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Biomedical subjects

K J Gerhardt

Publications and source records attributed to K J Gerhardt.

At least 55 records · Page 3Linked to original sources
PubMed1991

Behavioral state transition and local cerebral blood flow in fetal sheep.

Local cerebral blood flow in four near term fetal sheep was evaluated continuously before and after natural alternations in fetal behavioral state. Measurements were made in fetuses several days following an aseptic surgery to place electrodes for behavioral state recordings as well as heated and reference thermojunctions in cortical and subcortical tissue. These thermojunctions were used to qualitatively assess local cerebral blood flow. The time of transition between rapid eye movement sleep (REMS) and non-rapid eye movement sleep (NREMS) was based on visual inspection of strip chart recordings of electrocortical, electroocular, and neck electromyographic activity and application of published criteria for their assessment. To confirm that transition had occurred, the amplitude of the spectrum of the electrocorticogram in one-third octave bands centered around 1 Hz and 20 Hz was measured before and after the transition point. Mean cerebral blood flow rose significantly by 24 s (P less than 0.05) after the transition from NREMS to REMS and fell by 12 s after the transition from REMS to NREMS (P less than 0.05).

Animals↗
PubMed1990

Cocaine alters behavioral states in fetal sheep.

Intrauterine cocaine exposure causes subtle neurologic abnormalities in human newborn infants; however, the mechanism for these abnormalities is not known. To investigate whether cocaine alters fetal behavioral state, the electrocortical, electro-ocular and neck muscle electrical activity was monitored in 7 chronically instrumented fetal sheep before and during both saline and cocaine HCl infusions directly to the fetus. Saline infusion to the fetus had no effect on the percentage of time spent in rapid eye movement sleep compared to no infusion (37.5 +/- 11.6% vs 46.3 +/- 4.6%, mean +/- SD, P greater than 0.1). Cocaine infusion directly to the fetus had no effect on fetal arterial pO2, but did increase mean arterial pressure from 53.6 +/- 15 mmHg to 61.0 +/- 21 mmHg (P less than 0.01). In addition, during cocaine infusion, the percentage of time spent in rapid eye movement sleep dropped to 3.9 +/- 5.1% (P less than 0.0001) and the average duration of rapid eye movement epochs decreased from 10.1 +/- 3.0 min precocaine infusion to 1.9 +/- 2.6 min during cocaine infusion (P less than 0.02). The influence of cocaine was noted in a frequency analysis of the electrocorticogram. The amplitude of the energy centered at 1 Hz during cocaine infusion (73.8 +/- 4.0 dB) was greater than the amplitude during rapid eye movement sleep (65.5 +/- 4.7 dB) and less than the amplitude during non-rapid eye movement periods (79.9 +/- 4.5 dB) (P = 0.01). Cocaine appears to alter fetal behavioral state directly and this may play a role in the abnormal behavior in newborn infants exposed to cocaine in utero.

Animals↗
PubMed1990

Sound environment of the fetal sheep.

The internal sound pressure levels within the intact amnion of pregnant ewes surgically implanted with a hydrophone was determined during conditions of quiet and during sound field exposures to broadband and octave-band noise. Measurements were made of sound pressures outside and inside the ewe, and sound attenuation through maternal tissues and fluids was calculated. Sound pressures generated by low frequencies (less than 0.25 kHz) were 2 to 5 dB greater inside than outside the ewe. Above 0.25 kHz, sound attenuation increased at a rate of 6 dB per octave. For 4.0 kHz, sound attenuation averaged 20 dB. The sound pressure recorded at different locations within the amnion with respect to the sound source varied by up to 6 dB. The internal noise floor in the absence of externally generated sounds was as low as 50 dB (spectrum level) above 0.2 kHz. Thus the fetus is developing in an environment that is rich with internal and external sounds.

Acoustic Stimulation↗
PubMed1989

Effect of high-intensity sound on local cerebral glucose utilization in fetal sheep.

The effects of external noise on fetal sheep cerebral glucose utilization were determined with the [14C]deoxyglucose method. Seventeen animals were prepared at 130 days gestation with catheters and electrodes for assessing fetal behavioral state. Five to 7 days later, 7 animals were studied under normal laboratory sound conditions (65-70 dB), 5 animals were exposed to 105-120 dB broadband noise levels produced by two earphones applied to the abdomen of the ewe, and 5 fetuses were stimulated with an electronic artificial larynx (EAL), positioned on the abdomen directly over the fetal head. There were no significant differences between local cerebral glucose utilization in controls and earphone ewes, and no obvious alteration in behavioral states. However, there were marked, significant differences in glucose utilization along the central auditory pathway during EAL stimulation. These autoradiographs revealed isofrequency-like bands in medial geniculate body and irregular darkening of cortex of the temporal lobe. Total time spent in clearly defined high and low voltage electrocortical activity did not change during EAL stimulation.

Animals↗
PubMed1989

Phenomenologic aspects of the acoustic reflex following noise exposure.

Thirty-seven normal hearing subjects were exposed to broadband noise for 2 hours at 90 dB (Lp). Measures of behavioral thresholds and acoustic reflex activity were completed prior to and following the noise exposure. As expected, temporary threshold shift was obtained at several different frequencies (1.0-6.0 kHz). In addition, reflex threshold shift occurred for all pure-tone elicitors under evaluation (0.5 to 4.0 kHz). The observed shift in acoustic reflex threshold at 0.5 kHz without a concomitant change in behavioral sensitivity reveals the limitation of utilizing behavioral threshold testing as the sole measure of cochlear disruption. Changes in acoustic reflex onset latency and magnitude were not observed after compensating for shifts in acoustic reflex threshold.

Acoustic Stimulation↗
PubMed1988

Intrauterine noise levels produced in pregnant ewes by sound applied to the abdomen.

Sound pressure in amniotic fluid was created by application of either an electronic artificial larynx or a standard audiometric earphone to the abdominal surface of pregnant ewes. Sound transmission was assessed with a hydrophone placed near the ear of the fetus within the intact amnion. Sound pressures produced by the electronic artificial larynx located directly over the hydrophone averaged 134.9 dB, whereas the highest sound pressure produced by a similarly placed earphone was 95.2 dB. Sound transmission decreased dramatically with increasing distance between the sound source and the hydrophone; this may account for some of the variability in fetal response when the sound test is used clinically. Caution is recommended in administering the sound test until the effects of high noise levels on the fetus are better understood.

Abdomen↗
PubMed1988

Adaptation properties of the acoustic reflex in response to continuous-, intermittent- and industrial-noise stimulation.

Adaptation properties of the acoustic reflex (AR) were evaluated in a group of normal-hearing subjects. Signals used to elicit the AR included: (1) continuous broad-band noise; (2) broad-band noise interrupted every 7.5 s. with pauses of 250, 500 and 750 ms, and (3) industrial noise. AR activity was continuously monitored for a period of 5 min. Values of acoustic admittance at the beginning and end of the test period were used to calculate percent adaptation for all five elicitor conditions. The greatest adaptation occurred during continuous broad-band-noise stimulation and the least adaptation was observed under the industrial-noise condition. The AR system appears to be resistant to the process of adaptation when activated by stimuli with periodically changing temporal signatures. Results also support previous investigations that demonstrate that the AR is extremely active in certain industrial environments.

Acoustic Stimulation↗
PubMed1987

Local cerebral glucose utilization in fetal sheep exposed to noise.

Rate of local cerebral glucose utilization in fetal sheep was measured in conditions of normal laboratory ambient sound (65 to 70 dB sound pressure levels) and broadband noise (105 and 120 dB sound pressure levels) delivered by two earphones situated on the abdominal surface of the ewe. Broadband noise had neither a visible effect on the ewe nor specific effects on the fetal arterial pH, PaO2, PaCO2, and plasma glucose concentration. However, glucose utilization of many cerebral structures was elevated in noise-exposed fetuses.

Animals↗
PubMed1987

Ear canal volume and variability in the patterns of temporary threshold shifts.

Ear canal resonance is thought to be responsible for the prominent loss of hearing in the 3.0 to 6.0 kHz region caused by over-exposure to noise. Relationships were identified between frequency-of-maximum temporary threshold shift (TTS) and physical measurements of the ear canal in 56 subjects. TTSs were produced by two separate exposures to broadband noise of either 2 hr or 20 minute duration. Ear canal volume was estimated from admittance tympanograms, diameter of the canal opening was measured, and canal length was calculated. Thirty subjects suffered the greatest TTS at 4.0 kHz, while 17 and 9 subjects had the greatest shift at 3.0 and 6.0 kHz, respectively. With subjects grouped according to frequency-of-maximum TTS, mean ear canal volume was found to be significantly different. Subjects with larger volumes were more likely to suffer the greatest loss at 3.0 kHz. Accordingly, subjects with smaller ear canal volumes suffered peak hearing loss at 6.0 kHz. Significant correlations were identified between ear canal volume and calculated length (resonance). Also, modest correlations were noted between frequency-of-maximum TTS and volume as well as TTS and length.

Acoustic Stimulation↗
PubMed1986

Binaural acoustic reflex activity following monaural noise exposure in decerebrate chinchillas.

Acoustic reflex thresholds, latencies and magnitudes were recorded binaurally in decerebrate chinchillas suffering monaural, temporary noise trauma. Simultaneous recordings of cochlear microphonics from both cochleae during monaural reflex stimulation served as an index for acoustic reflex dynamics. Baseline values of threshold, latency and magnitude were recorded from ipsilateral and contralateral ears prior to a monaural 2-hour exposure to a 0.5-kHz octave-band noise at 100 dB SPL. Reflex dynamics were measured immediately after exposure and again following 1 h of recovery. Cochlear microphonic input-output functions for the exposed ear were completed prior to and after cessation of the noise and after 1 h of recovery. Acoustic reflex threshold and magnitude increased and latency decreased following exposure to noise for both the ipsilateral and contralateral recording conditions. Reflex activity in the nonexposed ear was unaffected. Reflex properties returned to pre-exposure values following recovery. These responses followed the same time course as changes in cochlear sensitivity. Alterations in reflex response following noise exposure is caused by changes at purely peripheral sites.

Animals↗
PubMed1983

Acoustic-reflex activity and behavioral thresholds following exposure to noise.

The relationships between properties of the acoustic reflex and temporary threshold shift (TTS) were examined in eight subjects exposed to a 95 dB SPL, 1.0-kHz octave-band noise for 4 h. The specific pattern of TTS obtained from this exposure was consistent with the expected sensitivity changes along the cochlear partition. Maximum decrease in behavioral thresholds was noted at 1.4 kHz with significant TTS also occurring at 2.0 and 4.0 kHz. Behavioral threshold at 0.5 kHz, one octave below the center frequency of exposure, was not affected. Significant reflex threshold shift (RTS) occurred at 1.4 and 2.0 kHz. In addition, RTS was noted at 0.5 kHz in the absence of any change in behavioral sensitivity at that frequency. Magnitude of the acoustic reflex was examined from threshold to 10-dB sensation level. A systematic reduction in magnitude was noted during the noise exposure for 1.4 and 2.0 kHz. The same pattern did not emerge for 0.5 kHz. Magnitude of the acoustic reflex pre-exposure was significantly correlated to TTS at 1.4 kHz. Changes in acoustic reflex thresholds and magnitudes followed the same time course as changes in behavioral thresholds during the growth and recovery periods.

Adult↗
PubMed1979

Reflex threshold shift in chinchillas following a prolonged exposure to noise.

The acoustic reflex is considered to reduce transmission across the middle ear and thereby protect the inner ear from intense sounds. The dynamic properties of this reflex seem to be a function of the duration of the eliciting stimulus. Assessment of the protective action afforded by middle-ear muscle contractions for long-term noise exposures requires the knowledge of how these dynamic properties change under such conditions. Round window electrodes were implanted in eight chinchillas. Changes in the threshold of the acoustic reflex were measured during an eight-hour exposure at 95 dB SPL to an octave-band noise centered at 0.5 kHz. The criterion measure of the acoustic reflex was a change in the amplitude of the cochlear microphonic generated by a 0.5 kHz eliciting tone. Thresholds of the acoustic reflex increased systematically throughout the noise exposure up to approximately 14 dB after 8 hours. The time course of the changes in the threshold of the acoustic reflex was nearly identical to the time course of behaviorally measured changes in the auditory sensitivity as reported by Carder and Miller (1972).

Acoustic Stimulation↗
PubMedDate not supplied

Cochlear microphonics recorded from fetal and newborn sheep.

PURPOSE: Sounds present within the uterus stimulate the fetal inner ear and central auditory pathway. This study was undertaken to determine the efficiency of transmission of exogenous airborne stimuli to the fetal inner ear. In this way, we may quantify the extent to which the fetal auditory system is isolated from sounds produced outside the mother. MATERIALS AND METHODS: Cochlear microphonics were recorded from fetal and newborn sheep to evaluate the extent to which the fetus is isolated from sounds exogenous to the ewe. Electrodes were surgically placed in contact with the round window membrane in nine near-term fetal sheep. Cochlear microphonics were recorded in response to 1/3 octave-band noises (0.125 to 2.0 kHz) delivered through a loudspeaker 1.8 m from one side of the pregnant ewe. Sound pressure levels generated by the noises were simultaneously recorded ex utero with a microphone and in utero with a hydrophone previously sutured to the fetal neck. After cochlear microphonic amplitudes were recorded, the fetus was delivered through an abdominal incision. Recordings were repeated from the newborn lamb. Fetal sound isolation was calculated as the difference between the sound pressure levels that were necessary to evoke equal cochlear microphonic amplitudes from the fetus and from the newborn lamb. RESULTS: The sound attenuation observed was variable for all frequencies. The fetus was isolated from external sounds by 11.1 dB for 0.125 kHz, 19.8 dB for 0.25 kHz, 35.3 dB for 0.5 kHz, 38.2 dB for 1.0 kHz, and 45.0 dB for 2.0 kHz. CONCLUSIONS: Other investigators have demonstrated that the immature auditory system is more susceptible to damage produced by noise exposure than is the mature auditory system. Low-frequency noise produces damaged cells that later in life code higher frequencies. A possibility of fetal hearing loss produced by intense noise exposure needs more careful evaluation.

Animals↗
PubMedDate not supplied

Relationship between outer ear resonance and permanent noise-induced hearing loss.

INTRODUCTION: Prolonged exposure to noise produces permanent hearing loss in the frequency region centered around 4,000 Hz despite differences in the spectral and temporal characteristics of the noise. It is unclear to what extent properties of the auditory system contribute to the development of permanent hearing loss in this frequency region. This study was designed to evaluate the relationship between the frequency of peak outer ear resonance and the frequency of maximum hearing loss in a population of patients with a history of noise exposure. MATERIALS AND METHODS: Analysis of the frequency of maximum hearing loss was completed with sweep frequency Bekesy audiometry with resolution of 100 Hz. Peak outer ear resonant frequency was determined with a resolution of 175 Hz. RESULTS: The mean frequency of maximum hearing loss was 4,481 Hz whereas the mean peak outer ear resonant frequency was 2814 Hz for 43 ears. Pearson product correlation coefficient equaled .64 (P < .0001). CONCLUSIONS: Despite differences in the type and duration of noise exposure reported by the patients, the positive correlation between outer ear resonant frequency and frequency of maximum hearing loss emphasizes the role that external ear properties play in the development of the 4,000 Hz audiometric notch.

Audiometry, Pure-Tone↗