Evaluation of the dispersional analysis method for fractal time series

Fractal signals can be characterized by their fractal dimension plus some measure of their variance at a given level of resolution. The Hurst exponent,H, is <0.5 for rough anticorrelated series, >0.5 for positively correlated series, and =0.5 for random, white noise series. Several methods are available: dispersional analysis, Hurst rescaled range analysis, autocorrelation measures, and power special analysis. Short data sets are notoriously difficult to characterize; research to define the limitations of the various methods is incomplete. This numerical study of fractional Brownian noise focuses on determining the limitations of the dispersional analysis method, in particular, assessing the effects of signal length and of added noise on the estimate of the Hurst coefficient,H, (which ranges from 0 to 1 and is 2-D, whereD is the fractal dimension). There are three general conclusions: (i) pure fractal signals of length greater than 256 points give estimates ofH that are biased but have standard deviations less than 0.1; (ii) the estimates ofH tend to be biased towardH=0.5 at both highH (>0.8) and lowH (<0.5), and biases are greater for short time series than for long; and (iii) the addition of Gaussian noise (H=0.5) degrades the signals: for those with negative correlation (H<0.5) the degradation is great, the noise has only mild degrading effects on signals withH>0.6, and the method is particularly robust for signals with highH and long series, where even 100% noise added has only a few percent effect on the estimate ofH. Dispersional analysis can be regarded as a strong method for characterizing biological or natural time series, which generally show long-range positive correlation.     

A comparison of analytical methods for the study of fractional brownian motion

Fractional Brownian motion (FBM) provides a useful model for many physical phenomena demonstrating long-term dependencies and 1/f-type spectral behavior. In this model, only one parameter is neeessary to describe the complexity of the data,H, the Hurst exponent. FBM is a nonstationary random function not well suited to traditional power spectral analysis however. In this paper we discuss alternative methods for the analysis of FBM, in the context of real-time biomedical signal processing. Regression-based methods utilizing the power spectral density (PSD), the discrete wavelet transform (DWT), and dispersive analysis (DA) are compared for estimation accuracy and precision on synthesized FBM datasets. The performance of a maximum likelihood estimator forH, theoretically the best possible estimator, are presented for reference. Of the regression-based methods, it is found that the estimates provided by the DWT method have better accuracy and precision forH>0.5, but become biased for low values ofH. The DA method is most accurate forH<0.5 for a 256-point data window size. The PSD method was biased for bothH<0.5 andH>0.5.     

Fractal Characteristics of End-Expiratory Lung Volume in Anesthetized Rats

To test the possibility that the variations in end-expiratory lung volume (EEV) might exhibit long-term correlation, we calculated the Hurst exponent H for EEVin anesthetized rats during sustained positive and negative transrespiratory pressure (i.e., (PTRP) and (NTRP), correspondingly) using the dispersional analysis method. In nine vagi-intact rats,H for EEV was 0.551±0.051 (mean±std) and 0.641±0.076 (mean±std) with PTRP and NTRP, respectively. In both cases Hwas significantly different from those obtained after random shuffling of the original time series, indicating a positive long-term correlation in EEV. Also H with NTRP wassignificantly greater than that with PTRP (P=0.015). In six vagotomized rats, there was no significant difference in H for EEV between NTRP andPTRP. We demonstrated that long-term correlation is present in EEV. We inferred that the stimulation of vagal afferents could change the fractal property in EEV since different vagal pulmonary receptors are activatedby PTRP and NTRP. Although the exact mechanisms for the fractal behavior are unknown, the results suggest that interaction of the pulmonary mechanoreceptor activity with the central respiratory pattern generator mightcontribute to the observed long-term correlation. (Supported by HL 40369) © 2000 Biomedical Engineering Society. PAC00: 8719Uv, 0545Df, 4753+n     

Neonatal control of nutritive sucking pressure: evidence for an intrinsic τ-guide

 Human newborns appear to regulate sucking pressure when bottle feeding by employing, with similar precision, the same principle of control evidenced by adults in skilled behavior, such as reaching (Lee et al., 1998a). In particular, the present study of 12 full-term newborn infants indicated that the intraoral sucking pressures followed an internal dynamic prototype – an intrinsic τ-guide. The intrinsic τ-guide, a recent hypothesis of general tau theory is a time-varying quantity, τ_g, assumed to be generated within the nervous system. It corresponds to some quantity (e.g., electrical charge), changing with a constant second-order temporal derivative from a rest level to a goal level, in the sense that τ_g equals τ of the gap between the quantity and its goal level at each time t. (τ of a gap is the time-to-closure of the gap at the current closure-rate.) According to the hypothesis, the infant senses τ_p, the τ of the gap between the current intraoral pressure and its goal level, and regulates intraoral pressure so that τ_p and τ_g remain coupled in a constant ratio, k; i.e., τ_p=kτ_g. With k in the range 0–1, the τ-coupling would result in a bell-shaped rate of change pressure profile, as was, in fact, found. More specifically, the high mean r ² values obtained when regressing τ_p on τ_g, for both the increasing and decreasing suction periods of the infants’ suck, supported a strong τ-coupling between τ_p and τ_g. The mean k values were significantly higher in the increasing suction period, indicating that the ending of the movement was more forceful, a finding which makes sense given the different functions of the two periods of the suck. Received: 2 October 1997 / Accepted: 26 August 1998     

Modelling of aerobic and anaerobic energy production during exhaustive exercise on a cycle ergometer

An extension of the original hyperbolic model (Model-2) was proposed by using power output required to elicit maximal oxygen uptake (P _t). This study aimed to test this new model (Model-α) using mechanical work produced during cycle ergometry. Model α assumed that power exceeding a critical power (P _c) was met partly by the anaerobic metabolism. The parameter α was the proportion of the power exceeding P _c provided by anaerobic metabolism, while power exceeding P _t was exclusively met by anaerobic metabolism. Aerobic power was assumed to rise monoexponentially with a time constant τ. The exhaustion was assumed to be reached when the anaerobic work capacity W′ was entirely utilised. Twelve subjects performed one progressive ramp test to assess the power at ventilatory threshold (P _VT) and P _t and five constant-load exercise to exhaustion within 2–30 min, with one to estimate the maximal accumulated oxygen deficit (MAOD). Parameters from Model α were fitted with τ equal to 0, 10, 20 and 30 s. Results in goodness-of-fit was better than Model-2 whatever the value assumed for τ (P < 0.05). The value of τ did not affect much the estimates for P _c and α. P _c estimates were significantly correlated with P _c from Model-2 and with P _VT. W′ estimates, which were dependent on the value ascribed to τ, were not statistically different than MAOD. These two variables were, however, not significantly correlated. In conclusion, Model α could provide useful information on the critical power and the anaerobic contribution according to exercise intensity, whereas W′ estimates should be used with care because of the sensitivity to the assumption on aerobic power kinetics τ.     

Membrane potential fluctuations of human T-lymphocytes have fractal characteristics of fractional brownian motion

The voltage across the cell membrane of human T-lymphocyte cell lines was recorded by the whole cell patch clamp technique. We studied how this voltage fluctuated in time and found that these fluctuations have fractal characteristics. We used the Hurst rescaled range analysis and the power spectrum of the increments of the voltage (sampled at 0.01-sec intervals) to characterize the time correlations in these voltage fluctuations. Although there was great variability in the shape of these fluctuations from different cells, they all could be represented by the same fractal form. This form displayed two different regimes. At short lags, the Hurst exponentH=0.76±0.05 (SD) and, at long lags,H=0.26±0.04 (SD). This finding indicated that, over short time intervals, the correlations were persistent (H>0.5), that is, increases in the membrane voltage were more likely to be followed by additional increases. However, over long time intervals, the correlations were antipersistent (H<0.5), that is, increases in the membrane voltage were more likely to be followed by voltage decreases. Within each time regime, the increments in the fluctuations had characteristics that were consistent with those of fractional Gaussian noise (fGn), and the membrane voltage as a function of time had characteristics that were consistent with those of fractional Brownian motion (fBm).     

Effect of the underwater torque on the energy cost,drag and efficiency of front crawl swimming

Underwater torque (T′) is defined as the product of the force with which the swimmer's feet tend to sink times the distance between the feet and the centre of volume of the lungs. It has previously been shown that experimental changes ofT′, obtained by securing around the swimmer's waist a plastic tube filled, on different occasions, with air, water or 2-kg lead, were accompanied by changes in the energy cost of swimming per unit of distance (C_S) at any given speed. The aim of this study was to investigate whether the observed increases of C_S withT′ during front crawl swimming were due to an increase of active body drag (D_b), a decrease of drag efficiency (η_d) or both. The effect of experimental changes ofT′ on C_S, D_b and η_d were therefore studied on a group of eight male elite swimmers at two submaximal speeds (1.00 and 1.23 m · s⁻¹). To compare different subjects and different speeds, the individual data for C_S, D_b,η_d andT′ were normalized dividing them by the corresponding individual averages. These were calculated from all individual data (of C_S, D_b, η_d andT′) obtained from that subject at that speed. It was found that, between the two extremes of this study (tube filled with air and with 2-kg lead),T′ increased by 73% and that C_S, D_b and η_d increased linearly withT′. The increase of C_S between the two extremes was intermediate ( ≈ 20%) between that of D_b (≈ 35%) and of η_d ( ≈ 16%). Thus, the actual strategy implemented by the swimmers to counteractT′, was to tolerate a large increase of D_b. This led also to a substantial (albeit smaller) increase of did, the effect of which was to reduce the increase of C_S that would otherwise have occurred.     

Fasciola hepatica and Fasciola gigantica: comparative morphometric studies on the redial stage of both species

Experimental infections of Galba truncatula with Fasciola gigantica or F. hepatica were carried out under laboratory conditions (20°C) to determine the characteristics of rediae of both species via their morphometry and to find reliable measurements that might be efficiently used to discriminate between the rediae of both species of Fasciola. These results were compared to those of another snail: Radix natalensis, infected with either F. gigantica or F. hepatica under the same protocol. At day 28 post-exposure, abortive infections with F. hepatica were found in a group of R. natalensis. By contrast, live rediae were observed in the other three groups. The group of infected snails and the redial category significantly influenced the mean values of the seven measurements studied and those of three indices. Using the PSLD Fisher test, it was found that the index, distance from the anterior end of the body to the collar/length of the body, was an efficient means of distinguishing the rediae of F. hepatica from those of F. gigantica [second-appearing mother rediae (R1b) of the first generation, 0.14 instead of 0.22; daughter rediae (R2a) produced by the first mother rediae, 0.19 instead of 0.24]. Another index, distance from the anterior end of the body to the collar/diameter of the collar, could also be used to discriminate between rediae (R1b, 0.80 for F. hepatica instead of 1.09 for F. gigantica; R2a, 0.90 instead of 1.26, respectively). Compared to measurements recorded for the rediae of F. hepatica, rediae of F. gigantica can be characterized by the following measurements: the diameter of the pharyngeal lumen and the distance from the anterior end of the body to the collar for larvae developed in R. natalensis, and the length of the body and the distance from the posterior end of the body to lateral projections for those found in G. truncatula. The species of snail host and, consequently, its growth, as well as the species of Fasciola, had a significant influence on the morphometric characters of the redial stage.     

Physiological time series: distinguishing fractal noises from motions

Many physiological signals appear fractal, in having self-similarity over a large range of their power spectral densities. They are analogous to one of two classes of discretely sampled pure fractal time signals, fractional Gaussian noise (fGn) or fractional Brownian motion (fBm). The fGn series are the successive differences between elements of a fBm series; they are stationary and are completely characterized by two parameters, sigma2, the variance, and H, the Hurst coefficient. Such efficient characterization of physiological signals is valuable since H defines the autocorrelation and the fractal dimension of the time series. Estimation of H from Fourier analysis is inaccurate, so more robust methods are needed. Dispersional analysis (Disp) is good for noise signals while bridge detrended scaled windowed variance analysis (bdSWV) is good for motion signals. Signals whose slopes of their power spectral densities lie near the border between fGn and fBm are difficult to classify. A new method using signal summation conversion (SSC), wherein an fGn is converted to an fBm or an fBm to a summed fBm and bdSWV then applied, greatly improves the classification and the reliability of H, the estimates of H, for the times series. Applying these methods to laser-Doppler blood cell perfusion signals obtained from the brain cortex of anesthetized rats gave H of 0.24+/-0.02 (SD, n=8) and defined the signal as a fractional Brownian motion. The implication is that the flow signal is the summation (motion) of a set of local velocities from neighboring vessels that are negatively correlated, as if induced by local resistance fluctuations.     

Detection ofChlamydia trachomatis in clinical specimens by the polymerase chain reaction

Sequences derived from the endogenous plasmid ofChlamydia trachomatis and from the genes coding for ribosomal 16S RNA ofChlamydia psittaci were used as primers and oligonucleotide probes for detection of chlamydiae by the polymerase chain reaction. The endogenous plasmid primers generated specific amplified products of 517 bp with all knownChlamydia trachomatis serovars. No specific products ofChlamydia psittaci andChlamydia pneumoniae could be detected using these primers. With the rRNA primers specific amplified products of 208 bp were generated withChlamydia psittaci, Chlamydia trachomatis andChlamydia pneumoniae. No specific amplified products were detected with DNA isolated from a variety of microorganisms from the urogenital and the respiratory tract. Of 156 clinical specimens used for evaluation of the polymerase chain reaction, 26 were found to be positive forChlamydia trachomatis on culture. All 26 culture positive samples were also found to be positive forChalmydia trachomatis DNA by the polymerase chain reaction with both primer sets. Two culture negative samples were also found to be positive by this technique. The polymerase chain reaction thus seems to be a sensitive and reliable method for detection ofChlamydia trachomatis.     

Graphic analysis of contraction-distension isotherms for pulmonary surfactant monolayers

A new method for analyzing the surface-active properties of pulmonary surfactants is proposed, based on calculating a “local stability index” and then plotting this index as a function ofS/S _o, whereS _o andS are, respectively, the initial and contracted areas of the surfactant monolayer formed at the interface. The course of the curves thus obtained allows for reasonably precise and easy comparisons of alterations in the surface-active properties of pulmonary surfactants in response to different factors acting on the body. Translated fromByulleten' Eksperimental'noi Biologii i Meditsiny, Vol. 120, N ^o 10, pp 370–372, October, 1995 Presented by B. I. Tkachenko, Member of the Russian Academy of Medical Sciences     

Quantitative trait loci (QTL) for circadian rhythms of locomotor activity in mice

The loomotor activity of male mice (Mus musculus) was monitored by infrared photoelectric beams under three lighting regimens: LD (12 h of light and 12 h of dark), DD (constant dark), and LL (constant broad-spectrum light, 10 lux). Circadian period of locomotor activioty (τ) was compared among 3 inbred strains of mice, C57BL/6J (B6), BALB/c (C), and DBA/2J (D2), and 26 recombinant inbred strains B×D (B6×D2). the τ under both continuous low-intensity light and continuous darkenss varied significantly among strains. Under DD the mean τ was 23.8 h for B6, 23.7 h for D2, and 23.6 h for C. Under LL the mean τ was 25.1 for B6, 23.9 h for D2, and 25.5 h for C. Frequency histograms of the mean τ of 26B×D RI mouse strains (three to seven animals per strain) in either DD or LL and the difference between them, Δτ, had distributions which appeared unimodal, suggesting polygenic inheritances. The narrow-sense heritability determined using 26 strains of B×D RI mice was about 55% for τ and about 38% for both τ in LL and Δτ. An estimated four loci contribute to the variance of τ in constant darkness and five to the variance of τ in constant low-intensity light among the strains studied. Quantitative trait locus (QTL) analysis identified several potential genetic loci associated with τ in constant darkness, τ in constant low-intensity light, and Δτ. The associations of highest probability for each of these traits were theD1Nds4 locus (p<0.001) on mouse chromosome 1, theD5Ncvs52 locus (p<.05) on mouse chromosome 5, and thePmv12 locus (p<.01) at 70 cM on mouse chromosome 5, respectively. A QTL identified for τ was associated (p<.05) with theD2NDS1 marker at 45 cM on chromsome 2 near the Ea 6 marker at 46 cM associated (p<.05) with that reported for the period of wheel running activity in seven C×B RI strains (Schwartz, W. J., and Zimmerman, P.,J. Neurosci. 10:3685 1990).     

Prospective dynamic balance control in healthy children and adults

Balance control during gait initiation was studied using center of pressure (CoP) data from force plate measurements. Twenty-four participants were divided into four age groups: (1) 2–3 years, (2) 4–5 years, (3) 7–8 years, and (4) adults. Movement in the antero-posterior (CoPy) direction during the initial step was tau-G analyzed, investigating the hypothesis that tau of the CoPy motion-gap (τ_CoPy), i.e., the time it will take to close the gap at its current closure rate, is tau-coupled onto an intrinsic tau-G guide (τ_G), by maintaining the relation τ_CoPy = Kτ_G, for a constant K. Mean percentage of tau-guidance for all groups was ≥99%, resulting in all r2 exceeding 0.95, justifying an investigation of the regression slope as an estimate of the coupling constant K in the tau-coupling equation. Mean K values decreased significantly with age and were for 2- to 3-year-olds 0.56, for 4- to 5-year-olds 0.50, for 7- to 8-year-olds 0.47, and for adults 0.41. Therefore, the control of dynamic balance develops from the youngest children colliding with the boundaries of the base of support (K > 0.5) to the older children and adults making touch contact (K ≤ 0.5). The findings may provide us with a measure for testing prospective balance control, a helpful tool in assessing whether a child is following a normal developmental pattern.     

Effect of cyclothiazide on spontaneous miniature excitatory postsynaptic currents in rat hippocampal pyramidal cells

 Spontaneous miniature excitatory postsynaptic currents (mEPSCs) were recorded from the CA1 region of slices using the whole-cell patch-clamp technique. Cyclothiazide (0.1 mM), a complete blocker of desensitization of (S)-α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) channels, was applied to determine the changes in amplitude and kinetics of mEPSCs occurring with complete suppression of desensitization. The amplitude of mEPSC (A) was not affected significantly by cyclothiazide, but both the rise (τ_r) and the decay time (τ_d) were consistently increased (from 2.3 to 6.5 ms and from 9.9 to 22.2 ms respectively). The amplitude dependence of both τ_d and τ_r became much greater, but there was no upward shift of the best-fitting lines. The slopes of the control best-fitting lines were (±SD; ms/pA; n=5) 0.39±0.05 for τ_d:A and 0.12±0.07 for τ_r:A, but, in the presence of cyclothiazide, the corresponding slopes were much steeper (2.1±0.60 and 0.68±0.21; holding potential was –50 mV and temperature 32°C). These changes, which were slow to develop, suggest that cyclothiazide blocks AMPA receptor channel desensitization, whilst having no effect on the closing rate of AMPA channels. Judging by the extent of change, the speed of diffusion of glutamate in the synaptic cleft is probably similar to that in water. In conclusion, this study provides evidence that: (1) under control conditions, desensitization of AMPA channels plays a major role in shaping the time course of synaptic currents in CA1; and (2) cyclothiazide prolongs their time course solely by abolishing desensitization. Received: 28 May 1998 / Received after revision: 16 July 1998 / Accepted: 3 September 1998     

Passive properties of neostriatal neurons during potassium conductance blockade

Voltage recordings from neostriatal projection neurons were obtained using in vitro intracellular techniques before and during K⁺-conductance blockade. Neurons were stained with the biocytin technique. Somatic surface area (A _S) was determined by both whole-cell recordings in isolated somata and by measuring stained somata recorded in slices. Dendritic measurements were done in reconstructed neurons. Average determinations of dendritic (A _D) and neuronal (A _N) surface areas coincided with previously reported anatomical data. Thus: A _S≈ 6.5 × 10^–6 cm²; A _D≈ 1.9 × 10^–4 cm²; A _N≈A _D + A _S≈ 2 × 10^–4 cm²; A _D/A _S≈ 30. Measurements were done before and after superfusion with K⁺-conductance blockers (K⁺-blockers). Cells whose neuronal morphology was not obviously distorted by K⁺-blockade were chosen for the present study. Electrotonic transients were matched to a somatic shunt equivalent cylinder model adjusted with the generalized correction factor (F _dga) that constrains the parameters for neuronal anatomy. Neuronal input resistance (R _N; mean ± SEM) increased when it was corrected for somatic shunt, from 49 ± 2 MΩ (n = 80) to 179 ± 7 MΩ (n = 32). A difference was also obtained between the slowest time constant, τ₀ = 16 ± 0.9 ms (n = 49), and the dendritic membrane time constant, τ_mD = 33 ± 1.6 ms (n = 36). When these electrophysiological measurements were used to calculate A _N, the value obtained was similar to the anatomical measurements. Combining anatomical and electrophysiological data, somatic and dendritic input resistances were determined: R _D = 182 ± 7 MΩ; R _S (with shunt) = 74 ± 4 MΩ (n = 32). The generalized correction factor, F _dga = 0.91 ± 0.007 (n = 10), implied a short effective electrotonic length for dendrites: L _D = 0.46 ± 0.014 (n = 32). Saturating concentrations of the K⁺-blockers tetraethylammonium, Cs⁺, and Ba²⁺ increased R _N and induced charging curves well fitted by single exponential functions in 56% of neostriatal neurons. Ba²⁺ greatly decreased the somatic shunt (n = 5): (R _N = 216 ± 21 MΩ, τ₀ = 46 ± 2 ms, R _D = 239 ± 25 MΩ, and R _S = 3.2 ± 0.5 GΩ), rendering values similar to those obtained with whole-cell recordings (e.g., R _N≈ 198 MΩ, R_S≈ 2.62 GΩ) (n = 52). Cs⁺ (n = 5) had less effect on the somatic shunt (R _N = 115 ± 19 MΩ, τ₀ = 49 ± 13 ms, R _S = 161 ± 8 MΩ), although dendritic conductance was equally blocked (R _D = 261 ± 16 MΩ). The Cs⁺-sensitive conductance exhibited inward rectifying properties not displayed by the Ba²⁺-sensitive conductance, suggesting that Cs⁺ preferentially acted upon inward rectifier conductances. In contrast, Ba²⁺ significantly acted upon linear conductances making up the somatic shunt. This suggests a differential action of different K⁺-blockers on the somato-dendritic membrane, implying a differential distribution of membrane conductances. Another action of K⁺-blockers, in about 40% of the cells, was to induce dye and probably electrical coupling between neighboring neurons. Received: 30 April 1997 / Accepted: 14 October 1997     

Non-occurrence ofMycoplasma genitalium in clinical specimens

Five hundred and thirteen clinical specimens, mainly from patients with urogenital inflammations, were examined forUreaplasma urealyticum and mycoplasmas, including cultures forMycoplasma genitalium. The study yielded 95 isolates ofUreaplasma urealyticum, 37 isolates ofMycoplasma hominis and two isolates ofMycoplasma fermentans, but no growth ofMycoplasma genitalium was obtained. It was concluded thatMycoplasma genitalium is a relatively rare inhabitant of the human urogenital tract in Israel.     

Characterization of abdominally acquired uterine electrical signals in humans,using a non-linear analytic method

The present work seeks to determine if a particular non-linear analytic method is effective at quantifying uterine electromyography (EMG) data for estimating the onset of labor. Twenty-seven patients were included, and their uterine EMG was recorded non-invasively for 30 min. The patients were grouped into two sets: G1: labor, N=14; G2: antepartum, N=13. G1 patients all delivered spontaneously within 24 h of recording while G2 patients did not. The uterine electrical signals were analyzed offline by first isolating the uterine-specific frequency range and then randomly selecting “bursts” of uterine electrical activity (each associated with a uterine contraction) from every recording. Wavelet transform was subsequently applied to each of the bursts’ traces, and then the fractal dimension (FD) of the resulting transformed EMG burst-trace was calculated (Benoit 1.3, Trusoft). Average burst FD was found for each patient. FD means for G1 and G2 were calculated and compared using t test. FD was significantly higher (P<0.05) for G1: 1.27±0.03 versus G2: 1.25±0.02. The wavelet-decomposition-generated fractal dimension can be used to successfully discern between patients who will deliver spontaneously within 24 h and those who will not, and can be useful for the objective classification of antepartum versus labor patients.     

Discordant Prevalence of Chlamydia trachomatis in Asymptomatic Couples Screened Using Urine Ligase Chain Reaction

 The following study was conducted to determine whether there would be an effect on the prevalence of Chlamydia trachomatis if both partners in a sexual relationship, rather than only one, underwent screening. First-void urine samples were collected from 1,690 asymptomatic women (mean age, 30 years; range, 15–70 years) and their male sex partners (mean age, 33 years; range, 16–71 years). The duration of sexual partnership for these subjects ranged from 2 months to more than 10 years.. At the time of testing, 687 of the women were pregnant. Ligase chain reaction testing revealed that 42 (2.5%) female and 63 (3.7%) male urine samples were positive. Detection rates for Chlamydia trachomatis differed for males and females, a difference that was found to be significant (P<0.0046, McNemar chi-square). Both partners tested positive in 27 (1.6%) couples, whereas at least one partner tested positive in 78 (4.6%) couples. Thus, screening males for Chlamydia trachomatis would have identified 63 (81%) of these 78 couples compared with only 42 (54%) couples had females been screened exclusively. In standard clinical practice, women most often undergo screening. The results of this study underscore the need to screen both males and females for Chlamydia trachomatis.     

Signal source impedance of implanted pacemaker electrodes estimated from the spectral ratio between loaded and unloaded electrograms in man

The signal source impedance Z of implanted pacemaker electrodes plays an important role for adequate sensing of electrogram signals and has so far been little explored. A frequency-domain method for thein vivo calculation of Z is described. Electrogram signals picked up between two electrodes were recorded unloaded, and loaded with resistors and capacitors, then amplified, and stored on magnetic tape. After time sampling and a.d. conversion, the signals were read into a digital computer as discrete time vectors with 2^P samples. Each vector was subjected to a fast Fourier transform (f.f.t.), and the ratio H(jω_i) between the transformed loaded and unloaded vectors was calculated. For the electrode impedance Z a linear model was chosen that consisted of the tissue and electrode resistance R_T, in series with a parallel coupling between Faraday resistance R_F, and Helmholtz capacitance C_H. By nonlinear regression analysis the parameters (∧) in this model were estimated from the ratios H(jjω_i) obtained in patients with permanent pacemaker electrodes, and in patients with temporary multicore wire electrodes after open heart surgery. As an estimate for R_T was also used the voltage/current ratio R′_T, 90 μs into the stimulation pulse from an external variable parameter pulse generator. The 3-component impedance model described the essential features of the electrode impedance in the frequency range up to 100 Hz. It was found that Ĉ_H, decreased slightly with increasing frequency, and that Ř_T in general was larger than R′_T. A modified model with frequency dependent parameters was found to obtain a better approach to the experimental data.     

Activatable T₁ and T₂ magnetic resonance imaging contrast agents

Magnetic resonance imaging (MRI) has become one of the most important diagnosis tools available in medicine. Typically MRI is not capable of sensing biochemical activities. However, recently emerged activatable MRI contrast agents (CAs), whose relaxivity is variable in response to a specific parameter change in the surrounding physiological microenvironment, potentially allow for MRI to indicate biological processes. Among the various factors influencing the relaxivity of a CA, the number of inner-sphere water molecules (q) directly coordinated to the metal center, the residence time of the coordinated water molecule (τ _m), and the rotational correlation time representing the molecular tumbling time of a complex (τ _R) contribute strongly to the relaxivity of an activatable CA. Tuning the ligand structure and properties has been the subject of intensive research for activatable MR CA designs. This review summarizes a variety of activatable MRI CAs sensitive to common variables in microenvironment in vivo, i.e., pH, luminescence, metal ions, redox, and enzymes, etc., with emphasis on the influence of ligand design on parameters q, τ _m, and τ _R.