Julian R. Henty

Measurement of CMRO2 in Neonates Undergoing Intensive Care Using Near Infrared Spectroscopy

Greater understanding of the rate of oxygen delivery and uptake in sick preterm and term infants undergoing intensive care is an important aim of brain-orientated neonatal medicine. Near infrared spectroscopy (NIRS) is a continuous, non-invasive and portable technique which can be used to measure cerebral blood flow (CBF) in infants. It is also possible to use spatially resolved spectroscopy to measure absolute mean cerebral oxygen saturation (SmcO2). The aim of this study was to investigate the derivation of cerebral metabolic rate for oxygen (CMRO2) from these two measurements. Nine preterm infants were studied, of median (range) gestational age 25 (23-37) weeks. A NIRO300 was used to measure CBF and SmcO2 simultaneously over the right and left hemisphere. Median (range) left and right cerebral hemisphere values for CMRO2 were 0.95 (0.79-1.53) ml 100g(-1) x min(-1) and 0.88 (0.69-1.46) ml 100g(-1) x min(-1), respectively. No significant difference was seen between the left- and right-sided values. These values are similar to median (range) values previously reported in infants using positron emission tomography or more invasive NIRS methods. Further work is necessary to define limits on the use of this technique, particularly in the assumption of the venous:arterial compartment volume ratio across different infants.

Quantification of adult cerebral blood volume using the NIRS tissue oxygenation index.

Near-infrared spectroscopy (NIRS) is increasingly used as a non-invasive technique for monitoring cerebral oxygenation and haemodynamics . Simple continuous-wave (CW) NIRS systems utilising differential spectroscopy can measure quantitative changes in oxyand deoxyhaemoglobin (∆[O2Hb], ∆[HHb]) but only from an arbitrary baseline. Numerous studies of changes in cerebral oxygenation and haemodynamics in adults have been published but only few absolute quantitative measurements have been reported. Recent advances in the NIRS technology have enabled quantitative assessment of haemoglobin concentration in tissue using near-infrared (NIR) phase and time resolved systems; and absolute measurements of tissue saturation using phase, time or spatially resolved spectroscopy (SRS) systems 3, 4, 5, 6 . This paper suggests a way to use a commercially available spectrometer, which has both CW and SRS capabilities in order to measure absolute tissue haemoglobin (Hbtc) and hence cerebral blood volume (CBV). The methodology is based on that of Wyatt et al. 7 who developed a method for measuring absolute CBV, using NIRS measurements during controlled changes in inspired O2 fraction. By using NIRS measured tissue ∆[O2Hb] and comparing it to changes in arterial saturation (SaO2) measured with a pulse oximeter it is possible to calculate absolute Hbtc concentration. This is the so-called ‘desaturation method’ or ‘O2 method’ or ‘SaO2 method’ 8, 9, 10, 11 . The purpose of the present study was to compare measurements of CBV made using the conventional ‘SaO2 method’ with those using a new method employing the SRS derived absolute cerebral tissue oxygenation index (TOI), which will be called the ‘TOI method’.

Simultaneous measurement of cerebral tissue oxygenation over the adult frontal and motor cortex during rest and functional activation.

Near-infrared spectroscopy (NIRS) has been shown to measure subtle haemodynamic changes in the human brain. Differential spectroscopy has been used to record changes in oxy-haemoglobin concentration 4[Hb02] and deoxy-haemoglobin concentration 4[HHb], following various kinds of stimulation including motor, visual, and cognitive. NIRS based topography systems have also been used to map cerebral haemodynamic responses around the evoked region. NIRS provides an alternative to more expensive techniques such as positron emission tomography and functional magnetic resonance imaging, in the understanding of brain function.

Investigation of Oxygen Saturation Derived from Cardiac Pulsations Measured on the Adult Head Using NIR Spectroscopy

Cardiac related pulsatile signals can be detected in different parts of the human body, including the finger, ear lobe and forehead [1] by using near infrared (NIR) monitoring. These pulsatile signals are due to attenuation of light by the increase of arterial blood volume during systole in the cardiac cycle. Pulse oximetry exploits these pulsatile signals to calculate oxygen saturation (SpO2) [2]. There are two types of pulse oximetry: (1) the transmission type where the light source and detector are facing each other across the measurement site (e.g. ear lobe or finger), and (2) the reflectance type where both the light source and detector are in the same plane (e.g. forehead or forearm) [1] with the source detector spacing typically less than 1 cm. With more sensitive optical instruments, it has been shown that these pulsatile signals can be measured on the forehead at a greater spacing using either a CCD spectrometer [3] or a phase resolved system [4]. Both of these two methods can be considered as operating in reflectance mode with large source detector spacings of 3 or 3.5 cm. These pulsatile signals were also thought to be mainly caused by the change in arterial blood volume. However, at source detector spacing larger than ~2 cm, NIR light can penetrate through the skull into the brain and the measured pulsatile signals are likely partly to include components caused by brain movement [5] as well as arterial blood pulsations in the scalp and the brain. The objective of this paper is to compare three algorithms used to calculate oxygen saturation from the head pulsatile signals, (signified by SpO2 to distinguish it from SpO2 measured at other sites) with large source detector spacing. Two of the algorithms implicitly allow the possibility of venous blood contributing to SpO2. We will show the SpO2 calculated by three algorithms in 8 adult subjects during normoxia and hypoxia. Examples of phase differences between the oxy and deoxy-haemoglobin (∆HbO2 and

Measurement of the optical properties of the adult human head with spatially resolved spectroscopy and changes of posture.

Absolute optical properties (i.e., absorption and reduced scattering coefficients, µ a and µ s ’) of human tissues such as the head, calf and arm have been measured using near-infrared (NIR) phase1, time2 or spatially3 resolved spectroscopy (SRS) systems. While a simple continuous-wave (CW) system can measure Δµ a , absolute µ a cannot be easily measured because of the complex geometry in which measurements are made in tissues. It has been shown that the SRS technique can be used to calculate a scaled absolute µ a , i.e. µ s ’ µ a where µ s ’ is considered as a time-invariant scaling factor4. This paper suggests a way to use a commercially available spectrometer, namely the NIRO-300 (Hamamatsu KK.) which has both CW and SRS capabilities, to calibrate an absolute µ a based on the changes of µ a (i.e., Δµ a , calculated from the CW data and a modified Beer-Lambert law) and the scaled µ a (i.e., µ s ’ µ a calculated from the SRS data). Using changes of posture from the supine to the head up position, the absolute optical properties of 15 adult human heads were calculated. Issues of errors due to the inhomogeneity in real tissues and methods to minimise them are also discussed.