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PubMed1993

The effect of frequency and mean airway pressure on volume delivery during high-frequency oscillation.

The performance of a commercially available oscillator (SensorMedics 3100) at different frequencies was assessed. A frequency response curve of a pneumotachograph system was constructed and this was used to measure the volume delivered by the oscillator to a lung model. The volume delivered by a constant diaphragm displacement was demonstrated to be inversely proportional to the frequency, but unaffected by increasing mean airway pressure from 15 to 25 cm H2O. The volume delivered during high frequency oscillation (HFO) was then assessed in 8 infants, median gestational age 29 weeks. The infants were studied at two frequencies, 10 and 15 Hz, both of which were used at two levels of mean airway pressure (MAP): 2 and 5 cm H2O above the MAP level previously used during conventional ventilation. The delivered volume was not significantly different at the two MAP levels, but was significantly greater at 10 than 15 Hz at both MAP levels (P < 0.03); at MAP +2 cm H2O above baseline the reduction in delivered volume was from a median of 1.54 mL/kg (range, 0.88-3.12) at 10 Hz to 1.18 mL/kg (range, 0.65-4.5) at 15 Hz. These results suggest that higher frequencies would require an increase in the oscillator displacement if effective gas exchange is to be maintained.

Blood Gas Analysis↗
PubMed1995

Frequencies of synonymous substitutions in mammals are gene-specific and correlated with frequencies of nonsynonymous substitutions.

The frequencies of synonymous substitutions of mammalian genes cover a much wider range than previously thought. We report here that the different frequencies found in homologous genes from a given mammalian pair are correlated with those in the same homologous genes from a different mammalian pair. This indicates that the frequencies of synonymous substitutions are gene-specific (as are the frequencies of nonsynonymous substitutions), or, in other words, that "fast" and "slow" genes in one mammal are fast and slow, respectively, in any other one. Moreover, the frequencies of synonymous substitutions are correlated with the frequencies of nonsynonymous substitution in the same genes.

Animals↗
PubMed1988

Frequency dependence of dead space during high-frequency ventilation in rhesus monkeys.

A rotary valve ventilator, designed to allow the direct measurement of expired gas volume and composition, was used to maintain gas exchange in anesthetized, paralyzed, rhesus monkey at ventilatory frequencies up to 20 Hz. A total of five studies were carried out on three animals. Determinations of the minute ventilation required to maintain a normal steady-state PCO2, together with the FECO2 and arterial PO2, PCO2, and pH, were made at a number of frequencies. The results so obtained were used to calculate the Bohr (physiological) dead space. Dead space remained approximately constant at close to the value determined during spontaneous ventilation for each individual animal in the range of ventilatory frequencies from below 1 to around 5 Hz, but decreased somewhat with increasing frequency above 5 Hz. The calculated physiological dead space at 15 Hz was about 70% of the value at normal respiratory frequencies. These findings in primates, obtained using a ventilator system which allows very accurate determinations of expired gas volume and content, when compared with those from our previous studies in rabbits and dogs, provide further evidence that the relationship between efficiency of gas exchange and ventilatory frequency during HFV is highly species-specific.

Animals↗
PubMed1977

Son-sire-regression based heritability estimates of chiasma frequency, using T70H mouse translocation heterozygotes, and the relation between univalence, chiasma frequency and sperm production.

T(1;13)70H/+ translocation heretozygous mice were used for assessing heritability values for chiasma frequencies and the epididymal sperm count. The chiasma frequency estimates were based on 15 son-sire pairs, the translocation heterozygotes being maintained in a Swiss random-bred genetic background. The chiasma frequencies were scored separately for the T70H/+ derived multivalent, specific pairing segments within the multivalent and the remaining bivalents. Chiasma counts within these specified parts of the genome were positively correlated. The heritability estimates, significantly greater than zero, ranged from 0.78-0.98, depending on the chromosome segments included. These results indicate a strong genetic control on a cellular basis for the formation of chiasmata in the mouse. Despite significantly positive correlations and regressions between the various chiasma frequencies and the sperm count (for which 29 pairs of observations were available), no significant heritability estimate for the sperm count was obtained. The relation between the chiasma frequency and the sperm count was weakest when the chiasma count was confined to a region of the translocation-caused multivalent in which the absence of a chiasma almost always resulted in the production of an univalent. This indicates that in the translocation heterozygotes used, the overall chiasma frequency has a greater predictive value for the sperm count than autosomal univalence alone.

Animals↗
PubMed1994

The effect of frequency on carbon dioxide levels during high frequency oscillation.

The aim of this study was to assess if, during high frequency oscillation (HFO), a frequency of 10 or 15 Hz was associated with lower carbon dioxide levels. Twelve infants were recruited, median gestational age 27 weeks. All infants were studied at both frequencies, applied in random order and the carbon dioxide level checked after 30 minutes on each frequency. Carbon dioxide levels were lower in 10 of the 12 infants at 10 compared to 15 Hz (p < 0.004) and, in the study population, overall were significantly lower at 10 Hz (p < 0.05). The difference in paCO2 levels at the two frequencies, however, was small, thus these results suggest the clinician should rely primarily on changes in oscillatory amplitude, rather than frequency, to effect changes in carbon dioxide levels.

Bronchopulmonary Dysplasia↗
PubMed1988

Effects of frequency and airway pressure on gas exchange during interrupted high-frequency, positive-pressure ventilation in ponies.

Cardiovascular effects and pulmonary gas exchange were compared during conventional mechanical ventilation (CMV) and interrupted high-frequency, positive-pressure ventilation (IHFPPV) in 6 anesthetized ponies in dorsal recumbency. When the peak airway pressure (Paw) was held constant at control values attained during CMV (18 to 20 cm of H2O), and the ventilator frequency of IHFPPV was varied over the range, 2.5 to 12.5 Hz, significant (P less than 0.05) changes from control values were observed only in the ratio of dead-space volume to tidal volume (VD/VT) and in the respiratory minute volume (VE). The mean (+/- SEM) carbon dioxide excretion (VCO2) was 2.12 +/- 0.1 ml/kg/min during IHFPPV. Dead-space ventilation ranged from 40 to 73.7% of total ventilation and increased directly with increasing frequency. The VE also increased, from 89 ml/kg/min at a ventilatory frequency of 2.5 Hz to 145 ml/kg/min at a frequency of 12.5 Hz. Maintaining the frequency of IHFPPV constant at 12.5 Hz and increasing the Paw over the range of 5 to 30 cm of H2O caused significant (P less than 0.05) changes in arterial partial pressure of O2 (PaO2), VCO2, pulmonary shunt fraction (QS/QT), VE, arterial-alveolar differences in oxygen tension (AaDO2), VD/VT, and cardiac output, compared with CMV. The PaO2 and the VCO2 increased linearly with increasing Paw. With increasing Paw, VD/VT decreased directly with increasing Paw from 98 to 69.3%. Gas exchange at a Paw of 15 cm of H2O during IHFPPV was equivalent to conditions at Paw of 20 cm of H2O during CMV. At a higher Paw during IHFPPV, improvements over control values were observed in gas exchange.

Airway Resistance↗
PubMed1996

Difference in frequency spectrum of extremely-low-frequency effects on the genome conformational state of AB 1157 and EMG2 E. coli cells.

The effect of weak extremely-low-frequency (ELF) magnetic fields (sinusoidal, 30 microT amplitude) on the genome conformational state (GCS) of E. coli mutant and wild type cells was studied by using the method of anomalous viscosity time dependency (AVTD) in the 6-37 Hz frequency range. We confirmed the existence of three resonance frequencies of 8.9, 15.5, and 29.4 Hz when mutant cells of K12 AB1157 strain were exposed. In the same frequency range, the wild type K12 EMG2 cells displayed only two effective windows, with resonance frequencies of 8.3 and 27 Hz. The resonance frequencies differed significantly (P < .001-.000001) in the strains studied, whereas other resonance parameters did not. It was concluded that mutations in the AB1157 strain resulted in a significant rearrangement in the ELF action spectrum, including the appearance of a new resonance.

DNA, Bacterial↗
PubMed1995

Frequency modulated sound pattern analysis in the lesser bulldog bat: the role of interactions between adjacent frequency elements of complex sounds.

A stereotyped approach phase vocalization response of Noctilio albiventris to artificial echoes simulating a virtual approaching object was used to assess the ability of the bat to analyze and extract distance information from the artificial echoes. The performance of the bats depended on the temporal pattern of frequency change of the continuously sweeping frequency modulated (FM) component of the signals. When the bats were presented with a CF/FM signal containing a time-reversed upward FM sweep, they responded with approach phase behavior at a performance level that was significantly below that seen with a CF/FM signal containing a naturally structured downward FM sweep. When the FM sweep was divided into a series of brief pure tone steps, the extent to which the bats showed a difference in their capability to process upward versus downward FM sweeps depended on the difference in frequency between the pure tone steps. The bats effectively processed downward but not upward FM sweeps when the difference in frequency between pure tone frequency elements of the FM sweeps was from about 100-200 Hz, but they effectually processed both downward and upward FM sweeps when the tonal elements composing the FM sweeps were separated by more than about 200 Hz. This suggests that the ability of the bats to effectively process downward but not upward FM sweeps is based on local interactions between adjacent frequency elements of the complex sounds.

Animals↗
PubMed1985

Temporal frequency discrimination in human vision: evidence for an additional mechanism in the low spatial and high temporal frequency region.

Temporal frequency discrimination at and above the detection threshold has been studied using gratings of low (0.2 c/deg) and medium (2 c/deg) spatial frequencies. At 2 c/deg the results of previous investigators are confirmed: The results being consistent with the existence of two broadly tuned and directionally selective temporal mechanisms (up to 32 Hz). For the lower spatial frequency an additional temporal frequency discrimination at threshold can be made between 4 and 32 Hz and enhanced temporal frequency discrimination at suprathreshold levels occurs above 24 Hz. One interpretation of this result is the existence of one or more additional temporal mechanisms with restricted spatial acuity responding to higher temporal frequencies.

Discrimination, Psychological↗
PubMed1996

Masking of spatial frequency in visual memory depends on distal, not retinal, frequency.

Spatial frequency discrimination thresholds measured in a two-interval forced choice paradigm are virtually constant across inter-stimulus intervals ranging from 400 to 30,000 msec, demonstrating that an accurate representation of spatial frequency is maintained in short-term memory. This representation can be degraded by briefly flashing a grating during the retention interval. Moreover, this memory masking effect varies with the spatial frequency of the mask, suggesting that the mechanisms used to store spatial frequency in memory are similar to low-level visual filters. In this paper we replicate those previous findings and extend them by showing (1) that accurate memory for spatial frequency lasts as long as 1 min; (2) that memory masking is based on distal (c/cm), not retinal (c/deg), spatial frequency.

Differential Threshold↗
PubMed1987

Effect of alterations in frequency, inspiratory time, and airway pressure on gas exchange during high frequency jet ventilation in dogs with normal lungs.

High frequency jet ventilation (HFJV) is becoming increasingly useful for providing respiratory support in patients with normal lungs during operative procedures, and also has been advocated as a technique for ventilating patients during cardiopulmonary resuscitation. We studied the effect of frequency, percent inspiratory time (I/E ratio), peak airway pressure, and airway pressure difference (peak-PEEP) during HFJV as operational variables on the efficacy of gas exchange in dogs with normal lungs. We observed that at a constant peak airway pressure and percent inspiratory time, PaCO2 generally increases as frequency rises above 100/min. In contrast, PaCO2 generally decreases as percent inspiratory time is reduced at a constant frequency and peak airway pressure. In addition, increasing peak airway pressure and airway pressure difference are associated with lower levels of PaCO2. Arterial oxygenation was adversely affected by frequencies above 300/min, but was otherwise not influenced by alterations in frequency, percent inspiratory time, or airway pressure.

Animals↗
PubMed1987

Frequency and time domain comparison of low-frequency auditory fiber responses in two anuran amphibians.

A comparative study of the phase-locked response of auditory nerve fibers was performed in two frog species, Eleutherodactylus coqui and Bombina orientalis. From the tuning characteristics and phase response of single auditory nerve fibers to low frequency tones (0.08-1.0 kHz) we attempt to deduce the mechanics of the auditory organ responsible for low-frequency hearing in the frog, the amphibian papilla (a.p.). The phase-locked responses of auditory nerve fibers in B. orientalis were essentially identical to those from cells with similar CFs in E. coqui, despite the presence of a conspicuous caudal extension of the a.p. in E. coqui (an apparently derived morphology), a feature completely absent in B. orientalis. The fine structure of the frequency-dependent phase behavior was examined in both species with a residual phase analysis. The most significant non-linear phase behavior was confined to neurons with CFs less than 0.3 kHz. The intensity dependence of the phase response in E. coqui revealed that the preferred firing phase of an auditory nerve fiber depends upon the relation of test frequency (TF) and CF of the neuron examined. For TFs greater than CF there was a progressive phase lag as stimulus level was increased; the inverse was true for TFs less than CF. Click latencies measured in E. coqui were inversely related to CF and were similar though systematically shorter than the response latencies estimated from the phase-frequency functions. The click response was similar to that documented in other species, showing a significant level dependence and the presence of multiple peaks, with the time between peaks related to the period of the neuron's CF. A 'neurogram' was compiled for a.p. fiber responses in both species in response to several pure tones. Based on the known tonotopy of the a.p. this measure reflects the phase response of the a.p. over the extent of its length. The population phase response in anurans is quite similar to that obtained from mammalian auditory nerve fibers for the same range of test frequencies (0.08-1.0 kHz). The similarity between the responses of auditory fibers in these two anuran species suggests the micromechanics of the a.p. rostral to the tectorial curtain is similar in both species and that it is the likely site for the origin of the CF-dependent time delays.

Acoustic Stimulation↗
PubMed2006

Classic electroencephalographic parameters: median frequency, spectral edge frequency etc.

Even today many anaesthesiologists rely on parameters of the autonomic nervous system, such as blood pressure and heart rate to decide if a patient is adequately anaesthetized. It is thought that the electroencephalogram (EEG) may provide more information on the state of anaesthesia. Because full EEG analysis is not possible in the operating room, processed EEG parameters have been developed comprising complex information into a single value. Time and frequency domain parameters are calculated. The power spectrum results from a Fourier analysis and can be described by parameters such as median frequency, spectral edge frequency and others. It was noted, however, that anaesthetics at low doses increase frequency of the EEG, whereas at high doses the EEG is depressed. This biphasic response makes it difficult to clearly distinguish the exact anaesthetic state of a patient. Median frequency and spectral edge frequency have been studied in numerous studies. However, no sole indicator has been derived from the EEG that could serve as a descriptor of anaesthetic depth.

Alpha Rhythm↗
PubMed2004

The effect of duty cycle and frequency on muscle torque production using kilohertz frequency range alternating current.

We investigated the frequency and duty cycle dependence of maximal electrically induced torque (MEIT) of the wrist extensors. Fifty hertz burst modulated sinusoidal alternating current (AC) in the frequency range 0.5-20 kHz was used, with duty cycles ranging from a minimum (one cycle) to maximum (continuous AC). MEITs were similar at low frequencies but decreased markedly above 2.5 kHz. MEITs also decreased markedly above a 20% duty cycle. Subjective reports of discomfort were fewest at 4 kHz and at duty cycles in the range 20-25%. Our conclusion is that for maximum torque production, a frequency of 1 kHz and a duty cycle of 20% are indicated. When comfort is a major consideration, a frequency of 2.5 kHz provides an acceptable trade-off between MEIT and comfort. The findings also suggest that low duty cycle, burst modulated AC stimulation may be more effective than stimulation using conventional low-frequency pulsed current.

Electric Stimulation Therapy↗
PubMed2005

The affection on the tissue lesions of difference frequency in dual-frequency high-intensity focused ultrasound (HIFU).

The lesions induced by dual-frequency high-intensity focused ultrasound (HIFU) in freshly excised porcine livers were investigated and compared with the lesions induced by conventional single-frequency HIFU. The results have shown that using different exposure time resulted in lesions of different sizes in both dual-frequency and single-frequency HIFU modes at the same intensity level (ISAL = 808 W cm(-2)), but the dimensions of lesions in dual-frequency mode were obviously larger than those in single-frequency mode. A much possible explanation was suggested for the results and the possibilities of applications in the future were also discussed in brief.

Animals↗
PubMed2000

Validity of nutrient estimates by food frequency questionnaires based either on exact frequencies or categories.

PURPOSE: The study was designed to assess if category-based Food Frequency Questionnaires (FFQ) could be substituted for exact-frequency based questionnaires. METHODS: Nutrient estimates on 203 subjects from Toronto, Canada, derived from recoding an exact frequency based 132 item food frequency questionnaire (E-FFQ) into categories were evaluated for validity against a 7-day food record (FR). RESULTS: Among men, the median Pearson's correlation coefficient (for energy adjusted nutrients) between FR and E-FFQ was 0.53, ranging from 0.26 for thiamin to 0.72 for calcium. The median correlation dropped to a value of 0.40 (range: 0.18 for thiamin to 0. 56 for carbohydrate) when a category-based FFQ (C-FFQ) and FR were compared. Data on women showed a similar pattern; a median correlation of 0.50 from E-FFQ and 0.42 from C-FFQ. CONCLUSIONS: While the results varied with individual nutrients, overall it is concluded that when validated against food records, the use of exact frequencies in FFQs yielded higher correlation coefficients for most nutrients, and slightly better agreement within quartiles of categories than recoded frequency categories.

Adult↗
PubMed2000

The variation in fatigue rate with frequency using kHz frequency alternating current.

The most commonly used stimulus for functional electrical stimulation is low frequency, short duration pulsed current. A disadvantage is the associated rapid fatigue. The present study investigated the effect of kHz frequency alternating currents on the rate of fatigue with electrically induced skeletal muscle contractions in normal subjects. Alternating current with frequencies between 1 and 15 kHz, interrupted at 50 Hz and applied in 3-s surges with an effective 1:1 duty cycle, was applied transcutaneously for a 10-min period, during which time electrically induced wrist extensor torque was measured. The decline in torque with time was analysed in terms of "fast" and "slow" fatigue components. A systematic frequency dependence was found in each. "Fast" torque decline is interpreted as reflecting fast-fatigue muscle fibre activity and "slow" torque decline, that of fatigue-resistant fibres. With this interpretation, over the frequency range 1-10 kHz, the proportion of fatigue-resistant fibres contributing to the torque increases. It is argued that this is due to selective dropout of fast-fatigue fibres. The findings suggest the potential usefulness of kHz frequency alternating current for functional electrical stimulation.

Adult↗