Search PubMed⌕ Search

Biomedical subjects

K Niizeki

Publications and source records attributed to K Niizeki.

At least 19 recordsLinked to original sources
PubMed2005

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 person's RR and HR with sleep posture changes and may be helpful for practical use in the home.

Adult↗
PubMed2000

New classes of quasicrystals and marginal critical states

One-dimensional quasilattices, namely, the geometrical objects that represent quasicrystals, are classified into mutual local-derivability (MLD) classes. Besides the familiar class, there exist an infinite number of new MLD classes, and different MLD classes are distinguished by the inflation rules of their representatives. It has been found that electronic properties of a new MLD class are characterized by the presence of marginal critical states, which are considered to be nearly localized states.

Journal Article↗
PubMed1999

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.

Adult↗
PubMed1997

Respiratory responses to passive and active recovery from exercise.

To investigate the effect of the neural components associated with leg movements on the control of ventilation during recovery from exercise, we recorded the minute ventilation (VE), oxygen uptake (VO2), and carbon dioxide output (VCO2) of eight normal volunteers during recovery from moderate, steady-state cycle exercise (170 W). The recovery phases were undergone separately under two different conditions: 5 min of rest (passive recovery) on a bicycle ergometer and 3 min of pedaling at a work rate of 0W (active recovery) followed by 2 min of rest. The phase-1 responses were observed in all the variables studied at the transition of passive recovery but not in the active recovery phase. The kinetics of VCO2, during the off-transition were significantly faster than those of VE in both recoveries, indicating that the decreases in VCO2 could precede the decreases in VE. Although the levels of VE and VCO2 during active recovery were significantly higher than those during passive recovery, the decline in VE was closely proportional to that of VCO2 under both recovery conditions, with resultant indications of similar VE-VCO2 regression lines. These findings suggest that the flux of CO2 to the lungs is an important determinant of ventilatory drive during recovery, and that neither central command nor neural afferents from contracting muscles are requisite for the control of ventilation during recovery from exercise.

Adult↗
PubMed1996

Ventilatory responses to CO2: constant fraction vs. constant inflow administration.

The purpose of this study was to determine whether the transfer function characteristics of the respiratory CO2 control system differs according to whether the CO2 administration method is constant fraction (CF) or constant inflow (CFlow). Ventilatory responses to CO2 changes were measured in seven healthy subjects during random PETCO2 perturbation by the CF and CFlow administration methods in normoxia and hyperoxia. The transfer function from PCO2 to VE was estimated in the frequency domain from 0.002 to 0.02 Hz. The transfer function characteristics showed a low-pass filter character in both of CFlow and CF. The impulse responses to both the methods persisted for > or = 60 sec, while the maximum amplitude (hmax) of the CFlow response was statistically greater than that of the CF response in normoxic condition. The time required until the peak (tmax) of the CFlow impulse response was shorter than that of CF in normoxia. Hyperoxia retarded the tmax and reduced hmax in both CFlow and CF, with the result that significant differences in the normoxic impulse responses were not observed between CFlow and CF in hyperoxia. To characterize the CO2 control system quantitatively, we determined the static transfer gain, oscillatory frequency, damping factor, and pure time delay by applying a second-order delay model to the observed transfer function. The static gain was not significantly different between CFlow and CF responses in both normoxia and hyperoxia. The pure time delay and damping factor were significantly decreased for CFlow only in normoxia. We suggest that inhalation of CO2 by CFlow modifies ventilatory response, probably mediating through the peripheral chemoreceptor activity.

Adult↗
PubMed1996

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.

Adult↗
PubMed1995

A model analysis of asymmetrical response of pulmonary VO2 during incremental and decremental ramp exercise.

The responses of gas exchange and ventilatory variables exhibit obvious asymmetry between the incremental and decremental phases of trapezoidal ramp exercise (29). Computer simulation was performed to explore the possible mechanisms producing this asymmetry of pulmonary O2 uptake (VO2) dynamics. The model consisted of three compartments (the lungs, the exercising muscles, and the other organs and tissues), which were perfused by respective regional circulations. Exercise was simulated by increasing the pulmonary blood flow (Q) and the muscle. VO2 in proportion to work rate. The dynamic responses of Q and the muscle VO2 were assumed to obey first-order linear kinetics. The ratio of blood flow in the muscle compartment to total cardiac output was assumed to increase linearly with work rate. Our simulation demonstrated that the muscle blood flow and mixed venous O2 content exhibited significant asymmetry between the incremental and decremental phases of exercise. The model suggested that the asymmetry of VO2 between incremental and decremental ramp exercise was dependent on the redistribution ratio of blood flow to the working muscles. The validity of the model was then evaluated by comparing the simulated results with experimental data obtained.

Computer Simulation↗
PubMed1995

Simultaneous measurement of pulmonary diffusing capacity for CO and cardiac output by a rebreathing method in patients with pulmonary diseases.

Pulmonary diffusing capacity for CO (DLCO) and cardiac output (Q) were simultaneously measured by a noninvasive rebreathing method (RB) in 15 normal subjects and in 60 patients, including cardiac diseases (CD), bronchial asthma (BA), chronic obstructive pulmonary disease (COPD) and idiopathic pulmonary fibrosis (PF). DLCO and Q were tested by the single breath method (SB) and the dye-dilution method (Dye), respectively. DLCO (RB) correlated well with DLCO (SB) (r = 0.890, p < 0.001). Q(RB) also correlated well with Q(Dye) (r = 0.909, p < 0.001). Factors affecting DLCO (RB) were age, height, body surface area, O2 consumption, Q, hematocrit (Het) and Q.Hct, among which Q.Hct was the prominent influential determinant. DLCO per lung volume was smaller in COPD and PF than in BA or healthy subjects, while, the ratio of DLCO/(Q.Hct) was significantly higher in COPD than in PF. Simultaneous measurements of DLCO and Q offer comprehensive characterization for functional changes in lung parenchyma.

Adult↗
PubMed1995

Ventilatory responses during incremental exercise in men under hyperoxic conditions.

The aim of the present study was to explore the role of the carotid chemoreceptors in the regulation of breathing during incremental ramp exercise. We measured minute ventilation (VE), oxygen uptake (VO2), carbon dioxide output (VCO2), end-tidal PO2 and PCO2 (PETO2 and PETCO2), and heart rate (HR) during incremental exercise in healthy young men breathing air and 50% O2. During incremental exercise (15 W/min, from 0 to 300 W) VCO2 in hyperoxia did not differ from the normoxic response, but VE in hyperoxia increased more linearly with an increasing load in comparison to the curvilinear rise of normoxic VE. The isocapnic buffering of PETCO2 observed in normoxia at the transition from moderate to heavy work did not appear in hyperoxia until a very heavy work load had been attained. This agrees with the observation that normoxic VE/VCO2 that was consistently falling with load and became flat near the isocapnic point and then turned upward with a further increase in work load, while VE/VCO2 in hyperoxia decreased continuously during heavy exercise. These results would suggest the delayed onset of anaerobic metabolism and the depression of VE under hyperoxic conditions. However, we found that VE and HR increased from a specific work rate with a steeper slope during incremental exercise under both normoxic and hyperoxic conditions. There was a significant correlation between the work rates at which the inflection points of the VE and HR slopes were observed. These findings suggest that factors unrelated to peripheral chemoreceptor activity and affecting both the ventilatory and circulatory systems may be responsible for hyperpnea during heavy exercise.

Adult↗
PubMed1994

Interactions between respiratory, cardiac and stepping rhythms in decerebrated cats: functional hierarchical structures of biological oscillators.

Interactions are described of central origin between respiratory, cardiac and stepping rhythms during fictive locomotion in paralyzed, vagotomized, and decerebrated cats. Fictive locomotion was induced by tonic electrical stimulation of the mesencephalic locomotor region (MLR). The coherence between heart beat fluctuation, the efferent discharges of the phrenic, and the lateral gastrocnemius nerves was used to evaluate the strength of the coupling between those three rhythms. The heart beat rhythm was modulated by the centrally generated respiratory and stepping rhythms. The central respiratory rhythm was modulated by the centrally generated stepping rhythm. Based on the present findings, we have proposed a new model concerning the functional hierarchical structures of the three biological oscillators.

Animals↗
PubMed1993

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.

Animals↗
PubMed1993

Interaction among cardiac, respiratory, and locomotor rhythms during cardiolocomotor synchronization.

The nature of entrainment between cardiac and locomotor rhythms was investigated while normal human subjects walked or ran on a treadmill. To detect the incidence of entrainment occurrence, the phase relationships among cardiac, respiratory, and locomotor rhythms were analyzed. The phase relationship between heartbeats and gait signals showed that entrainment of cardiac rhythm to locomotor rhythm occurred in all subjects at one or more treadmill speeds. To elucidate interactions among cardiac, respiratory, and locomotor rhythms during the cardiolocomotor synchronization, spectral and coherence analyses were done for these three rhythms. Spectral and coherence analyses on fluctuations in the heart period and respiratory rhythms revealed that the strength of coupling between cardiac and respiratory rhythms decreased in the presence of cardiolocomotor synchronization. In addition, the coupling of cardiac and locomotor rhythms appeared to induce dissociation of coupling between respiratory and locomotor rhythms. These results were similar to those observed when stepping was voluntarily synchronized with cardiac rhythm. Possible mechanisms to explain coordination and interaction among the neural oscillators innervating these three rhythms are discussed.

Adult↗