Kyuichi Niizeki

Unconstrained cardiorespiratory and body movement monitoring system for home care.

An unconstrained respiratory rate (RR) and heart rate (HR) monitoring system to be used during sleeping is proposed. The system consisted of eight polyvinylidene fluoride cable sensors, charge amplifiers and measuring software, together with an analogue-to-digital converter unit. The cable sensors were horizontally embedded into a textile sheet on a bed surface covering the upper half of the body. The digital infinite impulse response filters were constructed to extract cardiorespiratory signals from displacement of the sensors. The system software automatically searched the optimum sensor(s) based on the power of the respective filter outputs. Then, the system obtained the 5 s average HR and 15 s average RR by measuring the intervals between the peaks of the respective autocorrelation functions of the filtered output. If the subject changed his posture, the system captured the image of the body position as a time stamp using a CCD camera. To show the validity of this method, the HR and RR obtained by this monitor were compared with those simultaneously measured using respiratory flow and an electrocardiogram. The results showed that the mean frame-by-frame difference ranged from −1.2 to 0.2 beats min−1 for the HR and, for RR, ranged from −0.8 to 1.4 breath min−1 during the short-term recordings. Similar differences were obtained during the first 2 h of overnight recordings. The proposed system is feasible for the combined long-term monitoring of a persons RR and HR with sleep posture changes and may be helpful for practical use in the home.

Noninvasive imaging of human skin hemodynamics using a digital red-green-blue camera

In order to visualize human skin hemodynamics, we investigated a method that is specifically developed for the visualization of concentrations of oxygenated blood, deoxygenated blood, and melanin in skin tissue from digital RGB color images. Images of total blood concentration and oxygen saturation can also be reconstructed from the results of oxygenated and deoxygenated blood. Experiments using tissue-like agar gel phantoms demonstrated the ability of the developed method to quantitatively visualize the transition from an oxygenated blood to a deoxygenated blood in dermis. In vivo imaging of the chromophore concentrations and tissue oxygen saturation in the skin of the human hand are performed for 14 subjects during upper limb occlusion at 50 and 250 mm Hg. The response of the total blood concentration in the skin acquired by this method and forearm volume changes obtained from the conventional strain-gauge plethysmograph were comparable during the upper arm occlusion at pressures of both 50 and 250 mm Hg. The results presented in the present paper indicate the possibility of visualizing the hemodynamics of subsurface skin tissue.

Visualizing depth and thickness of a local blood region in skin tissue using diffuse reflectance images

A method is proposed for visualizing the depth and thickness distribution of a local blood region in skin tissue using diffuse reflectance images at three isosbestic wavelengths of hemoglobin: 420, 585, and 800 nm. Monte Carlo simulation of light transport specifies a relation among optical densities, depth, and thickness of the region under given concentrations of melanin in epidermis and blood in dermis. Experiments with tissue-like agar gel phantoms indicate that a simple circular blood region embedded in scattering media can be visualized with errors of 6% for the depth and 22% for the thickness to the given values. In-vivo measurements on human veins demonstrate that results from the proposed method agree within errors of 30 and 19% for the depth and thickness, respectively, with values obtained from the same veins by the conventional ultrasound technique. Numerical investigation with the Monte Carlo simulation of light transport in the skin tissue is also performed to discuss effects of deviation in scattering coefficients of skin tissue and absorption coefficients of the local blood region from the typical values of the results. The depth of the local blood region is over- or underestimated as the scattering coefficients of epidermis and dermis decrease or increase, respectively, while the thickness of the region agrees well with the given values below 1.2 mm. Decreases or increases of hematocrit value give over- or underestimation of the thickness, but they have almost no influence on the depth.

Estimation of Melanin and Hemoglobin Using Spectral Reflectance Images Reconstructed from a Digital RGB Image by the Wiener Estimation Method

A multi-spectral diffuse reflectance imaging method based on a single snap shot of Red-Green-Blue images acquired with the exposure time of 65 ms (15 fps) was investigated for estimating melanin concentration, blood concentration, and oxygen saturation in human skin tissue. The technique utilizes the Wiener estimation method to deduce spectral reflectance images instantaneously from an RGB image. Using the resultant absorbance spectrum as a response variable and the extinction coefficients of melanin, oxygenated hemoglobin and deoxygenated hemoglobin as predictor variables, multiple regression analysis provides regression coefficients. Concentrations of melanin and total blood are then determined from the regression coefficients using conversion vectors that are numerically deduced in advance by the Monte Carlo simulations for light transport in skin. Oxygen saturation is obtained directly from the regression coefficients. Experiments with a tissue-like agar gel phantom validated the method. In vivo experiments on fingers during upper limb occlusion demonstrated the ability of the method to evaluate physiological reactions of human skin.

Incoherent oscillations of respiratory sinus arrhythmia during acute mental stress in humans

Respiratory sinus arrhythmia (RSA) has been widely used as a measure of the cardiac vagal control in response to stress. However, RSA seems not to be a generalized indicator because of its dependency on respiratory parameter and individual variations of RSA amplitude (A(RSA)). We hypothesized that phase-lag variations between RSA and respiration may serve as a normalized index of the degree of mental stress. Twenty healthy volunteers performed mental arithmetic task (ART) after 5 min of resting control followed by 5 min of recovery. Breathing pattern, beat-to-beat R-R intervals, and blood pressure (BP) were determined using inductance plethysmography, electrocardiography, and a Finapres device, respectively. The analytic signals of breathing and RSA were obtained by Hilbert transform and the degree of phase synchronization (λ) was quantified. With the use of spectral analysis, heart rate variability (HRV) was estimated for the low-frequency (LF) and high-frequency (HF) bands. A steady-state 3-min resting period (REST), the first 3 min (ART1), and the last 3 min (ART2) of the ART period (ranged from 6- to 19 min) and the last 3 min of the recovery period (RCV) were analyzed separately. Heart rate, systolic BP, and breathing frequency (f(R)) increased and λ, A(RSA), and HF power decreased from REST to ART (P < 0.01). The λ was correlated with normalized A(RSA) and the HF power. The decrease in λ could not be explained solely by the increase in f(R). We conclude that mental stress exerts an influence on RSA oscillations, inducing incoherent phase lag with respect to breathing, in addition to a decrease in RSA.

Cardiac, Respiratory, and Locomotor Coordination during Walking in Humans

Interactions between locomotor, respiratory, and cardiac rhythms were investigated in human subjects (n = 11) walking on a treadmill. Investigation of the phase relationship between heart rate and gait signals revealed that cardiac rhythms were entrained to locomotor rhythms when both frequencies were close to an integer ratio. Coherence spectra were estimated between heartbeat fluctuation, respiratory, and gait signals, and their magnitudes were evaluated. The results suggest that the respiratory-induced fluctuation in heartbeat would vary depending on the strength of the cardiolocomotor coupling. The synchronization tends to occur for one or two specific phases in an individual subject, but there was some variation among subjects. When the subjects voluntarily synchronized their cadence with the cardiac rhythm, the heart rate and blood pressure varied depending on the phase lag within a cardiac cycle. The coordination of locomotor and cardiac rhythms is discussed.

Phase-dependent heartbeat modulation by muscle contractions during dynamic handgrip in humans

The influence of cardiac phase on the response of the cardiac pacemaker to dynamic hand contraction in eight healthy young men was studied to determine whether heart rate response to muscle contraction varied as a function of timing within the cardiac cycle. Changes in R-R interval (RRI) in response to muscle contraction were measured at various cardiac phases during heartbeat-synchronized handgrip at a rate of one contraction per two heartbeats. To extract the direct effect of the muscle contraction on the RRI, spontaneous slow variations and respiratory sinus arrhythmia were removed from the total RRI fluctuations in the frequency domain. Cross-correlograms between the extracted RRI fluctuations and muscle contraction showed that the coupling was strong when the muscle contraction occurred at the middle phase of the cardiac cycle. Muscle contraction at the systolic phase of the cardiac cycle had a tendency to produce a phase advance (shortening of RRI), whereas muscle contraction at the middle phase or later had a tendency to produce a phase delay (prolongation of RRI). The results showed the presence of a neuronal circuit that modulates the cardiac pacemaker activity depending on the timing of muscle contraction in the cardiac cycle.The influence of cardiac phase on the response of the cardiac pacemaker to dynamic hand contraction in eight healthy young men was studied to determine whether heart rate response to muscle contraction varied as a function of timing within the cardiac cycle. Changes in R-R interval (RRI) in response to muscle contraction were measured at various cardiac phases during heartbeat-synchronized handgrip at a rate of one contraction per two heartbeats. To extract the direct effect of the muscle contraction on the RRI, spontaneous slow variations and respiratory sinus arrhythmia were removed from the total RRI fluctuations in the frequency domain. Cross-correlograms between the extracted RRI fluctuations and muscle contraction showed that the coupling was strong when the muscle contraction occurred at the middle phase of the cardiac cycle. Muscle contraction at the systolic phase of the cardiac cycle had a tendency to produce a phase advance (shortening of RRI), whereas muscle contraction at the middle phase or later had a tendency to produce a phase delay (prolongation of RRI). The results showed the presence of a neuronal circuit that modulates the cardiac pacemaker activity depending on the timing of muscle contraction in the cardiac cycle.

Noninvasive spectral imaging of skin chromophores based on multiple regression analysis aided by Monte Carlo simulation.

In order to visualize melanin and blood concentrations and oxygen saturation in human skin tissue, a simple imaging technique based on multispectral diffuse reflectance images acquired at six wavelengths (500, 520, 540, 560, 580 and 600 nm) was developed. The technique utilizes multiple regression analysis aided by Monte Carlo simulation for diffuse reflectance spectra. Using the absorbance spectrum as a response variable and the extinction coefficients of melanin, oxygenated hemoglobin, and deoxygenated hemoglobin as predictor variables, multiple regression analysis provides regression coefficients. Concentrations of melanin and total blood are then determined from the regression coefficients using conversion vectors that are deduced numerically in advance, while oxygen saturation is obtained directly from the regression coefficients. Experiments with a tissue-like agar gel phantom validated the method. In vivo experiments with human skin of the human hand during upper limb occlusion and of the inner forearm exposed to UV irradiation demonstrated the ability of the method to evaluate physiological reactions of human skin tissue.

Active muscle oxygenation dynamics measured during high-intensity exercise by using two near-infrared spectroscopy methods.

Near-infrared spectroscopy is a noninvasive optical technique used to monitor tissue oxygenation. Generally, the modified Beer-Lamberts law (MBL) using continuous-wave light has been used to measure active muscle oxygenation during exercise; however, it cannot measure absolute changes in the oxy- (oxy-[Hb + Mb]), deoxy- (deoxy-[Hb + Mb]), and total hemoglobin/myoglobin concentrations (total-[Hb + Mb]) because the pathlength and scattering coefficient are not measured. In contrast, the time-resolved spectroscopy (TRS) using a ultra short pulsed laser can be used to determine absolute changes in the concentration, although the temporal resolution is inferior to that in MBL. This study evaluated the absolute changes in active muscle oxygenation and the optical mean pathlength and scattering and absorption coefficient during high-intensity exercise by using the TRS system. In addition, the difference between the changes determined using TRS and MBL measurements was assessed. When the TRS and MBL measurements obtained during high-intensity exercise were compared, the total-[Hb + Mb] and oxy-[Hb + Mb] dynamics differed markedly during high-intensity exercise, while the deoxy-[Hb + Mb] dynamics and kinetics did not differ.

Heart beat fluctuation during fictive locomotion in decerebrate cats : locomotor-cardiac coupling of central origin

Fictive locomotion was evoked by stimulation of the mesencephalic locomotor region (MLR) in immobilized, vagotomized and decerebrate cats. The coherence between heart beat fluctuation and efferent discharges of the hindlimb nerve was used to evaluate the strength of the coupling between the cardiac and locomotor rhythms during MLR-elicited fictive locomotion. This study demonstrated that there was a locomotor-cardiac coupling of central origin.