Stephen R. Quint

Heart period variability in sleep

Analysis of heart period variability is a dynamic noninvasive technique to quantify the autonomic control over the heart period. We recorded electroencephalographic, electro-oculographic, electromyographic and electrocardiographic data from 10 normal subjects during sleep using an ambulatory polysomnographic monitor. R-R intervals were determined for 10 min segments of electrocardiographic data from wakefulness, stage 2 sleep, slow wave sleep and REM sleep. Average heart period, instantaneous changes greater than 50 msec and fractal dimension were calculated and the time domain and phase plots were depicted. The R-R interval time domain plots were subsequently analyzed using the discrete Fourier transform. We found sleep stage specific, time domain and frequency domain changes in heart period variability, particularly using spectral analysis of heart period. Increased power in the 0.2-0.4 Hz band was associated with stage 2 sleep when compared to awake and slow wave sleep states. Power in the 0.0-0.04 and 0.04-0.12 Hz bands was increased in association with REM sleep when compared to non-REM sleep, and slow wave sleep had diminished power in all frequency bands. Our results support other investigations demonstrating stage 2 sleep is associated with increased parasympathetic influences and REM sleep is associated with increased sympathetic and neurohumoral influences. We feel that spectral analysis of heart period variability is an effective noninvasive method to quantify changes in the autonomic influences over the heart during sleep.

Heart period variability during vagal nerve stimulation

Vagal nerve stimulation is an emerging therapy for epilepsy, yet little is known regarding the effects of this stimulation on heart period variability. We selected 10 patients (two female, eight male) who were receiving high-frequency, high-intensity left vagal nerve stimulation for intractable epilepsy. Electrocardiogram data were recorded for a 7 min baseline, 2.5 min of stimulation and a 7 min post-stimulation period. We found no significant changes in average heart period, instantaneous changes of successive R-to-R intervals greater than 50 ms or fractal dimension. We also found no significant changes in the total power in the 0.0-0.04 Hz, 0.04-0.12 Hz and 0.2-0.4 Hz bands with stimulation of the left vagus nerve. This study suggests that left vagal nerve stimulation has little acute effect on the cardiac rhythm or heart period variability.

An Adaptive Framework for Multiprocessor Real-Time System

In this paper, we develop an adaptive scheduling framework for changing the processor shares of tasks - a process called reweighting - on real-time multiprocessor platforms. Our particular focus is adaptive frameworks that are deployed in environments in which tasks may frequently require significant share changes. Prior work on enabling real-time adaptivity on multiprocessors has focused exclusively on scheduling algorithms that can enact needed adaptations. The algorithm proposed in this paper uses both feedback and optimization techniques to determine at runtime which adaptations are needed.

Heart rate variability during interictal epileptiform discharges

RATIONALE Seizures may produce a variety of autonomic alterations. These alterations may occur due to evoked autonomic reflexes or as a direct cortical effect on autonomic control. In animal studies, lock step phenomena of interictal discharges to autonomic output have been repeatedly documented. However, the association of interictal discharges and autonomic output is not as well established in humans. METHODS RR intervals timely locked to interictal epileptiform discharge (RR(n)) were compared to RR intervals immediately following (RR(n+1)) interictal discharges in 40 patients with focal onset epilepsy and low baseline heart beat variability. RESULTS In 20 patients with 200 left sided interictal epileptiform discharges, RR(n) shortened in 100 and prolonged in 31 when compared to RR(n+1) intervals. While in 20 patients with 200 right sided interictal epileptiform discharges RR(n) intervals shortened in 17 and prolonged in 116 (Chi square P<0.001). No consistent differences in RR(n) intervals variability between frontal versus temporal localization of the interictal discharges from the same side was found. CONCLUSIONS Interictal discharges, may influence autonomic control over the cardiac cycle and agree with animal studies. Further study of the relationship of interictal discharges to autonomic output is needed to delineate the potential lateralized influences over autonomic nervous system.

Monitoring heart period variability changes during seizures. II. Diversity and trends

Seizures may prompt autonomically mediated changes in heart period. Heart period variability (HPV) is a sensitive tool for quantification of autonomic control over the cardiac cycle. We used analysis of HPV to evaluate changes in the time and frequency domain autonomic regulation of heart period during seizure activity in 44 seizures from 12 patients with epilepsy: 16% of seizures were associated with changes in heart rate and variability preceding behavioral and surface electroencephalographic (EEG) onset. Patients with consecutive partial seizures had variable HPV changes. Seizures of left-sided EEG onset were associated with no change or a decrease in the power of the 0.2- to 0.4-Hz band, whereas seizures of right-sided EEG onset were associated with no change or a decrease in the power of the 0.04- to 0.12-Hz band. We believe that time- and frequency-domain analysis of HPV may provide valuable information about the temporal and spatial onset of seizure activity. Additional study using invasive EEG recordings will be needed to evaluate the reliability of HPV to lateralize or localize seizure foci.

Myocardial electrical impedance as a predictor of the quality of RF-induced linear lesions

Production of complete (i.e. continuous and transmural) cardiac lesions by radiofrequency (RF) ablation can cure certain cardiac arrhythmias. However, a predictor of lesion completeness that is reliable and can be measured intraoperatively is needed in order to maximize effectiveness of ablation therapy. Predictors that require membrane excitation or response to stimulation are not always practical. This study tested whether changes of myocardial impedance across the lesion can predict completeness. RF energy was applied epicardially on perfused rabbit ventricles to produce linear lesions that were complete (n = 25) or incomplete (noncontinuous or nontransmural, n = 25). Before and after creation of each lesion, the magnitude and phase of impedance at 1 kHz were measured with a four-electrode epicardial array across the lesion. For 16 of the lesions, the translesion stimulus-excitation delay was also measured. Lesion completeness was evaluated with 2,3,5-triphenyltetrazolium chloride stain. Complete lesions increased resistivity by 26 Omega cm (21% of the preablation value, p = 0.0007, n = 17) when the inactive RF electrode remained on the epicardium during impedance measurements. When the RF electrode was removed during measurements, the rise of resistivity by complete lesions increased to 58 Omega cm (30% of the preablation value, p = 0.022, n = 8). For incomplete lesions, resistivity did not change significantly. Ablation did not significantly alter the phase of impedance. Accuracies of predictions of lesion completeness by the change in resistivity or the change in translesion stimulus-excitation delay were comparable (Youdens index 0.75 and 0.625, respectively, n = 16). Thus, RF ablation increases myocardial resistivity. The resistivity can predict lesion completeness and may provide an alternative to predictors based on excitation.

Monitoring heart period variability changes during seizures. I. Methods

Abstract The analysis of heart period variability (HPV) is a sensitive tool for detecting alterations in cardiac activity, which has been used to quantify changes in autonomic influences on cardiac function. We have developed a technique that utilizes time-and frequency-domain analysis of HPV to detect and quantify seizure-related alterations of cardiac autonomic regulation. Electrocardiographic (EKG) data obtained from video/EEG or ambulatory EEG recordings are processed off-line to provide graphic and tabular data regarding seizure-related changes in HPV. Using the preictal period as a baseline, ictal and postictal periods are analyzed to determine statistically significant changes in HPV during the ictal and postictal periods. This technique can detect seizure-related alterations of cardiac activity that are not apparent on routine inspection of the EKG. Some of these changes are evident prior to any apparent behavioral or EEG changes indicative of seizure activity. HPV analysis appears to be an important adjunct to video/EEG monitoring. It can allow detection and quantification of the cardiac effects of seizures that are likely to be mediated by changes in cardiac autonomic inputs. The method has promise in revealing important information regarding the mechanisms by which seizures may influence cardiac activity. In addition, HPV monitoring may provide an earlier and more reliable indication of the time of seizure onset than is apparent from inspection of video or EEG data alone.

Assessing autonomic activity from the EKG related to seizure onset detection and localization

Methods are presented for the detection and analysis of heart period variation (HPV) in both the time domain and the frequency domain. Fractal analysis and rate-change detection procedures are used for the recognition of transients in the HPV random variable. The power spectra of HPV are used to quantify relative autonomic input to the heart, with power at high frequencies (sinus arrhythmia) sensitive to parasympathetic input and low frequency power due to a combination of sympathetic and parasympathetic activities. Spectral analysis of HPV is used to study the dynamic activity of the autonomic nervous system, under conditions observed clinically which are considered to be risk factors for sudden death in epileptics. Results and conclusions from a study of a population of epilepsy patients are also presented.<<ETX>>

Heart period variability in seizure-related sinus arrest

Analysis of heart period variability (HPV) is a valuable tool for the evaluation of autonomic influences over the cardiac cycle in patients at risk of sudden death. Using both time and frequency domain techniques, we analyzed the variation in heart period for five episodes from a 3-month-old infant who had recurrent periods of asystole related to generalized tonic-clonic seizures (GTCS). The frequency domain analysis consistently showed increased power in the low-(0.0–0.04 Hz), middle-(0.04–0.12 Hz), and high-(0.2–0.4 Hz) frequency bands during the ictus as compared with preictal states, suggesting an increase in both parasympathetic and sympathetic influences on the heart period during these seizures. Similar power spectra changes in other patients with GTCS are associated with tachycardia. These findings are in contrast to HPV patterns detected in patients with cardiac disease at risk of sudden death. This case suggests that sinus arrest produced by seizures may be related to more complex autonomic influences on heart period than are postulated with cardiogenic sudden unexpected death.

A real-time system for the spectral analysis of the EEG

We have developed an inexpensive and portable system for processing multiple channels of electroencephalograms (EEG) in real-time to assist the electroencephalographer in identifying subtle changes in these data, particularly in the interpretation of long records. Up to 16 channels of data are spectrally decomposed with a selection of bandwidth, windows, scaling methods, epoch averaging, and smoothing options available. The resulting power spectral estimate may be displayed or printed in a variety of formats, including color encoding of selectable spectral bands. Both the sampled EEG and its spectra may be stored for off-line reprocessing, for archiving, or for statistical analysis in the time or frequency domains.