PubMed Health⌕ Search

Biomedical subjects

K A Ellenbogen

Publications and source records attributed to K A Ellenbogen.

At least 37 records · Page 2Linked to original sources
PubMed1999

Induction of ventricular fibrillation by T wave shocks: observations from monophasic action potential recordings.

INTRODUCTION: Shocks given during the vulnerable period of cardiac repolarization may induce ventricular fibrillation (VF). However, the relationship of the vulnerable period and the monophasic action potential (MAP) has not yet been reported in humans. The purpose of this study was, therefore, to determine how the monophasic action potential recorded from the right ventricle correlates with inducibility of VF using T wave shocks during ventricular pacing. METHODS: Eleven patients undergoing implantable cardioverter defibrillator (ICD) implantation had a MAP catheter positioned in the right ventricle (RV). The local monophasic action potential duration at 90% repolarization (MAP90) duration was measured during pacing at 400 ms. VF induction was attempted by pacing at 400 ms for 10 cycles and then giving a 1.0 joule monophasic T wave shock at varying coupling intervals (CI) to the last paced stimulus. The maximum and minimum CI that induced VF were determined and mapped in relation to the MAP90 recording. RESULTS: The average paced MAP duration was 275 +/- 20 ms. The minimum and maximum CI to induce VF were 255 +/- 24 ms and 325 +/- 36 ms respectively. This ranged from 93% to 118% of the MAP90 duration but because of delay in conduction time to the MAP catheter, shocks that induced ventricular fibrillation occurred between 74% and 99% of local repolarization time. CONCLUSION: VF is inducible with low energy T wave shocks falling during the last 25% of the right ventricular MAP90 recording. This corresponds with VF initiation during phase III repolarization.

Action Potentials↗
PubMed1999

The North American experience with the Ablate and Pace Trial (APT) for medically refractory atrial fibrillation.

The Ablate and Pace Trial (APT) was a prospective registry study of clinical outcomes and survival following ablation and pacing therapy for medically refractory atrial fibrillation. One hundred and fifty-six patients were enrolled at 16 centres in North America. The mean patient age was 66 +/- 11 years, with mean left ventricular ejection fraction of 48% +/- 18%. Seventy-eight percent of the patients had structural heart disease. During one year of follow up, multiple measures of quality-of-life showed significant and sustained improvement following ablation and pacing therapy. Also, left ventricular ejection increased significantly for patients with baseline left ventricular ejection fraction <45%. Metabolic exercise testing showed trends toward improved exercise tolerance; however, these did not achieve statistical significance. The one year overall survival was 85%, with 3% of patients experiencing sudden death. In summary, this large, non-randomized, trial showed significant improvement in quality of life and left ventricular function following ablation and pacing therapy. Ablation and pacing therapy is a viable strategy for palliative management of patients with medically refractory, highly symptomatic atrial fibrillation.

Aged↗
PubMed1999

Steroid eluting high impedance pacing leads decrease short and long-term current drain: results from a multicenter clinical trial. CapSure Z investigators.

Pacemaker lead technology has changed considerably over the past decades. The widespread use of low polarization highly porous electrodes and steroid elution electrodes has resulted in low chronic pacing thresholds, as well as a decrease in the incidence of exit block. Efforts to develop pacing leads with high impedance might theoretically lead to lower lead current drain, which is a component of battery capacity. Pulse generator longevity can be increased without sacrificing pacemaker capabilities if pacing current drain can be decreased. Decreasing the size of the stimulation electrode results in increased pacing impedance, and if pacing thresholds are unchanged, a decreased current drain is predicted by Ohm's law (I = V/R). There is limited data available on the pacing characteristics of large numbers of patients with high impedance leads, despite their recent general availability and increasing widespread use. This multicenter, controlled trial examined the differences in performance between standard steroid-eluting pacing leads in the atrium (Medtronic model 5524) and ventricle (Medtronic model 5024), and new high impedance steroid-eluting pacing leads in the atrium (Medtronic model 5534) and ventricle (Medtronic model 5034). Measurements of bipolar pacing thresholds at 2.5 V, pacing impedance, and sensing thresholds were determined within 24 hours of pacemaker implantation, and at 0.5, 1, 3, 6 and 12 months after pacemaker implantation in 609 patients. Pacing and sensing thresholds were similar for the control and high impedance leads at all times except for a slightly larger R wave with the high impedance leads at implantation and 12 months. The mean impedance of the high impedance pacing leads in the atrium and ventricle at 12 months was 992 +/- 175 and 1,080 +/- 220 omega, compared to 522 +/- 69 and 600 +/- 89 omega for the standard pacing leads in the atrium and ventricle (P < or = 0.001 for the high impedance leads compared to standard leads in each chamber). The mean atrial lead current (measured at 2.5 V) at 12 months was 2.6 +/- 0.5 mA with the high impedance lead, and 4.9 +/- 0.7 mA with the standard lead in the atrium (P < or = 0.001). In the ventricle, the mean lead current at 12 months was 2.4 +/- 0.4 mA with the high impedance pacing lead and 4.3 +/- 0.6 mA with the standard lead (P < or = 0.001). High impedance leads are associated with lower lead current drain than standard pacing leads in the atrium and ventricle for up to 1 year. No clinically important differences in sensing characteristics was noted with the high impedance leads in the atrium or ventricle compared to standard pacing leads. High impedance leads may result in increased pulse generator longevity.

Aged↗
PubMed1999

Optimal cardiac pacing after heart transplantation.

The transplanted heart is characterized physiologically by autonomic denervation, chronotropic incompetence, intermittent episodes of allograft rejection, and frequently by diastolic dysfunction. Sinus node dysfunction resulting in bradycardia is common in the early postoperative period following standard orthotopic cardiac transplantation. Bradycardia tends to remit spontaneously but there are no factors that accurately identify patients who will need long-term pacing. Patients in whom bradycardia persists beyond the second postoperative week despite treatment with theophylline require permanent pacemaker implantation. It has been observed that chronotropic incompetence and diastolic dysfunction are important determinants of exercise capacity following heart transplantation. Pacing that restores chronotropic competence improves exercise capacity, confirming the importance of impaired heart rate response. As in other settings, pacing that preserves atrioventricular (AV) synchrony results in increased cardiac output. For these reasons when pacing is necessary we recommend the DDDR mode (AAIR if intact AV nodal conduction is present) so that the 30%-50% of patients who remain pacemaker-dependent long-term obtain maximal benefit from their transplant.

Animals↗
PubMed1999

Variation of spectral power immediately prior to spontaneous onset of ventricular tachycardia/ventricular fibrillation in implantable cardioverter defibrillator patients.

INTRODUCTION: Short-term heart rate variability (HRV) may change immediately before onset of a ventricular arrhythmia (ONSET). METHODS AND RESULTS: Power spectrum analysis was performed on instantaneous heart rate (IHR; including all beats) and normal heart rate (NHR; excluding ectopics) curves obtained at equally spaced 0.5-second intervals using a cubic spline. The database consisted of 135 sets of 1,024 RR intervals leading to ventricular arrhythmia (VA) and controls from 78 patients. Total periodogram and time course of spectral power were obtained. Ten spectral bands of 0.1-Hz bandwidth (0 to 1 Hz) were analyzed. A simple threshold technique was retrospectively used to predict the onset of a VA. RR intervals that led to VA ONSET had significantly higher total spectral power than controls (P < 0.001 for both NHR and IHR for every band). Spectral power remained constant until 100 seconds before ONSET and then increased significantly in the time window immediately preceding ONSET (P < 0.02 compared with others). Using a simple threshold method, a predictive accuracy of 68%+/-1.4% was obtained with different window sizes. Using specific spectral bands, the predictive accuracy of VA ONSET could be improved to 76% for IHR and 71% for NHR (0.8- to 0.9-Hz band). CONCLUSION: Our results suggest that a sustained higher power increase in NHR and IHR occurs during the course of 12.11+/-.57 minutes, followed by a sudden elevation in spectral power within 100 seconds of ONSET, and may be a precursor to ventricular tachycardia/ventricular fibrillation episodes.

Algorithms↗
PubMed1998

Quality of life and clinical outcomes in elderly patients treated with ventricular pacing as compared with dual-chamber pacing. Pacemaker Selection in the Elderly Investigators.

BACKGROUND: Standard clinical practice permits the use of either single-chamber ventricular pacemakers or dual-chamber pacemakers for most patients who require cardiac pacing. Ventricular pacemakers are less expensive, but dual-chamber pacemakers are believed to be more physiologic. However, it is not known whether either type of pacemaker results in superior clinical outcomes. METHODS: The Pacemaker Selection in the Elderly study was a 30-month, single-blind, randomized, controlled comparison of ventricular pacing and dual-chamber pacing in 407 patients 65 years of age or older in 29 centers. Patients received a dual-chamber pacemaker that had been randomly programmed to either ventricular pacing or dual-chamber pacing. The primary end point was health-related quality of life as measured by the 36-item Medical Outcomes Study Short-Form General Health Survey. RESULT: The average age of the patients was 76 years (range, 65 to 96), and 60 percent were men. Quality of life improved significantly after pacemaker implantation (P<0.001), but there were no differences between the two pacing modes in either the quality of life or prespecified clinical outcomes (including cardiovascular events or death). However, 53 patients assigned to ventricular pacing (26 percent) were crossed over to dual-chamber pacing because of symptoms related to the pacemaker syndrome. Patients with sinus-node dysfunction, but not those with atrioventricular block, had moderately better quality of life and cardiovascular functional status with dual-chamber pacing than with ventricular pacing. Trends of borderline statistical significance in clinical end points favoring dual-chamber pacing were observed in patients with sinus-node dysfunction, but not in those with atrioventricular block. CONCLUSION: The implantation of a permanent pacemaker improves health-related quality of life. However, the quality-of-life benefits associated with dual-chamber pacing as compared with ventricular pacing are observed principally in the subgroup of patients with sinus-node dysfunction.

Aged↗
PubMed1998

Intravenous amiodarone for acute heart rate control in the critically ill patient with atrial tachyarrhythmias.

Control of heart rate in critically ill patients who develop atrial fibrillation or atrial flutter can be difficult. Amiodarone may be an alternative agent for heart rate control if conventional measures are ineffective. We retrospectively studied intensive care unit patients (n = 38) who received intravenous amiodarone for heart rate control in the setting of hemodynamically destabilizing atrial tachyarrhythmias resistant to conventional heart rate control measures. Atrial fibrillation was present in 33 patients and atrial flutter in 5 patients. Onset of rapid heart rate (mean 149 +/- 13 beats/min) was associated with a decrease in systolic blood pressure of 20 +/- 5 mm Hg (p <0.05). Intravenous diltiazem (n = 34), esmolol (n = 4), or digoxin (n = 24) had no effect on heart rate, while reducing systolic blood pressure by 6 +/- 4 mm Hg (p <0.05). The infusion of amiodarone (242 +/- 137 mg over 1 hour) was associated with a decrease in heart rate by 37 +/- 8 beats/min and an increase in systolic blood pressure of 24 +/- 6 mm Hg. Both of these changes were significantly improved (p <0.05) from onset of rapid heart rate or during conventional therapy. Beneficial changes were also noted in pulmonary artery occlusive pressure and cardiac output. There were no adverse effects secondary to amiodarone therapy. Intravenous amiodarone is efficacious and hemodynamically well tolerated in the acute control of heart rote in critically ill patients who develop atrial tachyarrhythmias with rapid ventricular response refractory to conventional treatment. Cardiac electrophysiologic consultation should be obtained before using intravenous amiodarone for this purpose.

Aged↗
PubMed1998

Suppression of inducible ventricular tachycardia by ibutilide in patients with coronary artery disease. Ibutilide Investigators.

BACKGROUND: Recent studies suggest that class III antiarrhythmic agents may have enhanced efficacy in the treatment of ventricular tachycardia. This study describes the first clinical assessment of the new class III agent ibutilide to suppress inducible monomorphic ventricular tachycardia (VT) in human beings. METHODS AND RESULTS: Fifty-five patients with coronary artery disease and inducible sustained monomorphic VT at baseline received either low (0.005 mg/kg + 0.001 mg/kg, load and maintenance infusion, respectively), middle (0.01 mg/kg + 0.002 mg/kg), or high dose (0.02 mg/kg + 0.004 mg/kg) infusions of ibutilide followed by repeat programmed ventricular stimulation. The mean age of the study group was 65.5 +/- 9.5 years and mean left ventricular ejection fraction was 36% +/- 11%. Of 48 evaluable patients, 21 (44%) were rendered noninducible after ibutilide, with no difference in efficacy among the three dosing groups (p = 0.83). Ventricular effective refractory periods, QTc interval, and ventricular monophasic action potential duration were prolonged over baseline at all tested cycle lengths. The QTc and action potential prolongation were dose related. Serious drug-related adverse reactions included sustained polymorphic VT in two patients (3.6%), spontaneous monomorphic VT in one patient (1.8%), heart block in one patient (1.8%), and hypotension in one patient (1.8%). CONCLUSIONS: Ibutilide prolongs ventricular repolarization in human beings and demonstrates efficacy in suppressing inducible monomorphic VT. Significant cardiovascular side effects occurred in 12.7% of patients.

Aged↗
PubMed1998

The Ablate and Pace Trial: a prospective study of catheter ablation of the AV conduction system and permanent pacemaker implantation for treatment of atrial fibrillation. APT Investigators.

BACKGROUND: The Ablate and Pace Trial (APT) prospectively assessed the effects of catheter ablation of the AV conduction system and permanent pacemaker implantation on health-related quality of life, survival, exercise capacity, and ventricular function in 156 patients with symptomatic atrial fibrillation. METHODS: All patients referred for catheter ablation and permanent pacemaker implantation because of medically-refractory atrial fibrillation at 16 centers were screened for enrollment in a prospective registry. Baseline assessment prior to ablation included measurement of quality of life, including the Health Status Questionnaire, the Quality of Life Index and the Symptom Checklist: Frequency and Severity. Exercise capacity was assessed with metabolic treadmill exercise testing and ventricular function was quantitated with echocardiography. The quality of life instruments, exercise capacity, and echocardiography were repeated at 3 and 12 months after catheter ablation. RESULTS: The APT population included 90 men and 66 women (66.1 +/- 11.5 years of age) with either chronic (n = 70), recurrent (n = 31), or paroxysmal atrial fibrillation (n = 55). Structural heart disease was present in 78.2% of patients. Successful ablation of AV conduction was achieved in 155 of 156 patients (99.4%). Survival at 1 year was 85.3%, with 5 of 23 deaths being sudden cardiac deaths. Survival over the first year of follow-up was significantly lower for patients with a baseline left ventricular ejection fraction (LVEF) < 0.45 (0.73) than for patients with a LVEF > or = 0.45 (0.88, p = 0.03). The NYHA functional class improved from 2.1 at baseline to 1.8 at 3 months and 1.9 at 12 months of followup (p = 0.0001). Significant improvement in quality of life scores were noted for all 8 subscales of the Health Status Questionnaire, for the overall rating of the Quality of Life Index, the Health and Function subscales; Arrhythmia-related symptoms were markedly reduced as measured by the Symptom Checklist: Frequency and Severity scale. The mean LVEF improved from 0.50 +/- 0.20 at baseline to 0.54 +/- 0.20 at 3 months (p = 0.03). The LVEF 12 months after ablation was 0.52 +/- 0.20, not statistically different from baseline. Individuals with reduced systolic function at baseline had the greatest improvement, from LVEF 0.31 +/- 0.20 at baseline to 0.41 +/- 0.20 at 3 months and 0.41 +/- 0.30 at 12 months (p = 0.0001). There were no significant changes in treadmill exercise duration (10.0 +/- 4.3 min at baseline and 11.6 +/- 3.6 min at 12 months) or VO2max (1467 +/- 681 ml O2 min baseline and 1629 +/- 739 ml O2 min at 12 months). CONCLUSIONS: Catheter ablation of the AV conduction system and permanent pacemaker implantation were associated with improved quality of life and left ventricular function in this population of highly symptomatic patients with atrial fibrillation refractory to medical therapy.

Aged↗
PubMed1998

Physiology of the escape rhythm after radiofrequency atrioventricular junctional ablation.

The physiology of the escape rhythm (ER) and its response to pharmacological modulation under varying autonomic conditions were studied 48 patients undergoing radiofrequency ablation of the atrioventricular junction (AVJ) for refractory atrial fibrillation. The QRS morphology and cycle length (CL) of the baseline ER were measured 15 minutes postablation. The CL of the ER was measured in response to doses of isoproterenol, atropine, adenosine, lidocaine, and verapamil. The ER QRS was narrow (QRS < 120 ms) in 20 patients and wide (QRS > 120 ms) in 28 patients. Of the 28 patients with wide QRS ER, 11 patients had a new bundle branch block (8 patients new right bundle branch block [RBBB] and 2 patients new left bundle branch block [LBBB]). The ERCL was similar in both narrow and wide ERs (1,593 +/- 376 ms and 1,516 +/- 296 ms, P = 0.44). In 23 patients receiving isoproterenol infusion, the ER CL decreased with increasing doses from 1 mcg/min to 2 mcg/min (1,378 +/- 200 to 1,240 +/- 229 ms, P < 0.001), but did not decrease further at 3 mcg/min (1,201 +/- 192 ms, P = 0.48 vs 2 mg/min). Seven patients received 0.02 mg/kg of atropine, and ER decreased significantly (1,572 +/- 408 ms to 1,319 +/- 333 ms, P = 0.028). In 30 patients who received intravenous boluses of adenosine (6-18 mg), the ER did not change significantly. In 28 patients who received 150 mg of lidocaine, the ER increased from 1,462 +/- 286 ms to 1,715 +/- 467 ms (P < 0.001), and one patient developed transient asystole. Nineteen patients received 7.5 mg of verapamil, and the ER did not change (1,488 +/- 313 ms to 1,513 +/- 666 ms, P = 0.80). There was no significant difference in response to isoproterenol, adenosine, lidocaine, or verapamil between the patients with wide and narrow QRS ERs. We conclude that patients may have stable ERs immediately following AVJ ablation even when a wide complex ER results. The ER is responsive to sympathetic stimulation and vagal blockade. The ER is prolonged after lidocaine but not after verapamil, suggesting response to sodium but not to calcium channel blockade. These data are consistent with an ER originating in the distal compact AV node or proximal His bundle.

Adenosine↗
PubMed1998

Immediate reproducibility of upper limit of vulnerability measurements in patients undergoing transvenous implantable cardioverter defibrillator implantation.

INTRODUCTION: Measurement of the upper limit of vulnerability (ULV) with monophasic T wave shocks has been proposed as a patient-specific measurement of defibrillation efficacy that results in fewer episodes of ventricular fibrillation (VF) than measurement of a defibrillation efficacy curve. METHODS AND RESULTS: We sought to determine the magnitude of variance in ULV in 63 consecutive patients undergoing implantation of an implantable cardioverter defibrillator (ICD). We measured ULV as the strength at or above which VF is not induced when a stimulus is delivered at 310 msec after an 8-beat ventricular pacing drive at 400 msec. Defibrillation threshold (DFT) was measured in patients with an active can device using a biphasic waveform and the binary search method beginning at 12 J. Sixty-three patients were studied; they had a mean age of 62 +/- 12 years and a mean ejection fraction of 35% +/- 15%. Three quarters of patients had an ischemic cardiomyopathy. Each patient underwent 4.5 +/- 0.8 measurements of ULV. Monophasic ULV correlated poorly with biphasic DFT (R between 0.19 and 0.28, P = 0.04 to 0.17). There was no change in ULV between second to third, third to fourth, and first to last measurement in 22% to 41% of patients. The reliability coefficient was 0.87. A ULV > or = 20 J was found in eight patients. The only predictor of high ULV was a high DFT. CONCLUSION: Monophasic ULVs do not closely predict biphasic active can DFTs using a standard protocol. High DFTs were predicted by high ULVs. There was little variation in the acute measurement of ULV between trials. These findings have important implications for using ULV measurements to determine changes in DFTs after interventions. The methodology of determining ULV is critical to its use for predicting DFTs and programming ICDs.

Defibrillators, Implantable↗
PubMed1997

Antiarrhythmic actions of intravenous ibutilide compared with procainamide during human atrial flutter and fibrillation: electrophysiological determinants of enhanced conversion efficacy.

BACKGROUND: The selective class III antiarrhythmic agent ibutilide prolongs action potential duration and terminates atrial flutter (AFL) and fibrillation (AF), but the mechanism of its antiarrhythmic efficacy in humans has not been fully characterized. This study compared the antiarrhythmic effects of ibutilide with the class IA agent procainamide in humans during AFL and AF. Antiarrhythmic drug actions and electrophysiological characteristics of AFL and AF that enhanced pharmacological termination were investigated. METHODS AND RESULTS: Right atrial monophasic action potentials were recorded during 148 episodes of AFL (n=89) or AF (n=59) in 136 patients treated with intravenous ibutilide (n=73) or placebo (n=22) as participants in randomized, double-blinded comparative studies or intravenous procainamide (n=53) in a concurrent open-label study. The conversion rates in AFL with ibutilide, procainamide, and placebo were 64% (29 of 45 patients), 0% (0 of 33), and 0% (0 of 11), respectively, whereas in AF the rates were 32% (9 of 28), 5% (1 of 20), and 0% (0 of 11), respectively. In AFL, ibutilide increased atrial monophasic action potential duration (MAPD) more (30% versus 18%, P<.001) and prolonged atrial cycle length (CL) less (16% versus 26%, P<.001) than procainamide. Ibutilide shortened and procainamide prolonged action potential diastolic interval during AFL (-12% versus 51%, P<.001). Ibutilide increased MAPD/CL ratio, whereas procainamide tended to decrease this ratio (13% versus -6%, P<.01). In AF, ibutilide and procainamide induced similar increases in atrial CL (48% versus 45%), but ibutilide induced a greater increase in MAPD (52% versus 37%, P<.05). Independent electrophysiological predictors of pharmacological arrhythmia termination were increase in MAPD/CL ratio (P=.005) in AFL and longer baseline mean MAPD (P=.011) in AF. Termination of AFL with ibutilide was characterized by significant increases in beat-to-beat atrial CL, MAPD, and diastolic interval variability. Ibutilide was significantly more effective in converting AF when the mean atrial CL was > or = 160 ms (64% versus 0%, P<.001) or MAPD was > or = 125 ms (57% versus 0%, P=.002) at baseline. CONCLUSIONS: Enhanced conversion efficacy of ibutilide compared with procainamide in AFL is correlated with a relatively greater prolongation of atrial MAPD than atrial CL, and termination of AFL by ibutilide is characterized by oscillations in atrial CL and MAPD. Conversion of AF by ibutilide is enhanced by a longer baseline mean atrial CL or MAPD.

Adult↗
PubMed1997

Ethylene oxide on electrophysiology catheters following resterilization: implications for catheter reuse.

Reuse of electrophysiology catheters is an important cost-saving option for many laboratories. However, to be reused safely, catheters must undergo resterilization with ethylene oxide (EtO). Residual EtO levels on resterilized catheters may be high and could pose a risk to patients. Resterilized diagnostic electrophysiology catheters were tested for residual EtO using headspace gas chromatography after both a standard resterilization with an aeration process and after a resterilization process that incorporated a detoxification period. The Food and Drug Administration's maximum permissible level of EtO for implantable products, 25 parts per million (ppm), was used as the cutoff for acceptable catheter residuals. At day 2 after standard resterilization, the residual level of EtO on catheters was high at 41 +/- 6 ppm. However, these levels decreased with shelf time, decreasing to 26 +/- 3 ppm by day 7 and to 14 +/- 2 ppm by day 14 after sterilization, at which time all catheters were <25 ppm (p <0.001). Detoxification periods of 6, 12, and 15 hours were tested and 15 hours was found to be optimal. After 15 hours of detoxification, residual EtO was 19 +/- 1 ppm by day 2 and all catheters were <25 ppm. In summary, electrophysiology catheters that have undergone resterilization have residual EtO levels that are twice the Food and Drug Administration's limit for implantable products. Residual EtO levels may be substantially reduced either by allowing a 14-day waiting period after resterilization or by incorporating a detoxification period immediately after EtO exposure.

Catheter Ablation↗
PubMed1997

Vagal and sympathetic mechanisms in patients with orthostatic vasovagal syncope.

BACKGROUND: Autonomic and particularly sympathetic mechanisms play a central role in the pathophysiology of vasovagal syncope. We report direct measurements of muscle sympathetic nerve activity in patients with orthostatic vasovagal syncope. METHODS AND RESULTS: We studied 53 otherwise healthy patients with orthostatic syncope. We measured RR intervals and finger arterial pressures and in 15 patients, peroneal nerve muscle sympathetic activity before and during passive 60 degree head-up tilt, with low-dose intravenous isoproterenol if presyncope did not develop by 15 minutes. We measured baroreflex gain before tilt with regression of RR intervals or sympathetic bursts on systolic or diastolic pressures after sequential injections of nitroprusside and phenylephrine. Orthostatic vasovagal reactions occurred in 21 patients, including 7 microneurography patients. Presyncopal and nonsyncopal patients had similar baseline RR intervals, arterial pressure, and muscle sympathetic nerve activity. Vagal baroreflex responses were significantly impaired at arterial pressures below (but not above) baseline levels in presyncopal patients. Initial responses to tilt were comparable; however, during the final 200 seconds of tilt, presyncopal patients had lower RR intervals and diastolic pressures than nonsyncopal patients and gradual reduction of arterial pressure and sympathetic activity. Frank presyncope began abruptly with precipitous reduction of arterial pressure, disappearance of muscle sympathetic nerve activity, and RR interval lengthening. CONCLUSIONS: Patients with orthostatic vasovagal reactions have impaired vagal baroreflex responses to arterial pressure changes below resting levels but normal initial responses to upright tilt. Subtle vasovagal physiology begins before overt presyncope. The final trigger of human orthostatic vasovagal reactions appears to be the abrupt disappearance of muscle sympathetic nerve activity.

Adolescent↗
PubMed1997

Effects of enhanced parasympathetic tone on atrioventricular nodal conduction during atrioventricular nodal reentrant tachycardia.

The effects of various physiologic and pharmacologic stimuli on the anterograde slow pathway in patients with atrioventricular nodal reentrant tachycardia are well characterized. We sought to further characterize the nature of anterograde and retrograde conduction during tachycardia and to define the differential input of the parasympathetic nervous system to these pathways. A custom-made neck suction collar was placed to stimulate the carotid body baroreceptors during supraventricular tachycardia. Neck suction at -60 mm Hg was applied and changes in tachycardia cycle length, AH, and ventriculoatrial intervals were measured in 20 patients. These measurements were repeated after intravenous administration of 10 mg of edrophonium to enhance vagal tone. We observed a 15 +/- 6 ms increase in tachycardia cycle length from baseline (p <0.0001) and a 14 +/- 6 ms increase in AH interval (p <0.0001), but no change in the VA interval with neck suction alone. The tachycardia cycle length prolonged 26 +/- 55 ms (p <0.0001) with edrophonium and an additional 12 +/- 43 ms (p <0.001) with neck suction after edrophonium. There was no change in the VA interval before or after edrophonium during neck suction. There were 10 tachycardia terminations in 8 patients during anterograde slow pathway block during neck suction, with tachycardia cycle length prolongation and mean AH prolongation before termination of 45 +/- 37 ms (vs 15 +/- 7 ms increase in AH interval without tachycardia termination, p = 0.10). There were 12 tachycardia terminations in 4 patients with retrograde block during neck suction, only after edrophonium, without any preceding change in tachycardia cycle length during 11 episodes. We conclude that anterograde slow pathway demonstrates gradual conduction slowing with parasympathetic enhancement, whereas retrograde fast pathway responds with abrupt block.

Atrioventricular Node↗
PubMed1997

Are more complex implantable cardioverter defibrillators associated with an increased mortality? Lessons learned from clinical trials.

We compared 1-year survival in patients receiving implantable cardioverter defibrillators (ICDs) that provide only shock therapy with more advanced ICDs that provide antitachycardia pacing, bradycardia pacing, low-energy cardioversion, and advanced detection algorithms. Outcome in patients with advanced-generation ICD systems was similar or improved compared with outcome in patients receiving ICDs with only monophasic shock.

Clinical Trials as Topic↗