Forced oscillatory impedance of the respiratory system at low frequencies

Respiratory mechanical impedances were determined during voluntary apnea in five healthy subjects, by means of 0.25- to 5-Hz pseudo/random oscillations applied at the mouth. The total respiratory impedance was partitioned into pulmonary (ZL) and chest wall components with the esophageal balloon technique; corrections were made for the upper airway shunt impedance and the compressibility of alveolar gas. Neglect of these shunt effects did not qualitatively alter the frequency dependence of impedances but led to underestimations in impedance, especially in the chest wall resistance (Rw), which decreased by 20-30% at higher frequencies. The total resistance (Rrs) was markedly frequency dependent, falling from 0.47 +/- 0.06 (SD) at 0.25 Hz to 0.17 +/- 0.01 at 1 Hz and 0.15 +/- 0.01 kPa X l-1 X s at 5 Hz. The changes in Rrs were caused by the frequency dependence of Rw almost exclusively between 0.25 and 2 Hz and in most part between 2 and 5 Hz. The effective total respiratory (Crs,e) and pulmonary compliance were computed with corrections for pulmonary inertance derived from three- and five-parameter model fittings of ZL. Crs,e decreased from the static value (1.03 +/- 0.18 l X kPa-1) to a level of approximately 0.35 l X kPa-1 at 2-3 Hz; this change was primarily caused by the frequency-dependent behavior of chest wall compliance.     

Characteristics of transverse electric and magnetic field transmission cells at extremely low frequencies

Transverse electric and magnetic field (TEM) cells are often designed to subject samples to electromagnetic radiation of intrinsic impedance (E/H) that is the same as in free space, 377 omega. Earlier work has shown this value to be correct for the RF region above about 2 kHz. In this study, measurements of magnetic fields in the extremely low frequency regions and at DC indicate the E/H ratio to be around 300 omega for frequencies less than 2 kHz in cells of a particular design. This lower value indicates that care should be taken in estimating AC magnetic field intensities from electric field measurements in TEM cells at frequencies below 2 kHz.     

Physiological basis for resonant frequencies in respiratory system impedances in dogs

The lumped six-element model of the respiratory system proposed by DuBois et al. (J. Appl. Physiol. 8: 587-594, 1956) has often been used to analyze respiratory system impedance (Zrs) data. This model predicts a resonance (relative minimum in Zrs) at fr between 6 and 10 Hz and an antiresonance (relative maximum in Zrs) at far at higher frequencies (greater than 64 Hz). The far is due to the lumped tissue inertance (Iti) and the alveolar gas compression compliance (Cg). An fr and far have been recently reported in humans, but the far was shown to be not related to Iti and Cg, but instead it is the first acoustic antiresonance of the airways due to their axial dimensions). Zrs data to frequencies high enough to include the far have not been reported in dogs. In this study, we measured Zrs in dogs for frequencies between 5 and 320 Hz and found an fr at 7.5 +/- 1.6 Hz and two far at 97 +/- 13 and 231 +/- 27 Hz (far,1 and far,2, respectively). When breathing 80% He-20% O2, the fr shifted to 14 +/- 2 Hz, far,1 did not change (98 +/- 9 Hz), and far,2 increased to greater than 320 Hz. The behavior of fr and far,1 is consistent with the structure-function implied by the six-element model. However, the presence of an far,2 is not consistent with this model, because it is the airway acoustic antiresonance not represented in the model. These results indicate that, for frequencies that include the fr and far,1, the six-element model can be used to analyze Zrs data and reliable estimates of the model's parameters can be extracted by fitting the model to the data. However, more complex models must be used to analyze Zrs data that include far,2.     

Lumped parametric model of the human ear for sound transmission

A lumped parametric model of the human auditoria peripherals consisting of six masses suspended with six springs and ten dashpots was proposed. This model will provide the quantitative basis for the construction of a physical model of the human middle ear. The lumped model parameters were first identified using published anatomical data, and then determined through a parameter optimization process. The transfer function of the middle ear obtained from human temporal bone experiments with laser Doppler interferometers was used for creating the target function during the optimization process. It was found that, among 14 spring and dashpot parameters, there were five parameters which had pronounced effects on the dynamic behaviors of the model. The detailed discussion on the sensitivity of those parameters was provided with appropriate applications for sound transmission in the ear. We expect that the methods for characterizing the lumped model of the human ear and the model parameters will be useful for theoretical modeling of the ear function and construction of the ear physical model.Supported by Oklahoma Center for the Advancement of Science and Technology.     

Respiratory impedance to ambient pressure changes at low frequencies

Respiratory impedance may be studied by measuring airway flow (Vaw) when pressure is varied at the mouth (input impedance) or around the chest (transfer impedance). A third possibility, which had not been investigated so far, is to apply pressure variations simultaneously at the two places, that is to vary ambient pressure (Pam). This provides respiratory impedance to ambient pressure changes (Zapc = Vaw/Pam). In that situation airway impedance (Zaw) and tissue impedance (Zt) are mechanically in parallel, and both are in series with alveolar gas impedance (Zg): Zapc = Zaw + Zg + Zaw.Zg/Zt. We assessed the frequency dependence of Zapc from 0.05 to 2 Hz in nine normal subjects submitted to sinusoidal Pam changes of 2-4 kPa peak to peak. The real part of Zapc (Rapc) was of 6.2 kPa.1(-1).s at 0.05 Hz and decreased to 1.9 kPa.1(-1).s at 2 Hz. Similarly the effective compliance (Capc), computed from the imaginary part of Zapc, decreased from 0.045 1.kPa-1 at 0.05 Hz to 0.027 1.kPa-1 at 2 Hz. Breathing against an added resistance of 0.46 kPa.1(-1).s exaggerated the negative frequency dependence of both Rapc and Capc. When values of airway resistance and inertance derived from transfer impedance data were introduced, Zapc was used to compute effective tissue resistance (Rt) and compliance (Ct). Rt was found to decrease from 0.32 to 0.15 kPa.1(-1).s and Ct from 1.11 to 0.64 1.kPa-1 between 0.25 and 2 Hz. Ct was slightly lower with the added resistance. These results are in good agreement with the data obtained by other approaches.     

Transfer function analysis between arterial pressure and renal sympathetic nerve activity at cardiac pacing frequencies in the rat.

This study examined the possible influence of changes in heart rate (HR) on the gain of the transfer function relating renal sympathetic nerve activity (RSNA) to arterial pressure (AP) at HR frequency in rats. In seven urethane-anesthetized rats, AP and RSNA were recorded under baseline conditions (spontaneous HR = 338 +/- 6 beats/min, i.e., 5.6 +/- 0.1 Hz) and during 70-s periods of cardiac pacing at 6-9 Hz applied in random order. Cardiac pacing slightly increased mean AP (0.8 +/- 0.2 mmHg/Hz) and decreased pulse pressure (-3.6 +/- 0.3 mmHg/Hz) while leaving the mean level of RSNA essentially unaltered (P = 0.680, repeated-measures ANOVA). The gain of the transfer function from AP to RSNA measured at HR frequency was always associated with a strong, significant coherence and was stable between 6 and 9 Hz (P = 0.185). The transfer function gain measured under baseline conditions [2.44 +/- 0.28 normalized units (NU)/mmHg] did not differ from that measured during cardiac pacing (2.46 +/- 0.27 NU/mmHg). On the contrary, phase decreased linearly as a function of HR, which indicated the presence of a fixed time delay (97 +/- 6 ms) between AP and RSNA. In conclusion, the dynamic properties of arterial baroreflex pathways do not affect the gain of the transfer function between AP and RSNA measured at HR frequency in the upper part of the physiological range of HR variations in the rat.     

Viscoelastic properties of gerbil tympanic membrane at very low frequencies

The tympanic membrane transfers sound waves in the ear canal to mechanical vibrations in the middle ear and cochlea. Good estimates of the mechanical properties of the tympanic membrane are important to obtain realistic models. Up till now, only limited resources about tympanic membrane viscoelastic properties are available in the literature. This study aimed to quantify the viscoelastic properties of gerbil tympanic membrane. Step indentations were applied with a custom indenter on four fresh, intact tympanic membranes and the resulting force relaxation was measured. The reduced relaxation functions were then fitted with two viscoelastic model representations: a 5-parameter Maxwell model and a model with a continuous relaxation spectrum. The average relaxation function is described by an initial rapid decrease of 6.5% with characteristic time 0.77 s, followed by a long term decrease with characteristic time 46 s that gradually tends stable till a total relaxation of 15%. The relaxation curves in the time domain were transformed to complex moduli in the frequency domain. It was found that these transformations yield information on strain-rate dependence only from quasi-static to the very lowest acoustic frequencies. Finally, relaxation and hysteresis were simulated in a finite element model with viscoelastic material properties.     

Measurement of pleural pressure at low and high frequencies in normal rabbits

In eight tracheotomized adult rabbits placed in the supine position, we employed a catheter-tip piezoresistive pressure transducer to measure esophageal pressure (Pes) and assessed the validity of taking the changes in Pes to be the changes in pleural pressure (Ppl). We applied an occlusion test in which the tracheal cannula was occluded during either spontaneous inspiratory efforts or body surface oscillations ranging from 3 to 50 Hz. The relationship between Pes and airway opening pressure (Pao) was recorded. In all instances, the changes in Pes and Pao were virtually identical in both amplitude and phase. We conclude that, as evaluated by the occlusion test, a catheter-tip pressure transducer placed in the esophagus of rabbits can give adequate estimation of local pleural changes up to at least 50 Hz.     

Adaptation in sound localization processing induced by interaural time difference in amplitude envelope at high frequencies

Background

When a second sound follows a long first sound, its location appears to be perceived away from the first one (the localization/lateralization aftereffect). This aftereffect has often been considered to reflect an efficient neural coding of sound locations in the auditory system. To understand determinants of the localization aftereffect, the current study examined whether it is induced by an interaural temporal difference (ITD) in the amplitude envelope of high frequency transposed tones (over 2 kHz), which is known to function as a sound localization cue.

Methodology/Principal Findings

In Experiment 1, participants were required to adjust the position of a pointer to the perceived location of test stimuli before and after adaptation. Test and adapter stimuli were amplitude modulated (AM) sounds presented at high frequencies and their positional differences were manipulated solely by the envelope ITD. Results showed that the adapter''s ITD systematically affected the perceived position of test sounds to the directions expected from the localization/lateralization aftereffect when the adapter was presented at ±600 µs ITD; a corresponding significant effect was not observed for a 0 µs ITD adapter. In Experiment 2, the observed adapter effect was confirmed using a forced-choice task. It was also found that adaptation to the AM sounds at high frequencies did not significantly change the perceived position of pure-tone test stimuli in the low frequency region (128 and 256 Hz).

Conclusions/Significance

The findings in the current study indicate that ITD in the envelope at high frequencies induces the localization aftereffect. This suggests that ITD in the high frequency region is involved in adaptive plasticity of auditory localization processing.     

Sample-hold technique for analysis of respiratory gas composition at high breathing frequencies

A gas sampling device is described for continuous monitoring of respiratory gas composition that is applicable to experimental conditions when the breathing frequency is very high (greater than 2 Hz) and the response time of conventional gas analyzers becomes a critical limiting factor. The system utilizes the principle of discontinuous gas collection at any selected point of the respiratory cycle facilitated by ultraspeed piezoelectric valves and includes provision for sample-hold characteristics. Two distinct modes of operation are supported. In phase-locked mode gas sampling is synchronous with breathing frequency. In scanning mode gas collection is asynchronous with breathing frequency. Phase-locked mode may be used for continuous monitoring of end-tidal gas concentrations, whereas scanning mode is intended for assessing the gas concentration profile throughout the respiratory cycle. The system may be applied to steady breathing encountered in mechanical ventilation at high frequency or during quasi-steady breathing observed in panting animals. Combined with a respiratory mass spectrometer, the system has been used for measurement of gas concentrations in alveolar gas mixtures at breathing frequencies ranging from 3 to 30 Hz that were otherwise not amenable to rapid measuring techniques.     

Dielectric changes in hyaluronate solutions at microwave frequencies as a function of concentration,system pH,and temperature

The dielectric response of human umbilical cord hyaluronic acid in various environments has been studied at microwave frquencies using a resonant microwave cavity as a probe. Both the real and imaginary parts of complex dielectric constant and the loss tangent for hyaluronate solutions are obtained by utilizing equations for perturbation of a resonant cavity. Dielectric changes at room temperature have been observed in aqueous solutions of hyaluronic acid as a function of concentration ranging from 0 to 350 mg/ml. The data indicate the existence of ordered phases in hyaluronate solutions at selective concentrations, that is, exhibiting lyotropic-type transitions. Hyaluronate solutions at 1.5 and 3 mg/ml concentrations have been studied at various pH in the range of 6–8 and at constant ionic strength 0.1. A temperature-dependent transition in hyaluronate solution of 120 mg/ml concentration has been observed at physiological temperature. It is shown that this temperature-dependent behavior can be related to the orientational polarizability term in the Debye theory of polar molecules in liquids.     

Coding interaural time differences at low best frequencies in the barn owl.

In birds and mammals, precisely timed spikes encode the timing of acoustic stimuli, and interaural acoustic disparities propagate to binaural processing centers. The Jeffress model proposes that these projections act as delay lines to innervate an array of coincidence detectors, every element of which has a different relative delay between its ipsilateral and contralateral excitatory inputs. Thus, interaural time difference (ITD) is encoded into the position of the coincidence detector whose delay lines best cancel out the acoustic ITD. Neurons of the avian nucleus laminaris and mammalian MSO phase-lock to both monaural and binaural stimuli but respond maximally when phase-locked spikes from each side arrive simultaneously, i.e. when the difference in the conduction delays compensates for the ITD. McAlpine et al. [Nat. Neurosci. 4 (2001) 396] identified an apparent difference between avian and mammalian ITD coding. In the barn owl, the maximum firing rate appears to encode ITD. This may not be the case for the guinea pig, where the steepest region of the function relating discharge rate to interaural time delay (ITD) is close to midline for all neurons, irrespective of best frequency (BF). These data suggest that low BF ITD sensitivity in the guinea pig is mediated by detection of a change in slope of the ITD function, and not by maximum rate. We review coding of low best frequency ITDs in barn owls and mammals and discuss whether there may be differences in the code used to signal ITD in mammals and birds.     

The respiratory metabolism of honey-bee colonies at low temperatures

Honey-bee colonies exposed to temperatures between 20° C and -39° C produced less CO₂ at 10° C than at higher or lower temperatures.

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Zusammenfassung Honigbienenvölker wurden im Winter aus ihren Stöcken genommen und in galvanisierten Eisenbehältern (Tanks) Temperaturen zwischen 20 und -39° C ausgesetzt. Das Kohlendioxyd der aus den Behältern abgesogenen Luft wurde absorbiert und gewogen. Die CO₂-Produktion sank, wenn die Temperatur von 20 auf 10° erniedrigt wurde und stieg bei niederen Temperaturen wieder an. Diese Veränderungen verliefen in umgekehrter Richtung, wenn die Temperatur erhöht wurde.

     

On the respiratory function of human blood platelets

A set of physiological parameters of the blood from healthy individuals and patients with hypochromic anemia was subjected to factor analysis in order to test the hypothesis that platelets perform a respiratory function in circulation. Platelets were shown to have no respiratory function comparable to that of erythrocytes; however, the state of the pool of circulating platelets was of significance for blood gas exchange and rheology. When interpreting the extracted factors and observing the type, strength, and dynamics of the correlations found, we suggested that the effects of platelets on blood gas exchange and rheology were indirect, mediated by the platelet pool of biologically active substances. Being involved in the regulation of microcirculation and vessel wall permeability, platelets modulate the erythrocyte transport function.     

Dielectric properties of human fetal organ tissues at radio frequencies

The in vitro dielectric properties of human fetal organ tissues were measured in the frequency range from 100 kHz to 500 MHz at 24 °C. The dielectric measurements were performed by using a network analyzer (HP4195A) and a coaxial line capacitive sensor. The tested samples, including skin, muscle, heart, lung, liver, kidney, spleen, and brain tissues, were obtained from the legal abortion of five women with 14–16 week gestation periods. © 1996 Wiley-Liss, Inc.     

Dielectric properties of human red blood cells in suspension at radio frequencies

Dielectric properties of human red blood cells (RBCs) in suspension (hematocrit 50%) from 243 healthy persons (120 males, 123 females) were measured at 25 °C in a frequency range of 1–500 MHz, with a coaxial transmission line reflection method (one-side measurement). The measuring system, controlled by an IBM-PC computer, was composed of a network analyzer (HP4195A), an impedance test adapter (HP41951-61001), a coaxial line sensor, and a temperature-controlling set. The data measured revealed a statistically significant age dependence, with a critical age of about 49 years, above which permittivity and conductivity of human RBCs in suspension decreased significantly. © 1994 Wiley-Liss, Inc.     

Hypothalamic autostimulation in rats at low frequencies of stimulation on the basis of locomotion]

A study was made of the hypothalamic autostimulation of rats with the impulse stimulation frequency of from 30 to 7 cycles/sec. The animals' brain was stimulated in Skinner's chamber and on the basis of locomotion. The duration of a series of impulses established by the animal on the basis of locomotion grew with reduction of the frequency of stimulation impulses. The pauses between the series of impulses and the duration of the series were also prolonged with a voltage reduction. With the help of stimulation frequency of 7 cycles it was possible to choose such a current that the sum of the total duration of pauses lay within 0 divided by 10% of the total experimental time. The duration of individual pauses was less than 0.5 sec.     

High infection rates at low transmission potentials in West African onchocerciasis

Onchocerciasis has been successfully controlled for many years in endemic countries but more than 120 million people are still at risk. Factors which stabilise the persistence of the parasite in the population must be studied to minimise the future risk of re-infection. Among these factors, the relationship between the annual transmission potential and the parasite establishment rate is a main determinant which has to date not been quantified. Using entomological information and palpation data collected by the Onchocerciasis Control Programme in West Africa prior to the initiation of control activities, we derive annual transmission potential-dependent estimates of the parasite establishment rate from statistical analyses and computer simulations. Even at very low transmission intensities, the filarial parasite Onchocerca volvulus can efficiently establish in the human population, originating from an infection process which is strongly limited with respect to the annual transmission potential. Implementing the estimates into a simplified transmission model predicts that the critical annual biting rate, below which transmission is not possible, is much lower than previously assumed. We conclude that under the current strategy of mass distribution of microfilaricides without additional measures of vector control, the risk of re-infection is higher than previously assumed.     

Simulation of the power transmission of bone-conducted sound in a finite-element model of the human head

Bone conduction (BC) sound is the perception of sound transmitted in the skull bones and surrounding tissues. To better understand BC sound perception and the interaction with surrounding tissues, the power transmission of BC sound is investigated in a three-dimensional finite-element model of a whole human head. BC sound transmission was simulated in the FE model and the power dissipation as well as the power flow following a mechanical vibration at the mastoid process behind the ear was analyzed. The results of the simulations show that the skull bone (comprises the cortical bone and diploë) has the highest BC power flow and thereby provide most power transmission for BC sound. The soft tissues was the second most important media for BC sound power transmission, while the least BC power transmission is through the brain and the surrounding cerebrospinal fluid (CSF) inside the cranial vault. The vibrations transmitted in the skull are mainly concentrated at the skull base when the stimulation is at the mastoid. Other vibration transmission pathways of importance are located at the occipital bone at the posterior side of the head while the transmission of sound power through the face, forehead and vertex is minor. The power flow between the skull bone and skull interior indicate that some BC power is transmitted to and from the skull interior but the transmission of sound power through the brain seem to be minimal and only local to the brain–bone interface.