PubMed Health⌕ Search

Biomedical subjects

G R Long

Publications and source records attributed to G R Long.

35 records · Page 2Linked to original sources
PubMed1988

Investigations into the nature of the association between threshold microstructure and otoacoustic emissions.

Three studies are described which investigate the nature of the association between threshold microstructure and otoacoustic emissions. In the first study, threshold dips (similar in shape to those seen in threshold microstructure) are produced by introducing a low-level masker. Threshold microstructure is not abolished when tonal probes are replaced by narrowband-noise probes, while dips induced by external tonal maskers are eliminated. These findings rule out a simple interpretation of the microstructure dips as an instance of masking by otoacoustic emissions. In the second study, ear-canal measurements of the interactions of external tones with spontaneous emissions indicate that, although beating is often detected near threshold maxima, stimuli close to threshold minima are perceived as tonal because the emission is frequency locked by the external tone. The last study shows that reduction of the levels of otoacoustic emissions by aspirin consumption is associated with an initial reduction of thresholds in regions of threshold microstructure, with the greatest reduction occurring at threshold maxima. This suggests that threshold maxima may be due, at least in part, to interference or masking by the nearby otoacoustic emissions. A simple analog (driven Van der Pol oscillator) of an external tone interacting with a spontaneous emission is used to interpret ear-canal pressure waveforms and associated psychophysical percepts (including threshold detection), for tones close in frequency to emissions.

Aspirin↗
PubMed1988

Measures of signal processing in persons with sensorineural hearing loss below 6 kHz.

Estimates of threshold, wide- and narrow-band noise masking, frequency and amplitude modulation detection, gap detection, and rate discrimination were obtained from 10 subjects with near-normal hearing at frequencies above 6 kHz, but severe-to-profound hearing losses at lower frequencies. The same measures were obtained from 10 young control subjects with normal hearing sensitivity for all frequencies up to 16 kHz. The hearing-impaired subjects were able to process sounds in the region of near-normal hearing sensitivity as well as the unimpaired control subjects. Performance in the low-frequency, impaired region depended on the lowest frequency of near-normal hearing sensitivity.

Adolescent↗
PubMed1988

Modification of spontaneous and evoked otoacoustic emissions and associated psychoacoustic microstructure by aspirin consumption.

The discovery that aspirin consumption can abolish spontaneous otoacoustic emissions [D. McFadden and H.S. Plattsmier, J. Acoust. Soc. Am. 76, 443-448 (1984)] provides a technique for further exploring the relation between otoacoustic emissions (spontaneous and evoked) and psychoacoustic threshold microstructure. Spontaneous emissions, delayed evoked emissions, synchronous evoked emissions, and threshold microstructure in four subjects were monitored before, during, and after consumption of 3.9 g of aspirin per day (three 325-mg tablets every 6 h) for 3 or 4 days. The changes in spontaneous emissions are consistent with the findings of McFadden and Plattsmier except that one spontaneous emission appeared to plateau at a reduced level above the noise floor during the last day and a half of the 3-day period of aspirin consumption. Evoked emissions and threshold microstructure were also reduced by aspirin consumption but persisted longer and recovered sooner. In most instances, the initial change in threshold microstructure was a trend to increased sensitivity (reduced thresholds), with a greater increase near threshold maxima than at threshold minima. Further reduction in the levels of the evoked emissions was accompanied by the eventual decrease in sensitivity (elevation of all thresholds).

Adult↗
PubMed1987

The effect of nitroprusside on pulmonary edema, oxygen exchange, and blood flow in hydrochloric acid aspiration.

In canine pulmonary capillary leak induced by intravenous oleic acid, reducing pulmonary wedge pressure (Ppw) reduces pulmonary edema, venous admixture (Qva/Qt), and cardiac output (Qt). The authors tested the possibility that in another canine model of pulmonary capillary leak, that induced by endobronchial instillation of hydrochloric acid, nitroprusside would reduce Ppw and edema without reducing Qt or oxygen delivery (QO2). In 18 dogs, the authors measured extravascular lung water (EVLW) by thermal-dye dilution and the hemodynamic and gas exchange variables before and at intervals (1, 1.5, 3, and 5 h) after .1 N HCl bronchial infusion. By 1 h, HCl increased EVLW from 175 to 250 ml and Qva/Qt from 11 to 21%. Immediately after the 1-h measurements, the dogs were divided into three equal groups: six controls (C) were maintained with a Ppw of 12 mmHg, while plasmapheresis (P) or nitroprusside (NP) reduced Ppw to 5 mmHg for the next 4 h. EVLW continued to increase to 548 ml in C, but did not increase further in P and NP. Weights of lungs excised at 5 h confirmed that P and NP reduced edema by 50% in 4 h. In C, Qva/Qt increased, but there was no reduction in Qt or QO2. In contrast, plasmapheresis reduced Qva/Qt, Qt, and QO2. With nitroprusside, Qt and QO2 were maintained despite reduced Ppw at 1.5 and 3 h, and Qva/Qt did not decrease as in Group P. We conclude that plasmapheresis-induced reduction in Ppw reduces the pulmonary capillary leak and venous admixture following acid aspiration, but this has the potentially adverse effect of reducing cardiac output and oxygen delivery.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Tissue oxygen extraction during hypovolemia: role of hemoglobin P50.

When the delivery of O2 to tissues (QO2 = blood flow X O2 content) falls below a critical threshold, tissue O2 uptake (VO2) becomes limited by QO2. The mechanism responsible for this extraction limitation is not understood but may involve molecular diffusion limitation as mean capillary PO2 drops below a critical minimum level in some capillaries. We tested this hypothesis by measuring the critical QO2 necessary to maintain VO2 independent of QO2 in anesthetized, paralyzed normal dogs (n = 7) and in a second group in which PO2 at 50% saturation of hemoglobin (P50) was reduced by exchange transfusion with low-P50 erythrocytes (n = 7). QO2 was reduced in stages by removing blood volume to reduce blood flow while VO2 was measured by spirometry at each step. To the extent that O2 extraction was limited by a critical capillary PO2, we reasoned that the onset of diffusion limitation should occur at a higher QO2 with low P50, since a lower end-capillary PO2 is required to achieve the same O2 extraction. The critical QO2 (7.8 +/- 1.2 ml X min-1 X kg-1) and extraction ratio (0.63 +/- 0.06) in dogs with reduced P50 were not different from controls. At the critical delivery, mixed venous PO2 was lower in low P50 (16.1 +/- 2.9 Torr) than controls (29.9 +/- 2.3 Torr). We concluded that diffusion limitation does not initiate the early fall in VO2 below the critical QO2 and offer an alternative model to explain the onset of supply dependency.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Canine bronchoconstriction, gas trapping, and hypoxia with methacholine.

The effects of an intravenous methacholine infusion on cardiovascular-pulmonary function were measured in seven mongrel dogs (22.0 +/- 2.8 kg), anesthetized with chloralose and urethan and beta-adrenergically blocked with propranolol. In a volume-displacement plethysmograph, physiological measurements were made at base line and 25 min after establishing a methacholine infusion (0.1-1.0 mg X kg-1 X h-1). Methacholine significantly (P less than 0.05) increased airways resistance (1.9 +/- 0.8 to 8.2 +/- 2.9 cmH2O X l-1 X s), decreased static lung compliance (84.7 +/- 18.5 to 48.2 +/- 9.4 ml/cmH2O), depressed arterial PO2 (81 +/- 17 to 56 +/- 10 Torr), and lowered blood pressure (132 +/- 10 to 69 +/- 18 Torr) and cardiac output (5.7 +/- 1.9 to 4.1 +/- 1.2 l/min). These effects persisted during a further 80 min of methacholine infusion conducted in five of the animals. During the initial 25-min period of methacholine, the end-expired volume (volume-displacement Krogh spirometer) rose in all animals, indicating an increase in functional residual capacity from 997 +/- 115 to 1,623 +/- 259 ml (P less than 0.0005). Analysis of pulmonary pressure-volume curves revealed no change in total lung capacity but an increase in residual volume from 489 +/- 168 to 1,106 +/- 216 ml (P less than 0.001). Thus methacholine caused 617 ml of gas trapping, which was not detected by the Boyle's law principle, presumably because gas was trapped at high transpulmonary pressure. We suggest that intravenous methacholine-induced canine bronchoconstriction, which causes gas trapping and hypoxia, may be a useful animal model of clinical status asthmaticus.

Animals↗
PubMed1986

Detection of pure-tone amplitude modulation as a function of sensation level from 8 to 14 kHz.

Amplitude modulation detection thresholds were obtained for pure-tone stimuli of 8, 10, 12 and 14 kHz at 5 dB intensity increments from 10 to 65 dB sensation level. Performance at 8 and 10 kHz was a non-monotonic function of sensation level for all four subjects with the largest difference limen measured near 30 dB sensation level and optimal performance at the highest sensation level (60 dB). Weber fractions at 12 and 14 kHz appear dependent on each subject's high frequency hearing profile; i.e., the difference limens remain high and either increase or remain essentially constant at high sensation levels only when the frequencies tested are near a particular subject's upper limit of hearing.

Auditory Threshold↗
PubMed1986

Rate discrimination of high-pass-filtered pulse trains.

Difference limens for trains of 30-microseconds pulses were determined for repetition rates of 50, 100, 200, 400, and 800 pulses per second under conditions of no filtering and high-pass filtering (115 dB/oct) with corner frequencies of 2.5, 5.0, 7.5, and 10 kHz. Low-pass-filtered noise was mixed with the trains of impulses to preclude discrimination on the basis of potential low-frequency signal components. Measures were obtained from four trained listeners at a signal level of 30 dB SL relative to individually determined thresholds for each filter condition and repetition rate. The data support the hypothesis that resolution of pulse-train repetition rate involves both temporal- and frequency-based processes--the latter becoming ineffective when frequency resolution of the ear is insufficient to resolve separate harmonics of the signal. Inter- and intra-individual differences are interpreted as reflecting frequency resolution capacity.

Auditory Threshold↗
PubMed1986

The effects of plasmapheresis and hemofiltration on canine acid aspiration pulmonary edema.

Aspiration of hydrochloric acid causes pulmonary capillary endothelial damage and edema. One approach to treatment is reduction of circulating volume and pulmonary wedge pressure (Ppw) to low values compatible with adequate cardiac output (QT) and oxygen delivery (QO2) to the tissues. We investigated this approach in 18 dogs after intratracheal instillation of HCl. One hour after injury, animals were randomized into 3 equal groups and studied for another 4 h. Control dogs (Group C) were maintained at a Ppw of 12 mmHg throughout the experiment. In a second group (Group H), Ppw was reduced over 30 min to 5 mmHg by hemofiltration and maintained at this amount throughout the experiment. In a third group (Group P) Ppw was reduced and maintained at 5 mmHg by plasmapheresis. In Group C, extravascular thermal volume (ETV), measured in vivo, increased to 520 ml by 5 h, and venous admixture (QVA/QT) increased to 30%. In contrast, plasmapheresis stopped ETV accumulation at the 1-h value of 250 ml, whereas QVA/QT decreased to 8% at 5 h. Hemofiltration did not significantly reduce either ETV or QVA/QT, which were 420 ml and 30%, respectively, at 5 h. The wet weights of lungs excised at 5 h and normalized to body weight (WW/BW) confirmed that plasmapheresis reduced edema (WW/BW, 19 ml/kg) compared (p less than 0.05 ANOVA) with that in the control group (WW/BW, 27), and that hemofiltration did not significantly reduce edema (WW/BW, 24).(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1985

Intensity difference limens at high frequencies.

Thresholds for amplitude modulation detection were obtained from four subjects at frequencies of 2, 4, 6, 8, and 10 kHz for sensation levels of 15, 30, 45, and 60 dB and modulation rates of 2, 4, and 8 Hz. High-frequency difference limens calculated from amplitude modulation thresholds were found to change nonmonotonically as a function of sensation level, independent of modulation rate. This nonmonotonic relation stemmed mainly from a gradual reduction of the difference limen at the lowest sensation level with increasing frequency. Difference limens for pulsed tone discrimination were also measured in two of the subjects at 2, 6, and 10 kHz and sensation levels of 15, 30, 45, and 60 dB. The relation between intensity discrimination and sensation level was similar to that found for amplitude modulation detection. These findings are interpreted as indicating that the nonmonotonic relation between sensation level and intensity resolution is a general characteristic of stimulus processing at higher frequencies.

Adult↗
PubMed1984

The microstructure of quiet and masked thresholds.

Factors leading to the microstructure of the audiogram (a constant pattern of threshold maxima and minima as a function of frequency) are shown to influence masked thresholds, changing the shape of masking functions when both constant and variable tonal maskers are used. Simultaneous masking with broadband noise gradually reduces the difference between threshold maxima and minima until no further differences can be seen when masked thresholds are above 40-50 dB SPL. Nonsimultaneous masking with broadband noise reveals a changed microstructure when thresholds are elevated above 30-40 dB SPL. Thus in both simultaneous and nonsimultaneous masking the rate of threshold growth with increasing masker level is different for tones from threshold minima than for tones from threshold maxima. These psychophysical measures are related to measures of evoked cochlear emissions and the results are discussed in terms of the implications for understanding the cochlear mechanisms responsible for the microstructure and for the interpretation of psychophysical measures with low level stimuli.

Audiometry, Pure-Tone↗
PubMed1984

Detection of frequency and rate modulation by the chinchilla.

Thresholds for detection of frequency modulation were obtained from three chinchillas at seven frequencies between 320 Hz and 12.75 kHz and at three sensation levels (20, 40, 60, dB SL). These were compared with human thresholds at 40 dB SL obtained under the same conditions. Our data show no significant impact of sensation level and confirm Nelson and Kiester 's [J. Acoust. Soc. Am. 64, 114-126 (1978)] report that the chinchilla has much poorer frequency discrimination than man. In an attempt to assess separately the chinchilla's capacity for temporal coding of frequency, thresholds for detection of a change in the modulation rate of amplitude modulated noise were obtained for nine modulation frequencies between 10 and 320 Hz. The chinchillas needed a larger rate change for detection than most humans but the relative difference was less than that found for frequency modulation detection.

Animals↗
PubMed1983

Independent influence of blood flow rate and mixed venous PO2 on shunt fraction.

We have tested the independent and combined effects of changes in mixed venous PO2 (PvO2) and blood flow (QT) on shunt fraction (Qs/QT) in isolated blood-perfused canine left lower lobes with edema. The lobes were ventilated with pure O2. Inflow (Pi) and outflow (Po) pressures always exceeded lobar alveolar pressure. PvO2 was varied by means of a clinical bubble oxygenator with appropriate mixtures of O2 and N2. QT was varied by changes in Pi and Po with care not to produce changes in lobar weight. Changes in QT did not influence Qs/QT. Increasing PvO2 from 40 +/- 6 to 88.4 +/- 40 Torr at constant QT significantly increased Qs/QT from 5.5 +/- 2.0 to 15.6 +/- 7.0%. Combined increases in QT and PvO2 from 66.4 +/- 2.7 to 135.6 +/- 21.5 ml/min and from 38.8 +/- 1.3 to 61.8 +/- 2.2 Torr, respectively, also produced a significant increase in Qs/QT from 7.33 +/- 2.27 to 15.43 +/- 4.45%. However, this combined change was explained exclusively by changes in PvO2. We therefore concluded that, under the conditions of our experiment, changes in PvO2 influence Qs/QT, and this may account for apparent dependence of Qs/QT on cardiac output in pulmonary edema.

Animals↗
PubMed1981

Tone-on-tone masking in the chinchilla.

Detailed measurements of tone-on-tone masking in the chinchilla with the signal (f2) higher in frequency than the masker (f1) were obtained with 50-dB SPL maskers of 1000 and 1500 Hz. Two regions of lower thresholds (notches) were found for each masker. The masker-notch interval was independent of masker frequency for one of these notches, but dependent on frequency for the other. The latter notch was though to be due to the detection of 2f1-f2 combination tones and to provide additional evidence for relatively poor frequency resolution in the chinchilla.

Acoustic Stimulation↗