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  • 1
    Keywords: Medicine ; Radiology, Medical ; Internal Medicine ; Cardiology ; Medicine & Public Health ; Cardiology ; Signal, Image and Speech Processing ; Imaging / Radiology ; Diagnostic Radiology ; Internal Medicine ; Springer eBooks
    Pages: : digital
    ISBN: 9781848825154
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  • 2
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1542-474X
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Background: This study was designed to evaluate the effects of autonomic tone on the QT interval, using conventional and heart rate independent analysis. Effects of autonomic tone on the QT interval have been studied either using rate correction formulae or during fixed rate atrial pacing, both of which have been associated with problems. Since most autonomic interventions are associated with heart rate changes, separation of “true” autonomic effects from rate related effects on the QT interval is essential.Methods: Electrocardiographic recordings were performed in 14 healthy volunteers during: (1) sympathetic stimulation (tilt, epinephrine infusion, isoproterenol infusion, and exercise); (2) β-adrenergic blockade; (3) parasympathetic blockade; (4) autonomic blockade; (5) tilt following autonomic blockade; (6) parasympathetic stimulation (phenylephrine infusion); and (7) isolated α-adrenergic stimulation (phenylephrine infusion following atropine). The QT interval was adjusted for heart rate using Bazett's formula. Heart rate independent analysis was performed between conditions with similar cycle lengths.Results: QT interval measurements were reproducible and exhibited the typical QT-RR relationship. Sympathetic stimulation decreased the RR interval and prolonged the QTc interval. Parasympathetic blockade also increased the QTc. Heart rate independent analysis of the effects of β-blockade showed a shortening of the QT (from 368.5 ± 20.5 ms to 355.9 ± 17.9 ms; n = 8). Alpha-adrenergic stimulation also decreased the QT interval from 302.4 ± 16.8 ms to 294.3 ± 17.7 ms (n = 7).Conclusion: Sympathetic stimulation prolongs the QT interval, while β-blockade shortens it. Alpha-adrenergic stimulation also shortens the QT interval. Autonomic effects on the QT interval as assessed by heart rate independent analysis may help separate the true autonomic effects from rate related effects.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1540-8159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Although the heterogeneity of electrophysiological properties is increased after myocardial infarction, the degree of this heterogeneity has not been well quantitoted and its relationship to the histoslogical changes that occur after infarction has not been carefully examined. The purpose of the present study was to test the hypothesis that alterations in electrophysiological properties in healing canine infarction are related to particular histological changes. Experimental infarction was produced by left anterior descending coronary ligation. Six dogs were used as controls, six were studied 5 days following, and six were studied 8 weeks following infarction. Pacing thresholds, effective refractory periods, and activation-recovery times were determined at 112 sites on the anterior left ventricle using a multiple electrode plaque. Conduction velocity, conduction-heterogeneity index—a measure of conduction disturbance—and histology of the epimyocardium underlying the plaque were assessed. The effective refractory periods and activation-recovery times were greater in both infarction groups, most prominently in the subacute group. In subacute infarction, significant postrepolarization refractoriness was present. In healed infarction, conduction velocity was decreased and the conduction-heterogeneity index was increased compared to controls and subacute infarction. Dispersion of excitability and repolarization was associated with more extensive local scarring. Dispersion of myocardial fiber angles was associated with the conduction-heterogeneity index. Some but not all of the electrophysiological changes noted in the animals with infarction were also seen in sham operated animals. Thus, heterogeneity in repolarization and refractoriness is greatest in the subacute phase of myocardial infarction and is associated with the extent of local cell death. In contrast, disturbances in conduction are greatest in healed infarction and associated with disarray of myocardial fibers.
    Type of Medium: Electronic Resource
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  • 4
    ISSN: 1540-8159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Inappropriate shocks delivered by implantable cardioverter-defibrillators may occur in 15%–40% of patients treated with these devices. This article describes a rare cause for inappropriate shocks. Two patients received inappropriate shocks due to the presence of premature ventricular complexes during attempted reconfirmation of ventricular fibrillation. Knowledge of device algorithms for detection and reconfirmation of ventricular fibrillation, designed to have a high degree of sensitivity and therefore lower specificity, allows for reprogramming to avoid further inappropriate shocks.
    Type of Medium: Electronic Resource
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  • 5
    Electronic Resource
    Electronic Resource
    Oxford, UK : Blackwell Publishing Ltd
    ISSN: 1540-8159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Type of Medium: Electronic Resource
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  • 6
    ISSN: 1540-8159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Heart rate variability analysis has been used to derive indices of sympathetic tone. As different sympathetic stimuli may give rise to divergent changes in heart rate variability, this study was designed to characterize the factors responsible for these divergent responses. Twelve healthy subjects (7 males, age 24.8 ±3.1 years) were evaluated. Five-minute electrocardiographic recordings were obtained at baseline, following upright tilt, and during isoproterenol infusion (25 ng/kg per min) under control conditions and following parasympathetic blockade. Data were acquired during spontaneous respiration and when breathing was timed with a metronome (15 breaths/min). Under control conditions, both upright tilt and isoproterenol infusion resulted in significant decreases in the SD and MSSD from baseline values of 69 ± 3 ms and 64 ± 5 ms to 48 ± 4 ms and 21 ± 5 ms during tilt and 44 ± 4 ms and 20 ± 5 ms during isoproterenol infusion, respectively. LF power also significantly increased from 0.47 ±0.17 In (beats/min)2 at baseline to 1.90 ± 0.20. In (beats/min)2 and 1.34 ± 0.18. In (beats/min)2 during tilt and isoproterenoi infusion, respectively. No change in HF power was noted. Following parasympathetic blockade, all heart rate variability parameters were significantly decreased. No significant change from baseline in the SD, MSSD, or HF power was noted with either tilt or isoproterenol infusion. The LF power increased only with tilt from a baseline value of -3.17 ± 0.17 in (beats/min)2 to -0.41 ± 0.19 in (beats/min)2. Similar changes were noted during spontaneous respiration and metronome breathing. These findings demonstrate that the response of the sinus node to β–adrenergic stimulation depends on the mode of stimulation. In addition, the associated level of parasympathetic tone affects the observed changes in heart rate variability that are associated with sympathetic stimulation.
    Type of Medium: Electronic Resource
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  • 7
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Stimulus Intensity Effect on SA Tissue. Introduction: Prior studies of sinus node function in man stated that the stimulus intensity of overdrive pacing has no effect on (he response of the sinus node to overdrive suppression; however, data documenting these statements were lacking. Previous studies have also suggested that drive train stimulus intensity can alter ventricular refractoriness, but similar studies have not been performed on the human atrium. The purpose of this study was to evaluate the effects of drive train stimulus intensity on atrial effective refractory period and sinus node recovery time. Methods and Results: The effect of drive train stimulus intensity on atrial effective refractory period and sinus node recovery time was studied in 42 patients undergoing clinical electrophysiologic tests. The atrial effective refractory period was shorter at 10 mA (221 ± 20 msec) and 5 mA (232 ± 25 msec) than at a drive train stimulus intensity of 1.5 times late diastolic threshold (248 ± 24 msec, P 〈 0.05 for pairwise comparison). The sinus node recovery time did not demonstrate a similar effect in the baseline state, following beta-adrenergic blockade, or following combined parasympathetic and beta-adrenergic blockade. However, following isolated parasympathetic blockade with atropine, the corrected sinus node recovery time shortened from 88 ± 51 msec at 1.5 times late diastolic threshold to 48 ± 55 msec at 10 mA (P 〈 0.05). Significant variability was present in sinus node recovery time measurements a(baseline and following beta blockade; this variability decreased following parasympathetic blockade. Conclusion: These data suggest that drive train stimulus intensity can affect the electrophysiologic properties of sinus node and atrial tissue. This effect appears to he mediated by local catecholamine and acetylcholine release and provides further evidence that the interaction between pacing stimuli and the cardiac autonomic system may need to be considered in evaluating electrophysiologic effects.
    Type of Medium: Electronic Resource
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  • 8
    ISSN: 1540-8167
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Integrated Versus Dedicated Bipolar Sensing of VF. Introduction: Dedicated bipolar sensing has been suggested to be safer than integrated bipolar sensing due to an increased incidence of failure to redetect ventricular fibrillation after an unsuccessful shock with leads that use integrated bipolar sensing. We compared sensing charactersitics during ventricular fibrillation of simultaneously recorded dedicated and integrated bipolar electrograms. Methods and Results: Thirty patients undergoing transvenous defibrillator implantation with a Transvene lead were studied. Simultaneous recordings were made from the dedicated bipole and the integrated bipole from the distal tip to the coil (interelectrode distance 18.3 mm). The mean detection time and number of undetected beats for the initial episode of ventricular fibrillation were 2804 ± 569 msec and 0.9 ± 0.8 using the dedicated recordings and 2938 ± 546 msec and 1.4 ± 1.1 (P = 0.026) using the integrated recordings. The mean redetection times and number of undetected beats following a failed first shock (n = 13) were 2468 ± 225 msec and 0.8 ± 1.1 for the dedicated recordings and 3042 ± 498 msec (P 〈 0.0003) and 4.2 ± 4.2 (P 〈 0.005) for the integrated recordings. Frequency analysis of the ventricular fibrillation electrograms demonstrated that the signal energy in the dedicated electrograms was significantly greater than the energy in the integrated electrograms (P 〈 0.0001). There was a significant negative relationship between detection times and the ventricular fibrillation signal energy. There was no independent effect of recording type (dedicated versus integrated). Conclusion: There are only minor differences in detection/redetection of ventricular fibrillation between dedicated and integrated (with tip to coil spacing of 18.3 mm) recording configurations. Detection times during ventricular fibrillation are related to the signal variance or energy recorded. Differences in the sensing performance of the two recording configurations can be explained by the differences in signal energy between the dedicated and integrated recordings that occur during ventricular fibrillation.
    Type of Medium: Electronic Resource
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  • 9
    ISSN: 1540-8159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: The purpose of this study was to assess the effects of autonomic stimulation and blockade on noise levels and to compare the noise measurements in the ST and TP segments of the signal-averaged ECG. Five-minute electrocardiographic data were recorded in 14 normal volunteers (8 males and 6 females; mean age 28.5 ± 5.0 years) on two separate days (day 1—baseline, epinephrine infusion, isoproterenol infusion, β-blockade, and combined adrenergic and parasympathetic blockade; day 2—baseline, phenylephrine infusion, parasympathetic blockade, and during phenylephrine infusion following atropine). Signal averaging was done off-line on 100 beats and noise was measured in both the ST and TP segments as the standard deviation of voltage in the segment of interest. For all conditions tested, the mean noise level measured in the ST segment (0.46 ± 0.16 μV) was significantly less than that measured in the TP segment (0.52 ± 0.24 μV; P = 0.0003). but there was good correlation between the noise measured in the STand the TP segment (R2= 0.62, P 〈 0.0001). Noise increased with isoproterenol infusion and decreased following adrenergic blockade. In addition, day 2 baseline noise was less than baseline noise on day 1. Finally, neither parasympathetic stimulation or blockade nor α-adrenergic stimulation significantly affected signal-averaged electrocardiography (SAECG) noise levels. Thus, the data support the notion that enhanced sympathetic tone increases noise levels and β-adrenergic blockade may decrease noise levels, likely due to effects from muscle sympathetic nerve activity. These findings are important since the target population for the SAECG are patients with myocardial infarction and congestive heart failure, conditions associated with increased sympathetic tone, which may in turn impact on the reproducibility or technical aspects of the SAECG. In addition, because noise in the ST and TP segments are highly correlated and the noise measured in the ST segment is less than that in the TP segment, uniform adoption of noise measurement in the ST segment seems most appropriate.
    Type of Medium: Electronic Resource
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  • 10
    ISSN: 1540-8159
    Source: Blackwell Publishing Journal Backfiles 1879-2005
    Topics: Medicine
    Notes: Adenosine has been reported to cause polymorphic VT in patients with congenital or acquired prolonged QT intervals and to cause pause dependent VT in a patient with a normal QT interval. There have been no previous reports of adenosine causing nonpause dependent polymorphic VT in patients without structural heart disease. We report two patients, with normal hearts and normal QT intervals, who developed nonsustained polymorphic VTthat was not pause dependent. Adenosine induced polymorphic VT does not appear to be a marker of the long QT syndrome, but may be an uncommon yet not unexpected pharmacological response to drug administration.
    Type of Medium: Electronic Resource
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