Kazuhisa Tanabe

Study of an algorithm based on model experiments and diffusion theory for a portable tissue oximeter.

A portable tissue oximeter that uses light-emitting diodes and two-wavelength near infrared spectroscopy has been developed. The tissue oximeter is compact enough to be portable and it is therefore expected to make better use of the advantages of NIRS-based oximetry and to expand the scope of applications of monitoring tissue oxygen. The algorithm for this instrument was deduced through systematic experiments by varying blood volume and scattering intensity in a tissuelike phantom. The experimental results were compared with theoretical results obtained from diffusion theory. Experimentally determined coefficients of the algorithm were in close agreement with the theoretically derived coefficients. From evaluation tests of the algorithm applied to in vitro and in vivo measurements, it was confirmed that a good linear response to the concentration of oxygenated and deoxygenated blood can be obtained by this algorithm within a range of about a 50% change in concentration from an initial state.

Development of a portable tissue oximeter using near infra-red spectroscopy

NON-INVASIVE AND continuous monitoring of physiological conditions during cxcrcise is very important for clinical diagnosis, sports medicine, healthcare monitoring etc. For the continuous monitoring of cardiovascular conditions, several types of instruments have been developed, such as the Holter ECG (HOLTER, 1961; ISHn3A et al., 1990) and ambulatory blood pressure monitors (YAMAKOSHI, 1991); these instruments are now widely accepted for clinical and research use. On the other hand, for monitoring metabolic conditions which also provide important physiological information, only a few methods have been developed, such as MRS (WANG et al., 1990; ZATINA et al., 1986); they are expensive and complicated systems, and are not appropriate for use during exercise. The optical method is more convenient to use and is capable of detecting tissue metabolism, as has been shown in previous work (CHANCE et al., 1992). Therefore, this method may be useful as a portable monitor of metabolic conditions during exercise. Many attempts have been made to use this method for research. Study by the optical method began with the work of Kramer and Millikan, followed by the work of Chance and Weber on isolated tissue metabolism monitoring (KR~IER, 1935; MmLm~aq, 1937; CHANGE and WEBER, 1963). Jobsis first showed that near infra-red (NIR) region light has the characteristic of excellent penetration through biological tissues (JOBStS; 1977) after this the NIR method then attracted attention. Cope and Delpy and Rolfe et aL applied the multiwavelength NIR method to neonates, and Chance et al. applied simple ~jal wavelength spectroscopy for muscle studies (COPE and DELPY, 1988; ROLFE et aL, 1991; CHANCE et al., 1992). In recent years, Chance et. al. have developed quantitative noninvasive methods by using timeand frequency-resolved spectroscopy to determine the optical path length (CHANCE et aL, 1988; 1990). By using time-resolved spectroscopy, Delpy et al. esftmated the optical path length through tissue, and 9 Ferrari et a/. examined the spectral properties of muscle tissue (DELPY et al., 1988; FEBRARI et aL, 1992). However, many of these instruments, which have complex

Compression of the photoprobe is effective in reducing Hb signals of the frontal skin in near infrared spectrophotometic cerebral tissue oximetry

The cutaneous hemoglobin of the forehead can contribute to near infrared spectrophotometric cerebral tissue oximetry in both light scattering and absorption. Wide distance between the light source and detector was theoretically proved to increase substantial signal from cerebral tissue, however increase in signal to noise ratio was practically undesirable. The tight compression of the probe onto the head expels the subcutaneous blood, which is supposed to improve cerebral signal detection. The spectrophotometric effect of probe compression with a head band was examined by NIRS monitor having temporal resolution of 10 Hz which provided detailed information in tissue hemoglobin signals including pulsatile fluctuations. The measurement for healthy volunteers was done in both supine and sitting positions. The oxygenated Hb and total Hb levels significantly decreased with probe compression in supine position. Compared with pre-compression state, the amplitude of Oxy-Hb pulsation was reduced to 61.0 +/- 10.0 (mean +/- SD)% in sitting position, and 53.3 +/- 68% in supine position (p < 0.01). The postischemic hyperemia was also observed in every measurement. The results indicate that cutaneous blood in the scalp significantly contributes pulsatile Hb signals in cerebral NIRS, and probe compression is a good measure to reduce scalpel effect especially in supine position.

Higher temporal resolution is necessary for continuous-wave near -infrared spectrophotometric monitors in both cerebral and muscular tissue oximetry

Conventional near infrared spectrophotometric monitors have temporal resolution of less than about 1 Hz. However, physiological Hb signals such as pulsation and muscle contraction have higher frequency than 1 Hz. Insufficient sampling rates inevitably lead aliasing of the recorded signals in tissue oximetry for both brain and muscle. Cerebral Hb signals (57 y.o. female artificially ventilated under general anesthesia) and thigh muscle (22 y.o. male with 20 W - 240 W exercise at 1 Hz cycling in semirecumbent ergometer) were measured with NIRS monitor with temporal resolution of 10 Hz (OMRON Co. Ltd., Japan). The detail of physiological fluctuations such as pulsation, ventilation, and muscle pumping was clearly recognized with a 10 Hz sampling. The comparison with recalculated waveforms at slower sampling rate (0.5 Hz, 1 Hz, 2 Hz) revealed that with slower sampling than 1 Hz cerebral respiratory waves were deformed by pulsation, and that magnitudes of muscle pumping could not be properly evaluated in dynamic exercise. In both pulsatile and muscle contractile cycle a phase delay between oxygenated component and deoxygenated one was also detected, which has been overlooked by conventional NIRS monitoring.

Real time monitoring of pulsatile change in hemoglobin concentrations of cerebral tissue by a portable tissue oximeter with a 10-Hz sampling rate

A portable CW tissue oximeter of a 10-Hz sampling rate was developed for examination of pulsatile components of the output signals as a mean of checking the signal reliability during long-term monitoring. Feasible studies were performed on a healthy subject. Changes in Hb and HbO2 signals of cerebral tissue were continuously measured by placing a photoprobe on the forehead during 6-hour sleep. Pulsatile changes in Hb and HbO2 were steadily observed over a whole period of the recording. The phase relation of pulsation in Hb and HbO2 was almost inverse. Not only information for reliable monitoring but also physiological parameters with respect to cerebral circulation and metabolism could be obtained by measuring the pulsatile components.