Comparison of adenosine effects on atrioventricular node reentry and atrioventricular reciprocating tachycardias

BACKGROUND

Adenosine is an established first line therapy for the treatment of narrow complex tachycardias. The two most common etiologies of paroxysmal supraventricular tachycardia (SVT) are atrioventricular node reentry tachycardia (AVNRT) and atrioventricular reciprocating tachycardia (AVRT).

HYPOTHESIS

We postulated that adenosine might have different effects on the termination of AVNRT vs. AVRT, and that these differences might assist in the noninvasive differentiation between these diagnoses.

METHODS

Fifty-nine patients referred for the diagnosis and treatment of SVT were included in the study. All patients had SVT induced during electrophysiology testing, and each patient received adenosine during SVT. The adenosine dose, time to tachycardia termination, and site of tachycardia termination were recorded. Seventeen patients required isoproterenol administration to initiate SVT. This subset of patients was compared with those not requiring isoproterenol.

RESULTS

There was no statistically significant difference in the adenosine dose or time to tachycardia termination when comparing patients with AVNRT with those with AVRT. All patients with AVNRT had termination of tachycardia in the antegrade direction with final activation in the atria. Patients requiring isoproterenol for tachycardia initiation experienced tachycardia termination significantly faster than those not requiring isoproterenol, although there was no difference in the dose of adenosine required for termination.

CONCLUSION

These data demonstrate that patients with dual AV node physiology and AVNRT do not have altered sensitivity to adenosine compared with patients with AVRT and normal AV nodes. Further investigation will be required to determine the clinical utility of the significantly shorter time to tachycardia termination for patients receiving isoproterenol.

Daily exercise reduces measures of heart rate and blood pressure variability in hypertensive rats

This study was designed to test the hypothesis that daily spontaneous running (DSR) reduces measures of heart rate and blood pressure variability in spontaneously hypertensive rats (SHR). After 8 weeks of DSR or sedentary control, rats were chronically instrumented with arterial catheters. Daily exercise reduced most measures of heart rate (HR) and blood pressure variability. Specifically DSR decreased heart rate, Low Frequency Power (LF: 0.19-0.61 Hz), and Low Frequency/High Frequency (HF: 1.2-2.5 Hz) ratio of HR. Furthermore, Total Power (TP), LF power, and LF/HF ratio of systolic blood pressure were reduced by daily spontaneous running. Finally, TP, LF and HF powers and LF/HF ratios of diastolic blood pressure were reduced by daily spontaneous running. These data demonstrate that daily exercise reduces sympathetic activity and possibly increases cardiac reserve in hypertensive animals.

Allopurinol improves myocardial efficiency in patients with idiopathic dilated cardiomyopathy

BACKGROUND

Dilated cardiomyopathy is characterized by an imbalance between left ventricular performance and myocardial energy consumption. Experimental models suggest that oxidative stress resulting from increased xanthine oxidase (XO) activity contributes to this imbalance. Accordingly, we hypothesized that XO inhibition with intracoronary allopurinol improves left ventricular efficiency in patients with idiopathic dilated cardiomyopathy.

METHODS AND RESULTS

Patients (n=9; ejection fraction, 29+/-3%) were instrumented to assess myocardial oxygen consumption (MVO(2)), peak rate of rise of left ventricular pressure (dP/dt(max)), stroke work (SW), and efficiency (dP/dt(max)/MV O(2) and SW/MVO(2)) at baseline and after sequential infusions of intracoronary allopurinol (0.5, 1.0, and 1.5 mg/min, each for 15 minutes). Allopurinol caused a significant decrease in MVO(2) (peak effect, -16+/-5%; P<0.01; n=9) with no parallel decrease in dP/dt(max) or SW and no change in ventricular load. The net result was a substantial improvement in myocardial efficiency (peak effects: dP/dt(max)/MVO(2), 22+/-9%, n=9; SW/MVO(2), 40+/-17%, n=6; both P<0.05). These effects were apparent despite concomitant treatment with standard heart failure therapy, including ACE inhibitors and beta-blockers. XO and its parent enzyme xanthine dehydrogenase were more abundant in failing explanted human myocardium on immunoblot.

CONCLUSIONS

These findings indicate that XO activity may contribute to abnormal energy metabolism in human cardiomyopathy. By reversing the energetic inefficiency of the failing heart, pharmacological XO inhibition represents a potential novel therapeutic strategy for the treatment of human heart failure.

Transfer function analysis of the autonomic response to respiratory activity during random interval breathing

We report a new method for the noninvasive characterization of the frequency response of the autonomic nervous system (ANS) in mediating fluctuations in heart rate (HR). The approach entails computation of the transfer function magnitude and phase between instantaneous lung volume and HR. Broad band fluctuations in lung volume were initiated when subjects breathed on cue to a sequence of beeps spaced randomly in time. We studied 10 subjects in both supine and standing positions. The transfer function, averaged among all the subjects, showed systematic differences between the two postures, reflecting the differing frequency responses of the sympathetic and parasympathetic divisions of the ANS.

Analysis of long term heart rate variability: methods, 1/f scaling and implications

The use of spectral techniques to quantify short term heart rate fluctuations on the order of seconds to minutes has helped define the autonomic contributions to beat-to-beat control of heart rate. We used similar techniques to quantify the entire spectrum (0.00003-1.0 Hz) of heart rate variability during 24 hour ambulatory ECG monitoring. The ECG from standard Holter monitor recordings from normal subjects was sampled with the use of a phase locked loop, and a heart rate time series was constructed at 3 Hz. Frequency analysis of the heart rate signal was performed after a nonlinear filtering algorithm was used to eliminate artifacts. A power spectrum of the entire 24 hour record revealed power that was inversely proportional to frequency, 1/f, over 4 decades from 0.00003 to 0.1 Hz (period approximately 10 hours to 10 seconds). Displaying consecutive spectra calculated at 5 minute intervals revealed marked variability in the peaks at all frequencies throughout the 24 hours, probably accounting for the lack of distinct peaks in the spectra of the entire records.

Electrical induction of ventricular fibrillation for resuscitation from postcountershock pulseless and asystolic cardiac arrests

BACKGROUND

There is increasing evidence that defibrillation from prolonged ventricular fibrillation (VF) before CPR decreases survival. It remains unclear, however, whether harmful effects are due primarily to initial countershock of ischemic myocardium or to resultant postdefibrillation rhythms (ie, pulseless electrical activity [PEA] or asystole).

METHODS AND RESULTS

We induced 15 dogs into 12 minutes of VF and randomized them to 3 groups. Group 1 was defibrillated at 12 minutes and then administered advanced cardiac life support (ACLS); group 2 was allowed to remain in VF and was subsequently defibrillated after 4 minutes of ACLS; group 3 was defibrillated at 12 minutes, electrically refibrillated, and then defibrillated after 4 minutes of ACLS. All group 1 and 3 animals were defibrillated into PEA/asystole at 12 minutes. After 4 minutes of ACLS, group 2 and 3 animals were effectively defibrillated into sinus rhythm. The extension of VF in group 2 and 3 subjects paradoxically resulted in shorter mean resuscitation times (251+/-15 and 245+/-7 seconds, respectively, versus 459+/-66 seconds for group 1; P<0.05) and improved 1-hour survival (10 of 10 group 2 and 3 dogs versus 1 of 5 group 1 dogs; Fisher's exact, P<0.005) compared with more conservatively managed group 1 subjects.

CONCLUSIONS

Precountershock CPR during VF appears more conducive to resuscitation than CPR during postcountershock PEA or asystole. The intentional induction of VF may prove useful in the management of PEA and asystolic arrests.

Lead configuration for defibrillator implantation in a patient with congenital heart disease and a mechanical prosthetic tricuspid valve

The authors devised a nonthoracotomy defibrillation system for a patient with a prosthetic tricuspid valve using existing technology and previously established implantation techniques. Their lead configuration deviates substantially from existing designs in its primary use of a coronary sinus defibrillation coil and a left-sided subcutaneous array to distribute current across the ventricular myocardium.

Usefulness of magnesium sulfate in stabilizing cardiac repolarization in heart failure secondary to ischemic cardiomyopathy

Experimental heart failure is associated with cardiac magnesium loss, causing increased beat-to-beat variability in the action potential. Unstable repolarization contributes to sudden death, but no therapy has been shown to reduce repolarization variability safely. We sought to test whether a prolonged infusion of magnesium sulfate (MgSO(4); 40 mmol/24 hours) would normalize QT interval variability in patients with compensated heart failure. Fifteen patients (New York Heart Association class II to III; mean age 63 years) were enrolled in a placebo-controlled, double-blind study. Surface electrocardiograms were recorded and digitized at entry and at 48 and 168 hours (drug washout). Repolarization stability was assessed using an automated method measuring each QT interval in a 5-minute epoch. The QT variability index was derived as the ratio of normalized QT-to-normalized heart rate variability. Seven of 15 patients received MgSO(4). Mean heart rate and QT did not change in either group. The QT variability index was stable in the placebo group (-0.69 +/- 0.15 at entry, -0.71 +/- 0.22 at 48 hours, -0.70 +/- 0.18 at 168 hours), but decreased significantly in the treated group at 48 hours (-0.95 +/- 0.19 to -1.36 +/- 0.13, p <0.05 repeated-measures analysis of variance), returning to baseline at 168 hours (-0.84 +/- 0.18). Regression analyses showed that administration of MgSO(4) resulted in a stronger correlation between the QT and RR interval (p <0.01). Thus, MgSO(4) stabilizes cardiac repolarization in patients with compensated heart failure due to ischemic heart disease. Magnesium therapy may be useful in altering the proarrhythmic substrate in heart failure.

Laser ablation of the pulmonary veins by using a fiberoptic balloon catheter: implications for treatment of paroxysmal atrial fibrillation

BACKGROUND AND OBJECTIVE

Focal sources of paroxysmal atrial fibrillation may be treatable by electrical isolation of the pulmonary veins from the left atrium. A new fiberoptic balloon catheter was tested as an alternative to radiofrequency catheter ablation for creation of circumferential thermal lesions at the pulmonary vein orifice.

STUDY DESIGN/MATERIALS AND METHODS

In vitro and in vivo experiments were conducted in canine hearts to demonstrate efficacy and optimize ablation dosimetry. Continuous-wave, 1.06-microm, Nd:YAG laser radiation was delivered radially through diffusing optical fiber tips enclosed in a balloon catheter. During in vivo studies, the catheter was placed at the pulmonary vein orifice through a left atrial appendage sheath under X-ray fluoroscopic guidance during an open-chest procedure. Additionally, circumferential lesions in the left atrial appendage were correlated with epicardial electrograms demonstrating elimination of electrical activity.

RESULTS

The pulmonary veins were successfully ablated by using laser powers of 30--50 W and irradiation times of 60--90 seconds. Transmural, continuous, and circumferential lesions were produced in the pulmonary veins in a single application without evidence of tissue vaporization or endothelial disruption.

CONCLUSION

Laser ablation by using a fiberoptic balloon catheter may represent a promising alternative to radiofrequency catheter ablation for electrical isolation of the pulmonary veins from the left atrium during treatment of paroxysmal atrial fibrillation. Further development and testing of the fiberoptic catheter is warranted for possible clinical studies.

Prospective randomized comparison of the safety and effectiveness of placement of endocardial pacemaker and defibrillator leads using the extrathoracic subclavian vein guided by contrast venography versus the cephalic approach

The purpose of this prospective randomized study was to compare the safety and efficacy of the cephalic approach versus a contrast-guided extrathoracic approach for placement of endocardial leads. Despite an increased incidence of lead fracture, the intrathoracic subclavian approach remains the dominant approach for placement of pacemaker and implantable defibrillator leads. Although this complication can be prevented by lead placement in the cephalic vein or by lead placement in the extrathoracic subclavian or axillary vein, these approaches have not gained acceptance. A total of 200 patients were randomized to undergo placement of pacemaker or implantable defibrillator leads via the contrast-guided extrathoracic subclavian vein approach or the cephalic approach. Lead placement was accomplished in 99 of the 100 patients randomized to the extrathoracic subclavian vein approach as compared to 64 of 100 patients using the cephalic approach. In addition to a higher initial success rate, the extrathoracic subclavian vein medial approach was determined to be preferable as evidenced by a shorter procedure time and less blood loss. There was no difference in the incidence of complications. In conclusion, these results demonstrate that lead placement in the extrathoracic subclavian vein guided by contrast venography is effective and safe. It was also associated with no increased risk of complications as compared with the cephalic approach. These findings suggest that the contrast-guided approach to the extrathoracic portion of the subclavian vein should be considered as an alternative to the cephalic approach.

Linear lesions in myocardium created by Nd:YAG laser using diffusing optical fibers: in vitro and in vivo results

BACKGROUND AND OBJECTIVE

Linear lesions may be necessary for successful catheter ablation of cardiac arrhythmias such as atrial fibrillation. This study uses laser energy delivered through diffusing optical fibers as an alternative to radiofrequency energy for the creation of linear lesions in cardiac tissue in a single application.

STUDY DESIGN/MATERIALS AND METHODS

Samples of canine myocardium were placed in a heated, circulating saline bath and irradiated with a 1.06-microm, continuous-wave Nd:YAG laser during in vitro studies. Laser ablation was then performed in vivo on the epicardial surface of the right ventricle during an open-chest procedure by using similar ablation parameters. Laser energy was delivered to the tissue by being diffused radially through flexible optical fiber tips oriented parallel to the tissue surface. Histology and temperature measurements verified transmurality, continuity, and linearity of the lesions.

RESULTS

Peak tissue temperatures measured in vitro remained low (51 +/- 1 degrees C at the endocardial surface, 61 +/- 6 degrees C in the mid-myocardium, and 55 +/- 6 degrees C at the epicardial surface) with no evidence of tissue charring or vaporization. Lesion dimensions produced in vitro and in vivo were similar (depth, 6 mm; width, 8-10 mm; length, 16-22 mm), demonstrating that tissue perfusion in vivo did not significantly alter the heating.

CONCLUSION

Long linear lesions, necessary for duplication of the surgical maze procedure during catheter ablation of atrial fibrillation, may be created by using laser radiation delivered through flexible diffusing optical fiber tips. Further development of steerable catheters for endocardial atrial ablation and studies correlating thermal damage zones with electrophysiologic indicators of irreversible conduction block are warranted.

The effects of rust and anthracnose on the photosynthetic competence of diseased bean leaves

ABSTRACT The effects of rust (caused by Uromyces appendiculatus) and anthracnose (caused by Colletotrichum lindemuthianum) and their interaction on the photosynthetic rates of healthy and diseased bean (Phaseolus vulgaris) leaves were determined by gas-exchange analysis, in plants with each disease, grown under controlled conditions. The equation P(x)/P(0) = (1 - x)() was used to relate relative photosynthetic rate (P(x)/P(0)) to proportional disease severity (x), where beta represents the ratio between virtual and visual lesion. The beta values obtained for rust were near one, indicating that the effect of the pathogen on the remaining green leaf area was minimal. The high values of beta obtained for anthracnose (8.46 and 12.18) indicated that the photosynthesis in the green area beyond the necrotic symptoms of anthracnose was severely impaired. The impact of anthracnose on bean leaf photosynthesis should be considered in assessments of the proportion of healthy tissue in diseased leaves. The accurate assessment of the healthy portion of the leaf could improve the use of concepts such as healthy leaf area duration and healthy leaf area absorption, which are valuable predictors of crop yield. The equation used to analyze the interaction between rust and anthracnose on the same leaf was P(z) = P(0) (1 - x)(x) x (1 - y)(y), where P(z) is the relative photosynthetic rate of any given leaf, P(0) is the maximum relative photosynthetic rate, x is anthracnose severity, y is rust severity, betax is the beta value for anthracnose in the presence of rust, and betay is the beta value for rust in the presence of anthracnose. From the resulting response surface, no interaction of the two diseases was observed. Dark respiration rate increased on diseased leaves compared with control leaves. The remaining green leaf area of leaves with both diseases was not a good source to estimate net photosynthetic rate because the effect of anthracnose extended far beyond the visual lesions, whereas the effect of rust on photosynthesis was essentially limited to the pustule plus halo.

Left ventricular or biventricular pacing improves cardiac function at diminished energy cost in patients with dilated cardiomyopathy and left bundle-branch block

BACKGROUND

Left ventricular or biventricular pacing/stimulation can acutely improve systolic function in patients with dilated cardiomyopathy (DCM) and intraventricular conduction delay by resynchronizing contraction. Most heart failure therapies directly enhancing systolic function do so while concomitantly increasing myocardial oxygen consumption (MVO(2)). We hypothesized that pacing/stimulation, in contrast, incurs systolic benefits without raising energy demand.

METHODS AND RESULTS

Ten DCM patients with left bundle-branch block (ejection fraction 20+/-3%, QRS duration 179+/-3 ms, mean+/-SEM) underwent cardiac catheterization to measure ventricular and aortic pressure, coronary blood flow, arterial-coronary sinus oxygen difference (DeltaAVO(2)), and MVO(2). Data were measured under sinus rhythm or with left ventricular or biventricular pacing/stimulation at the same heart rate. These results were then contrasted to intravenous dobutamine (n=7) titrated to match systolic changes during LV pacing. Systolic function rose quickly and substantially from LV pacing (18+/-4% rise in arterial pulse pressure, which correlates with cardiac output, and 43+/-6% increase in dP/dt(max); both P<0.01). However, DeltaAVO(2) and MVO(2) declined -4+/-2% and -8+/-6.5%, respectively (both P<0.05). Similar results were obtained with biventricular activation. In contrast, dobutamine raised dP/dt(max) 37+/-6%, accompanied by a 22+/-11% rise in per-beat MVO(2) (P<0.05 versus pacing).

CONCLUSIONS

Ventricular resynchronization by left ventricular or biventricular pacing/stimulation in DCM patients with left bundle-branch block acutely enhances systolic function while modestly lowering energy cost. This should prove valuable for treating DCM patients with basal dyssynchrony.

Targeted disruption of the Kvlqt1 gene causes deafness and gastric hyperplasia in mice

The KvLQT1 gene encodes a voltage-gated potassium channel. Mutations in KvLQT1 underlie the dominantly transmitted Ward-Romano long QT syndrome, which causes cardiac arrhythmia, and the recessively transmitted Jervell and Lange-Nielsen syndrome, which causes both cardiac arrhythmia and congenital deafness. KvLQT1 is also disrupted by balanced germline chromosomal rearrangements in patients with Beckwith-Wiedemann syndrome (BWS), which causes prenatal overgrowth and cancer. Because of the diverse human disorders and organ systems affected by this gene, we developed an animal model by inactivating the murine Kvlqt1. No electrocardiographic abnormalities were observed. However, homozygous mice exhibited complete deafness, as well as circular movement and repetitive falling, suggesting imbalance. Histochemical study revealed severe anatomic disruption of the cochlear and vestibular end organs, suggesting that Kvlqt1 is essential for normal development of the inner ear. Surprisingly, homozygous mice also displayed threefold enlargement by weight of the stomach resulting from mucous neck cell hyperplasia. Finally, there were no features of BWS, suggesting that Kvlqt1 is not responsible for BWS.

Visualization and temporal/spatial characterization of cardiac radiofrequency ablation lesions using magnetic resonance imaging

BACKGROUND

The purpose of this study was to describe a system and method for creating, visualizing, and monitoring cardiac radiofrequency ablation (RFA) therapy during magnetic resonance imaging (MRI).

METHODS AND RESULTS

RFA was performed in the right ventricular apex of 6 healthy mongrel dogs with a custom 7F nonmagnetic ablation catheter (4-mm electrode) in a newly developed real-time interactive cardiac MRI system. Catheters were positioned to intracardiac targets by use of an MRI fluoroscopy sequence, and ablated tissue was imaged with T2-weighted fast spin-echo and contrast-enhanced T1-weighted gradient-echo sequences. Lesion size by MRI was determined and compared with measurements at gross and histopathological examination. Ablated areas of myocardium appeared as hyperintense regions directly adjacent to the catheter tip and could be detected 2 minutes after RF delivery. Lesions reached maximum size approximately 5 minutes after ablation, whereas lesion signal intensity increased linearly with time but then reached a plateau at 12.2+/-2.1 minutes. Lesion size by MR correlated well with actual postmortem lesion size and histological necrosis area (55.4+/-7.2 versus 49.7+/-5.9 mm(2), r=0.958, P<0.05).

CONCLUSIONS

RFA can be performed in vivo in a new real-time interactive cardiac MRI system. The spatial and temporal extent of cardiac lesions can be visualized and monitored by T2- and T1-weighted imaging, and MRI lesion size agrees well with actual postmortem lesion size. MRI-guided RFA may be a useful approach to help facilitate anatomic lesion placement and to provide insight into the biophysical effects of new ablation techniques and technologies.

Resuscitation after prolonged ventricular fibrillation with use of monophasic and biphasic waveform pulses for external defibrillation

BACKGROUND

Survival after prolonged ventricular fibrillation (VF) appears severely limited by 2 major factors: (1) low defibrillation success rates and (2) persistent post-countershock myocardial dysfunction. Biphasic (BP) waveforms may prove capable of favorably modifying these limitations. However, they have not been rigorously tested against monophasic (MP) waveforms in clinical models of external defibrillation, particularly where rescue from prolonged VF is the general rule.

METHODS AND RESULTS

We randomized 26 dogs to external countershocks with either MP or BP waveforms. Hemodynamics were assessed after shocks applied during sinus rhythm, after brief VF (>10 seconds), and after resuscitation from prolonged VF (>10 minutes). Short-term differences in percent change in left ventricular +dP/dt(max) (MP -16+/-28%, BP +9.1+/-24%; P=0.03) and left ventricular -dP/dt(max) (MP -37+/-26%, BP -18+/-20%; P=0.05) were present after rescue from brief VF, with BP animals exhibiting less countershock-induced dysfunction. After prolonged VF, the BP group had lower mean defibrillation thresholds (107+/-57 versus 172+/-88 J for MP, P=0.04) and significantly shorter resuscitation times (397+/-73.7 versus 488+/-74.3 seconds for MP, P=0.03).

CONCLUSIONS

External defibrillation is more efficacious with BP countershocks than with MP countershocks. The lower defibrillation thresholds and shorter resuscitation times associated with BP waveform defibrillation may improve survival after prolonged VF arrest.

Predictors of systolic augmentation from left ventricular preexcitation in patients with dilated cardiomyopathy and intraventricular conduction delay

BACKGROUND

VDD pacing can enhance systolic function in patients with dilated cardiomyopathy and discoordinate contraction; however, identification of patients likely to benefit is unclear. We tested predictors of systolic responsiveness on the basis of global parameters as well as directly assessed mechanical dyssynchrony.

METHODS AND RESULTS

Twenty-two DCM patients with conduction delay were studied by cardiac catheterization with a dual-sensor micromanometer to measure LV and aortic pressures during sinus rhythm and LV free-wall pacing. Pacing enhanced isovolumetric (dP/dt(max)) and ejection-phase (pulse pressure, PP) systolic function by 35+/-21% and 16.4+/-11%, respectively, and these changes correlated directly (r=0.7, P=0.001). %DeltadP/dt(max) was weakly predicted by baseline QRS (r=0.6, P<0.02), more strongly by baseline dP/dt(max) (r=0.7, P=0.001), and best by bidiscriminate analysis combining baseline dP/dt(max) < or =700 mm Hg/s and QRS > or =155 ms to predict %DeltadP/dt(max) > or =25% and %DeltaPP > or =10% (P<0.0005, chi(2)), with no false-positives. Benefit could not be predicted by %DeltaQRS. To test whether basal mechanical dyssynchrony predicted responsiveness to LV pacing, circumferential strains were determined at approximately 80 sites throughout the LV by tagged MRI in 8 DCM patients and 7 additional control subjects. Strain variance at time of maximal shortening indexed dyssynchrony, averaging 28.0+/-7.1% in normal subjects versus 201.4+/-84.3% in DCM patients (P=0.001). Mechanical dyssynchrony also correlated directly with %DeltadP/dt(max) (r=0.85, P=0.008). Conclusions-These results show that although mechanical dyssynchrony is a key predictor for pacing efficacy in DCM patients with conduction delay, combining information about QRS and basal dP/dt(max) provides an excellent tool to identify maximal responders.

Temporal repolarization lability in hypertrophic cardiomyopathy caused by beta-myosin heavy-chain gene mutations

BACKGROUND

Certain genetic mutations associated with hypertrophic cardiomyopathy (HCM) carry an increased risk of sudden death. QT variability identifies patients at a high risk for sudden death from ventricular arrhythmias. We tested whether patients with HCM caused by beta-myosin heavy-chain (beta-MHC) gene mutations exhibit labile ventricular repolarization using beat-to-beat QT variability analysis.

METHODS AND RESULTS

We measured the QT variability index and heart rate-QT interval coherence from Holter monitor recordings in 36 patients with HCM caused by known beta-MHC gene mutations and in 26 age- and sex-matched controls. There were 7 distinct beta-MHC gene mutations in these 36 patients; 9 patients had HCM caused by the malignant Arg(403)Gln mutation and 8 patients had HCM caused by the more benign Leu(908)Val mutation. The QT variability index was higher in HCM patients than in controls (-1.24+/-0.17 versus -1. 58+/-0.38, P<0.01), and the greatest abnormality was detected in patients with the Arg(403)Gln mutation (-0.99+/-0.49 versus -1. 46+/-0.43 in controls, P<0.05). In keeping with this finding, coherence was lower for the entire HCM group than for controls (P<0. 001). Coherence was also significantly lower in patients with the Arg(403)Gln mutation compared with controls (P<0.05).

CONCLUSIONS

These findings suggest that (1) patients with HCM caused by beta-MHC gene mutations exhibit labile repolarization quantified by QT variability analysis and, hence, may be more at risk for sudden death from ventricular arrhythmias, and (2) indices of QT variability may be particularly abnormal in patients with beta-MHC gene mutations that are associated with a poor prognosis.

Prospective comparison of lesions created using a multipolar microcatheter ablation system with those created using a pullback approach with standard radiofrequency ablation in the canine atrium

The aim of this study was to compare the lesions created using a multipolar microcatheter (MICRO) ablation system in the right canine atrium to a pullback approach with a standard radiofrequency (STND RF) ablation and to determine the value of electrogram amplitude and pacing threshold in predicting transmurality of lesions. Ten dogs underwent right atrial ablation using a MICRO (6 dogs) or STND RF (4 dogs) ablation system in each animal. Attempts were made to create linear RF lesions at four predetermined atrial sites. RF energy was delivered for 60 seconds using closed-loop, temperature control to achieve a target temperature of 60 degrees C for STND RF and 50 degrees C for MICRO. Unipolar atrial electrogram amplitude and atrial pacing threshold were obtained before and after ablation. Pathological analysis was determined at 4 weeks after ablation. Lesions created with MICRO were narrower, more likely to be continuous, and more likely to be anchored to an anatomic structure than those lesions which were created using a STND RF. No difference was observed in overall lesion length or in the proportion of lesions that were transmural over at least 50% of their length. Of lesions created using MICRO, a significant relation was observed between transmurality of lesion and unipolar electrogram amplitude as well as pacing threshold. Further studies are needed to determine if this type of ablation technique and parameters during ablation may facilitate a successful catheter-based MAZE procedure.

Ventricular pacing with premature excitation for treatment of hypertensive-cardiac hypertrophy with cavity-obliteration

BACKGROUND

Hypertensive left ventricular hypertrophy with supranormal systolic ejection and distal cavity obliteration (HHCO) can result in debilitating exertional fatigue and dyspnea. Dual-chamber pacing with ventricular preactivation generates discoordinate contraction, which can limit cavity obliteration and thereby increase potential ejection reserve. Accordingly, we hypothesized that pacing may improve exercise tolerance long-term in this syndrome.

METHODS AND RESULTS

Dual-chamber pacemakers were implanted in 9 patients with exertional dyspnea caused by HHCO. Intrinsic atrial rate was sensed, and ventricular preactivation was achieved by shortening the atrial-ventricular delay. Pacing was on or off for successive 3-month periods (randomized, double-blind, crossover design), followed by 6 additional pacing-on months. Metabolic exercise testing, quality-of-life assessment, and rest and dobutamine-stress echocardiographic/Doppler data were obtained. After 3 months of pacing-on, exercise duration rose from 324+/-133 to 588+/-238 s (mean+/-SD; P=0.001, with 7 of 9 patients improving >/=30%), and maximal oxygen consumption increased from 13.6+/-2.9 to 16.7+/-3.3 mL of O(2). min(-1). kg(-1) (P<0.02). Both parameters were little changed from baseline during the pacing-off period. Improved exercise capacity persisted at 1-year follow-up. Clinical symptoms and activities of daily living improved during the pacing-on period and stayed improved at 1 year, but they were little changed during the pacing-off period. Despite similar basal values, stroke volume (P<0.001) and cardiac output (P<0.02) increased with dobutamine stimulation 2 to 3 times more after 1 year of follow-up as compared with baseline.

CONCLUSIONS

Long-term dual-chamber pacing can improve exercise capacity, cardiac reserve, clinical symptoms, and activities of daily living in patients with HHCO. This therapy may provide a novel alternative for patients in whom traditional pharmacological treatment proves inadequate.

Incidence-Severity Relationships in the Pathosystem Coffea arabica-Hemileia vastatrix

Incidence-severity relationships for coffee rust were studied to determine if the easily assessed incidence could be used to evaluate host resistance and fungicide treatment. At two locations in each of 3 years, the incidence of rust on 300 leaves was compared with two assessments of severity: (i) the average number of sporulating pustules per leaf, and (ii) the estimated leaf area with rust. For nine or 10 assessments in time at one location and pooled over 3 years, the average number of sporulating pustules per leaf (Y _sp) was well related with the incidence of leaves with rust (X) as Y _sp = 0.02982+ 0.017035X +0.000573X ²; R ² = 0.87. The leaf area with rust (Y _la) was also well related with incidence of leaves with rust as Y _la = 0.001 - 0.01076X +0.008376X ²; R ₂ = 0.92. For two independent data sets from a second location obtained over two seasons, the above models satisfactorily fit the relationships for the average sporulating pustules per leaf (R ² = 0.97 and 0.96) and for the estimated leaf area with rust (R ² = 0.95 and 0.98). Therefore, the readily determined incidence can be used to estimate both measures of disease severity of coffee rust.

Plectosporium tabacinum, a Pathogen of the Invasive Aquatic Weed Hydrilla verticillata in Florida

Plectosporium tabacinum, the anamorph of Plectosphaerella cucumerina, was isolated in 1996 from Hydrilla verticillata (hydrilla), an invasive aquatic weed in Florida. P. tabacinum, applied as a suspension of conidia, was pathogenic to hydrilla shoots maintained in aqueous solutions in test tubes. Koch's postulates were fulfilled in several repeated experiments. Infected shoots became slightly chlorotic within 24 h after inoculation. Infected leaves remained intact and were supported by water pressure but collapsed upon removal from water. Histological studies of leaves stained with malachite green and acid fuchsin revealed fungal hyphae within plant cells. The disease developed over a range of temperatures from 15 to 30°C. At 25°C, symptoms were most severe in 5% Hoagland's solution, followed by river water, deionized water, 0.5% Hoagland's, tap water, and spring water. Disease severity increased as inoculum concentration was increased from 10⁵ to 10⁷ conidia ml^-1. This is the first report of P. tabacinum as a pathogen of hydrilla, a fully submerged aquatic plant species.

Interaction between neuronal nitric oxide synthase and inhibitory G protein activity in heart rate regulation in conscious mice

Nitric oxide (NO) synthesized within mammalian sinoatrial cells has been shown to participate in cholinergic control of heart rate (HR). However, it is not known whether NO synthesized within neurons plays a role in HR regulation. HR dynamics were measured in 24 wild-type (WT) mice and 24 mice in which the gene for neuronal NO synthase (nNOS) was absent (nNOS-/- mice). Mean HR and HR variability were compared in subsets of these animals at baseline, after parasympathetic blockade with atropine (0.5 mg/kg i.p.), after beta-adrenergic blockade with propranolol (1 mg/kg i.p.), and after combined autonomic blockade. Other animals underwent pressor challenge with phenylephrine (3 mg/kg i.p.) after beta-adrenergic blockade to test for a baroreflex-mediated cardioinhibitory response. The latter experiments were then repeated after inactivation of inhibitory G proteins with pertussis toxin (PTX) (30 microgram/kg i.p.). At baseline, nNOS-/- mice had higher mean HR (711+/-8 vs. 650+/-8 bpm, P = 0.0004) and lower HR variance (424+/-70 vs. 1,112+/-174 bpm2, P = 0.001) compared with WT mice. In nNOS-/- mice, atropine administration led to a much smaller change in mean HR (-2+/-9 vs. 49+/-5 bpm, P = 0.0008) and in HR variance (64+/-24 vs. -903+/-295 bpm2, P = 0.02) than in WT mice. In contrast, propranolol administration and combined autonomic blockade led to similar changes in mean HR between the two groups. After beta-adrenergic blockade, phenylephrine injection elicited a fall in mean HR and rise in HR variance in WT mice that was partially attenuated after treatment with PTX. The response to pressor challenge in nNOS-/- mice before PTX administration was similar to that in WT mice. However, PTX-treated nNOS-/- mice had a dramatically attenuated response to phenylephrine. These findings suggest that the absence of nNOS activity leads to reduced baseline parasympathetic tone, but does not prevent baroreflex-mediated cardioinhibition unless inhibitory G proteins are also inactivated. Thus, neuronally derived NO and cardiac inhibitory G protein activity serve as parallel pathways to mediate autonomic slowing of heart rate in the mouse.

Beat-to-beat repolarization lability identifies patients at risk for sudden cardiac death

INTRODUCTION

Recent studies have implicated repolarization lability in the genesis of malignant ventricular arrhythmias. However, few data exist on assessment of temporal QT interval variability and its relation to arrhythmogenesis. We tested the ability of the QT variability index (QTVI), a measure of beat-to-beat QT interval fluctuations measured on a single ECG lead, to identify patients presenting with malignant ventricular arrhythmias and predict their subsequent occurrences.

METHODS AND RESULTS

We measured the QTVI in 95 patients presenting for electrophysiologic study (EPS). The ability of the QTVI to identify patients with sudden cardiac death (SCD) or sustained monomorphic ventricular tachycardia (MVT) on presentation and during follow-up of 23.7+/-14.3 months was compared with spatial QT dispersion, T wave alternans ratio during atrial pacing, MVT inducibility at EPS, signal-averaged ECG, heart rate variability, and ejection fraction. The QTVI was higher in patients with heart disease than in controls (-0.7+/-0.7 vs -1.1+/-0.5, P < 0.05), and higher in patients presenting with SCD than in other patients with heart disease (0.0+/-0.6 vs -0.8+/-0.5, P < 0.05). The QTVI was the only clinical variable that identified patients who presented with SCD (P = 0.004, odds ratio = 12.5) on stepwise, logistic multiple regression. Fourteen patients had arrhythmic events during follow-up. In a Kaplan-Meier analysis of arrhythmic events, QTVI> or =0.1 was a discriminator for higher risk of arrhythmic events (P < 0.05).

CONCLUSIONS

(1) This noninvasive measure of temporal repolarization lability identified patients with SCD and predicted arrhythmia-free survival. (2) Further studies are needed to determine the mechanisms that mediate beat-to-beat QT interval variability.

Predictors of fluoroscopy time and estimated radiation exposure during radiofrequency catheter ablation procedures

The objective of this study was to identify factors that predict fluoroscopy duration and radiation exposure during catheter ablation procedures. The patient population included 859 patients who participated in the Atakr Ablation System clinical trial at 1 of 9 centers (398 male and 461 female patients, aged 36 +/- 21 years). Each patient underwent catheter ablation of an accessory pathway, the atrioventricular junction, or atrioventricular nodal reentrant tachycardia using standard techniques. The duration of fluoroscopy was 53 +/- 50 minutes. Factors identified as independent predictors of fluoroscopy duration included patient age and sex, the success or failure of the ablation procedure, and the institution at which the ablation was performed. Catheter ablation in adults required longer fluoroscopy exposure than it did in children. Men required longer durations of fluoroscopy exposure than did women. The mean estimated "entrance" radiation dose was 1.3 +/- 1.3 Sv. The dose needed to cause radiation skin injury was exceeded during 22% of procedures. The overall mean effective absorbed dose from catheter ablation procedures was 0.025 Sv for female patients and 0.017 Sv for male patients. This degree of radiation exposure would result in an estimated 1,400 excess fatal malignancies in female patients and 2,600 excess fatal malignancies in male patients per 1 million patients.

Dispersal of Spores of Fusarium culmorum in Aquatic Systems

ABSTRACT The dispersal of spores of Fusarium culmorum, a biological control agent for the aquatic weed Hydrilla verticillata, was investigated in aquatic systems. Macroconidia and chlamydospores that were applied to the surface of the liquid settled rapidly in deionized water, tap water, 5% Hoagland's solution, natural spring water, or river water held in glass containers. The rate of fall, as measured for 50% of the spores, was determined to be 9 cm h(-1). Rapid lateral dispersal of spores from a point source occurred in still water. This initial spore movement occurred at a rate of >9 m h(-1) (15 cm min(-1)), approximately 100 times faster than the rate of settlement. The spores attained an even lateral distribution in a still, closed system. Spores dispersed rapidly in moving water and were transported with the water current. Spores were determined to carry positive electrostatic charges as they migrated towards the negative pole during electrophoresis. The physical components of dispersal of F. culmorum spores were defined in a still aquatic system to consist of rapid lateral dispersal and sinking due to gravity. In moving water, the dynamics of water movement was superimposed over the other two factors.

Human heart rate variability relation is unchanged during motion sickness

In a study of 18 human subjects, we applied a new technique, estimation of the transfer function between instantaneous lung volume (ILV) and instantaneous heart rate (HR), to assess autonomic activity during motion sickness. Two control recordings of ILV and electrocardiogram (ECG) were made prior to the development of motion sickness. During the first, subjects were seated motionless, and during the second they were seated rotating sinusoidally about an earth vertical axis. Subjects then wore prism goggles that reverse the left-right visual field and performed manual tasks until they developed moderate motion sickness. Finally, ILV and ECG were recorded while subjects maintained a relatively constant level of sickness by intermittent eye closure during rotation with the goggles. Based on analyses of ILV to HR transfer functions from the three conditions, we were unable to demonstrate a change in autonomic control of heart rate due to rotation alone or due to motion sickness. These findings do not support the notion that moderate motion sickness is manifested as a generalized autonomic response.

Comparison of heart rate variability in patients with chronic fatigue syndrome and controls

Recent studies have reported a close association between chronic fatigue syndrome and neurally mediated hypotension. We hypothesized that this association may result from an abnormality in autonomic function among patients with chronic fatigue syndrome, which may be detectable using an analysis of heart rate variability. We prospectively studied 19 patients who fulfilled the Centers for Disease Control criteria for chronic fatigue syndrome and 11 controls. Each subject underwent a two-stage tilt-table test while wearing a Holter monitor. Heart rate variability was assessed in the supine baseline position and during upright tilt using frequency domain parameters. In the baseline supine position, high frequency (HF) power, low frequency (LF) power, and the ratio of low frequency power to high frequency power (LF/HF ratio) were similar. In both patient groups, upright tilt resulted in a similar decrease in HF power, increase in LF power, and increase in the LH/HF ratio. In conclusion, autonomic function, as assessed using an analysis of heart rate variability, does not differ in the baseline supine state, nor in response to upright tilt among patients with chronic fatigue syndrome and healthy controls.

Lesion expansion as an epidemic component

ABSTRACT A simulator for the enlargement of cohorts of circular lesions on cohorts of host tissue was used to examine five epidemiological parameters: radial rate (mm day(-1)) of lesion expansion, k (exp); maximum basic infection rate, R (m); proportion of lesion area as infectious, f; initial lesion size (mm(2)), z; and proportion of susceptible host sites, s. Based on the proportion of disease severity at day 50 and the proportion of the total disease that originated solely from lesion expansion, k(exp) was the most sensitive of the five parameters. A radial rate of only 0.1 mm day(-1) resulted in a proportion of >0.7 of the diseased area that came from lesion expansion. In an extensive survey of phytopathological literature, many plant pathogens had radial rates greater than 0.1 mm day(-1), which would result in a proportion of >0.95 of the total disease that comes from lesion expansion. Susceptible host sites, s, was a sensitive parameter, as this determined the host area into which lesions could expand. Naturally, R(m) was a sensitive parameter for the proportion of disease on day 50, as it controlled the overall speed of the epidemic. Initial lesion size was a relatively insensitive parameter, although z interacted significantly with s. The greatest proportion of disease that originated from lesion expansion occurred with fast k(exp), small z, and low values of s, R(m), and f. The model was validated with lesion numbers and severities obtained in natural epidemics of Cercospora medicaginis on alfalfa and Exserohilum turcicum on maize. We recommend that the 'epidemic quintuplet' used to describe polycyclic epidemics be expanded to the 'epidemic sextuplet' with the inclusion of k(exp), since lesion expansion is a major component of many polycyclic epidemics.

Beat-to-beat QT interval variability: novel evidence for repolarization lability in ischemic and nonischemic dilated cardiomyopathy

BACKGROUND

Dilated cardiomyopathy (DCM) is associated with a high incidence of malignant ventricular arrhythmias and sudden death. Abnormalities in repolarization of ventricular myocardium have been implicated in the development of these arrhythmias. Spatial heterogeneity in repolarization has been studied in DCM, but temporal fluctuations in repolarization in this setting have been largely ignored. We sought to test the hypothesis that beat-to-beat QT interval variability is increased in DCM patients compared with control subjects.

METHODS AND RESULTS

Eighty-three patients with ischemic and nonischemic DCM and 60 control subjects served as the study population. Beat-to-beat QT interval variability was measured by automated analysis on the basis of 256-second records of the surface ECG. A QT variability index (QTVI) was calculated for each subject as the logarithm of the ratio of normalized QT variance to heart rate variance. The coherence between heart rate and QT interval fluctuations was determined by spectral analysis. In patients, ejection fractions were assessed by echocardiography or ventriculography, and spatial QT dispersion was determined from the standard 12-lead ECG. DCM patients had greater QT variance than control subjects (60.4+/-63.1 versus 25.7+/-24.8 ms2, P<.0001) despite reduced heart rate variance (6.7+/-7.8 versus 10.5+/-10.4 bpm2, P=.01). The QTVI was higher in DCM patients than in control subjects, with a high degree of significance (-0.43+/-0.71 versus -1.29+/-0.51, P<10[-12]). QTVI did not correlate with ejection fraction or spatial QT dispersion but did depend on New York Heart Association functional class. QTVI did not differ between DCM patients with ischemic and those with nonischemic origin. Coherence between heart rate and QT interval fluctuations at physiological frequencies was lower in DCM patients compared with control subjects (0.28+/-0.14 versus 0.39+/-0.18, P<.0001).

CONCLUSIONS

DCM is associated with beat-to-beat fluctuations in QT interval that are larger than normal and uncoupled from variations in heart rate. QT interval variability increases with worsening functional class but is independent of ejection fraction. These data indicate that DCM leads to temporal lability in ventricular repolarization.

Disease Progress, Yield Loss, and Control of Xanthomonas fragariae on Strawberry Plants

The progress of angular leaf spot, caused by the bacterium Xanthomonas fragariae, was examined in field plots of strawberry in the 1994 and 1995 seasons. Disease severity increased intermittently to maxima of circa 25% in 1994 and 15% in 1995. Angular leaf spot reduced marketable yield 8.6% in 1994 and 7.7% in 1995, despite differences in disease severity and base marketable yields for the two seasons. Minimal spread of the pathogen occurred from field plots with inoculated plants to plots with non-inoculated plants. A mixture of cupric hydroxide plus mancozeb was applied at the label rate (1×) at 7- to 14-day intervals and at a reduced rate (0.1×) at 2- to 4-day intervals. The bactericidal mixture at the 1× rate significantly decreased disease, but this mixture was phytotoxic; both plant size and yield were reduced. The 0.1× rate was nonphytotoxic and it reduced disease severity in both years and increased yield in 1994. Lesions of angular leaf spot were detected on strawberry transplants imported from nurseries in Canada and northern United States in both 1993 and 1994. X. fragariae was isolated from those lesions.

Angular leaf spot of phaseolus beans: relationships between disease, healthy leaf area, and yield

ABSTRACT Five field experiments were conducted to investigate the relationship between the severity of visible disease (X), area under the disease progress curve (AUDPC), healthy leaf area index on any given day (HLAI), radiation intercepted by healthy leaf area on any given day (HRI), healthy leaf area duration (HAD), total healthy leaf area absorption (HAA), and yield of Phaseolus beans, cultivars Rosinha and Carioca, inoculated with Phaeoisariopsis griseola at several doses. In general, yield was not related to disease severity (X) or AUDPC. In contrast, the highest yields were always related to the highest values of HAD and HAA. The relationship between yield and HAD was linear in each of five trials (29.9 < R(2) < 70.2%, P < 0.001). The relationship between yield and HAA was linear in four of the trials (52.3 < R(2) < 70.3%, P < 0.001) and exponential in one of them (in which the plant canopy was the largest). Singlepoint models using HRI to estimate yield at various times during the crop season were developed. The slope of the yield-HRI relationship proved to be stable (26.8 +/-2.4 g MJ(-1)), regardless of cultivar, locale, planting date, and bean growth stage (from R5 to R8). The yield-HLAI relationship proved to be less consistent. HRI is proposed as a key explanatory variable for a transportable system of disease management; it may be useful in producing precise recommendations at the farm level.

Signal-averaged intracardiac electrograms: a new method to detect kent potentials

INTRODUCTION

In patients with manifest accessory pathways, Kent potentials are often difficult to identify even at sites of successful catheter ablation, due largely to signal noise and catheter instability. We hypothesized that signal averaging the intracardiac electrogram recorded from the ablation catheter over a number of beats would improve the signal-to-noise ratio of the electrogram and aid in the detection of Kent potentials at accessory pathway locations.

METHODS AND RESULTS

We retrospectively analyzed distal-pair electrograms recorded from 9 successful, 6 transiently successful, and 10 failed ablation sites in 10 patients with manifest accessory pathways who underwent catheter ablation. We developed custom software to finely align 20 to 30 consecutive sinus beats and compute the signal average of the electrogram (SAE) for each site. Kent potentials were classified as probable, possible, or absent in the raw ablation site electrogram and the SAE base on morphologic criteria. A measure of beat-to-beat signal instability, the variability quotient (VQ), was also computed for each site. Probable Kent potentials were found in the raw ablation site electrogram at only 2 of the 15 successful and transiently successful sites, but were found in the SAE at 10 of these sites (P = 0.008). Eight of the 9 successful sites had VQ < 0.2, suggesting stable catheter-tissue contact, while 3 of the 6 transiently successful sites had VQ > 0.2, indicating unstable contact.

CONCLUSIONS

Signal averaging the intracardiac ablation site electrogram enhances detection of Kent potentials at accessory pathway locations. Catheter instability can be quantified by signal variability analysis and, when high, may predict lack of successful ablation even at sites where Kent potentials are present.

Recurrence of conduction following radiofrequency catheter ablation procedures: relationship to ablation target and electrode temperature. The Atakr Multicenter Investigators Group

INTRODUCTION

More than 1 in 10 patients may develop recurrence of conduction after undergoing a successful radiofrequency catheter ablation procedure. The physiologic basis for recurrence following successful ablation procedures remains uncertain. The purpose of this study was to evaluate the role of electrode temperature as a predictor of recurrence following radiofrequency catheter ablation procedures.

METHODS AND RESULTS

The subjects of this study were 538 patients who underwent a successful attempt at radiofrequency catheter ablation of AV nodal reentrant tachycardia, an accessory pathway, and/or the AV junction. Patients were followed for a mean of 215 +/- 138 days. Conduction recurred in 35 (6.5%) of the 538 patients. Recurrence of conduction occurred in 25 (9.3%) of 270 patients undergoing ablation of an accessory pathway, 7 (3.5%) of 201 patients undergoing ablation of AV nodal reentrant tachycardia, and in 3 (4.5%) of 67 patients undergoing ablation of the AV junction. The electrode temperature achieved at successful sites associated with recurrence was not different from the temperature achieved at successful sites without recurrence (61.1 +/- 8.9 vs 61.6 +/- 9.1; P = 0.8). The likelihood of developing a recurrence was higher following ablation of accessory pathways than following ablation of AV nodal reentrant tachycardia or the AV junction (P = 0.03). Patients experiencing a recurrence following ablation of an accessory pathway had longer procedure durations (P = 0.0001). Ablation of left free-wall pathways was associated with a lower incidence of recurrence as compared with all other locations (P = 0.008).

CONCLUSION

The results of this study suggest that electrode temperature at the successful ablation site cannot be used to identify patients at highest risk of recurrence.

Catheter ablation of coexistent bundle branch and interfascicular reentrant ventricular tachycardias

INTRODUCTION

Bundle branch reentry and interfascicular reentry are potential mechanisms of ventricular tachycardia in the setting of a dilated cardiomyopathy. We report a patient with myotonic dystrophy who presented with near syncope, in whom both of these mechanisms were present, leading to two different wide complex tachycardias.

METHODS AND RESULTS

Electrophysiologic study demonstrated infra-Hisian conduction system disease and inducible bundle branch reentrant ventricular tachycardia. Catheter ablation of the right bundle eliminated bundle branch reentry. However, following this, the patient had inducible interfascicular reentry, which subsequently occurred spontaneously while still hospitalized. Catheter ablation of the left posterior fascicle successfully eliminated this second tachycardia, and the patient has had no further arrhythmias.

CONCLUSIONS

This report is of an unusual patient with coexistent bundle branch reentry and interfascicular reentry producing two different forms of sustained ventricular tachycardia. This is the first report of catheter ablation of the left posterior fascicle for elimination of conduction system reentry.

Temperature monitoring during radiofrequency ablation

Thermal injury is the primary mechanism of lesion formation during radiofrequency catheter ablation procedures. Irreversible tissue injury requires heating to approximately 50 degrees C. Temperatures above 100 degrees C result in coagulum formation. Because of this importance of temperature during radiofrequency catheter ablation procedures, temperature monitoring has been proposed as a tool to facilitate catheter ablation procedures. The results of recent clinical studies demonstrate that electrode temperatures do not differ at successful and failed ablation sites, electrode temperature does not predict or eliminate the possibility of arrhythmia recurrence, and closed-loop temperature control decreases but does not eliminate the development of coagulum nor guarantees that target temperatures will be achieved. These observations are due in large part to the important distinctions between electrode temperature, the temperature at the electrode-tissue interface, and the temperature at the ablation target. Nonetheless, temperature monitoring and temperature control are valuable tools during radiofrequency ablation procedures as they provide important information regarding the adequacy of tissue heating, minimize the development of coagulum, and maximize lesion size.

Role of atrial contraction in diastolic pressure elevation induced by rapid pacing of hypertrophied canine ventricle

The mechanism of diastolic pressure elevation induced by acute rapid pacing in pressure-load hypertrophied left ventricles (LVs) remains incompletely understood. It has been ascribed to abnormalities of coronary flow, metabolism, and calcium cycling. However, rapid pacing also alters the timing of atrial and ventricular stimulation relative to the diastolic filling period, and this could also influence diastolic pressures. To test the role of such mechanical factors, LV pressure-volume hemodynamics were measured in closed-chested anesthetized dogs during and after abrupt cessation of rapid atrial pacing. Twenty-one dogs were studied: 6 dogs with LV hypertrophy (LVH) induced by perinephritic hypertension, 5 sham-operated normotensive dogs, and 10 acute normotensive control dogs. In LVH dogs, but not in sham-operated or control dogs, end-diastolic pressure rose progressively with increasing heart rate from 5.6 +/- 3.1 mm Hg at baseline to 22.6 +/- 8.1 mm Hg at 220 beats per minute. In all hearts, rapid pacing shifted the timing of left atrial contraction so that it occurred near the onset of LV filling rather than at end diastole. However, in LVH hearts, early LV diastolic pressure and peak atrial pressure were also markedly elevated. Most striking, immediately after terminating the pacing, diastolic pressure declined to near baseline. This rapid pressure decline occurred just when atrial systole would have ensued and before ventricular activation would have followed had pacing continued. Thus, diastolic pressure elevation resolved before a change in ventricular pacing rate. The role of atrial contraction was further explored by simultaneous atrioventricular pacing. This shifted the time of atrial systole so that it occurred during LV isovolumic contraction, while maintaining the identical LV pacing rate. This change eliminated the diastolic pressure elevation found previously. Further analysis revealed that the pressure increase during rapid pacing was not due simply to partial LV filling imposed on a relaxing ventricle or to hypertension or an intact pericardium. These data indicate that mechanical effects of atrioventricular interaction play an important role in tachycardia-induced diastolic dysfunction in this model of LVH and can be more causative than ischemia or metabolic factors in this setting.

Sudden cardiac death in heart failure. The role of abnormal repolarization

Congestive heart failure is a common, highly lethal cardiovascular disorder claiming over 200,000 lives a year in the United States alone. Some 50% of the deaths in heart failure patients are sudden, and most of these are probably the result of ventricular tachyarrhythmias. Methods designed to identify patients at risk have been remarkably unrewarding, as have attempts to intervene and prevent sudden death in these patients. The failure to impact favorably on the incidence of sudden death in heart failure patients stems largely from a lack of understanding of the underlying mechanisms of arrhythmogenesis. This article explores the role of abnormalities of ventricular repolarization in heart failure patients. We will examine evidence for the hypothesis that alteration of repolarizing K+ channel expression in failing myocardium predisposes to abnormalities in repolarization that are arrhythmogenic. The possible utility of novel electrophysiological and ECG measures of altered ventricular repolarization will be explored. Understanding the mechanism of sudden death in heart failure may lead to effective therapy and more accurate identification of patients at greatest risk.

A time domain approach for the fluctuation analysis of heart rate related to instantaneous lung volume

This paper presents a time domain technique for estimating transfer characteristics from fluctuations of instantaneous lung volume (ILV) to heart rate (HR). An effective procedure for estimating the impulse response of HR to ILV is proposed. Pre- and post-processing procedures, including prefiltering of the HR signal, preenhancement of the high frequency content of the ILV signal, and post-filtering of the estimated impulse response, together with a random breathing technique, are shown to effectively reduce spurious transfer gain so as to get a stable estimate of the impulse response. Analysis of the data collected from fourteen healthy male subjects in various conditions revealed that there are three components in the impulse response: fast positive, delayed slow negative, and oscillatory. The effects of the autonomic blocking agents propranolol and atropine on these transfer characteristics are also described.

Transfer function analysis of the circulation: unique insights into cardiovascular regulation

We have demonstrated previously that transfer function analysis can be used to precisely characterize the respiratory sinus arrhythmia (RSA) in normal humans. To further investigate the role of the autonomic nervous system in RSA and to understand the complex links between respiratory activity and arterial pressure, we determined the transfer functions between respiration, heart rate (HR), and phasic, systolic, diastolic, and pulse arterial pressures in 14 healthy subjects during 6-min periods in which the respiratory rate was controlled in a predetermined but erratic fashion. Pharmacological autonomic blockade with atropine, propranolol, and both, in combination with changes in posture, was used to characterize the sympathetic and vagal contributions to these relationships, as well as to dissect the direct mechanical links between respiration and arterial pressure from the effects of the RSA on arterial pressure. We found that 1) the pure sympathetic (standing + atropine) HR response is characterized by markedly reduced magnitude at frequencies greater than 0.1 Hz and a phase delay, whereas pure vagal (supine + propranolol) modulation of HR is characterized by higher magnitude at all frequencies and no phase delay; 2) both the mechanical links between respiration and arterial pressure and the RSA contribute significantly to the effects of respiration on arterial pressure; 3) the RSA contribution to arterial pressure fluctuations is significant for vagal but not for sympathetic modulation of HR; 4) the mechanical effects of respiration on arterial pressure are related to the negative rate of change of instantaneous lung volume; 5) the mechanical effects have a higher magnitude during systole than during diastole; and 6) the mechanical effects are larger in teh standing than the supine position. Most of these findings can be explained by a simple model of circulatory control based on previously published experimental transfer functions from our laboratory.

Growth, Development, Yield, and Seed Quality of Florunner Peanut Affected by Late Leaf Spot1

Late leaf spot, induced by Cercosporidium personatum (Berk. & Curt.) Deighton, causes serious yield losses of peanut (Arachis hypogaea L.) in the southeastern United States. A two-year study was conducted to observe progress of late leaf spot and to evaluate subsequent effects of late leaf spot on accumulation of dry matter, leaf area index (LAI), and pod production of Florunner peanut in fungicide-treated and non-treated plots. Disease severity, which is an expression of both disease-induced defoliation and necrotic leaf area, was used as an indicator of disease progression in the field. The leaf dry weight, LAI, and the dry weight of the total biomass were significantly different at 93 days after planting (DAP) in 1986, and at 78 DAP in 1987 between fungicide-treated and non-treated plots. Late leaf spot reduced the potential yield (harvested and dropped pods) of Florunner peanut by 37% in 1986 and 46% in 1987. In non-treated plots, the abscission of pods was initiated later but progressed faster in 1986 than in 1987. The predictions of pod yield with the measures of healthy leaf area duration (HAD) and healthy area absorption (HAA) were adequate for fungicide-treated plots where pod losses were minimal. However, HAD and HAA were inadequate for predicting pod yield of a peanut crop severely infected by late leaf spot, primarily because this predictive approach does not account for losses of dropped pods.

Spectral analysis of heart rate in diabetic autonomic neuropathy. A comparison with standard tests of autonomic function

Power spectral analysis of the heart rate was compared with standard tests of autonomic function performed on 15 insulin-dependent diabetic patients with symptoms of autonomic dysfunction. The high-frequency power, low-frequency power, and total power of the heart rate power spectrum measured in the supine position--possible measures of parasympathetic nervous system function--were significant predictors of the expiratory-inspiratory ratio, the beat-to-beat heart rate difference on deep respiration, the standard deviation of the resting heart rate, the 30:15 ratio, and the Valsalva ratio. The change in low-frequency power on moving from the supine to upright position, a possible measure of sympathetic nervous system function, was a modest predictor of the systolic and diastolic blood pressure fall in response to postural change, but a poor predictor of the blood pressure response to isometric exercise and a cold stimulus. This study supports the role of heart rate power spectral analysis as a clinical test of autonomic function in patients with known or suspected autonomic failure.

Heart rate and muscle sympathetic nerve variability during reflex changes of autonomic activity

Low-frequency (less than 0.15 Hz) fluctuations of heart rate are increased by maneuvers, such as standing or hemorrhage, that increase sympathetic outflow to the heart and vasculature. To test the hypothesis that low-frequency heart rate fluctuations provide an index of sympathetic efferent activity, we compared power spectral measures of heart rate variability with two measures of sympathetic outflow, peroneal nerve sympathetic activity and antecubital vein plasma norepinephrine concentrations. Autonomic outflow was varied with graded stepwise infusions of nitroprusside and phenylephrine, which lowered or raised average diastolic pressures by approximately 15 mmHg. Before vasoactive drug infusions, no spectral measure of heart rate variability correlated significantly with muscle sympathetic activity, plasma norepinephrine concentration, average heart rate, or arterial pressure. During increases of muscle sympathetic activity and probable reductions of cardiac vagal activity induced by nitroprusside, the fraction of heart rate spectral power at low frequencies, but not the absolute value, correlated significantly with muscle sympathetic activity and plasma norepinephrine. However, during reductions of muscle sympathetic activity and probable elevations of cardiac vagal activity induced by phenylephrine, no measure of heart rate variability correlated significantly with muscle sympathetic activity. These findings can be explained by a model of heart rate control in which low-frequency heart rate fluctuations result from changing levels of both the sympathetic and parasympathetic inputs to the sinoatrial node.