases exponentially as the concentration of the solute increases 8 . A decrement in transmitted light intensity is equivalent to the quantity of the substance and the amount of light absorbed by a unit quantity of that substance, defined as the extinction coefficient, a factor that varies with the substance and the incident -light wavelenght 4,8 . In mammalian tissue, only three compounds change their spectra when oxygenated: cytochrome aa3, myoglobin, and hemoglobin. Because the absorption spectra of oxyhemoglobin and deoxyhemoglobin differ, their relative concentrations change within tissue with oxygenation, and the relative concentrations of the types of hemoglobin can be determined 12 .
All NIRS devices emit light at wavelengths within the above mentioned spectrum, and analyze photons returning to the transducer. Because the change in the intensity of the reflected light is dependent upon the oxyhemoglobin to deoxyhemoglobin ratio, oxyhemoglobin saturation can be derived 13 . The ratio between oxygenated and deoxygenated hemoglobin is known as a tissue oxygenation index (TOI). The value of the regional oxygenation or oxygen saturation (StO 2 ) acquired by a NIRS-based device is a combination of intravascular oxygenated/deoxygenated venous and arterial (capillary) hemoglobin in a ratio estimated by the manufactor 8,9 . Pulse oximetry deliberately considers only the arterial compartment at the time getting the measurements 14 . Fractional oxygen extraction (FOE) is the ratio between oxygen consumption and oxygen delivery. This is calculated using the formula FOE = (SpO 2 -StO 2 ) / SpO 2 , without measuring the flow. Assuming constant cerebral oxygen consumption, the value will rise as oxygen delivery to the brain falls, until maximum oxygen extraction is achieved 15,16 . Unlike pulse oximetry, NIRS is not dependent on pulse wave, and it uses more wavelengths for spectroscopic transillumination, therefore characterizing more chromophores 10,14 . The extremely low level of absorption of NIR light by hemoglobin is the dominant factor in achieving measurement requiring low light intensity. While benchtop co-oximeters utilize multiple wavelengths to differentiate various dyshemoglobins in vitro, noise reduction remains the most important factor in improving accuracy and precision in vivo 17 .

Cerebral tissue oxygenation in Neonatology
The neonatal period is very unique, as the infant undergoes dramatic physiologic changes during transition from intra to extra uterine life. This process involves changes in hemodynamics and affects oxygenation, while reflected in StO 2 values 11 . Measuring cerebral oxygenation using NIRS has taken on an increasing important role in neonatal care. Several companies have developed commercial devices, and more publications are reporting absolute boundary NIRS values or percentiles for neonates 1 . NIRS is sensitive to all tissues penetrated by the light, for example, on the head these represent skin, scalp, skull, sub-arachnoid space, and grey and white brain matter. The skin, the scalp and the skull are much thinner in neonates, and instruments with a multi-distance approach cancel out the influence of the superficial tissue. Furthermore, NIR spectroscopy is less sensitive in larger compared to smaller blood vessels 18,19 .
A large number of studies on NIRS focused on functional brain imaging has been published. Compared to electroencephalography (EEG), NIRS is less susceptible to data corruption by movement artifacts, and offers more highly spatially resolved images of activation. Blasi et al. demonstrated that spatial mapping and size of activation in infants have a high degree of reliability 20 . Several studies using different NIRS devices have described the changes in cerebral regional tissue oxygenation during the first minute of life. Reference ranges and centile charts have already been established for cStO 2 and cFTOE during the first 15 min after birth 21 .

Reference ranges
The current literature indicates correlation between irreversible mitochondrial damage and the total energy disruption that lasts 30 to 120 min at values ranging from 33 to 45%. The studies were conducted on piglets models 22,23 . Since the premature brain is extremely vulnerable, especially in hemodynamically unstable neonates, monitoring cerebral oxygenation has a high priority and is increasingly combined with other brain monitoring devices such as amplitude-integrated EEG (ref. 19 ). Lemmers  heart rate (HR) were significantly higher in the first 5 min and StO 2 in the first 10 min, but then they did not differ from those in infants after cesarean section 27 . The study performed by Kratky et al. demonstrated that cStO 2 increased rapidly from minute 2. Cerebral fractional oxygen extraction which was calculated for each minute, showed a significant decrease from minute 2 until min 4 and it increased within the first 14 min after delivery. cStO 2 significantly showed no further changes after 5 min 28 .
Pichler et al. defined reference ranges and percentile charts for cStO 2 and cFTOE using INVOS TM 5100 (Somanetics, Troy, Michigan, USA) in a large cohort of term and preterm neonates without any need of medical support during the first 15 min after birth, and they found no significant differences comparing term and preterm neonates 29 . The normal reference range of cStO 2 for preterm infants depends on multiple factors such as instrument design, postnatal age, or current clinical status 30 . Cerebral hemoglobin oxygenation correlates with chronological age, but not with postmenstrual age. cStO 2 is not correlated with postmenstrual age, but varies with chronological age and hemoglobin concentration in the blood, suggesting that it depends on systemic changes and does not reflect changes associated with brain development 31 . This is consistent with the findings that cStO 2 may correlate with the heart and respiratory rate and with arterial SO 2 in newborns 32 . Significantly lower SpO 2 and heart rate values have been reported in infants born by cesarean section 33 . In the studies evaluating cStO 2 measurements in different brain regions in stable preterm and term neonates in the first week of life, it was shown that limits of agreement were quite large and varied between ±14% and ±18% (INVOS TM 5100; Somanetics, Troy, Michigan, USA) (ref. 34,35 ). These results suggest that single site recording of the regional cerebral oxygen saturation and cerebral fractional tissue oxygen extraction can monitor trends in individual patients to detect changes larger than the limits of agreement, but lacks the precision to be used as a robust quantitative variable of cerebral oxygenation 36 .
McNeill et al. confirmed that continuous, long-term StO 2 monitoring of premature infants in the NICU is both safe and feasible. Neonatal daily baseline StO 2 values measured by the INVOS TM 5100 (Somanetics, Troy, Michigan, USA) not only change with postmenstrual age, but also decrease in the variability of StO 2 . The group mean cStO 2 fell within the expected range of 60-80%, although several subject demonstrated cerebral values consistently above 80% during the first week of life before declining 37 .

Cerebral oxygenation and HIE
Neonatal HIE (hypoxic-ischemic encephalopathy) is a major cause of mortality, morbidity, and long-term neurological deficits 38 . Despite the progress in diagnostic modalities, accurate prediction of outcome in neonates with HIE remains a great challenge. MRI can predict outcome reliably only in severe HIE cases. Early EEG findings (24 h) do not provide additional support for outcome prediction and are nonspecific, possibly in relation to medication use 38 . cStO 2 was significantly higher between 24 and 48 weeks of age in neonates with HIE with adverse outcomes as compared to those with favourable outcomes, suggesting a decrease in cerebral oxygen consumption during secondary energy failure. Newborns with proven brain injury caused by HIE have higher values of cStO 2 than newborns without brain injury 39 . Plomgaard et al. investigated the benefits and harms of monitoring cerebral oxygenation by NIRS combined with an evidence-based treatment guideline versus no NIRS data and treatment as usual in the group during the first 72 h of life. Treatment guided by NIRS reduced the burden of cerebral hypoxia without affecting the selected EEG or blood biomarkers 40 .

Cerebral oxygenation and patent ductus arteriosus (PDA)
The ductus arteriosus (DA) is an important vascular connection between main pulmonary artery and the aorta. After birth, the DA undergoes active constriction and eventual obliteration. PDA occurs when the ductus fails to close completely after delivery 41 . The incidence of hemodynamically relevant DA has been known to affect perfusion of important organ systems such as the brain. Lemmers et al. measured NIRS values in 20 infants with PDA who underwent treatment with indomethacin (gestational age under 32 weeks). The mean arterial blood pressure and cStO 2 were significantly lower, and FTOE was significantly higher before the treatment of PDA compared to the control group. Indomethacin had no additional negative effect on cerebral oxygenation. Hemodynamically significant PDA had a negative effect on cerebral oxygenation in the premature infant 42 . NIRS can be used as a screening tool for PDA, it shows encouraging efficiency in identifying ELBW infants who are likely to benefit from early echocardiography and subsequent intervention to close a PDA (ref. 43 ). Vanderheagen et al. analysed a significant increase in cStO 2 with concomitant decrease in FTOE at the time of surgical ligation followed by return to baseline values. As a result, the ductal clipping has no negative effect on the cerebral oxygenation 44 . Ductal ligation poses a risk for a further decrease in already compromised cerebral oxygenation in preterm infants. During surgery, median range of cStO 2 dropped very slightly. Eleven infants showed a drop in blood pressure and 13 infants a drop in cStO 2 during surgery accompanied by a decrease in EEG amplitude 45 .

Cerebral oxygenation and respiratory distress syndrome (RDS)
During artificial ventilation due to RDS, systemic hemodynamics can be disturbed with consequent compromise of cerebral oxygenation. RDS is strongly related to low and fluctuating arterial blood pressure 18 . Lemmers et al. investigated the relationship between mean arterial blood pressure (MABP), cStO 2 and FTOE, during the first 72 h of life. RDS infants showed more periods of positive correlation between MABP and cStO 2, and negative correlation between MABP and FTOE. Although Lemmers et al. found that the patterns of cerebral oxygenation and extraction in RDS infants were not different when compared to infants without RDS. They suggested that the frequent periods with possible lack of cerebral autoregulation in RDS infants may cause these infants to be more vulnerable to cerebral damage 46 . Because cerebral oxygenation is essential, cStO 2 monitoring can be an important tool to avoid disturbances between carbon dioxide tensions and impact on complications of this process 29 .

Cerebral oxygenation and peri/intraventricular hemorrhage
Sorensen et al. found that there was a significant negative correlation between the severity of the intraventricular hemorrhage and the cerebral oxygenation 47 . Zhang et al. investigated the relationship between intraventricular hemorrhage (IVH) and cStO 2, in preterm infants. They founded higher cStO 2 values in the first 3 h after birth in neonates who later developed IVH (ref. 48 ). Higher cStO 2 and lover cFTOE values within 24 h before detection of IVH have been reported in another study referring compliant results in very preterm infants 49 . It has been proposed that monitoring of cStO 2 can potentially reduce the damage to the vulnerable preterm brain.

CONCLUSION
NIRS is a fascinating and dynamically evolving technology with over 40 years of history. This non-invasive bedside method is both safe and feasible. It enables continuous, long-term cStO 2 monitoring of NICU patients. Currently available NIRS based devices use the above technology principles. They only differ is in the number of absolute values of wavelength, as well as in computational algorithms translating measured changes into light attenuation. The use of various optical probes complicates comparison of the results referred in older NIRS studies with the currently published data.
Because large infant population-based normative data are lacking, further studies are needed in order to establish normative data for newly available commercial NIRS instruments. In the future, it is essential to provide the StO 2 measurement by means of NIRS based devices with improved quantitative precision, so that this technology will become consistent and usable for NICU worldwide.
However, NIRS provides a unique insight into the possibilities of diagnosing and treating diseases which are typically initially asymptomatic. This promising technology can be used in combination with EEG and ultrasonography in order to provide coherent information, thus improving non-invasive monitoring.

Search strategy and selection criteria
Scientific databases (PubMed and/or Web of Science) were searched for articles describing methods and applications of NIRS in continuous monitoring in neonates. Articles were searched from 1999 to April 2016 and were selected according to their revelance to clinical use, results and devices used. Articles describing the historical evolution and physical principles of NIRS, which were published earlier, were also included. Only English language papers were reviewed.

ABBREVIATIONS
cFTOE, Cerebral fractional tissue oxygen extraction; cStO 2 , Regional cerebral oxygen saturation; DA, Ductus arteriosus; EEG, Electroencephalography; ELBW, Extremely low birth weight; FDA, Food and drug administration; FTOE, Fractional tissue oxygen extraction; HIE, Hypoxic-ischemic encephalopathy; IVH, Intraventricular hemorrhage; HR, Heart rate; MABP, Mean arterial blood pressure; MRI, Magnetic resonance; NICU, Neonatal intensive care unit; NIRS, Near-infrared spectroscopy; PDA, Patent ductus arteriosus; RDS, Respiratory distress syndrome; SpO 2, Arterial oxygen saturation measured by pulse oximetry; StO 2 , Regional oxygen saturation; SO 2 , Oxygen saturation; THI, Tissue hemoglobin index; TOI, Tissue oxygenation index; USA, United States of America. Acknowledgment: The work was supported by a VEGA grant (No.1/0100/15) provided by Scientific Grant Agency of the Ministry of Education, science, research and sport of the Slovak Republic and the Slovak Academy of Sciences. The study was performed as a part of the Center of Excellence of Perinatology Research (CEPV II) project which is co-financed from European Union sources. The VEGA grant and the CEPV II project played no role in the design of the study and collection, analysis and interpretation of data and in writing the manuscript.