Biomedical subjects

S Akselrod

Publications and source records attributed to S Akselrod.

At least 55 records · Page 3Linked to original sources

PubMed1998

Step baroreflex response in awake patients undergoing carotid surgery: time- and frequency-domain analysis.

Step baroreceptor stimulation can provide an insight into the baroreflex control mechanism, yet this has never been done in humans. During carotid surgery under regional anesthesia, a step increase in baroreceptor stimulation occurs at carotid declamping immediately after removal of the intra-arterial atheromatous plaque. In 10 patients, the R-R interval and systolic and diastolic blood pressures (BP) were continuously recorded, and signals obtained within the time window from 10 min before until 10 min after carotid declamping were analyzed. Mean +/- SD time signals, power spectra, and transfer and coherence functions before and after declamping were calculated. Immediately after carotid declamping, both heart rate (HR) and BP declined in an exponential-like manner lasting 10.3 +/- 5.9 min, and their power spectra increased in the entire frequency range. Transfer function magnitude and coherence functions between BP and HR increased predominantly in the midfrequency region (approximately 0.1 Hz), with no change in phase function. Thus, in carotid endarterectomy patients, step increase in baroreceptor gain elicits a prolonged decline in HR and BP. Frequency analyses support the notion that the baroreflex control mechanism generates the midfrequency HR and BP variability, although other frequency regions are also affected.

Aged↗

PubMed1997

Autonomic response to change of posture among normal and mild-hypertensive adults: investigation by time-dependent spectral analysis.

In this study, we applied the time-dependent spectral analysis approach (SDA) to investigate the autonomic changes occurring during a transition from supine to standing position (CP), in normal and unmedicated mild hypertensive subjects. The SDA method enables an accurate follow-up of the instantaneous changes in autonomic activity, even during the unsteady phase of the transition, where sudden changes in heart rate (HR) and arterial blood pressure (ABP) are observed. We were able to quantify the vagal withdrawal (reflected in the high frequency component of the time-dependent spectrum of HR fluctuations) in the immediate response to CP and the more slowly following sympathetic increase (reflected in the low frequency component of ABP). This general pattern was observed in both groups. In addition, our results identified an altered sympathetic response to CP in mild-hypertensives, as compared to normal adults. Their basal sympathetic activity is enhanced (higher mean HR and increased low frequency fluctuations in ABP) and their response to CP is reduced, as reflected only in the LF content of ABP fluctuations, relative to normals. No difference was observed in HR fluctuations, showing that there is no parasympathetically mediated alteration of the baroreflex control of HR in mild-hypertension.

Adult↗

PubMed1997

The diastolic decay constant in spontaneously hypertensive rats versus WKY rats as an indicator for vasomotor control.

This research uses the diastolic decay constant (tau) to investigate the short term mechanism responsible for vasomotor control, mainly the alpha-sympathetic control system. Previous studies have shown that vasomotor control is altered in spontaneously hypertensive rats (SHR) preceding the phase of overt hypertension. The diastolic decay, according to the two element Windkessel model, displays an exponential shape with a decay constant, tau, depending on both the vascular resistance and the compliance. In our experiments, we used tau to characterize vasomotor activity and its control in the normotensive rats as well as in the spontaneously hypertensive rats (SHR) prone to hypertension. The beat to beat value of tau was evaluated from a continuous arterial blood pressure (ABP) signal, measured in the tail artery of the conscious, unrestrained rat. Four months old prehypertensive SHR were compared to their age matched normotensive controls (WKY). To study vasomotor regulation, we computed gains and delay times by investigating the compensatory response in tau to changes in mean ABP (MBP). These parameters are expected in the short term to be neurally controlled by the sympathetic system, mainly alpha-sympathetic. Our set of experiments consisted of changing MBP by performing successive injections in bolus of increasing doses of the vasoconstrictor angiotensin II. This procedure was repeated under double cardiac autonomic blockade of the vagal and beta 1 = sympathetic limbs. Our results show that, under baseline conditions, the absolute gain and delay times of tau are reduced in SHR compared to WKY. Double cardiac blockade decreases the absolute gain in both strains, while abolishing the baseline strain differences. These results reinforce our assumption that, in SHR, the alpha-sympathetic system is in a basic state of excitation even prior to the onset of overt hypertension and therefore reacting with reduced sensitivity (lower gain) to changes in MBP.

Adrenergic beta-Antagonists↗

PubMed1997

Measurement of strain exerted on blood vessel walls by double-quantum-filtered 2H NMR.

In this study 2H double quantum-filtered (DQF) NMR spectroscopy of water molecules inside the blood vessel wall was used to monitor structural changes after the application of strain. This method was applied to a variety of bovine large blood vessels, including aorta, coronary artery, carotid artery, and vena cava. Inducing strain in coronary arteries by inflating them by air or water revealed dramatic changes in the 2H DQF spectral lineshapes. Uniaxial extensions of longitudinal and circumferential strips led to distinct spectral responses for the different blood vessels. While the spectral lineshapes for the coronary and the carotid arteries were very sensitive to longitudinal strain, they were not affected by circumferential stretching. For the aorta there were no changes in the lineshape upon strain. For the vena cava a large effect of strain was found that was independent of the choice of elongation axis. The effect of elongation on the spectra was assigned to the outer layer, the adventitia, whereas spectral lineshapes that originated from inner layers, media and intima, were almost insensitive to strain. Angular dependence experiments proved the presence of macroscopic order in the strained state superimposed on a broad distribution of the local directors. Our results demonstrate the sensitivity of 2H DQF NMR as a probe for dynamic processes occurring on the microscopic scale in a molecular network.

Animals↗

PubMed1997

Failure of erythropoietin treatment in a case with primary autonomic failure.

Previous studies have reported that anemia is a frequent occurrence in patients with severe autonomic failure, that it can be corrected by exogenous erythropoietin and that upright blood pressure improves while on erythropoietin. The objective of this study was to determine the alteration of autonomic control during erythropoietin therapy in a patient with severe autonomic failure and severe symptomatic orthostatic hypotension, evaluated by spectral analysis of heart rate variability. The autonomic response to standing was evaluated before, after 1 month and after 6 months of erythropoietin therapy. The results were compared to an age- and sex-matched control adult. There was no improvement in the orthostatic hypotension during and at the end of the erythropoietin treatment despite an increase in hemoglobin from 9.6 g/dl before treatment to 12.5 g/dl during treatment. The spectral estimates of heart rate variability displayed a low variability at baseline, a paradoxical vagal enhancement and a lack of sympathetic increase on standing (before, during and at the end of the treatment). There was no improvement of baseline activity, nor of the response to standing during and at the end of the treatment with erythropoietin. We conclude that erythropoietin did not improve the autonomic response to standing, although it corrected anemia. Erythropoietin did not alter sympathetic activity, as reflected in the low frequency content of the power spectrum of heart rate fluctuations during and at the end of treatment.

Autonomic Nervous System Diseases↗

PubMed1997

Spectral analysis of heart rate fluctuations and optimum thermal management for low birth weight infants.

Spectral analysis of heart rate variability is studied in 10 healthy growing premature infants to investigate the changes in autonomic balance achieved as a function of changes in skin temperature. Heart rate is obtained from ECG recordings and the power spectrum of beat-to-beat heart rate fluctuations is computed. The infants maintain mean rectal temperature within 36.3-37.2 degrees C, while skin temperature changes. The respiratory rate does not change at the different servocontrol set points. Heart rate is found to increase slightly, but consistently. The low-frequency band (0.02-0.2 Hz), reflecting the interplay of the sympathetic and parasympathetic tone and known to be maximum at the thermoneutral zone, is maximum at 35.5 and 36 degrees C and decreases gradually to a lower level at a servocontrol temperature of 36.5-37 degrees C. The high-frequency band (0.2-2.0 Hz), coinciding with the respiratory peak and reflecting parasympathetic activity, is significantly elevated at 36 degrees C (p < 0.01). The minimum low: high ratio, indicating the minimum sympathetic-parasympathetic balance and possibly reflecting the most comfortable conditions, occurs at 36 degrees C, although the differences are not statistically significant. Servocontrol skin temperature may thus be adapted, and possibly selected at 36 degrees C for growing premature infants in an attempt to achieve thermal comfort and more balanced autonomic activity.

Electrocardiography↗

PubMed1997

The source of NMR-detected motional anisotropy of water in blood vessel walls.

2H Double quantum-filtered (DQF) NMR spectroscopy of deuterated water is sensitive to the presence of order in biological systems. This is because the only nuclei that are detected are those with residual quadrupolar interactions due to their anisotropic motion. In the present study, samples of aorta, coronary and carotid arteries, and vena cava were studied in parallel by 2H DQF NMR and by light microscopy. The average quadrupolar splitting, calculated from the NMR data, varies considerably among the different blood vessels, with high reproducibility for each type of vessel. Polarization microscopy examinations using collagen-specific staining with picrosirius red, have shown a variety of color profiles for the different blood vessels. These reflect different physical modes of aggregation (packing and thickness) of collagen fibers. A correlation was found between the NMR parameters and the color profiles of the picrosirius red-stained sections. Treating the blood vessels with 90% formic acid resulted in the elimination of the 2H DQF NMR signal. Histological analysis demonstrated a complete degradation of collagen and muscle, whereas the elastin filaments were preserved. Evidence is given that the 2H DQF NMR signal is dominated by the contribution of water molecules interacting with the collagen fibers.

Animals↗

PubMed1997

Decay constant of Doppler flow waveform as a possible indicator of ovarian malignancy.

The objectives of this study were to analyze the decay constant (tau) of the Doppler flow waveform in ovarian tumors; to determine if differences in this constant can discriminate between malignant and benign ovarian tumors; and to compare the decay constant to the known resistive index (RI), in order to determine its potential prognostic application. Patients with ovarian masses (46) were evaluated in a retrospective study; 13 had malignant tumors, 7 showed tumors with low malignant potential (LMP), 11 had benign masses, 4 had secondary ovarian metastases and 11 had functional ovarian masses. Doppler flow waves measured in the ovary before operation were analyzed from archival videotapes. The RI was calculated preoperatively, and the decay constant of the flow waveform was analyzed retrospectively. We approximated the decaying portion of the flow waveform from the systolic peak to the diastolic level to an exponential curve. Then, the decay constant associated with the flow signal was compared for different types of ovarian pathology. Ovaries with malignancies showed significantly higher mean values for the decay constant (89.7; 95% confidence interval 60.0-119.3) than those with benign tumors (41.8; 25.7-57.9) (p < 0.007), where tau is provided in pixels (in this study each pixel equals approximately 11.4 ms). The mean RI value for malignant tumors was 0.44 +/- 0.12 whereas, in benign tumors, it was 0.622 +/- 0.11. For the benign tumors, both tau and RI did not differ significantly from the measured indices in LMP tumors, metastases and functional ovarian findings. In addition, when the cutoff value of tau was set at 48, 92.3% of all malignancies were identifiable using only tau. This preliminary study indicates that the decay constant of the Doppler flow waveform is able to discriminate between malignant and benign masses and may, thus, provide substantial assistance as an additional parameter in the diagnosis of malignant ovarian tumors in postmenopausal patients.

Adult↗

PubMed1997

Nonlinear high pass filter for R-wave detection in ECG signal.

A simple and easily implemented method for R-wave detection from ECG signals is presented. The method is based on the subtraction of a filtered version of the signal. The filter we used is a nonlinear median filter. The median filter is applied to the ECG signal. It results in a smoothed version of the signal, without any reminder of the R waves. This smoothed signal is then subtracted from the original and the resulting signal presents undistorted R-waves, without baseline drift. A simple threshold detection can then be performed on the filtered signal. Results are presented for simulated signals, with sinusoidal and step baseline drifts, as well as ECG complex shape change. The detection is accurate and the average error we obtained for 300 s length signals was of the order of 10(-8)s, for RR intervals of 1 s. Results are also presented for a real experimental signal with strong baseline drift, and the accuracy of the detection can be observed.

Algorithms↗

PubMed1996

Laser irradiation of biological tissue through water as a means of reducing thermal damage.

BACKGROUND AND OBJECTIVE: Reduction of thermal damage inbred with laser surgery is an ongoing challenge. The cavitation effect has been shown to facilitate the transmission of a laser beam through the otherwise opaque water layer. We suggest that by immersing the target tissue in water during laser surgery, thermal damage will be diminished. STUDY DESIGN/MATERIALS AND METHODS: A method of irradiating tissue by CO2 laser through a layer of a few millimeters of water is described. A series of experiments was conducted on fresh bovine cornea immersed in water or in air, histology of thermal damage was compared. RESULTS: We were able to show that irradiation of tissue immersed in water reduces the thermal damage caused to the area surrounding the incision as compared to the damage caused during irradiation of tissue in air (P < 0.001). CONCLUSION: Laser surgery of tissue immersed in water can reduce the inbred thermal damage. Application of this method to clinical use may result in precise, clean cutting, and enable the use of CO2 laser through water.

Animals↗

PubMed1996

The effects of lidocaine on cardiac parasympathetic control in normal subjects and in subjects after myocardial infarction.

It has been widely accepted that lidocaine has little or no effect on the autonomic nervous system. However, we have previously shown that in dogs with vagally induced atrial fibrillation, lidocaine has a pronounced parasympatholytic effect. To study the possible effect of lidocaine on autonomic cardiac control in humans, we performed spectral analysis of heart rate fluctuations in 19 healthy volunteers, who received an i.v. bolus of lidocaine (1.4 mg/kg), as well as in 13 patients suffering from acute inferior myocardial infarction (IMI) and 13 patients suffering from acute anterior myocardial infarction (AMI), who received therapeutic doses of i.v. lidocaine infusion (4 mg/min). Heart rate variability and respiratory pattern were monitored according to a predetermined protocol, with and without lidocaine. Computing the heart rate power spectrum and integrating over predetermined frequency bands, we focused mainly on the respiratory frequency band, known to predominantly reflect parasympathetic control. The administration of lidocaine resulted in a significant overall increase in mean heart rate: for the healthy control group an increase of 5.5 +/- 2.2% (mean +/- SE), for the IMI group an increase of 9.4 +/- 3.5%, and for the AMI group an increase of 8.1 +/- 2.9% (p < 0.01 for all). Simultaneously, following the administration of lidocaine, there was a decrease in the power of respiratory fluctuations: for the healthy control group a decrease of 38.4 +/- 12.5%, for the IMI group a decrease of 46.3 +/- 32.9%, and for the AMI group a decrease of 33.9 +/- 16.2% (p < 0.01 for all). These findings indicate that lidocaine has a consistent and significant parasympatholytic effect on the human heart, in healthy volunteers as well as in patients in the acute phase of myocardial infarction.

Adult↗

PubMed1996

Estimation of fast vagal response by time-dependent analysis of heart rate variability in normal subjects.

In this study, the selective windowed discrete Fourier transform algorithm (SDA) for time-frequency analysis was applied on non-stationary heart rate signals, recorded during vagal perturbations. These perturbations were achieved in healthy subjects (aged 6-42 years) by inducing the oculocardiac reflex and the diving response. The results showed that the SDA can detect and quantify the expected, although brief, increase in vagal tone, by displaying a marked transient increase in the respiratory peak of the time-dependent spectrum. It allowed us to demonstrate an age-dependent reduction of the vagal response, obtained from the respiratory peak of the heart rate time-dependent spectrum. The SDA approach is thus an adequate tool for the evaluation of an instantaneous change in vagal activity, as well as steady-state vagal activity, including the detection of a malfunction or an exaggerated response of the parasympathetic tone. Since changes in heart rate control are expressed by a transient response, they would not have been detected by a standard, steady-state spectral analysis of heart rate variability, which requires the consideration of long and steady trace lengths and would therefore cause smearing of such fast changes. Time-dependent, or transient changes in autonomic control can thus be detected and quantified by SDA time-frequency analysis.

Adult↗

PubMed1996

The distribution of the local entropy in ultrasound images.

In this article, a model for the amplitude statistics of the backscattered ultrasonic signal is presented. We propose to view a tissue as being composed of a large number of small units, each having slightly different characteristics. This variability within the tissue is expressed by fluctuations in the parameters of the local probability distribution function (PDF). Based on analogous expressions derived for radio propagation and optical scintillations, the local PDF is modulated by a lognormal distribution of the local standard deviation. Integrating the local contributions yields the amplitude PDF for the entire tissue. We show that the local entropy is a normal variable, since for four different local PDFs it is linearly related to the logarithm of the local standard deviation. When the local entropy histogram exhibits distinct and multiple peaks, the local entropy distribution can be used for region segmentation. This fact is demonstrated for ultrasonic images of ovarian cysts.

Entropy↗

PubMed1996

A two-dimensional extension of minimum cross entropy thresholding for the segmentation of ultrasound images.

Segmentation is often an important step in medical image analysis. The local entropy is a possible variable for segmenting ultrasound images containing fluid surrounded by a soft tissue. A commonly used tool for image segmentation is thresholding. Recently, a new thresholding technique, known as "minimum cross entropy thresholding" (MCE), has been proposed. We present a multivariate extension of MCE in which the segmented variable (gray level) is replaced by a weighted combination of several image parameters. We propose to use a bivariate extension of MCE, which uses a linear combination of the gray level and the local entropy. The results obtained are demonstrated for ultrasound images of ovarian cysts.

Algorithms↗

PubMed1996

Selective discrete Fourier transform algorithm for time-frequency analysis: method and application on simulated and cardiovascular signals.

The Selective Discrete Fourier transform (DFT) Algorithm [SDA] method for the calculation and display of time-frequency distribution has been developed and validated. For each time and frequency, the algorithm selects the shortest required trace length and calculates the corresponding spectral component by means of DFT. This approach can be extended to any cardiovascular related signal and provides time-dependent power spectra which are intuitively easy to consider, due to their close relation to the classical spectral analysis approach. The optimal parameters of the SDA for cardiovascular-like signals were chosen. The SDA perform standard spectral analysis on stationary simulated signals as well as reliably detect abrupt changes in the frequency content of nonstationary signals. The SDA applied during a stimulated respiration experiment, accurately detected the changes in the frequency location and amplitude of the respiratory peak in the heart rate (HR) spectrum. It also detected and quantified the expected increase in vagal tone during vagal stimuli. Furthermore, the HR time-dependent power spectrum displayed the increase in sympathetic activity and the vagal withdrawal on standing. Such transient changes in HR control would have been smeared out by standard heart rate variability (HRV), which requires consideration of long trace lengths. The SDA provides a reliable tool for the evaluation and quantification of the control exerted by the Central Nervous System, during clinical and experimental procedures resulting in nonstationary signals.

Adult↗

PubMed1996

An automatic contour extraction algorithm for short-axis cardiac magnetic resonance images.

In this article an automatic process is presented for the extraction of the left ventricular contours. The only operator-supplied information is the location of a single point within the left ventricle. The raw data for processing are short-axis cardiac MR images, obtained using various MRI protocols. In this study we considered only protocols for which the blood appears dark. The proposed algorithm performs smoothing and thresholding of the original MR images, and uses the segmented images to form edge images. From those edge images the endocardial curve is extracted using a modified contour following approach. The algorithm can accommodate moderate noise and small gaps in the contour. A smooth curve that approximates the epicardium is then obtained using a method combining radial search and FFT based smoothing. The quality of the obtained contours was visually assessed by three observers. Their evaluation shows that the algorithm is successful when images of reasonable quality are processed. Further evaluation work is required in order to fully assess the performance and limitation of the suggested method.

Algorithms↗

PubMed1996

Nonlinear analysis of BP signal. Can it detect malfunctions in BP control?

In the present study, our aim was to evaluate the applicability of the nonlinear technique to the investigation of cardiovascular control. We applied an approach known as the "surrogate data method" to test for nonlinear components in the blood pressure (BP) signal. Our results strongly indicate that there are nonlinear components in the BP time series taken from a Wistar-Kyoto rat (WKY), suggesting that the use of nonlinear methods may provide new information about the BP control system. We developed a procedure appropriate for the stable and reliable calculation of the Grassberger-Procaccia correlation dimension (D) of the arterial BP signal. The saturation value D was 5.48 +/- 0.30 for the WKY group and 5.92 +/- 0.26 for the spontaneously hypertensive rat (SHR) group, with P < 0.001. We also found that in the WKY group D displays a significant response to complete alpha 1 blockade and bleeding, whereas no response is observed in the SHR group. These results imply that differences in the control mechanisms may be detected by the nonlinear dynamics approach both under baseline conditions and when interfering with cardiovascular control.

Adrenergic alpha-Antagonists↗

PubMed1995

Insight into blood-pressure control in SHR via the response to acute hemorrhage: a spectral analysis approach.

In this study we investigated, by means of the spectral analysis approach, the possible alterations in the activity of the pressor-control mechanisms in relation to the development of essential hypertension. Since the maintenance of controlled arterial blood pressure (ABP) levels is achieved by a continuously fluctuating control system, instantaneous ABP varies continuously in direct correlation with the activation of various control branches. The power spectrum of ABP fluctuations thus provides a quantitative measure of the activity of the various controlling mechanisms. Two strains of rats, Spontaneously Hypertensive Rats (SHR) and Wistar-Kyoto normotensive rats (WKY), were subjected to acute hemorrhage, a procedure known to trigger a strong response, of both the neural (autonomic nervous system) and the hormonal (renin-angiotensin and vasopressin) systems. ABP was continuously recorded from the caudal artery in conscious, 1-month-old SHR and WKY. Three groups of rats were studied. Group 1, acute 2-ml hemorrhage; Groups 2, injection of prazosin (2.5 mg/kg) or Group 3, captopril (4 mg/kg), each followed by bleeding as in the group 1. Spectral analysis of ABP fluctuations was performed on time traces of 20 min duration. The low-frequency part of the power spectrum was analyzed. Three frequency bands were investigated: 0.004-0.04 Hz, 0.04-0.07 Hz and 0.07-0.1 Hz. In SHR, although the baseline mean ABP levels were similar to those of WKY, ABP fluctuations were significantly dampened in SHR in each of the three frequency ranges. Hemorrhage induced, in both strains, a similar fall in mean ABP accompanied by an increase in the slow ABP fluctuations. However, in SHR, the response was significantly greater that that of WKY. The steepest response was observed in the slowest, 0.004-0.04 Hz frequency band, 8.7 +/- 1.7 vs. 1.5 +/- 0.4 times the baseline levels. However, this intense increase in power after hemorrhage brought the two strains to similar levels. The difference in the response to bleeding was eliminated, in the three frequency ranges, by alpha 1 blockade. Captopril reduced the response to bleeding in SHR, to the level observed in WKY in all three frequency bands. Spectral analysis of the spontaneous oscillations in ABP unmasks abnormalities which conventional blood pressure measurements cannot detect in 1-month-old, still normotensive SHR. It thus, provides a tool to study the dynamics of the abnormalities which precede the development of hypertension. Bleeding amplifies the malfunction observed in SHR under baseline conditions, since the mean ABP levels were similar in both strains before and after bleeding, SHR seem to retain the ability to respond to a fall in blood volume by requiring a greater recruitment of the control mechanisms than WKY.

Acute Disease↗