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S Akselrod

Publications and source records attributed to S Akselrod.

At least 91 records · Page 5Linked to original sources
PubMed1991

A combined heat clearance method for tissue blood flow measurement.

Tissue Blood Flow is measured by applying a combined procedure of two independent approaches based on heat clearance: the Pulse Decay Method and the Continuous Method. The Pulse Method allows absolute assessment of tissue BF with no need for calibration, and can be applied only if the tissue BF is steady during the period of measurement. On the other hand, the Continuous Method enables the observation of rapid changes in tissue BF, and can be applied under non steady-state conditions. Using the combined method, a continuous quantitative measurement of transient changes in tissue BF can be obtained. For this purpose, we have developed two experimental systems consisting of independent electronic units: a Pulse Unit and a Continuous Unit. A micro-computer with dedicated software controls the operation of the electronic units and calculates tissue BF on-line. In vitro measurements are performed and demonstrate the reliability of the methods. In vivo measurements in rat brain tissue are also performed and include physiological and pharmacological changes of local tissue BF. The results of the two heat clearance methods correlate well with tissue BF values measured by a third independent method, the Hydrogen Clearance Method.

Animals↗
PubMed1990

Some aspects of the wideband recording of the electrocardiogram.

The physiological mechanisms responsible for the high frequency components of the electrocardiographic signals in different frequency ranges are still unknown. These components have been studied in the frequency domain as well as in the time domain. Numerous clinical and experimental studies have focused upon the incidence and interpretation of high frequency notches and slurs under different conditions in the myocardium. However, the analysis of the high frequency components appeared to be tedious and imprecise, and the interpretation of its results difficult and controversial, and strongly dependent upon empirically determined standards which still require thorough evaluation. Digital methods for processing electrocardiographic signals developed during the last decade have given rise to a variety of fast and reliable approaches for analyzing the information carried by the electrocardiogram. Many researchers have applied such methods to various clinical situations in human studies and to different kinds of interventions in animal studies. These developments prompted us to review research in the field of wideband recording of the electrocardiogram and to briefly discuss some of the aspects that still require elucidation.

Animals↗
PubMed1990

Parasympathetically modulated antiarrhythmic action of lidocaine in atrial fibrillation.

Clinical experience has shown that the antiarrhythmic effect of lidocaine on atrial arrhythmias, and specifically for the conversion of atrial fibrillation to normal sinus rhythm, is minimal. This study summarizes our experience in 30 dogs in which atrial fibrillation was initiated and sustained (greater than or equal to 15 minutes) under increased vagal tone achieved by either alpha-chloralose anesthesia (26 dogs) or pentobarbital sodium anesthesia combined with direct external electrical vagal stimulation (four dogs). Under increased vagal tone (regardless of the procedure), an intravenous bolus of lidocaine (2 to 3 mg/kg) was 100% effective (101 of 101 episodes) in pharmacologically converting atrial fibrillation to normal sinus rhythm. This was associated with marked slowing of intra-atrial electrical activity, as shown by fast Fourier analysis of intra-atrial electrograms. Over a period of 3 to 5 minutes, lidocaine progressively shifted the peak frequency content from 84 +/- 18 mV2/Hz in the 10 to 20 Hz frequency band during the pre-lidocaine phase to 110 +/- 34 mV2/Hz in the 0 to 10 Hz frequency band immediately prior to conversion to normal sinus rhythm. When atropine was administered or electrical vagal stimulation was discontinued, the conversion of atrial fibrillation to normal sinus rhythm followed a similar electrophysiologic pattern. When isoproterenol was infused, it was difficult to induce atrial fibrillation; when the arrhythmia was initiated, it could not be sustained even with concomitant electrical vagal stimulation. Thus in this model of parasympathetically sustained atrial fibrillation, lidocaine was 100% effective in converting atrial fibrillation to normal sinus rhythm.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1990

A planning method for 125I implants in cancer therapy.

A method for planning implants of 125I seeds has been developed. The treatment dose prescribed by the physician is delivered to the tumour, as uniformly as possible, by a minimal number of seeds. The number of seeds and their locations are derived by requesting that, at any point in the tumour, the dose will be equal to or higher than the prescribed dose. As a result, the total implanted activity is lower for small volumes and higher for large volumes, relative to other implantation protocols which derive their planning from requests on the minimum peripheral dose. Results obtained from computer simulations performed on different tumour shapes and volumes, show a linear dependence between the total implanted activity and the tumour volume. The total activity does not depend on the shape of the tumour. A description of the algorithm of our procedure and a detailed example of its application are presented.

Algorithms↗
PubMed1990

Spectral analysis of canine epicardial electrogram. Short-term variations in the frequency content induced by myocardial ischemia.

Power spectra of epicardial electrograms were studied in 13 anesthetized dogs subjected to occlusion of the left anterior descending coronary artery. Electrograms were obtained from a bipolar electrode placed on the epicardial surface of the left ventricle and recorded before and after coronary occlusion. After digitization, power spectra of the first and every 50th subsequent waveform were evaluated and compared with the power spectrum of the average waveform obtained from the baseline recording. In particular, we examined variations in relative power content in three frequency ranges: 150-250 Hz, previously shown to be directly affected by myocardial ischemia; 40-150 Hz, presumably corresponding to the fine notches and slurs on the body surface QRS; and 2-40 Hz, the low-frequency range. Apart from a mild initial rise during the first 50 heart beats, the power in the high-frequency range gradually decreased, reaching 5% of the control value at wave 500. The power in the mid-frequency range showed a monotonous decrease after the occlusion and reached 16% of the control. The power in the low-frequency range showed a gradual buildup to 140% of the control after 500 heart beats. Therefore, ischemia causes a shift of the high-frequency spectral components of the local electrographic waveform to lower frequencies. Our findings and the fact that body surface ECG is produced by spatial summation of local electric potentials over the different regions of the myocardial tissue may explain two previously described effects of acute myocardial ischemia on the body surface ECG complex. First, local loss of spectral components in the range 150-250 Hz may produce a zone of reduced amplitude within a QRS complex band-pass filtered in this range. Second, displacement of power in the frequency domain may suggest an explanation to the increased incidence of visible notches and slurs on the surface QRS complex, characteristic to various myocardial pathologies.

Animals↗
PubMed1989

Frequency content of the QRS notching in high-fidelity canine ECG.

High-fidelity ECGs, defined as ECG signals including high-frequency components, have been studied and variations in the incidence of fine notches and slurs on the QRS complex were reported in different myocardial pathologies. These observations might be of clinical importance since they suggested a noninvasive marker for cardiac dysfunctions. We studied high-fidelity ECG waveforms displaying pronounced notches and slurs. Signals were obtained from 12 anesthetized dogs. Computer analysis included digital averaging, followed by digital filtering in different frequency bands in order to determine the frequency range corresponding to notches and slurs. Low-pass filtering of the low-noise average waveforms was performed while gradually lowering the upper frequency limit, until the fine notch (or slur) could no longer be visually detected, thus determining the lower limit of its frequency content. A band cutoff filter was then applied to the original average waveform. The lower limit of the band cutoff filter was set at the frequency previously determined as the lower limit of the notch (or slur), and its upper limit was determined by gradual raising until the notch (or slur) was visually indistinct. Following this approach, the notches were found to contribute to a frequency range of 40-185 Hz, whereas the slurs contributed only to the lower subrange of this frequency band below 100 Hz.

Animals↗
PubMed1988

Effect of localized surface cooling of the heart muscle on the high-frequency content of ECG waveforms in dogs.

Hypothermia is known to affect the electrophysiology of the myocardium in various ways. A marked increase in action potential duration, combined with a decrease in rate of depolarization and conduction velocity, has been observed. We studied the effect of localized hypothermia of the ventricular myocardium on the high-frequency (HF) components of the ECG waveform. Signals were obtained from 6 anesthetized dogs using simultaneous recording of three orthogonal body surface leads before, during, and following surface cooling of different areas of the epicardium. Computer analysis included digital averaging and filtering in a frequency range of 150-250 Hz. For each intervention in each animal, the variance of the average nonfiltered QRS complex was used for a quantitative estimate of the total power, whereas the variance of the derived filtered wave (HF QRS complex) expressed the power content in the HF range. The total power increased during localized cooling of the anterior as well as the inferior epicardial surface, while a clear reduction of power was observed in the HF range. This reduction was shown to be lead dependent and nonuniformly distributed during the course of the QRS. In all cases, hypothermia of either anterior or inferior ventricular epicardium produced a zone of reduced amplitude in the HF QRS complex of at least one lead. Thus, typical changes in the morphology of the HF QRS complex are reliable markers for cooling-induced localized electrophysiological (EP) variations. Therefore, the HF analysis of the body surface ECG may provide noninvasive insight into the EP properties of the myocardium.

Animals↗
PubMed1988

Heart-rate spectral analysis: a noninvasive probe of cardiovascular regulation in critically ill children with heart disease.

Children may die suddenly and unexpectedly following cardiac surgery, even while receiving intensive medical care, when current noninvasive and invasive monitoring techniques fail to warn us of an impending problem. This report examines the efficacy of heart-rate spectral analysis, a noninvasive method that quantitates the beat-to-beat fluctuations in heart rate, in characterizing and tracking cardiovascular regulatory status. A total of 45 heart-rate fluctuation studies, performed on selected patients, at the bedside, using heart-rate spectral analysis, were retrospectively correlated with clinical events. The spectral pattern of 18 patients who sustained a cardiac arrest differed significantly from the others in the study group (p less than 0.0001).

Adolescent↗
PubMed1987

Altered cardiac repolarization in some victims of sudden infant death syndrome.

Abnormal prolongation of cardiac repolarization, as reflected by a long QT interval with respect to the RR interval on the electrocardiogram, is known to be associated with ventricular tachyarrhythmias. To test the hypothesis that prolonged cardiac repolarization may characterize some babies who die of sudden infant death syndrome (SIDS), we studied the dependence of the QT interval on the preceding RR interval in 10 babies with SIDS and 29 healthy control babies. We analyzed approximately 5000 pairs of QT and RR intervals in each subject over a wide range of RR intervals. We found that the QT intervals demonstrated less dependence on the preceding RR intervals in 5 of 10 babies who subsequently died of SIDS than in normal controls. No ventricular arrhythmias were observed, however, during the six-hour recording period. Our data suggest that in some babies with SIDS the ability to shorten the QT interval as the heart rate increases is impaired. These observations are consistent with the hypothesis that relatively prolonged cardiac repolarization may predispose such babies to ventricular arrhythmias.

Electrocardiography↗
PubMed1987

Spectral analysis of heart rate fluctuations. A non-invasive, sensitive method for the early diagnosis of autonomic neuropathy in diabetes mellitus.

Early detection and a quantitative evaluation of the degree of diabetic autonomic neuropathy were performed in 23 diabetic patients and 22 controls by computerized spectral analysis of beat-to-beat R-R interval variations on a continuous electrocardiogram. Simultaneous recording of cardiac and respiratory activity, R-wave detection by a fast peak detection algorithm and spectrum computation by Fast Fourier transform enabled the study of the power spectrum of heart rate fluctuations. The power of fluctuations at different frequencies is the result of sympathetic and vagal input into the sinoatrial node: this input is derived from vasomotor, baroreceptor and respiratory control loops. A marked reduction in the power of heart rate (HR) fluctuations, at all frequencies, was found in the diabetic patients as compared to controls. This indicates a depression of both parasympathetic and sympathetic activity. The difference was especially pronounced in subjects below age 65. The lowest activity was found in diabetics with concomitant peripheral neuropathy. The method described here is simple, objective, quantitative and very sensitive. It may facilitate the screening of diabetic patients for autonomic neuropathy and enable a convenient quantitative follow-up.

Adult↗
PubMed1987

Evaluation of autonomic dysfunction in familial dysautonomia by power spectral analysis.

We examined the nature and extent of the autonomic control defect in patients with autonomic dysfunction using power spectral analysis of heart rate fluctuations. Heart rate variability and respiratory patterns were monitored and discrete blood pressure measurements were made during supine and standing positions in 10 ambulatory patients with familial dysautonomia and in controls. Postural hypotension without compensatory tachycardia was confirmed in the patients upon standing. The balance between sympathetic and parasympathetic activity was compared in both positions by quantifying the power of the low (0.04-0.095 Hz) and high (respiratory) frequency fluctuations in instantaneous heart rate. After changing from supine to standing position there was a small decrease in the low frequency power of heart rate fluctuations in the patients as opposed to a significant increase in controls. The mean power of fluctuations occurring at high frequency decreased only slightly in the patients compared to a marked decrease in the controls. We conclude that the fall in blood pressure, lack of appropriate heart rate modulation, and failure to increase low frequency heart rate power which occurred in the patients upon standing, are all due to lack of increased sympathetic output under the influence of gravity. The failure to decrease power in the respiratory frequency peak in the patients, suggests an abnormal retention of parasympathetic activity. This may be explained by parasympathetic compensation for the substantial sympathetic loss, or by a lack of appropriate inhibition of parasympathetic tone from baroreceptors or supraspinal structures.

Adult↗
PubMed1987

Hemodynamic regulation in SHR: investigation by spectral analysis.

This work is based on the premise that hypertension in the spontaneously hypertensive rat (SHR) is a consequence of an imbalance in autonomic cardiovascular control. Spontaneous, instantaneous fluctuations in heart rate (HR) and arterial blood pressure (ABP) were studied by spectral analysis in age-matched groups of SHR and normotensive Wistar-Kyoto (WKY) rats. Continuous recordings of ABP and HR made in conscious, unrestrained, young 1-, 2-, 3-, and 6-mo-old rats show that power spectra of HR fluctuations were similar in both strains, whereas ABP fluctuations were significantly different in the low, presumably vasomotor frequency range being reduced in SHR as compared with age-matched WKY rats. In 1-mo-old rats, the difference between strains became smaller with urethan anesthesia and disappeared with spinalization, suggesting that it might have originated in some disparity between the strains at the level of the central nervous system. Low doses of phentolamine elicited a fall in low-frequency ABP fluctuations of WKY but a rise in SHR, whereas larger doses produced a fall in the power of both. In SHR, reduction in the power spectrum of ABP fluctuations in the presence of unaltered HR fluctuations may indicate an impaired control of sympathetic drive to resistance vessels compared with WKY. Note that in young SHR, hypertension also cannot be detected by conventional measurement. Fluctuations in ABP in WKY may reflect an optimal level of sympathetic activity; reduction in fluctuations may be associated with an increase or decrease in sympathetic activity at the vascular bed.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1987

Effects of coronary occlusion on high-frequency content of the epicardial electrogram and body surface electrocardiogram.

The very high-frequency content (150 to 250 Hz) of epicardial electrogram waveforms was studied in 19 anesthetized dogs subjected to occlusion of left anterior descending coronary artery. Computer techniques of digital averaging and digital band-pass filtering were applied. Signals were obtained from epicardial electrodes placed in the ischemic left ventricular region and on the noninjured right ventricular surface, and from the body surface electrocardiogram. All recordings were made simultaneously before, during, and after coronary occlusion and subjected to the same analysis. The waveforms obtained from the ischemic left ventricular region showed a considerable decrease in high-frequency content, while those obtained from the noninjured right ventricular surface remained unchanged. The results correlated with the appearance of a zone of reduced amplitude in the body surface high-frequency QRS complex. Therefore, this macroscopic phenomenon measured noninvasively from the body surface is explained by local reduction of high-frequency activity in the ischemic region of the myocardium.

Animals↗
PubMed1987

Increased respiratory frequency and variability in high risk babies who die of sudden infant death syndrome.

We have tested the hypothesis that autonomic instability, reflected in increased variability of heart rate and respiratory frequency, characterized high risk babies who died of sudden infant death syndrome. Using computer-based methods, we compared the power spectra of instantaneous heart rate and respiration on coded tape recordings from seven asymptomatic siblings and 10 babies with symptomatic apnea who died of sudden infant death syndrome to 34 age- and sex-matched controls. We confirmed our previous observation of increased respiratory bandwidth, an index of variability in respiratory frequency (p = 0.009) but failed to confirm our finding of increased low frequency fluctuations in heart rate (p = 0.18). In addition, we found an increase in mean respiratory frequency during quiet breathing (p = 0.001) and a significant relationship between respiratory bandwidth and mean respiratory frequency (r = 0.604, p = 0.0002). These variables along with those from a previous analysis of the same data base yield a discriminant function with 82% sensitivity and 100% specificity. These results confirm previous suggestions that high risk babies who die of sudden infant death syndrome exhibit autonomic instability.

Autonomic Nervous System↗
PubMed1987

A new technique to determine the correlation between the QT interval and heart-rate for control and SIDS babies.

The ability of the autonomic nervous system to alter the QT interval in response to heart rate changes is essential to cardiovascular control. An accurate way to determine the relation between QT intervals and their corresponding RR intervals is described. A computer algorithm measures the RR intervals using digital filtering and cross-correlating the QRS sections of consecutive waveforms. The QT intervals is calculated by choosing a section of, the ECG that includes the T wave and cross-correlating it with all the consecutive T waves. At least 4000 pairs of QT-RR intervals are computed for each subject and a best fit correlation function determines the relations between the QT and RR intervals. This technique enables to establish a precise correlation between RR and QT in order to distinguish between control and SIDS babies.

Algorithms↗