Cerebral and muscle oxygen saturation measurement by frequency-domain near-infra-red spectrometer

Tissue oxygen saturation quantification was obtained using a frequency-domain multi-source method based on two wavelength light-emitting diodes. Brain saturation was 60·3±1·1% (n=12). Brachioradial muscle saturation declined during forearm ischaemia and maximal voluntary contraction from 73·7±1·8 and 74·7±1·8% at rest to 44·2±3·3 and 61·4±2·9%, respectively.

1 Introduction NON-INVASlVE near-infra-red (IR) spectroscopy has bcen found to be a useful means of monitoring intraccrebral oxygenation and haemodynamics in the fetus and newborn infants (BRAZY et al., 1985; W]CKRAMASINGHE et al., 1985;WYATt et al., 1986;PRYD$ et al., 19901LWERA et aL, 1991;PEEBLES et aL, 1992), and oxygen transport in muscle (DE BLASI et aL, 1993).Near-infra-red radiation (700-1100 rim) penetrates several ccntimetres into living tissue.Strong scattering in tissue results in diffuse transmission and a lack ofapriori knowledge of the optical path length required for quantifying oxy-and dcoxyhaemoglobin concentrations.
Expensive and complex picosecond laser source and fast detectors have made it possible to measure the path length (DELPY et aL, 1988).An alternative and ch~ approach for the measurement of optical path length and vascular haemoglobin saturation is suggested by the development of frequcneyMomain spectrophotorneters (FlSrlKIN et aL,199t).
In a previous freqtmney-domair~ study, we showed that, using a number of different ~'c~ di~c~, it is possible to derive an analytical expression for the absorption and scattering coefficients in a strongly scattering medium ind~ently of a priori assumptions about the path length (FAmTm et aL, 1994a).On the basis of this multi-distance method, a two wavelength insmmaent for tiss~ oxygen satu~on (St) monitoring has been developed (FANT~ et aI., 1995), This approach dots not require any a pr/ori assumptions about the valu~ of the scattering coe~cicnts as this co~ffi~.nt is directly m~sured.Two near.IR bands, peaked at 715" and 850 nm t, were generated by two arrays of four intensity-modulated lightemitting diodes (LEDs).The average optical power emitted by LEDs is less than a few mW and is distnbuted over a wide solid angle.These LEDs are sinusoidally modulated at a frequency of t20 MHz by driving them with a low-voltage oscillating signal.The eight LEDs are driven by a multiplexing circuit which turned them on one at a time under computer control.The light, collected by a fibre-optic bundle (diameter 0.3 era) positioned 1.7 and 3-5 em from the frst and the last LED, restmctively, is connected to it photomultiplier tubew The LEDs and the bundle were included in a holder positioned on the left of the forehead, 3 em from the middle line, or on the brachioradial musele of the left forearm.
The 120 MHz frequency provides the best comtnomisr among the three requirements of high modulation of the source, high sensitivity of the ckaector, and high sensitivi~ of the quantities (slope., phaseshift, "average light intensity, and amplitude of intensity oscillations).The photomultiplier tube gain is modulated at a froqucncy of 120,0004 Mhz.The photomul~plier tube output is fed to a dam acquisition card** inserted in the personal computer, featuring current/voltage convertors, the signal amplification stage and a 12-bit A/D convertor.
The 400 Hz low-frequency component of the photomultiplier output is isolated using a variable-bandwidth digital filter and is digitised through a fast Fourier transform routine.Each of the eigth light sources is turned on for a time multiple of the 400 I-Iz wave period.All the collected periods at 400 Hz are averaged together, giving an average wave consisting of 16 points.At the end of this acquisition process, the 16-point wave is transformed using a fast Fourier transform algorithm to give the values of the DC, AC and phase of the fundamentals harmonic frequency of 400 Hz for all 8 LEDs.The slopes associated with the DC, AC and phase are computed as described previously (FANq'INI et al., 1994b).Once the slopes are known, the absolute values of the scattering and absorption coefficients at the two source wavelengths can be obtained (FANTINI et aL, 1994b).
The entire process is then repeated to provide continuous monitoring of the scattering and absorption coefficients' values.Light source equilibration was accomplished by driving each LED with different currents.The light intensity and the phase were calibrated by placing the measurement head on a solid block of a substance with absorption and scattering coefficients similar to the tissue values.These values of the optical properties of the calibration block are measured by using a single intensity-modulated light source and the diffusion model for the semi-infinite geometry (FAr~q'INI et aL, 1994b).
The overall accuracy of the measurement in the macroscopically homogeneous semi-infinite medium in the diffusion model was better than 4% for the absorption coefficient (in the range 0.02--0-04 cm -1) and better than 15% for the reduced scattering coefficient (in the range 4--16 cm -I) (FANTINI et al., 1995).Oxy-and deoxyhaemoglobin concentrations were derived from extinction coefficients and absorption coefficients as previously described (FlstIYdN et al., 1995).The ratio of oxyhaemoglobin/total haemoglobin was utilised to calculate St. Muscle St was attributed only to haemoglobin saturation changes.The low partial pressure of myoglobin and the fact that myoglobin concentration in skeletal muscle is about three times lower than haemoglobin suggests only a small contribution from this compound (O'BRmN et al., 1992).
Seven healthy unu'ained male and five female subjects were recruited from the laboratory (age range between 24 and 44 years).No subject was overweight.The study was performed in a warm, quiet room with the subject lying supine in a comfortable bed.The study was carried out according to the principles of the Helsinki Declm~on, and informed consent was obtained.Measurements were obtained on the forehead and on the forearm.Forearm measurements were made during thr~ consecutive protocols.
In the first protocol, an abrupt vascular occlusion was achieved by inflating a pneumatic cuff to a pressure of 240-260 mm Hg.The cuffocclusion was maintained for 8 min with the muscle resting.After cuff release, a 3 rain recovery followed.In the second protocol, two isometric maximal voluntary contractions (MVC) of 30 s length were executed 30 and 90 s, respectively, after the beginning of ischaemia.The euffwas released 175 s after the start of the occlusion.In the third protocol, two isometric MVC of 30 s length were executed without vascular occlusion at 60 aad 120 s, respectively, after the start of protocol.Data are represented as means 4-SE.A paired t-test was used for statistical analysis.

Discussion
Non-invasivr ne.ar-IR s~opy has been found to be a useful means of monitoring intmc, e~bral oxygenation and haemodynamics in different circumstances (B~zYet al., 1985;PEEaLES et al., 1992).The available technology does not permit the direct quantification of tissu~ oxy-and d~oxyhaemoglobin concentrations without combining ~nuation n~a-s~ents with optical path length.
The results of this study indicate that a ftequency-domaln spe~ome~er can monitor brain and muscle oxygen saturation non-invasively.Quantification is obtained on the basis of a multi-distance method that permits the quantification of tissue s~tt~ng and absorption coefficients.This new instrument has been utilised to measure brain St and forearm St during ischaemia and exercise (Table I).Brain St values fit with jugular bulb saturation data, as well as with brain regional oxygen extraction values measured by positron emission tomogmphy (MARCHAL et al., 1992).Similar changes of the relative concentration of oxy-and deoxyhaemoglobin during ischaemia and exercise have been reported and discussed elsewhere (DE BLASl et al., 1993).
Taking into account that St reflects the balance between oxygen supply and energetic requests, the possibility of quantifying and monitoring St by a low-cost frequency-domain near-IR spectrophotometer might be extremely attractive for the management of tissue oxygenation in different critical conditions. Fig.1