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At least 991 records · Page 55Linked to original sources
PubMed1991

Rebreathing during oxygen treatment with face mask. The effect of oxygen flow rates on ventilation.

The influence of different oxygen flow rates on ventilation and arterial blood gases was investigated in ten healthy volunteers during oxygen treatment with the Hudson mask. Respiratory parameters were calculated using inductive plethysmography calibrated against pneumotachography. The minute ventilation was greater when using the mask with oxygen flow rates less than 5 l/min compared to when no mask was used. With an oxygen flow rate of 3 l/min, minute ventilation was about 140% of minute ventilation without face mask. With 0 l/min, minute ventilation increased to about 160%. The increase in minute ventilation was mainly due to an increase in tidal volume. No change was seen, however, in PaCO2 with different oxygen flow rates. Secondary objective signs following an increase in respiratory work (changes in heart rate, systolic blood pressure and oxygen saturation) were not seen. We recommend 5 l/min as the lowest oxygen flow rate to be used during oxygen therapy with the Hudson mask, in order to avoid rebreathing and excessive respiratory work.

Adult↗
PubMed1994

Use of the laryngeal mask for fibrescope-aided tracheal intubation in an awake patient with a deviated larynx.

I report a case in which fibrescope-aided awake tracheal intubation was achieved using a laryngeal mask, in a patient with a mediastinal goitre in whom tracheal intubation with both a laryngoscope and a fibreoptic bronchoscope had failed. The tumour extended to the upper part of the mediastinum. The larynx and the upper segment of the trachea were displaced by the tumour. Awake tracheal intubation with both a laryngoscope and a fibreoptic bronchoscope failed. The laryngeal mask was then inserted without difficulty. After a fibreoptic bronchoscope had been covered by a plastic tube, the combination was passed through the laryngeal mask into the trachea. The fibreoptic bronchoscope and the laryngeal mask were removed, and a reinforced endotracheal tube was then inserted over the plastic tube into the trachea. The time for tracheal intubation was about 70 s. The laryngeal mask may allow easier location of the laryngeal inlet with a fibreoptic bronchoscope, and this technique is a useful alternative to the conventional technique of tracheal intubation in the patient with a deviated larynx.

Aged↗
PubMed1997

Fibreoptic laryngotracheoscopy via the laryngeal mask airway in children.

BACKGROUND: Fibreoptic laryngotracheoscopy via the laryngeal mask airway-previously reported in adults but not in children-gives a better endoscopic view of the upper airway than does endoscopy via an endotracheal tube. METHOD: The endoscopic procedure was carried out via a size 2 laryngeal mask in 4 spontaneously breathing children between 1 and 6 years old. Anaesthesia was induced and maintained with halothane in 50-100% oxygen. RESULTS: The laryngeal mask was found to enable laryngotracheoscopy with a flexible 5.0-mm fibreoptic endoscope with no technical difficulties. Spontaneous ventilation could be readily preserved throughout the endoscopic procedure. CONCLUSIONS: In children anaesthetized with halothane, flexible fibreoptic laryngotracheoscopy via a laryngeal mask is a useful method-offering technical advantages not achieved otherwise-provided that generally approved restrictions to the use of a laryngeal mask airway are taken into account.

Anesthesia, Inhalation↗
PubMed2004

Surgical masks: operative field contamination following visor-to-visor contact.

BACKGROUND: Clashing of surgical visor masks frequently occurs when two surgeons bend over an operative field simultaneously; however, it is unknown whether this results in contamination. The purpose of the present study was to determine the potential for operative field contamination following surgical visor-mask clashes. The nature of bacterial contamination was also assessed. METHODS: Thirty sham operative procedures were performed under normal operating conditions for a specified time period. The number of surgical visor mask clashes during each procedure was determined by randomization (0, 1, 2, 3, 4 or 6 clashes). All procedures were performed over a standard blood agar plate array. The degree of bacterial contamination was assessed by counting c.f.u. that developed after 24 h of incubation. Bacterial types were also determined. RESULTS: Surgical visor mask clashes resulted in increased contamination of the operative field; however, this was found to be independent of the number of clashes. 95% of pathogens were coagulase negative Staphylococcus species. Other bacteria included Micrococcus species, Bacillus species, Corynebacterium species, various Gram negative bacilli and Staphylococcus aureus (< 1%). CONCLUSION: Surgical visor mask clashes increase the risk of bacterial contamination of the operative field.

Bacteria↗
PubMed1996

Nasal mask positive pressure ventilation in paediatric patients with type II respiratory failure.

We report our experience with nasal mask ventilation in children and adolescents with type II respiratory failure admitted to the paediatric intensive care unit (PICU) over an 18-month period. Seven patients were treated with nasal mask ventilation during part of their PICU stay. All showed significant improvement in arterial pH, PaCO2, and PaO2/FiO2 from presentation to discharge, although at discharge PaCO2 and PaO2/FiO2 fell outside of the normal range. Complications occurred in four patients. When compared to 11 patients with type II respiratory failure not treated with nasal mask ventilation, the nasal mask ventilation group had a similar PICU length of stay and incidence of complications. We conclude that nasal mask ventilation may be useful in maintaining near normal alveolar ventilation in selected children with type II respiratory failure and that a prospective study of this technique is indicated.

Adolescent↗
PubMed1997

Blood splashes to the masks and goggles during caesarean section.

A prospective study was performed of 100 caesarean sections (31 elective, 69 emergency) to determine the incidence of blood splashes to the masks and goggles of surgeons, assistants and scrub nurses. The rate of blood splashes was 40% on surgeons' masks, 49.29% on surgeons' goggles, 31% on assistants' masks, 21.12% on assistants' goggles, 23.1% on scrub nurses' masks and 15.71% on scrub nurses' goggles. The use of masks and protective goggles is strongly recommended to avoid body fluid contamination and acquiring disease in obstetrics.

Blood↗
PubMed2001

A masking level difference due to harmonicity.

The role of harmonicity in masking was studied by comparing the effect of harmonic and inharmonic maskers on the masked thresholds of noise probes using a three-alternative, forced-choice method. Harmonic maskers were created by selecting sets of partials from a harmonic series with an 88-Hz fundamental and 45 consecutive partials. Inharmonic maskers differed in that the partial frequencies were perturbed to nearby values that were not integer multiples of the fundamental frequency. Average simultaneous-masked thresholds were as much as 10 dB lower with the harmonic masker than with the inharmonic masker, and this difference was unaffected by masker level. It was reduced or eliminated when the harmonic partials were separated by more than 176 Hz, suggesting that the effect is related to the extent to which the harmonics are resolved by auditory filters. The threshold difference was not observed in a forward-masking experiment. Finally, an across-channel mechanism was implicated when the threshold difference was found between a harmonic masker flanked by harmonic bands and a harmonic masker flanked by inharmonic bands. A model developed to explain the observed difference recognizes that an auditory filter output envelope is modulated when the filter passes two or more sinusoids, and that the modulation rate depends on the differences among the input frequencies. For a harmonic masker, the frequency differences of adjacent partials are identical, and all auditory filters have the same dominant modulation rate. For an inharmonic masker, however, the frequency differences are not constant and the envelope modulation rate varies across filters. The model proposes that a lower variability facilitates detection of a probe-induced change in the variability, thus accounting for the masked threshold difference. The model was supported by significantly improved predictions of observed thresholds when the predictor variables included envelope modulation rate variance measured using simulated auditory filters.

Adult↗
PubMed2001

Informational and energetic masking effects in the perception of multiple simultaneous talkers.

Although many researchers have examined the role that binaural cues play in the perception of spatially separated speech signals, relatively little is known about the cues that listeners use to segregate competing speech messages in a monaural or diotic stimulus. This series of experiments examined how variations in the relative levels and voice characteristics of the target and masking talkers influence a listener's ability to extract information from a target phrase in a 3-talker or 4-talker diotic stimulus. Performance in this speech perception task decreased systematically when the level of the target talker was reduced relative to the masking talkers. Performance also generally decreased when the target and masking talkers had similar voice characteristics: the target phrase was most intelligible when the target and masking phrases were spoken by different-sex talkers, and least intelligible when the target and masking phrases were spoken by the same talker. However, when the target-to-masker ratio was less than 3 dB, overall performance was usually lower with one different-sex masker than with all same-sex maskers. In most of the conditions tested, the listeners performed better when they were exposed to the characteristics of the target voice prior to the presentation of the stimulus. The results of these experiments demonstrate how monaural factors may play an important role in the segregation of speech signals in multitalker environments.

Adult↗
PubMed2002

The effect of spatial separation on informational and energetic masking of speech.

The effect of spatial separation of sources on the masking of a speech signal was investigated for three types of maskers, ranging from energetic to informational. Normal-hearing listeners performed a closed-set speech identification task in the presence of a masker at various signal-to-noise ratios. Stimuli were presented in a quiet sound field. The signal was played from 0 degrees azimuth and a masker was played either from the same location or from 90 degrees to the right. Signals and maskers were derived from sentences that were preprocessed by a modified cochlear-implant simulation program that filtered each sentence into 15 frequency bands, extracted the envelopes from each band, and used these envelopes to modulate pure tones at the center frequencies of the bands. In each trial, the signal was generated by summing together eight randomly selected frequency bands from the preprocessed signal sentence. Three maskers were derived from the preprocessed masker sentences: (1) different-band sentence, which was generated by summing together six randomly selected frequency bands out of the seven bands not present in the signal (resulting in primarily informational masking); (2) different-band noise, which was generated by convolving the different-band sentence with Gaussian noise; and (3) same-band noise, which was generated by summing the same eight bands from the preprocessed masker sentence that were used in the signal sentence and convolving the result with Gaussian noise (resulting in primarily energetic masking). Results revealed that in the different-band sentence masker, the effect of spatial separation averaged 18 dB (at 51% correct), while in the different-band and same-band noise maskers the effect was less than 10 dB. These results suggest that, in these conditions, the advantage due to spatial separation of sources is greater for informational masking than for energetic masking.

Audiometry, Pure-Tone↗
PubMed2003

Forward masking in different cochlear implant systems.

The goal of this study was to evaluate, from a psychophysical standpoint, the neural spread of excitation produced by the stimulation of different types of intracochlear electrode arrays: the Ineraid, the Clarion S-Series on its own or with the Electrode Positioning System (EPS), and the Clarion HiFocus-I with the EPS. The EPS is an independent silicone part designed to bring the electrode array close to the modiolus. Forward masking was evaluated in 12 adult subjects (3 Ineraid, 4 Clarion S-Series, 3 Clarion S-Series+EPS, 3 HiFocus-I+EPS) by psychophysical experiments conducted using trains of biphasic stimuli (813 pulses per second, 307.6 micros/phase). Masker signals (+8 dB re: threshold, 300 ms) were applied to the most apical electrode. Probe signals (30 ms, 10-ms postmasker) were delivered to more basal electrodes. Masked and unmasked detection thresholds of probe signals were measured. For both Clarion HiFocus-I subjects, measurements were conducted in both monopolar and bipolar stimulus configurations. No major differences were found in forward masking between the different intracochlear electrode arrays tested in the monopolar configuration at suprathreshold levels equivalent to those used in speech-coding strategies, but significant differences were found between subjects. A significant negative correlation also was found between the level of forward masking and the consonant identification performance. These measurements showed that the neural spread of excitation was more restricted in the bipolar configuration than in the monopolar configuration for HiFocus-I subjects. It was found that CIS strategies implemented without using apical electrodes, which showed high levels of masking, could improve consonant identification.

Adult↗
PubMed2004

The effect of overlap-masking on binaural reverberant word intelligibility.

Reverberation interferes with the ability to understand speech in rooms. Overlap-masking explains this degradation by assuming reverberant phonemes endure in time and mask subsequent reverberant phonemes. Most listeners benefit from binaural listening when reverberation exists, indicating that the listener's binaural system processes the two channels to reduce the reverberation. This paper investigates the hypothesis that the binaural word intelligibility advantage found in reverberation is a result of binaural overlap-masking release with the reverberation acting as masking noise. The tests utilize phonetically balanced word lists (ANSI-S3.2 1989), that are presented diotically and binaurally with recorded reverberation and reverberation-like noise. A small room, 62 m3, reverberates the words. These are recorded using two microphones without additional noise sources. The reverberation-like noise is a modified form of these recordings and has a similar spectral content. It does not contain binaural localization cues due to a phase randomization procedure. Listening to the reverberant words binaurally improves the intelligibility by 6.0% over diotic listening. The binaural intelligibility advantage for reverberation-like noise is only 2.6%. This indicates that binaural overlap-masking release is insufficient to explain the entire binaural word intelligibility advantage in reverberation.

Acoustics↗
PubMed2005

Speech intelligibility and spatial release from masking in young children.

Children between the ages of 4 and 7 and adults were tested in free field on speech intelligibility using a four-alternative forced choice paradigm with spondees. Target speech was presented from front (0 degrees); speech or modulated speech-shaped-noise competitors were either in front or on the right (90 degrees). Speech reception thresholds were measured adaptively using a three-down/one-up algorithm. The primary difference between children and adults was seen in elevated thresholds in children in quiet and in all masked conditions. For both age groups, masking was greater with the speech-noise versus speech competitor and with two versus one competitor(s). Masking was also greater when the competitors were located in front compared with the right. The amount of masking did not differ across the two age groups. Spatial release from masking was similar in the two age groups, except for in the one-speech condition, when it was greater in children than adults. These findings suggest that, similar to adults, young children are able to utilize spatial and/or head shadow cues to segregate sounds in noisy environments. The potential utility of the measures used here for studying hearing-impaired children is also discussed.

Adult↗
PubMed2005

Word recognition in noise at higher-than-normal levels: decreases in scores and increases in masking.

Under certain conditions, speech recognition in noise decreases above conversational levels when signal-to-noise ratio is held constant. The current study was undertaken to determine if nonlinear growth of masking and the subsequent reduction in "effective" signal-to-noise ratio accounts for this decline. Nine young adults with normal hearing listened to monosyllabic words at three levels in each of three levels of a masker shaped to match the speech spectrum. An additional low-level noise equated audibility by producing equivalent masked thresholds for all subjects. If word recognition was determined entirely by signal-to-noise ratio and was independent of overall speech and masker levels, scores at a given signal-to-noise ratio should remain constant with increasing level. Masked pure-tone thresholds measured in the speech-shaped maskers increased linearly with increasing masker level at lower frequencies but nonlinearly at higher frequencies, consistent with nonlinear growth of upward spread of masking that followed the peaks in the spectrum of the speech-shaped masker. Word recognition declined significantly with increasing level when signal-to-noise ratio was held constant which was attributed to nonlinear growth of masking and reduced "effective" signal-to-noise ratio at high speech-shaped masker levels, as indicated by audibility estimates based on the Articulation Index.

Adult↗
PubMed2005

Informational masking of speech in children: effects of ipsilateral and contralateral distracters.

Using a closed-set speech recognition paradigm thought to be heavily influenced by informational masking, auditory selective attention was measured in 38 children (ages 4-16 years) and 8 adults (ages 20-30 years). The task required attention to a monaural target speech message that was presented with a time-synchronized distracter message in the same ear. In some conditions a second distracter message or a speech-shaped noise was presented to the other ear. Compared to adults, children required higher target/distracter ratios to reach comparable performance levels, reflecting more informational masking in these listeners. Informational masking in most conditions was confirmed by the fact that a large proportion of the errors made by the listeners were contained in the distracter message(s). There was a monotonic age effect, such that even the children in the oldest age group (13.6-16 years) demonstrated poorer performance than adults. For both children and adults, presentation of an additional distracter in the contralateral ear significantly reduced performance, even when the distracter messages were produced by a talker of different sex than the target talker. The results are consistent with earlier reports from pure-tone masking studies that informational masking effects are much larger in children than in adults.

Acoustic Stimulation↗
PubMed2006

Effect of electrode configuration on psychophysical forward masking in cochlear implant listeners.

Bipolar stimulation has been thought to be more beneficial than monopolar stimulation for speech coding in cochlear implants, on the basis of its more restricted current flow. The present study examined whether bipolar stimulation would indeed lead to reduced channel interaction in a behavioral forward masking experiment tested in four Nucleus 24 users. The masker was fixed on one channel and three masker levels that were balanced for loudness between the configurations were chosen. As expected, masking was maximal when the masker and probe channels were spatially close and decreased as they were separated. However, overall masking patterns did not consistently demonstrate sharper tuning with bipolar stimulation than monopolar. This implies that the spatial extent of a bipolar current field is not consistently narrower than that of an equally loud monopolar stimulus; therefore, it should not be assumed that bipolar stimulation leads to reduced channel interaction. Notably, bipolar masking patterns appeared to display more variations across channels, possibly influenced more by anatomical and neural irregularities near electrode contacts than monopolar masking patterns. The present psychophysical results provide a theoretical basis regarding the widespread use (and success) of monopolar configurations by implant users.

Adult↗
PubMed1979

Masking of filtered noise bursts by synthetic vowels.

The present investigation assessed the simultaneous and temporal masking produced by computer-generated synthetic vowels. The durations (100 and 200 ms) of each of four vowel-like maskers were employed. The masker was presented at 70 dB SPL. The probe signals were three filtered noise bursts whose spectral distributions corresponded to regions of high spectral energy in three English stop consonants. Quiet and masked thresholds were determined using the method of adjustment. Data are reported for two experienced listeners who participated in all the listening conditions. The results were generally in accord with the results of masking experiments using nonspeech signals in that both the frequency specificity of masking and temporal masking effects were demonstrated.

Computers↗
PubMed1979

Forward masking with equal-energy maskers.

Temporal masking of chicks by noise was investigated using a forward-masking paradigm. The temporal separation delta T between the click and the noise ranged from 0.03 to 100 msec. The duration of the noise varied from 3 to 500 msec while its energy remained fixed. For fixed delta T (delta T greater than 3 msec), the masking effect may actually increase for the longer, less intense noises despite the fact that, for long maskers, there is less masker energy near the signal in time. These results are interpreted in terms of the modified version of the running-average hypothesis [M. J. Penner, J. Acoust. Soc. Am. 63, 195--201 (1978)] in which it is argued that the auditory system compresses the intensity of the stimulus prior to integrating it. If the temporal integrator depends on stimulus intensity, then these results may be easily predicted. As an alternative explanation we show that compression may reduce the effective intensity of short maskers to such an extent that they do less masking than the longer ones. Such reduction in masking effectiveness will occur if the time between the masker and the signal is long enough so that the effects of compression and integrator shape do not counterbalance each other.

Adult↗
PubMed1979

Psychophysical frequency resolution in the cat as determined by simultaneous masking and its relation to auditory-nerve resolution.

Critical bandwidths were measured behaviorally at 1 and 2 kHz by simultaneous masking in four cats. Three methods were used. They were (i) the masking of a tone by noise of variable bandwidth, (ii) the masking of a narrow-band signal by two tones, and (iii) the masking of a tone by noise of rippled spectrum. The three methods agreed closely and gave a mean critical bandwidth of 410 Hz at 1 kHz and 690 Hz at 2 kHz. These values were about three times as great as the electrophysiologically-determined effective bandwidths of single fibers of the auditory nerve at the same frequencies, both as measured in other animals, and in one of the animals that had been tested behaviorally as well. Psychophysical tuning curves were also determined behaviorally; in contrast, they agreed closely with auditory-nerve fibers in both bandwidth and slope. The results suggest that the critical band as measured by simultaneous masking is not a close relation of the frequency-threshold curve of auditory-nerve fibers, but that the psychophysical tuning curve possibly may be. Possible reasons and implications are discussed.

Animals↗