Hilmi R. Dajani

Continuous positive airway pressure improves nocturnal baroreflex sensitivity of patients with heart failure and obstructive sleep apnea.

Objectives To determine the acute effects of continuous positive airway pressure (CPAP) on baroreceptor reflex sensitivity (BRS) for heart rate during sleep in congestive heart failure (CHF) patients with obstructive sleep apnea (OSA). Design and methods In eight CHF patients with OSA not previously treated with CPAP, spontaneous BRS was assessed during overnight polysomnography prior to the onset of sleep, and during stage 2 non-rapid eye movement sleep (NREM) before, during and after application of CPAP. Results CPAP alleviated OSA and acutely increased the slope of BRS (median, 25%,75%) [from 3.9 (3.5, 4.8) to 6.2 (4.6, 26.2) ms/mmHg, P < 0.05]. Increases in the slope of BRS persisted following withdrawal of CPAP [4.9 (4.3, 6.9) ms/mmHg, P < 0.05]. CPAP also lowered heart rate (from 81.3 ± 4.9 to 76.0 ± 5.7 bpm, P < 0.05), an effect which persisted after its withdrawal (76.7 ± 5.7 bpm, P < 0.05). Systolic blood pressure at the midpoint of the pressure range of BRS sequences fell while on CPAP (from 139 ± 8 to 120 ± 7 mmHg, P < 0.05), and remained lower following CPAP withdrawal (124 ± 9 mmHg, P < 0.05). Conclusions In CHF patients with OSA, CPAP increases acutely BRS during sleep, lowers heart rate and resets the operating point for BRS to a lower blood pressure. These effects of CPAP persist after its withdrawal, suggesting that nocturnal CPAP therapy may cause sustained improvement in the neural control of heart rate.

Sympathetic Alternans Evidence for Arterial Baroreflex Control of Muscle Sympathetic Nerve Activity in Congestive Heart Failure

Alternation in the amplitude of muscle sympathetic nerve activity (MSNA) was documented in three patients with severe heart failure. In the index patient with pulsus alternans, the amplitude of MSNA was inversely related to changes in the preceding diastolic pressure with a lag time of 1.2 to 1.3 seconds, indicating that oscillations in burst amplitude are determined primarily by changes in this component of blood pressure. Spectral analysis of the blood pressure and MSNA signals identified two spectral peaks, one at the cardiac frequency and a second peak, with greater spectral power, at the alternans frequency (ie, at half the heart rate). The latter peak for both blood pressure and MSNA disappeared when alternans was abolished by nitroglycerin. The presence of sympathetic alternans in synchrony with pulsus alternans and the rapid transduction of changes in the diastolic blood pressure afferent signal to the amplitude of sympathetic outflow indicate that the arterial baroreflex control of MSNA must be active and rapidly responsive in human heart failure.

Recording human evoked potentials that follow the pitch contour of a natural vowel

We investigated whether pitch-synchronous neural activity could be recorded in humans, with a natural vowel and a vowel in which the fundamental frequency was suppressed. Small variations of speech periodicity were detected in the evoked responses using a fine structure spectrograph (FSS). A significant response (P/spl Lt/0.001) was measured in all seven normal subjects even when the fundamental frequency was suppressed, and it very accurately tracked the acoustic pitch contour (normalized mean absolute error <0.57%). Small variations in speech periodicity, which humans can detect, are therefore available to the perceptual system as pitch-synchronous neural firing. These findings suggest that the measurement of pitch-evoked responses may be a viable tool for objective speech audiometry.

Dynamics of real time DPOAE contralateral suppression in chinchillas and humans Dinámica de la supresión contralateral de las DPOAE en tiempo real en chinchillas y humanos

The dynamics of contralateral acoustic suppression were studied using real time (millisecond resolution) distortion product otoacoustic emissions (DPOAEs) in chinchillas and humans. Latency of DPOAE suppression onset is 26u2005ms in chinchillas and 45u2005ms in humans. After onset, suppression builds over time before tending to plateau, reflecting a temporal integration process with a time constant of 100u2005ms (chinchillas). In chinchillas, suppression persists for 40u2005ms even when elicited by stimuli as short as 5u2005ms. With stimuli >40u2005ms, offset and onset latencies are similar and duration of suppression equals that of the contralateral stimulus. A comparison of DPOAE suppression onset latency with neural latency data from the pathways involved suggests the following timing scheme: stimulus onset to activity in (ventral) cochlear nucleus, 4u2005ms (15% of delay); transfer to olivocochlear efferents, 9u2005ms (35%); efferent conduction to presynaptic OHC site, 4u2005ms (15%); synaptic and mechanical events at OHCs, 9u2005ms (35% of delay).

Neural and Hypotensive Effects of Angiotensin II Receptor Blockade

Angiotensin II participates in the neural regulation of the heart and circulation at both central and peripheral sites. To explore the role of endogenous angiotensin II in blood pressure regulation, we conducted a randomized double-blind crossover trial in nine young healthy men (aged 33+/-3 [mean+/-SE] years) studied in the absence of salt restriction, comparing the effect of 1 week treatment with the angiotensin II receptor antagonist losartan (100 mg daily) against placebo with respect to the following variables, recorded during supine rest: intra-arterial blood pressure (BP), heart rate (HR), forearm vascular resistance and norepinephrine appearance rate, total body norepinephrine spillover, variability of BP and HR (spectral analysis), and baroreflex sensitivity for HR (gain of the transfer function from systolic BP to HR). Blood pressure was 119+/-7/66+/-4 mm Hg (systolic BP/diastolic BP) after 1 week of placebo and 112+/-6/61+/-3 mm Hg after 1 week of losartan (P<.05). Forearm vascular resistance tended to fall, from 42.3+/-6.9 U on placebo to 32.8+/-5.0 U with losartan treatment (P=.07). Losartan had no effect on HR (60+/-3 on placebo versus 59+/-2 beats per minute with losartan), total body norepinephrine spillover (3.0+/-0.8 versus 3.3+/-1.2 nmol/min), forearm norepinephrine appearance rate (3.8+/-1.1 versus 5.3+/-1.1 pmol/100 mL forearm tissue per minute), power in the high- or low-frequency components of the HR variability and BP variability spectra or on baroreflex sensitivity for HR. Endogenous angiotensin II contributes to the maintenance of supine BP in normal subjects, studied in the absence of sodium restriction. The fall in BP caused by losartan is accompanied by a resetting of the baroreflex regulation of HR and sympathetic outflow, but baroreflex sensitivity for heart rate is not altered. Therefore, the reduction in BP observed after short-term angiotensin type 1 receptor antagonism may be achieved through a direct effect on vascular tone rather than through a primary reduction in sympathetic outflow.

Human auditory steady-state responses to changes in interaural correlation

Steady-state responses were evoked by noise stimuli that alternated between two levels of interaural correlation rho at a frequency fm. With rho alternating between +1 and 0, responses at fm dropped steeply above 4 Hz, but persisted up to 64 Hz. Two time constants of 47 and 4.4 ms with delays of 198 and 36 ms, respectively, were obtained by fitting responses to a transfer function based on symmetric exponential windows. The longer time constant, possibly reflecting cortical integration, is consistent with perceptual binaural sluggishness. The shorter time constant may reflect running cross-correlation in the high brainstem or primary auditory cortex. Responses at 2fm peaked with an amplitude of 848+/-479 nV (fm=4 Hz). Investigation of this robust response revealed that: (1) changes in rho and lateralization evoked similar responses, suggesting a common neural origin, (2) response was most dependent on stimulus frequencies below 1000 Hz, but frequencies up to 4000 Hz also contributed, and (3) when rho alternated between [0.2-1] and 0, response amplitude varied linearly with rho, and the physiological response threshold was close to the average behavioral threshold (rho=0.31). This steady-state response may prove useful in the objective investigation of binaural hearing.

Real-time method for the measurement of noise exposure from communication headsets

Abstract A method for measuring noise exposure from communication headsets is presented. It is based on an accurate acousto-mechanical model of the human head, and a filter to transform the data from the eardrum level to equivalent diffuse free-field values. The filter fits an existing integrating sound-level meter (Bruel and Kjaer type 2231), allowing measurements in real-time. The measurements were validated by comparing them with those obtained with a sound-level meter in the free field and with a miniature microphone. The method is accurate, objective, relatively simple, and works with any kind of communication headset. Measurements of noise exposure from communication headsets were taken at eight sites. The noise exposure level Lex,8(the equivalent sound level Leq normalized to 8 h) varied from 64 to 81 dBA in quiet office settings and from 77 to 88 dBA in noisy environments, with one measurement at 95 dBA.

Fine structure spectrography and its application in speech

A filterbank-based algorithm for time-varying spectral analysis is proposed. The algorithm, which is an enhanced realization of the conventional spectrogram, consists of hundreds or thousands of highly overlapping wideband filter/detector stages, followed by a peak detector that probes the filter/detector outputs at very short time intervals. Analysis with synthetic modulated signals illustrates how the proposed method demodulates these signals. The resulting spectrogram-like display, referred to as a fine structure spectrogram, shows the fine structure of the modulations in substantially higher detail than is possible with conventional spectrograms. Error evaluation is performed as a function of various parameters of a single- and two-component synthetic modulated signal, and of parameters of the analysis system. In speech, the fine structure spectrogram can detect small frequency and amplitude modulations in the formants. It also appears to identify additional significant time-frequency components in speech that are not detected by other methods, making it potentially useful in speech processing applications.

Binaural hearing in the presence of a low frequency magnetic field

The authors propose the binaural auditory system as a candidate neural system that may be disrupted by exposure to relatively weak LF magnetic fields. Extracellular currents, induced by time-varying magnetic fields, may change the timing of action potentials in the auditory nerve, thereby disrupting sound localization when interaural time differences are very small. Three subjects were exposed to a 1,000 Hz magnetic field - with a maximum rate of change of 2.3 T/s at the location of the cochlea - while presenting two identical 1,000 Hz tones randomly delayed to the left or right ear by less than 10 /spl mu/s. The subjects were asked whether the signal was perceived to be displaced to the left or right side of midline. After a total of over 20,000 trials, conducted at different phase angles between the field signal and the tones, there was no clear evidence for a consistent change in performance when the magnetic field was present. This, however, does not rule out an effect at other combinations of magnetic and acoustic frequencies.<<ETX>>

An audition‐inspired method for producing high resolution spectrograms

Inspired by the ability of the auditory system to dynamically track frequency and amplitude changes in modulated signals, an algorithm for high resolution time‐varying spectral analysis is proposed. The main feature of the algorithm is an auditory‐inspired filterbank followed by a peak detector. Mathematical analysis, with various types of synthetic modulated signals, demonstrates that the proposed method correctly demodulates these signals. The resulting time‐frequency display, referred to as a fine structure spectrogram, shows the frequency and amplitude modulations in higher detail than is possible with conventional spectrograms. With recorded consonant–vowel syllables, the fine structure spectrogram detects small frequency and amplitude modulations in the initial glides and in the steady vowel formants. These modulations are not clearly seen in conventional time‐frequency representations.