REVIEW ARTICLE

Near-Infrared Spectroscopy in Newborn Infants Kurt von Siebenthal, MD, Gunther Bernert, MD and Paul Casaer, MD, PhD

Neonatal encephalopathy of early onset, plausibly related to hypoxia and ischemia remains one of the main problems in perinatal medicine. Efforts are necessary to find new non-invasive methods for assessing brain oxygenation. Near-infrared spectroscopy (NIRS) provides information on the concentrations of the oxygenated and reduced forms of hemoglobin, as well as the redox state of cytochrome aa3. Different important variables can be derived through hemoglobin measurement, such as cerebral blood volume and flow, and the responses of these to changes in pC0 2 • Changes in cytochrome aa3 may provide immediate information on intracellular oxygen utilization. Various studies have shown the feasibility of NIRS in pre term infants. Methodological and technical problems of this method are discussed. Key words: Near-infrared spectroscopy, oxygenated and reduced hemoglobin, cytochrome 003, cerebral blood volume, cerebral blood flow, neonatal care. von Siebenthal K, Bernert G, Casaer P. Near-infrared spectroscopy in newborn infants. Brain Dev 1992;14:135-43

Six in 1,000 full-term infants develop a clinical picture of neonatal encephalopathy that may be related to hypoxia and ischemia of the brain. 1 in 1,000 of these infants dies or becomes severely handicapped [1] . The incidence of this form of neonatal encephalopathy in pre term infants is still unknown. At present we have to admit also that the underlying mechanisms of hypoxic ischemic events remain poorly understood. Therefore, a lot of effort has been made to fmd new, non-invasive methods for assessing oxygenation and hemodynamics in neonates. In neonatal special care units, transcutaneous p02 and pC0 2 monitors, and pulse-oxymetry are commonly used to measure oxygenation of the blood [2-4]. Systemic circulation is monitored with an electrocardiograph as well as with non-invasive or invasive blood pressure registration equipment. All these techniques are continuously applicable at the bedside. They do not provide us, however, with direct information on oxygenation and metabolism in the brain. Cerebral magnetic resonance spectroscopy (MRS) and positron emission tomography (PET) scanning provide From the Department of Paediatrics and Neonatal Medicine, Developmental Neurology Research Unit, University Hospital Gasthuisberg, Leuven. Received for publication: January 9,1992. Accepted for publication: March 20, 1992. Correspondence address: Prof. Paul Casaer, Developmental Neurology Research Unit, Department of Paediatrics and Neonatal Medicine, University Hospital Gasthuisberg, B-3000 Leuven, Belgium.

more specific information on cerebral metabolism, but it is impossible to perform continuous cotside recording with these techniques [5-9] . 133Xenon clea;ance has increased our understanding of cerebral blood flow, but it involves a radioactive tracer and thus cannot be considered a non-invasive technique

[10-12] . Doppler flow velocity studies, especially when the behavioural state and gestational age are taken into account, can be very helpful [13]. This method, however, provides a measure of blood flow velocity but not of blood flow . Imaging techniques such as computed tomography (CT), magnetic resonance imaging (MRI), especially cranial ultrasonography are very helpful diagnostic tools, but they only show brain tissue damage. In this paper we review the value of a new technique, near-infrared spectroscopy (NIRS), as a tool for continuous, non-invasive bedside monitoring of aspects of brain circulation and oxygenation. METHODOLOGY Basic principles Optical methods for measuring tissue oxygenation are generally called "oxymetry" and are well established in clinical medicine (e.g., photometric measure of bilirubin, pulsoxymetry). Until recently the lack of sensitivity of photon-detection has limited the measurement of brain oxygenation by optical methods. J6bsis et al [14, 15] showed that in the near-infrared frequency range (700-1,000 nm), myocardial and brain

Glucose

Beer·lambert's law

Pyruvate

in vitro resp. chain electron transfer

e- - C - eCytochrome b

Ii

•.......__

0, aa, ~ ereduced Cyt aa, oxidized

It -+--1~ log A = log Ii/It = a*d*p*C + G

H,o A Ii It a d

Fig 1 Simplified diagram illustrating that cytochrome aa 3 catalyzes the last step of electron transport in the oxidative phosphorylation of ADP to ATP.

e

Absorption Incident light Transmitted light Absorption coefficient Distance Concentration

G Geometrical Factor P Path length Factor

in vivo Ii

tissue of the cat is relatively transparent to light. He found two natural chromophores exhibiting oxygenation dependent absorption spectra: hemoglobin (Hb) and cytochrome aa3 (Cyt aa3). The latter is also called cytochrome c oxidase. Hemoglobin is only present in red blood cells and its absorption properties alter when it changes from its oxygenated to its deoxygenated form. Its oxygenation state can therefore be used as an indicator of blood oxygenation. Absorption spectra of Hb were obtained in cuvette studies on lysed human red blood cells [16] . Cyt aa3 plays a major role in energy metabolism in a cell. Together with cytochrome band c, Cyt aa3 is located in the inner membrane of mitochondria and represents the terminal electron accepting enzyme in the oxidative metabolic pathway (Fig 1). Cyt aa3 catalyzes the last step of electron transport in the oxidative phosphorylation of ADP to ATP. It reacts directly with molecular oxygen. If there is enough oxygen available, the enzyme is in its oxidized state, because it has shifted its electrons to the oxygen molecule. A lack of oxygen, however, results in a decreased flow of electrons and therefore the energy-rich components are depleted. The cytochrome is then in its reduced state. Monitoring of the redox-state of Cyt aa3 provides immediate information on the intracellular availability of oxygen [17, 18] . The exact structure ofCyt aa3 remains unknown. It is an enzyme complex comprising phospholipids, two copper atoms (copper A and B) and two hems (hem a and a3). In their reduced forms all three cytochromes (b, c and aa3) exhibit absorption maxima between 500 and 650 nm, which are shifted towards lower wavelengths when they are in their oxidated forms. Conversely, Cyt aa3 in its oxidized state shows an additional flat absorption spectrum at 830 nm. Copper A is probably responsible for about 85% of the cytochrome oxidase absorption in the near-infrared region [19] . In vivo and in vitro studies have yielded different absorption values for cytochrome [20] . Wray et al [16] conducted experiments on rats in which the blood was replaced by fluorocarbon. The obtained spectra of Cyt aa3 were dif· ference spectra between animals ventilated with 100% oxygen and with 100% nitrogen, respectively.

136 Brain & Development, Vol 14, No 3, 1992

log A =

log I i/It

= a*d*C

+G

Fig 2 fliustration of in vivo and in vitro application of BeerLambert's law.

The analysis procedure converts the obtained optical densities (OD) to concentrations of oxygenated (Hb0 2 ), deoxygenated hemoglobin (Hbred) and oxidized cytochrome aa3, expressed in millimoles per liter per optical pathlength (mmol x 1-1 x cm), using Beer-Lambert's law. Beer-Lambert's law states that the light attenuation on a logarithmic scale caused by solutions is proportional to the product of the concentration (c), the absorption coefficient (a) and the light pathlength (d) (Fig 2). log A

=

log

li It =

a x c x d

+G

A: absorption, li: incident light, It: transmitted light, a: absorption coefficient, c: concentration of substance (= chromophore), d: interoptode distance, G: geometric factor. Absorption spectra (a) have been determined, as mentioned above [16]. Due to the scattering of light in various tissues, the pathway of the transmitted light is longer than the geometrical spacing between the opt odes. Modifying the Beer-Lambert law by introducing a constant pathlength factor (p) makes calculation of the concentrations of the different compounds possible. Different methods have been used to define this pathlength factor [21-24] . In preterm infants this factor was measured post mortem and found to be 4.39 [23]. In a recent study [24], a mean value of 3.85 and a standard deviation of 0.57 were obtained. The geometric factor , G, reflects densities and tissue geometries. Light intensity decreases logarithmically, and is mainly dependent on the concentrations of the chromophores and factor G. In vivo it is almost impossible to quantify this factor. As we are mea. suring only changes in concentration, G can be assumed

Block Diagram of the System

Fig 3

System block diagram of the NIR 1000 (Hamamatsu).

to be constant. This is expressed by the following formula: I Ii LlC = -a-x-d-=--x-p x log It

p: pathlength factor, LlC: change in concentration. Near-infrared spectroscope and measurement procedure Instrumentation-developments since the early seventies have now made it possible to perform continuous cotside NIRS measurements in neonates [25-29]. Different types of near infrared spectrophotometers have been developed, working at 4 or 6 wavelengths, involving various systems for detecting the transmitted light. In Leuven we use a spectrophotometer (NIR 1000 Hamamatsu Photonics KK) which was developed by Delpy and Cope in the Departments of Medical Physics and Paedi~trics of University College, London, UK and built by Hamamatsu Photonics KK, Japan [4,8,28,30] (Fig 40). Six laser diodes emit bundled light of 6 different wavelengths (775, 802,824,846,867 and 905 nm). Each laser diode can produce a peak optical output of about one watt for a pulse duration of only 100 nanoseconds (ns). The six laser diodes are fired in sequence and repetitively pulsed at 4 kilohertz (kHz), thus every pulse has a duration of 250 microseconds (ps). The light is conducted by an optical fibre to the head of the baby. The transmitted light is passed through a second optical fibre from the baby's head to a photomultiplier tube, which is connected to a multichannel photon counter, operating in the so-called "photon counting mode." This enables one to count single photons. The number of photons at each wavelength is compared with the light output of the lasers (31, Hamamatsu Photonics KK, Japan, Internal report, 1989) (Fig 3).

The optical density (00) is calculated on a logarithmic scale in terms ofloss of light intensity per centimeter. The data can be displayed on a colour screen and stored on disc for further off-line analysis. The inbuilt photomultiplier tube (PMT) is very sensitive and needs absolute stable temperatures for measurement. Thus cooling of the system is started at least half an hour before the measurement. The system drift over a 5-hour period has to be less than 0.0200. Without previous cooling a much bigger shift in the optical densities of cytochrome aa3 can occur and interpretation of the results becomes difficult. The optodes are fixed biparietally ( transmission mode) with an elastic band and with double-sided adhesive rings (Fig 4A, B). If the biparietal diameter exceeds 7-8 cm the optodes have to be fixed frontally and parietally (reflection mode). Careful fixation is important, especially when the measurement is performed in older or unsedated infants, as artifacts may be caused by spontaneous movements. During the procedure, especially in long-term recordings, the interoptode distance should be controlled with calipers. A further important point concerns shielding from ambient light (Fig 4C). Because of the high sensitivity of the equipment, this has to be done with much care. In our unit, we work with special opaque clothes, usually used in photo-laboratories. The light intensity at the skin surface is within the range permitted by the laser standard authority (International Electrotechnical Commission Standard Publication 825, first edition, 1984). Although the light intensity is below the upper level set for safe retina exposure, mOving away of the optodes has to be avoided. This is controlled by an internal safety device, that measures the reflected light from the skin surface and signals, through an alarm system, that the fibres are becoming detached. To ensure accuracy of the measurement, the system has several internal control devices. The amount of ambient light and the reflection of the emitted light are measured. Thus, artifacts can be detected throughout the measurement period or by detailed off-line analysis. Quantification of circulatory variables A possible application of the method consists of the quantification of hemodynamic variables. For that purpose, changes in chromophore concentrations during induced changes in arterial oxygen saturation (Sat0 2 ) are measured . Further analysis of the data provides absolute values of cerebral blood volume (CBY), expressed in ml x 100 g-I, and cerebral blood flow (CBF), expressed in ml x 100 g-I X min -I. CBY and CBF alterations related to changes in pC0 2 can provide us with information on cerebrovascular autoregulation.

von Siebenthal et al: Near-in/rared spectroscopy 137

Fig 4 Optode fixation and light shielding. A: The optodes are fixed, using double-sided adhesive rings. Example of a measurement in the reflection mode; the optodes are fixed frontally and parietally. B: The positioning of both optodes is ensured with an elastic band. In this example the optodes are fixed biparietally (transmission mode). C: Shielding from ambient light with an opaque cloth. D: View of the whole measurement setting; the NIR 1000 (Hamamatsu) on the patient 's right side, and the pulsoxymeter (Radiometer) and the combined transcutaneous pO. and pCO, monitor (Kontron) on the patient's left side.

Suctioning 10

18

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14

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v

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-4

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-6

2

-8

-10 10

20

30

40

lime (min}

50

Cerebral blood volume During a period of about 10 minutes a small increase in Sat0 2 in the range of 5% is induced. CBV can be derived with the following equation [32] : CBV =

~(Hb02-Hbred)

2 x [Hb] x R x ~Sat02

~(HbOrHbred):

change in Hb difference, [Hb]: central

138 Brain & Development, Vol 14, No 3, 1992

60

70

80

-2

Fig 5 Example of a NIRS measurement during suctioning (t): A newborn infant with hyaline membrane disease grade II, with a gestational age of 29 weeks, was examined on day 4. A decrease in HbO. (-) and an increase of Hbred (__), and nearly stable cytochrome aa 3 values can be observed.

Hb concentration, R: cerebral-to-Iarge vessel hematocrit ratio (= 0.69), ~Sat02: change in peripheralSat0 2. Cerebral blood flow NIRS seems to be the first method of choice for obtaining data on CBF, which is applicable at the bedside and can be performed without radioactive tracers. The Fick princi. pie is followed, which states that the rate of accumulation

Start cooling the photomultiplier Calibration of transcutaneous monitor

Patient selection ' Indication Discussion with nursing and medical staff Parent's permission

Preparation: Patient data collection (especially head size, etc,) Attachment of optodes and lightshielding Setting of NIR-1000 system parameters Attachment of other used equipment Start measurement

Measurement: Normalization after the system has stabilized (after 12- 15 minutes) Control of measured data and external inputs Attachment OK? Infant comfortable? Recording of special events, clinical data and behavioural states on a separate sheet

is altered over 8-10 seconds. This manoeuvre causes a change in Sat0 2 , that is measured by pulsoxymetry. The alteration in Sat0 2 induces a change in Hb0 2 , which is measured by NIRS. Now the calculation can be performed, with the following equation: CBF = _...:o.Q..>....(tL.)ftpart(t)/dt o ilHb0 2 xK CBF rilSat0 2 /dt x [Hb] o It is of great importance that during the manoeuvre, cerebral hemodynamics and oxygen consumption have to remain constant. Consequently there are a number of prerequisites, some are related to the infant's stability and some to the quality of the collected data. The induced changes have to be within a strict range to avoid autoregulatory phenomena.

Aspects of autoregulation The circulation in arterioles is strongly influenced by CO 2 tension, also called CO 2 reactivity. The increased CO 2 tension level results in a proportional increase in CBV (and CBF). Following a baseline observation, a small alteration in pC0 2 in the range of approximately 1-2 kPa is induced over about 10 minutes by a small change in the ventilation rate [12,33,34]. pC0 2 was maintained within a range of 4-7 kPa. The response of CBV to changes of about 1 kPa in pC0 2 can be observed by means of NIRS. CLINICAL APPLICATIONS

After measurement: Control of attachment sites Data transmission to a personal computer Analysis procedure Discussion

Fig 6 Actual measurement procedure: from patient selection to analysis.

(dQ/dt) of a tracer substance in a organ is equal to the difference between the rate of arrival (arterial content) and the rate of departure (venous content). This is expressed by the following formula: dQ/dt = CBF x (Part(t) - Pven(t)) The rates of arrival and departure are the product of blood flow and content of the tracer in arterial (Part) and venous (Pven) blood, respectively. The principle in humans is to use oxygen as a tracer. The inspired oxygen concentration

Since 1985 NIRS has been mainly used in preterm infants receiving intensive care therapy. Changes in Hb, Hb0 2 and Cyt aa3 have been measured under various common conditions, such as decreased oxygenation, episodes of bradycardia, before and after surgical ligation of a ductus arteriosus [25], during spontaneous hypertensive peaks [26], crying episodes [27], hypoxemia and bradycardia [35]. Quantitative measurements of CBV have been performed in prematures after spontaneous as well as induced changes in Sat0 2 and pC0 2 [28,34], and in infants receiving indomethacin for closure of a patent ductus arteriosus [36,37] . Monitoring of clinical events A first clinical study, performed in 1985 on 3 patients [25] , concentrated on changes in Cyt aa3' Short hypoxic states with or without bradycardia were found to cause a slight shift in Cyt aa3 to a more reduced state, whereas the change in reduced Hb was more pronounced. Prolonged hypoxic episodes led to simultaneous decreases in Hb0 2 and Cyt aa3" In a recent study [35], during spontaneous or in-

von Sieben thai et al: Near-in/rared spectroscopy 139

duced decreases in Sat0 2 in 17 infants, similar results were obtained: total Hb concentration (Hbtot), the sum of oxidized and reduced Hb, showed an increase, if hypoxia was not accompanied by bradycardia. This effect was mainly due to an increase in reduced Hb. The opposite change, a decrease in Hbtot, was seen when hypoxia occurred together with bradycardia. For that phenomenon, a marked decrease in oxidized Hb was the main contributing factor. In adults, the effect of hypoxia has been measured, using a rebreathing technique [38]. Normocapnic and hypocapnic hypoxia were produced in each of 8 male volunteers. Hypoxia resulted in a steady decrease in cerebral oxidized Hb and in Cyt aa3. Hbtot increased significantly. These effects were strongly influenced by the pC0 2 level, a greater decrease of Cyt aa3 and a smaller increase in Hbtot being seen in hypocapnic-hypoxic subjects. The influence of blood pressure peaks on Hbtot was investigated in 1986 [26]. These measurements, performed in three ventilated, nonparalyzed very low birthweight infants, showed a marked increase in Hbtot during movement associated hypertensive peaks. This increase mainly reflected an increase in reduced Hb, whereas there was no or only little change in oxidized Hb. Cyt aa3 paralleled the relative contribution of Hb0 2 on these changes, e.g., it became more reduced, if the rise in Hbtot resulted from an increase in Hbred, and more oxidized when the rise in Hbtot was the result of an increase in Hb0 2 • In a study on the effects of crying on Hbtot and Cyt aa3 [27], the changes of these chromophores in infants with and without respiratory problems were compared. The authors showed that baseline Hbtot rose in 86% of crying episodes and remained at that level during the crying, possibly indicating a periodic obstruction to cerebral venous return. In contrast, Cyt aa3 showed marked variability with the presence of respiratory problems. A reduction of Cyt aa3 occurred significantly more often in infants with respiratory problems than in healthy ones. Aspects of nonnal and abnonnal brain circulation, and its autoregulation A clinical application of NIRS [28], related to aspects of normal and abnormal brain circulation, gave us the results of 11 sick newborns. Measurements and quantification of the results have been achieved after induced or spontaneous small changes in Sat0 2 and pC0 2 • Using NIRS, CBV values were calculated for the first time. Following this concept, new values of mean CBV were reported in 1990 [32]. In 12 preterm and fullterm infants under intensive care, who did not suffer from cerebral lesions, mean CBV was 1.7 ml x 100 g-l, whereas in 10 comparable infants with hypoxic ischemic brain injury, mean CBV was significantly higher, 3.0 ml x 100 g-l. The highest values were found in infants who had sustained severe birth asphyxia.

140 Brain & Development, Vol 14, No 3, 1992

Cyl aa3

l~l·~~~ g.,

o

U -2

-3 -

-~ 31

'=-

2 1

:

0 c:: -1 o

U

-2-1 -3 -

i

-40

I

-30

I

-20

i

-10

I

... 0

I

10

I

20

I

30

I

40

I

50

I

60

Time (min)

LP

Fig 7 Changes in cyt aa, in a neonate born at 28 weeks of gestational age with and without clinical signs of increased ICP. The top curve represents the results obtained at the age of 3 weeks with clear signs of increased ICP. The bottom curve represents the results obtained at the age of 5 weeks, when the neonate showed no signs of increased ICP. Changes in Cyt aa, are expressed in micromols per liter. The time scale is in minutes, before and after the lumbar puncture (time zero).

The response of CBV to small changes in pC0 2 is a matter of increasing interest [39], because results could have significant implications for therapeutic strategies and management in intensive care units. Preliminary results having been published in 1986 [28], a recent study [35] on infants with normal brains yielded values about 0.5 ml x 100 g-1 X kPa- 1. In contrast, in infants with birth asphyxia a pronounced reduction in or even the absence of this cerebrovascular response was observed. As mentioned above, cerebral blood flow (CBF) can be measured using the Fick principle. 31 measurements in 9 ill, primarily very preterm infants gave values about 18 ml x 100 g-1 X min -1. These values seem comparable with those obtained with 133Xenon clearance and PET [12, 40,41]' and with venous occlusion plethysmography [42]. Assessment of therapeutical intervention The first study involving NIRS to monitor drug therapy concerned the effect of indomethacin on the brain circulation. The results, in 13 very low birthweight infants, showed a pronounced decrease in CBF, as well as a reduction in CBV and in its response to changes in arterial pC0 2 • These data have been confirmed recently [37]. In Leuven we concentrated on Cyt aa3 changes in infants with well defined clinical problems. A first study [43] was performed in order to examine the effect of lumbar punctures in infants suffering from posthemorrhagic hydrocephalus (Fig 7). Two groups of infants were compared, both with

marked ventricular dilatation, the diagnosis was made on the basis of cranial ultrasonography and regular lumbar punctures were performed in order to prevent the rapidly growing hydrocephalus. The first group showed clinical signs of raised intracranial pressure (ICP), such as bulging of the anterior fontanelle and diastasis of the sutures. The control group also exhibited ventricular dilatation but not signs of increased ICP. NIRS measurement was started 30 minutes before and was continued for at least 45 minutes after each lumbar puncture. The results showed interesting differences between the 2 groups: all neonates with clinical signs of raised ICP showed a significant increase in Cyt aa3 starting with or shortly after the beginning of the lumbar puncture. In contrast, no increase in Cyt aa3 was found in neonates without the clinical signs mentioned above. DISCUSSION Development of the method For discussion of the state of the art of this relatively new method , a brief portraiture of its stepwise development might be useful. After the first description of NIRS in animals studies [14,15], it took almost one decade until the first clinical studies were performed [25] . In this first period, the quantification of changes in concentration was not possible and the results had to be expressed as changes in optical density. Only the detection of the pathlength factor [21, 22, 24] made it possible to measure changes in concentrations of HbO z and Hbred, and the oxidized state of Cyt aa3' It should be realized, however, that the pathlength is negatively correlated with the different wavelengths. Furthermore, the pathlength of light through scattering tissues depends on a number of factors, e.g., myelinization and bone mineralization. This implies that it increases with the age of the infants. The appropriate use of manipulation of inhalated gases made it possible to quantify CBF and CBY. One of the chromophores, HbO z , is used as a tracer after the induction of a stepwise change in SatO z . This elegant methodological approach, however, has its limitations: it is mainly used in ventilated babies, and in that group only in those with not too severely impaired lung function. Some other questions have to be answered, before we can accept, that the measurements of HbO z and Hbred really quantify CBY and CBF. In contrast to pulsoxymetry, NIRS does provide a measure not only related to SatO z. Due to light scattering in the brain NIRS provides information on changes in HbO z and Hbred in all vascular compartments. The main part, however, from the venous vessels, making up 60-70% of the blood volume. In the short-lasting recordings NIRS measures CBF in arterial vessels, since HbO z is measured before it appears

in the venous efflux as the time of the measurement is shorter than the assumed minimum transit time of blood through the brain in newborns. Estimation of CBY is based on a change of blood oxygenation over a longer period of about 10 minutes and therefore the increased amount of oxygen will mainly be derived from the venous compartment. A major problem arises when using Hbtot as an estimate of CBY during long-term recordings, thus outside the experimental procedures, as described above. Changes in blood plasma would not be detected by NIRS. An estimate of CBY is thus only allowed, when the hematocrit remains constant during the whole measurement period. Finally, CBY results are only acceptable, when there are no changes in CBF and oxygen consumption. Therefore, simultaneous recording of behavioural states, heart rate, oxygen saturation, transcutaneous pOz and CO z , together with the cerebral variables measured by NIRS, is mandatory. Although Cyt aa3 is conceptually the most promising of the chromophores, a number of open questions remain: there is still a lack of knowledge on its role in vivo. Additionally, some methodological problems make the analysis difficult: in contrast to the absorption spectra of HbO z and Hbred, one has to bear in mind that only the difference between the oxidized and reduced forms of Cyt aa3 can be determined. Furthermore, the magnitude of its expression is only one tenth of the two others and shows a very flat peak. Present applications of the method The cerebral vascular autoregulation in premature babies remains a topic of considerable concern. NIRS seems to be a potent tool for studying this problem [35] . The response of CBY or CBF to changes in arterial pCO z can be examined. The results confirm those of earlier studies involving CBF velocity measured with Doppler ultrasound, a positive linear relationship between the CO 2 response and conceptional age [44,45] being shown. In patients who are not ventilated, a pCO z increase can be induced by reb rea thing manoeuvres or spontaneous breathing of Oz/ CO 2 gas mixtures [38]. After the administration of indomethacin to infants, suffering from open ductus arteriosus, CBF is diminished [36,37]. This provides a new insight into the broad circulatory effects of this drug and it is thus of great practical value. Whether these changes in CBY are a result of diminished cardiac output or of central vascular responses remains doubtful, even after the combination Of NIRS measurements with Doppler ultrasound measuring blood flow velocity in the carotis [37] . A promising clue to the investigation of central nervous drug effects, however, is given. Furthermore, NIRS was performed before and after a lumbar puncture. More observations are necessary to con-

von Sieben thaI et al: Near-infrared spectroscopy 141

firm these preliminary results. Our experience in this study with neither intubated nor sedated patients stresses the importance of exact fixation and precise light shielding. An increased amount of spontaneous motor activity can easily compromise the accuracy of measurements. As a consequence of the absence of a "golden" standard, comparison with established methods, all involving different approaches, requires caution. There is the question of whether or not comparison of plasma flow, measured by 133Xenon clearance, with Hb flow, measured by NIRS, is valid. Finally, NIRS is, in contrast to 133Xenon clearance and NMR spectroscopy, a technique that makes bedside determination and repeated measurements of circulatory variables possible. As a further step to a monitoring system, there is a need for smaller and lighter apparatus generations. More flexible light cables and an opt ode design which allows reliable fixation , even on moving heads, are required. Long-term measurement during critical surgical intervention will need good compatibility with other technical equipment and less sensitivity to ambient light.

CONCLUSIONS From the presently available evidence we conclude that NIRS allows us, to some degree, to monitor various clinical conditions. Future studies should concentrate on improving the methodology and on testing of precisely formulated hypotheses related to the mechanisms underlying brain circulation and oxygenation, both in the normal condition and in specific disease states. ACKNOWLEDGMENTS The Developmental Neurology Research Project KU Leuven, Belgium, is mainly supported by a grant from the Medical Research Council, Belgium (FGWO), by a grant from the Janssen Research Foundation, Beerse, Belgium and by a travel grant of the British Council, Brussels. The authors wish to express their thanks to Professor Hugo Devlieger and the neonatological staff of the University Hospital, Gasthuisberg Leuven, to Drs. Hans-Ullrich Bucher and Martin Wolff, Department of Neonatology, University Hospital, Zurich, Switzerland and to Drs. David Edwards and Ma:;:c Cope, Departments of Paediatrics, Medical Physics and Bioengineering, University College, London, UK.

REFERENCES 1. Levene MI, Kornberg J, Williams THe. The incidence and severity of post-asphyxial encephalopathy in full-term infants. Early Hum Dev 1985;11:21-8. 2. Huch R, Huch A, Lubbers DW, eds. Transcutaneous PO,. Stuttgart: Georg Thieme Verlag, 1981. 3. Severinghaus JW. Introduction to skin pCO, measurement. In: Huch R, Huch A, eds. Continuous transcutaneous blood gas monitoring. New York·Basel: Marcel Dekker, 1983: 17784 . 4. Delpy DT. Developments in oxygen monitoring. J Biomed

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Brain & Development, Vol 14, No 3, 1992

List of abbreviations ADP ATP CBF* CBV* CT

Cyt aa 3 Hb HbO, Hbred Hbtot ICP kHz MRI MRS NIRS nm ns OD PET pCO, pO, p

PMT SatO, JotS

Adenosine diphosphate Adenosine triphosphate Cerebral blood flow Cerebral blood volume Computed tomography Cytochrome aa 3 Hemoglobin Oxidized hemoglobin Reduced hemoglobin Total hemoglobin concentration Intracranial pressure Kilohertz Magnetic resonance imaging Magnetic resonance spectroscopy Near-infrared spectroscopy Nanometer Nanosecond Optical density Positron emission tomography Partial pressure of carbon dioxide tension Partial pressure of oxygen tension Pathlength factor Photomultiplier tube Arterial oxygen saturation Microsecond

Bold: measured by near infrared spectroscopy, *calculated from variables measured by NIRS.

Eng 1988;10:53340. 5. Delpy DT, Gordon RE, Hope PL. Non invasive detection of cerebral ischaemia by phosphorus nuclear magnetic resonance. Pediatrics 1982;70:310-3. 6. Volpe JJ, Herscovitch P, Perlman JM, Raichle ME. Positron emission tomography in the newborn: extensive impairment of regional blood flow with intraventricular haemorrhage and haemorrhagic intracerebral involvement. Pediatrics 1983; 72: 589-601. 7. Hope PL, Reynolds EOR. Investigation of cerebral energy metabolism in newborn infants by phophorus magnetic resonance spectroscopy. Clin Perinatol 1985; 12:261-75. 8. Reynolds EOR, Wyatt JS, Azzopardi D, et -al. New noninvasive methods for assessing brain oxygenation and haemodynamics. Br Med Bull 1988;44: 1052-75 . 9. Wyatt JS, Edwards AD, Azzopardi D, Reynolds EOR. Magnetic resonance and near infrared spectroscopy for investigation of perinatal hypoxic-ischaemic brain injury. Arch Dis Child 1989;64:953-63. 10. Greisen G, Trojaburg W. Cerebral blood flow, PaCO, changes and VEP's in mechanical ventilated, preterm infants. Acta Pediatr Scand 1987;76:394400. 11. Lipp AE, Muller A, Ilichschmid P, Duc G. Oxygen affinity of Hb modulates CBF in premature infants. Acta Pediatr Scand 1989;360(suppO :26-32. 12. Pryds 0, Greisen G, Skov LL, Friis-Hansen B. Carbon dioxide-related changes in cerebral blood volume and cerebral blood flow in mechanical ventilated preterm neonates: comparison of near infrared spectrophotometry and 133Xenon clearance. Pediatr Res 1990;27:445-9. 13. Ramaekers VT, Casaer P, Daniels H, Smet M, Marchal G. The influence of behavioural states on -cerebral blood flow velocity patterns in stable preterm infants. Early Hum Dev 1989;

20:229-46. 14. J6bsis van der Vliet FF. Noninvasive, infra-red monitoring of cerebral and myocardial oxygen sufficiency and circulatory parameters. Science 1977;198:1264-7. 15. J6bsis van der Vliet FF, Keizer JH, LaManna JC, Rosenthal M. Reflectance spectrophotometry of cytochrome aa 3 in vivo. J Appl Physiol: Respirat Envir Exercise Physiol 1977; 43:858-72. 16. Wray S, Cope M, Delpy DT, Wyatt JS, Reynolds EOR. Characterization of the near infrared spectra of cytochrome aa 3 and haemoglobin for the non-invasive monitoring of cerebral oxygenation. Biochim Biophys Acta 1988;933: 184-92. 17. Lehninger AL. Bioenergetics. 2nd ed. Menlo Park, California· Reading, Massachusetts· London· Amsterdam · Don Mills· Ontario· Sydney: Benjamin/Cummings Publ Co, 1973: 82-92. 18. Kadenbach B, Kuhn-Nentwig L, Biige U. Evolution of a regulatory enzyme. Current Topics in Bioenergetics 1987; 15:113~1.

19. Beinert H, Shaw RW, Hansen RE, Hartzell CR. Studies on the origin of the near infrared (800-900 nm) absorption of cytochrome c oxidase. Biochim Biophys Acta 1980;591: 458-70 . 20. Jobsis van der Vliet FF, Piantadosi CA, Sylvia AL, Lucas SK, Keizer HH. Near infrared monitoring of cerebral oxygen sufficiency. Neurol Res 1988; 10: 7-17. 21. Delpy DT, Cope M, Van Der Zee P, Arridge S, Wray S, Wyatt J. Estimation of optical pathlength through tissue from direct time of flight measurement. Phys Med BioI 1988;33 : 1433-42. 22. Delpy DT, Arridge S, Cope M, et al. Quantitation of pathlength in optical spectroscopy. Adv Exp Med BioI 1989;247: 41-6. 23. Wyatt JS, Cope M, Delpy DT, et al. Measurement of optical pathlength for cerebral near infrared spectroscopy in newborn infants. DevNeurosci 1989;12:140-4. 24. Van der Zee P, Cope M, Arridge SR, et al. Experimentally measured optical pathlengths for the adult head, calf and forearm and the head of the newborn infant as a function of inter optode spacing.Adv Exp Med BioI 1992 (in,press). 25. Brazy JE, Lewis DV, Mitnick MH, Jobsis van der Vliet FF. Noninvasive monitoring of cerebral oxygenation in preterm infants: preliminary ob servations. Pediatrics 1985; 75: 21 7 -25. 26. Brazy 1£, Lewis DV. Changes in cerebral blood volume and cytochrome aa 3 during hypertensive peaks in preterm infants. J Pediatr 1986; 108:983-7. 27. Brazy JE. Effects of crying on cerebral blood volume and cytochrome aa 3 • J Pediatr 1988;112:457-61. 28. Wyatt JS, Cope M, Delpy DT, Wray S, Reynolds EOR. Quantification of cerebral oxygenation and haemodynamics in sick newborn infants by near infrared spectrophotometry. Lancet 1986;ii: 1063-6. 29. Edwards AD, Wyatt JS, Delpy DT, Cope M, Reynolds EOR. Cotside measurement of cerebral blood flow in ill newborn infants by near infrared spectroscopy. Lancet 1988;ii:770-1. 30. Cope M, Delpy DT. A system for long term measurement of cerebral blood flow and tissue oxygenation in newborn infants by near-infrared transillumination. Med BioI Eng Com-

put 1988;26:289-94. 31. Cope M, Delpy DT, Reynolds EOR, Wray S, Wyatt J, van der Zee P. Methods of quantitating cerebral near infrared spectroscopy data. Adv Exp Med BioI 1988;222: 183-9. 32. Wyatt JS, Cope M, Delpy DT, et al. Quantification of cerebral blood volume in human infants by near-infrared spectroscopy. J Appl Physiol 1990;68: 1086-91. 33. Wolf M, Bucher HU, Keel M, Duc G. Cerebral CO 2 response in neonates studied by near infrared spectroscopy. Proceedings of the 4th International Conference on Fetal and Neonatal Physiological Measurements, Noordwijkerhout. Amsterdam·London·New York·Tokyo: Elsevier, 1991. 34. Wyatt JS, Edwards AD, Cope M, et al. Response of cerebral blood volume to changes in arterial carbon dioxide tension in preterm and term infants. Pediatr Res 1991 ;29:553-7. 35. Livera LN, Spencer SA, Thorniley MS, Wickramasinghe YABD, Rolfe P. Effects of hypoxaemia and bradycardia on neonatal cerebral haemodynamics. Arch Dis Child 1991 ;66: 376-80. 36. Edwards AD, Wyatt JS, Richardson C, et al. Effects of indomethacin on cerebral haemodynamics in very preterrn infants. Lancet 1990;i: 1491-5_ 37 . Djien Liem K, Oeseburg K, Hopman JCW, Kollee LAA. Effects of indomethacin on cerebral oxygenation in preterm newborns: combined near infrared spectrophotometry and ultrasound doppler study (abstract). Proceedings of the 4th International Conference on Fetal and Neonatal Physiological Measurements, Noordwijkerhout. Amsterdam· London· New York ·Tokyo: Elsevier, 1991. 38. Hampson NB, Camporesi EM, Stolp BW, et al. Cerebraloxygen availability by NIR spectroscopy during transient hypoxia in humans. J Appl Physiol 1990;69:907-13. 39. Lipp AE, Morales CG, Duc G. Prognostic value of cerebrovascular CO 2 -reactivity in premature infants (abstract). European Society for Pediatric Research Annual Meeting, Ziirich, 1991. 40. Greisen G. Cerebral blood flow in preterm infants in the fust week of life. Acta Paediatr Scand 1986 ;75:43-51. 41. Volpe 11, Herscovitch P, Perlman JM, Kreusser KL, Raichle ME. Positron emission tomography in the asphyxiated term born: · parasagittal impairment of cerebral blood flow . Ann NeuroI1985;17:287-96. 42 . Edwards AD, Reynolds EOR, Richardson CE, Wyatt IS. Estimation of blood flow using near infra-red spectroscopy. J PhysioI1989;410:50. 43. Casaer P, von Siebenthal K, van der Vlugt A, et al. Cytochrome aa 3 and intracranial pressure in newborn infants; a near infrared spectroscopy study. Neuropediatrics 1992;23: 111. 44. Levene MI, Short land D, Gibson N, Evans DH. Carbon dioxide reactivity of the cerebral circulation in extremely premature infants: effects of postnatal age and indomethacin. Pediatr Res 1990;24: 175-9. 45. Ramaekers VTH, Casaer P, Daniels H, Marchal G. Upper limits in brain blood flow autoregulation in stable infants of various conceptional age. Early Hum Dev 1990;24: 249-58.

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