Time-frequency dynamics in neurally mediated syncope.

In this study, the responses during syncope were determined by noninvasive beat-to-beat analysis during passive orthostasis. Twenty patients with recurrent unexplained syncope (13 men and seven women) and ten healthy aged-matched control subjects were studied during 80 degrees head-up tilt for 25 min. Time-frequency mapping of R-R intervals, systolic and diastolic pressures and respiration was used to determine the responses to tilt. The spectral estimation was based on a modified Wigner distribution and the frequency content was evaluated on a beat-to-beat basis. Ten patients developed syncope (tilt-positive group) during tilt, while the remaining ten were asymptomatic (tilt-negative group). Control subjects reacted to tilt by the immediate shortening of R-R intervals to a plateau with an accompanying moderate increase in diastolic pressure. In the tilt-negative group the responses to tilt were similar, but of greater amplitude. In contrast, in the tilt-positive group, R-R intervals gradually and continuously decreased with tilt while systolic and diastolic pressures increased until shortly before syncope, when an abrupt fall in blood pressure followed by R-R intervals lengthening occurred. Furthermore, the R-R intervals fluctuations at both respiratory and nonrespiratory frequencies were the highest at rest as well as during tilt in the tilt-positive group. Nonrespiratory fluctuations in blood pressure increased more during tilt in both tilt-positive and negative groups compared to the control group. The nonrespiratory fluctuations in R-R intervals and blood pressure reached a maximum at syncope, simultaneously with hypotension and bradycardia. Time-frequency mapping has demonstrated that an elevated parasympathetic tone at rest which persists during orthostasis identifies patients prone to vasodepressor syncope. The counteracting sympathetic activation is not sustained and results in hypotension followed by cardioinhibition and loss of consciousness.

Periodic amplitude modulation of EEG.

Time variation of the EEG spectral parameters was analyzed during a 10 min resting period in 40 healthy subjects. Spectral band powers over the theta and alpha bands were calculated for each non-overlapping 2.5 s long EEG segment. The time variation of the band powers was further analyzed by computing the power spectra. The results showed that both theta and alpha band powers oscillate at an average frequency 0.024 Hz and 0.057 Hz. This indicates, that the background EEG activity is modulated by periodical slow components. We hypothesize that this modulation reflects spontaneous periodic changes of cortical excitability with control at the brainstem level.

Slow modulation of EEG.

Time variation of the successive 2.5 s long EEG theta and alpha band powers during 5 min rest and 4 min hyperventilation (HV) was analyzed by modified Wigner distribution (WD) in 22 subjects. Two main peaks at frequency 0.02 Hz and 0.068 Hz were detected in the WDs of both alpha and theta powers at rest. The HV increased the magnitude of the faster (0.068 Hz) component of both theta and alpha WD's while the slower (0.02 Hz) component was increased only in the theta WD. We hypothesize that this slow brain activity reflects spontaneous periodic changes of cortical excitability with control at the brain stem level.

Positive pressure on neck reduces baroreflex response to apnoea.

This study was designed to evaluate the arterial blood pressure and heart rate responses to positive pressure applied to the neck during repetitive inspiratory apnoea. Twenty-five subjects (aged 20-40 years) were trained to exert a positive pressure on the neck by actively contracting the neck muscles and pressing the chin in the jugular notch. Blood pressure and heart rate were evaluated during 5-min long periods at rest, at the beginning and end of a 25-min period of apnoea with and without positive pressure and after a second period of rest. Positive pressure diminished the initial hypotensive and bradycardiac reactions to apnoea and augmented the heart rate and blood pressure increase towards the end of apnoea. Both systolic and diastolic pressures and heart rate were significantly elevated during both apnoeic sequences, and also remained significantly elevated after the release of pressure. Spectral analysis (FFT) and auto-regressive model showed the entrainment of the slow 0.03 Hz oscillations by repetitive apnoea and the occurrence of 0.1 Hz and respiratory 0.2 Hz components in the heart rate and blood pressure in both types of apnoea. It is suggested, since the positive pressure decreases the baroreflex and the increased sympathetic tone persists after apnoea, that such effects may contribute to the development of cardiac complications in prediposed individuals with obstructive apnoea syndrome.

Increase of slow periodic modulation of EEG in a patient with Alzheimer's disease.

The recently described slow oscillations of amplitude of theta and alpha waves of the EEG (with a frequency below 0.08 Hz) in healthy subjects are attributed to the autonomic nervous system with control at the brain stem level. In the present pilot study, the slow brain rhythms were analyzed in a patient with Alzheimer's disease and were compared to a healthy subject. Dynamic analysis of the EEG was performed using time-frequency mapping which gives simultaneous time and frequency representation of the brain signal. This method comprises a transform of the filtered EEG signal into its analytic form and application of the Wigner distribution modified by time and frequency smoothing. It has been shown that the envelope of both theta and alpha activities oscillates at 0.04 Hz and 0.07 Hz in the healthy subject and at 0.03 Hz and 0.06 Hz in a patient with Alzheimer's disease. The amplitude of the slow oscillations of theta activity was substantially higher in the patient with Alzheimer's disease as compared with the healthy subject. It is being proposed that the increase of slow brain rhythms in the patient with Alzheimer's disease reflects an abnormal activity of the autonomic nervous system. However, the underlying pathophysiological mechanisms need to be further studied.

Hathayogic exercise jalandharabandha in its effect on cardiovascular response to apnoea.

Jalandharabandha (JB) is the important constituent of apnoea (kumbhaka) in hathayogic breathing exercises. It is performed by pressing the chin into the jugular notch and creating thus the positive pressure on the neck region. The influence of JB on the heart rate and vasomotor response was studied in relationship to different lung volumes. The course of R-R intervals is highly significantly different according to the type of apnoea. JB leads to the diminution of bradycardia, but does not change the position of the maximum and minimum in comparison to the apnoea without JB. Application of JB increases the number of vasodilatations and shortens the latencies of vasodilatations, duration and amplitude of reactions. JB during breath holding decreases the vagal reflex changes and may thus work as a stabilizing component in yogic breathing exercises.