Epinephrine, vasodilation and hemoconcentration in syncopal, healthy men and women.

Healthy young people may become syncopal during standing, head up tilt (HUT) or lower body negative pressure (LBNP). To evaluate why this happens we measured hormonal indices of autonomic activity along with arterial pressure (AP), heart rate (HR), stroke volume (SV), cardiac output (CO), total peripheral resistance (TPR) and measures of plasma volume. Three groups of normal volunteers (n = 56) were studied supine, before and during increasing levels of orthostatic stress: slow onset, low level, lower body negative pressure (LBNP) (Group 1), 70 degrees head up tilt (HUT) (Group 2) or rapid onset, high level, LBNP (Group 3). In all groups, syncopal subjects demonstrated a decline in TPR that paralleled the decline in AP over the last 40 s of orthostatic stress. Ten to twenty seconds after the decline in TPR. HR also started to decline but SV increased, resulting in a net increase of CO during the same period. Plasma volume (PV, calculated from change in hematocrit) declined in both syncopal and nonsyncopal subjects to a level commensurate with the stress, i.e. Group 3 > Group 2 > Group 1. The rate of decline of PV, calculated from the change in PV divided by the time of stress, was greater (p < 0.01) in syncopal than in nonsyncopal subjects. When changes in vasoactive hormones were normalized by time of stress, increases in norepinephrine (p < 0.012, Groups 2 and 3) and epinephrine (p < 0.025, Group 2) were greater and increases in plasma renin activity were smaller (p < 0.05, Group 2) in syncopal than in nonsyncopal subjects. We conclude that the presyncopal decline in blood pressure in otherwise healthy young people resulted from declining peripheral resistance associated with plateauing norepinephrine and plasma renin activity, rising epinephrine and rising blood viscosity. The increased hemoconcentration probably reflects increased rate of venous pooling rather than rate of plasma filtration and, together with cardiovascular effects of imbalances in norepinephrine, epinephrine and plasma renin activity may provide afferent information leading to syncope.

Gender differences in autonomic cardiovascular regulation: spectral, hormonal, and hemodynamic indexes.

The autonomic nervous system drives variability in heart rate, vascular tone, cardiac ejection, and arterial pressure, but gender differences in autonomic regulation of the latter three parameters are not well documented. In addition to mean values, we used spectral analysis to calculate variability in arterial pressure, heart rate (R-R interval, RRI), stroke volume, and total peripheral resistance (TPR) and measured circulating levels of catecholamines and pancreatic polypeptide in two groups of 25 +/- 1.2-yr-old, healthy men and healthy follicular-phase women (40 total subjects, 10 men and 10 women per group). Group 1 subjects were studied supine, before and after beta- and muscarinic autonomic blockades, administered singly and together on separate days of study. Group 2 subjects were studied supine and drug free with the additional measurement of skin perfusion. In the unblocked state, we found that circulating levels of epinephrine and total spectral power of stroke volume, TPR, and skin perfusion ranged from two to six times greater in men than in women. The difference (men > women) in spectral power of TPR was maintained after beta- and muscarinic blockades, suggesting that the greater oscillations of vascular resistance in men may be alpha-adrenergically mediated. Men exhibited muscarinic buffering of mean TPR whereas women exhibited beta-adrenergic buffering of mean TPR as well as TPR and heart rate oscillations. Women had a greater distribution of RRI power in the breathing frequency range and a less negative slope of ln RRI power vs. ln frequency, both indicators that parasympathetic stimuli were the dominant influence on women's heart rate variability. The results of our study suggest a predominance of sympathetic vascular regulation in men compared with a dominant parasympathetic influence on heart rate regulation in women.

Usefulness of midodrine in patients with severely symptomatic neurocardiogenic syncope: a randomized control study.

INTRODUCTION: The efficacy of midodrine for the management of patients with neurocardiogenic syncope was assessed prospectively in a randomized control study. METHODS AND RESULTS: Patients who had at least monthly occurrences of syncope and a positive tilt-table test were included in the study. A total of 61 patients were randomly allocated to treatment either with midodrine or with fluid, salt tablets, and counseling. Midodrine was given at a starting dose of 5 mg three times a day and increased up to a dose of 15 mg three times a day when required. Midodrine was given during the daytime every 6 hours. Thirty-one patients were assigned to treatment with midodrine; the other 30 patients were advised to increase their fluid intake and were instructed to recognize their prodromes and abort the progression to syncope. Patients were followed-up for at least 6 months. A quality-of-life questionnaire was administered at the time of randomization and 6 months after. At the 6-month follow-up, 25 (81%) of 31 midodrine-treated patients and 4 (13%) of the 30 fluid-therapy patients had remained asymptomatic (P < 0.001). One patient had to discontinue taking midodrine due to severe side effects and another six patients experienced minor side effects that did not require drug discontinuation. CONCLUSION: Midodrine appeared to provide a significant benefit in patients with neurocardiogenic syncope. To prevent recurrence of symptoms, dose adjustments were required in about one third of patients.

Ablation of incisional atrial tachycardias using a three-dimensional nonfluoroscopic mapping system.

Incisional atrial reentrant tachycardias are macroreentrant arrhythmias in which surgical scars or prosthetic material constitute one of the constraining barriers of the circuit. Accurate reconstruction based on fluoroscopy-guided endocardial mapping of the reentrant circuit is often incomplete and time consuming explaining, at least in part, the modest long-term results of this technique. Mapping and ablation of these arrhythmias using a three-dimensional nonfluoroscopic mapping system that allows electroanatomic reconstruction of the reentrant circuit could help in identifying the ablation targets and improve long-term outcome. The study included 20 patients (12 men, mean age 45+/-18 years) with corrected congenital heart disease (4 patients), coronary artery bypass surgery (7 patients), mitral or aortic valve replacement or reconstruction (6 patients), valve replacement and coronary revascularization (2 patients), and mitral valve replacement with maze procedure for atrial fibrillation (1 patient). Endocardial mapping with this novel system was complemented by standard electrophysiological techniques used to identify a critical isthmus of conduction. Two or more nonconductive areas of atrial tissue or surgical prosthetic material delimiting a critical isthmus of conduction were identified in every patient. Radiofrequency linear applications spanning two to more boundaries successfully eliminated the tachycardia in every patient. At a follow-up of 11.5+/-5.1 months (range 17-5 months), two (10%) patients developed a new clinical arrhythmia. The remaining 18 had no recurrences off medical therapy. Mean fluoroscopy time was 45.7+/-15.2 minutes for patients with a single scar and 89+/-41.2 minutes in patients with two or more scars. In conclusions, this new nonfluoroscopic mapping system offers the opportunity to achieve a high rate of cure of complex macroreentrant atrial tachycardias by facilitating reconstruction of the macroreentrant circuit and its boundaries.

Time-frequency representation of epicardial electrograms during ventricular fibrillation.

In the present study we quantified changes in dominant frequency, which is reciprocal of activation interval or cycle period, during ventricular fibrillation (VF). We used a Smoothed Pseudo Wigner Distribution (SPWD) to estimate time-frequency representations of epicardial electrograms recorded in swines. We used a sock with 64 electrodes spaced equally to record electrograms during 30 seconds of electrically induced VF. Results from 29 trials in three animals showed a mean dominant frequency of 6.64 Hz. We observed considerable variation in dominant frequencies during VF. Temporally, the frequencies varied by as much as +/- 1.24 Hz (2 standard deviations). Spatial variation in frequencies was +/- 1.20 Hz. Cycle periods were computed as the reciprocal of dominant frequencies obtained from the SPWD. These cycle periods were verified to be numerically similar to the cycle periods estimated using activation times detected from differentiated electrograms. Results of recent studies by others have shown that cycle periods during VF are correlated with refractory periods. Our results show that a non-stationary analysis technique such as the SPWD can be used to track spatio-temporal variation in cycle periods. These changes can be used to investigate spatio-temporal variation in cellular properties such as the effective refractory periods during VF. The substantial temporal variation in dominant frequencies that we observed suggest the possibility that the excitable gap at any epicardial location also varies considerably from one instance to another during a VF episode.

Time averaged spatial distribution of epicardial dominant frequencies during ventricular fibrillation.

Recent evidence suggests that the dominant frequencies during ventricular fibrillation (VF) may be used as indicators of dispersion in repolarization and in activation patterns. In the present study, we quantified dominant frequencies from multiple epicardial electrodes to investigate if there are differences in the averaged frequencies within the electrograms recorded from the left and the right ventricles. Further, we quantified whether the difference in average frequency between the two ventricles changed during 30 seconds of VF. Results from eighteen trials in two pigs showed that during the entire duration of VF the average dominant frequencies of all electrodes over the left ventricle were higher than those over the right ventricle (p < 0.005). The dominant frequencies are reciprocal of cycle periods or activation intervals during VF. Our results show that on average, activations in the left ventricle occurred at a faster rate than those in the right ventricle. Activation intervals at any site are determined by the refractory period at that site and the arrival time of next activation. Although differences in cellular properties may have contributed to the observed differences in activation intervals between the ventricles, it is possible that activation arrival times may be different as well. It is possible that the increased tissue mass of the left ventricle may increase the probability that any site will get excited at a faster rate after it is recovered from previous activation.

False positive head-up tilt: hemodynamic and neurohumoral profile.

OBJECTIVES: This study examined differences in mechanisms of head-up tilt (HUT)-induced syncope between normal controls and patients with neurocardiogenic syncope. BACKGROUND: A variable proportion of normal individuals experience syncope during HUT. Differences in the mechanisms of HUT-mediated syncope between this group and patients with neurocardiogenic syncope have not been elucidated. METHODS: A 30-min 80 degrees HUT was performed in eight HUT-negative volunteers (Group I), eight HUT-positive volunteers (Group II) and 15 patients with neurocardiogenic syncope. Heart rate and blood pressure (BP) were monitored continuously. Epinephrine and norepinephrine plasma levels, as well as left ventricular dimensions and contractility determined by echocardiography, were measured at baseline and at regular intervals during the test. RESULTS: The main findings of this study were the following: 1) All parameters were similar at baseline in the three groups; and 2) During tilt: a) the time to syncope was shorter in Group III than in group II (9.5 +/- 3 vs. 17 +/- 3 min p < 0.05); b) there was an immediate, persisting drop in mean BP in Group III; c) the decrease rate of left ventricular end-diastolic dimensions was greater in Group III than in Group II or Group I (-1.76 +/- 0.42 vs. -0.87 +/- 0.35 and -0.67 +/- 0.29 mm/min, respectively, p < 0.05); d) the leftventricular shortening fraction was greater in Group III than in the other two groups (39 +/- 1 vs. 34 +/- 1 and 32 +/- 1%, respectively, p < 0.05); and e) although the norepinephrine level remained comparable among the groups, there was a significantly higher peak epinephrine level in Group III than in Group II and Group I (112.3 +/- 34 vs. 77.6 +/- 10 and 65 +/- 12 pg/ml, p < 0.05). CONCLUSIONS: Mechanisms of syncope during HUT appeared to be different in normal volunteers and patients with neurocardiogenic syncope. In the latter, there was evidence of an impaired vascular resistance response from the beginning of the orthostatic challenge. Furthermore, in the patients there was more rapid peripheral blood pooling, as indicated by the echocardiographic measurements of left ventricular end-diastolic changes, leading to more precocious symptoms. In syncopal patients, the higher level of plasma epinephrine probably mediated the increased cardiac contractility and possibly contributed to the impaired vasoconstrictive response.

Frequency of superior vena cava syndrome following radiofrequency modification of the sinus node and its management.

In a series of 35 consecutive patients, the presence of a permanent pacemaker appears to be a strong risk factor for developing superior vena cava syndrome after radiofrequency modification of the sinus node. Treatment of this complication with balloon venoplasty is as effective as surgical repair.

Energy steering of biphasic waveforms using a transvenous three electrode system.

The optimal electrode configuration for endocardial defibrillation is still a matter of debate. Current data suggests that a two pathway configuration using the right ventricle (RV) as cathode and a common anode constituted by a superior vena cava (SVC) and a pectoral can (C) is the most effective combination. This may be related to the more uniform voltage gradient created by shocks delivered using this configuration. We hypothesized that more effective waveforms could be obtained by varying the distribution of the shock current between the two pathways of a three electrode endocardial defibrillation system. In 12 pigs, we compared the characteristics and the defibrillation efficacy of six biphasic waveforms discharged using either a two (RV-->C) or a three (RV-->SVC + C) electrode combination with the following configurations: Configuration 1 (W1): the RV apical coil was used as a cathode and the subcutaneous C as anode (RV-->C). Configuration 2 (W2): The RV was used as cathode and the combination of the atriocaval coil (SVC) and the subcutaneous C as anode (RV-->SVC + C). Configuration 3 (W3): The RV-->C was used for the first 25% of f+ and RV-->SVC + C for the remainder of the discharge including f 2 Configuration 4 (W4): The RV-->C was used for the first 50% of f+ and RV-->SVC + C for the remainder of the discharge including f 2 Configuration 5 (W5): The RV-->C was used for the first 75% of f+ and RV-->SVC + C for the remainder of the discharge including f 2. Configuration 6 (W6): The RV-->C was used for f+ and RV-->SVC + C for f 2. As an increasing fraction of the waveform was discharged using the RV-->SVC + C pathways, the impedance and the pulse width decreased while the tilt, the peak, and the average current significantly increased. The waveforms delivered using the RV-->SVC + C configuration for 100% or 75% of their duration had significantly lower stored energy DFT than the other waveform. Current distribution between three endocardial electrodes can be altered during the shock and generates waveforms with different characteristics. Shocks with 75% or more of the current flowing to the RV-->SVC + C required the lowest stored energy to defibrillate. This method of energy steering could be used to optimize current delivery in a three electrodes system.

Human histopathologic findings following radiofrequency ablation of the tricuspid-inferior vena cava isthmus.

Radiofrequency (RF) ablation of the tricuspid valve-inferior vena cava isthmus is now the first line of treatment in the management of typical atrial flutter. Successful ablation is associated with conduction block in this region, although the histopathologic changes following this procedure have never been reported. We describe the pathologic changes following RF ablation of this region in an explanted heart of a patient undergoing heart transplantation 4 months after successful atrial flutter ablation. The findings confirm the ability of RF ablation to create in the isthmus a chronic full thickness fibrosis, which represents the histopathologic counterpart of the conduction block demonstrated at the end of procedure.

Epicardial mapping and radiofrequency catheter ablation of ischemic ventricular tachycardia using a three-dimensional nonfluoroscopic mapping system.

Endocardial radiofrequency catheter ablation of ischemic left ventricular tachycardia has been of variable success due to multiple factors. Two such factors include the location of the reentrant circuit in the deep myocardium or on the epicardial surface and the inherent limitations of fluoroscopy as a guide for target localization. We report a patient in whom successful epicardial mapping and radiofrequency catheter ablation of an ischemic left ventricular tachycardia was performed using pericardial access and the CARTO electroanatomic mapping system.

The effects of endocardial defibrillation on left ventricular function: a transoesophageal echocardiographic study.

This study evaluates the immediate effects of the endocardial electrical shocks delivered by a transvenous defibrillation system on left ventricular (LV) function in a pig model. A triple-lead system consisting of two endocardial electrodes, in the right ventricular apex and the junction of superior cava-right atrium, and a custom-made defibrillation can implanted subcutaneously in the thorax was set up in 10 close-chest pigs. Transesophageal echocardiography with two dimensional image, m-Mode, and pulse Doppler was performed at baseline and after several episodes of fibrillation/defibrillation (F/DF). Each animal underwent an average of 8 (range 6 to 11) episodes of ventricle F/DF for a total of 210 (range 165 to 290) joules of biphasic-waveform defibrillation shocks. Heart rate, blood pressure, LV end-systolic area, end-diastolic area and fractional area contraction, isovolumic relaxation time, and both ratios of velocities and time-velocity integrals in transmitral Doppler flow E and A waves were unchanged after the shocks. This animal study suggests that multiple countershocks up to 210 joules delivered by a transvenous defibrillation system do not cause LV global systolic and/or diastolic dysfunction.

Selective interference with pacemaker activity by electrical dental devices.

OBJECTIVE: We sought to determine whether electromagnetic interference with cardiac pacemakers occurs during the operation of contemporary electrical dental equipment. STUDY DESIGN: Fourteen electrical dental devices were tested in vitro for their ability to interfere with the function of two Medtronics cardiac pacemakers (one a dual-chamber, bipolar Thera 7942 pacemaker, the other a single-chamber, unipolar Minix 8340 pacemaker). Atrial and ventricular pacemaker output and electrocardiographic activity were monitored by means of telemetry with the use of a Medtronics 9760/90 programmer. RESULTS: Atrial and ventricular pacing were inhibited by electromagnetic interference produced by the electrosurgical unit up to a distance of 10 cm, by the ultrasonic bath cleaner up to 30 cm, and by the magnetorestrictive ultrasonic scalers up to 37.5 cm. In contrast, operation of the amalgamator, electric pulp tester, composite curing light, dental handpieces, electric toothbrush, microwave oven, dental chair and light, ENAC ultrasonic instrument, radiography unit, and sonic scaler did not alter pacing rate or rhythm. CONCLUSIONS: These results suggest that certain electrosurgical and ultrasonic instruments may produce deleterious effects in medically fragile patients with cardiac pacemakers.

Systolic and diastolic effects of variable atrioventricular delay in patients with complete heart block and normal ventricular function.

This study was designed to demonstrate the effects of varying the atrioventricular delay (AVD) on ventricular diastolic filling dynamics and the resultant stroke volume in patients with complete heart block and normal cardiac function. We studied 7 patients with normal cardiac function in whom a dual chamber pacemaker had been implanted because of complete heart block. Doppler and M-mode echocardiography was performed at 70, 100, 140, 180, and 220 ms, AVD with the device in DDD mode at a rate of 80 beats/min. The effects of these variable intervals on the contribution of the E and A waveform to the diastolic filling, on the stroke volume, and on the systolic intervals were evaluated. Optimization of this interval, with a 19% increase in stroke volume was achieved in the group of patients at an AVD of 140 ms. When considered individually, the AVD associated with the largest stroke volume, was 100 ms in 2 patients and 140 ms in the remaining 5. At this individual optimal AVD the ventricular septal contraction occurred 31 +/- 14 ms, before the end of the transmitral flow. The optimal AVD is, therefore, the one which synchronizes the ventricular and atrial systole so that the first ventricular septal contraction occurs after the peak of the A wave, just before the end of the transmitral flow. Because of the different functional cardiovascular status of the single patient, this parameter should be individualized; this can be clinically important as it may lead, in this patient population, to an improvement of the stroke volume up to 42%.

The vanishing defibrillator syndrome: incidence, mechanism, and clinical relevance.

Intraperitoneal migration of an abdominally implanted cardioverter defibrillator is a complication not yet fully described. In a consecutive series of 195 patients, migration occurred between 1 and 20 months in 5 (8%) of the 63 patients in whom a subrectus abdomini placement of the generator was chosen. It was unrelated to the patients' clinical characteristics or the defibrillator model. Dysuria and inability to interrogate the device were present in every subject, and the diagnosis was confirmed by the characteristic abdominal x-ray appearance and the findings at the time of surgery. Adhesions involving the omentum, and in one case, the small bowels, were present in three patients and seem to be related to the length of intraabdominal permanence of the generator. Because this complication may be due to specific anatomical characteristics of the aponeurosis of the abdominal muscles, it is likely that its incidence will be unchanged by the use of smaller devices. A close follow-up of the generators implanted deep to the rectus fascia is therefore advisable.

Natural history, determinants, and clinical relevance of conduction abnormalities following orthotopic heart transplantation.

To study the long-term evolution, determinants, and clinical relevance of the conduction abnormalities after orthotopic heart transplantation, 87 patients, followed for a mean of 105 +/- 72 weeks, were divided into 3 groups according to the characteristics of their electrocardiograms compared with their initial electrocardiogram recorded at study entry. The first group consisted of 24 patients whose initial electrocardiogram was normal, and subsequent electrocardiograms remained normal throughout the study. The second group included 27 patients who developed electrocardiographic evidence of progressive conduction system damage. The third group comprised 36 patients whose initial electrocardiogram was abnormal and subsequent electrocardiograms remained unchanged during follow-up. Although the hemodynamic and echocardiographic evaluation of right and left ventricular function were initially similar among the 3 groups, groups 2 and 3 demonstrated a significant deterioration of left ventricular ejection fraction (62 +/- 12% to 55 +/- 16% and 62 +/- 8% to 57 +/- 14%, respectively; p < 0.05) and cardiac index (2.7 +/- 0.6 to 2.3 +/- 0.5 and 3.0 +/- 0.9 to 2.5 +/- 0.9 L/min/m2, respectively; p < 0.05) while patients in group 1 maintained their normal baseline indices. Incidence and progression of coronary artery disease, as well as frequency of rejection episodes, were comparable among the groups. Mortality was higher in the 2 groups with evidence of conduction defects. Sudden death associated with complete heart block (2 patients) or ventricular arrhythmias (3 patients) was exclusively confined to patients with evidence of progressive electrocardiogram abnormalities. We conclude that, following orthotopic heart transplantation, stable or progressive conduction system damage on the electrocardiogram is associated with left ventricular dysfunction and increased mortality. Sudden death is not uncommon among patients demonstrating worsening cardiac conduction and, in some cases, is related to the development of potentially preventable complete heart block.

A short duration, small capacitor, biphasic waveform has lower defibrillation thresholds than a clinically available biphasic waveform

A biphasic waveform delivered from an 85 uF capacitor was compared to a biphasic waveform delivered from a commercially available implantable defibrillator (ICD). The test waveform had a phase one (O1) duration of 4 ms, a phase two (O2) duration of 2.5 ms, and an innitial O2 voltage equal to the terminal O1 voltage. The control was delivered from a 150 uF capacitor, had adjustable pulse widths (O1, O2 = 8 ms for a 50 ohm load), and an initial O2 voltage equal to 50 por cento of the O1 terminal volge. The short duration, small capacitor waveform reduced stored energy defibrillation thresholds (DFT) by 18 por cento and increased O1 leading edge voltage by 20 por cento when compared to the control.

Defibrillation thresholds are lower with smaller storage capacitors.

Present implantable cardioverter defibrillators use a wide range of capacitance values for the storage capacitor. However, the optimal capacitance value is unknown. We hypothesized that a smaller capacitor, by delivering its charge in a time closer to the heart chronaxie, should lower the defibrillation threshold (DFT). We compared the energy required to defibrillate 10 open-chest dogs, after 15 seconds of ventricular fibrillation, with a monophasic, time-truncated waveform delivered from either a 85-microF or a 140-microF capacitor. Shocks were delivered through a pair of 14-cm2 epicardial patch electrodes: The two capacitors were randomly tested twice with each dog using a modified 3-reversal method for each DFT determination. The average stored and delivered DFT energies for the 85-microF capacitor were 6.0 +/- 1.7 joules and 5.2 +/- 1.5 joules, respectively, compared to 6.7 +/- 1.7 joules and 6.0 +/- 1.5 joules for the 140-microF capacitor (P = 0.01 and P = 0.004, respectively). The mean leading edge voltages were higher, the pulse duration shorter, and the mean impedance lower for the 85-microF capacitor. The impedance was inversely related to the pulse duration and the voltage decay suggesting that, at least in part, the mechanism of improved defibrillation could be accounted for by the waveform electrical characteristics. There was an equal number of episodes of postshock bradyarrhythmias and tachyarrhythmias following discharges from each capacitor. Moreover, there was no relationship between the likelihood of these arrhythmias and either the initial voltage or the delivered current nor there was a higher number of episodes of postshock hypotension following the smaller capacitor discharges.(ABSTRACT TRUNCATED AT 250 WORDS)

Woven wire patches are superior to solid disks for subcutaneous electrodes: implications for active can defibrillation.

The housing of the implantable cardioverter defibrillator (ICD) is being considered for a remote electrode to replace the conventional subcutaneous woven wire patch. It is not clear that the solid smooth and rigid metal surface of the ICD housing will provide the same performance as does the woven wire patch. We compared a solid titanium disk to a titanium woven wire patch for defibrillation performance in a canine model. The patch had a smaller outline area, a slightly smaller conductive perimeter, and slightly less of a small feature surface area than did the disk. The remote electrode (disk or patch) was inserted at the point of maximal apical cardiac impulse. A commercially available endocardial electrode was placed in the right ventricle (RV). Conventional biphasic shocks (140-microFrench capacitor and 65% tilt) were delivered between the RV and subcutaneous electrode. The patch had significantly lower resistances than did the disk (81.6 +/- 8.0 omega vs 90.0 +/- 11.6 omega P < 0.006). The patch also had significantly lower stored energy defibrillation thresholds than did the disk (8.0 +/- 2.6 J vs 9.3 +/- 3.3 J, P < 0.007). In spite of smaller values for every geometrical dimension, the woven wire patch out performed the solid disk for defibrillation with conventional biphasic waveforms. Since the ICD housing is typically smooth titanium, the use of waveforms better suited for the active can configuration may deserve a systematic evaluation.

A long thin electrode is equivalent to a short thick electrode for defibrillation in the right ventricle.

We hypothesized that a long thin right ventricular (RV) electrode would have equivalent defibrillation threshold (DFT) performance to a short thick electrode with approximately the same surface area. This could lead to thinner transvenous lead systems, which would be easier to implant. A thin (5.1 French) lead was compared to a standard control (10.7 French). The thin lead had an 8-cm RV electrode length with a surface area of 4.26 cm2. The standard lead had a RV electrode length of 3.7 cm and a surface area of 4.12 cm2. A 140-mu French capacitor 65%/65% tilt biphasic defibrillation shock was delivered between the RV electrode and a 14-cm2 subcutaneous patch. DFTs were determined following 10 seconds of fibrillation in 11 dogs by a triple determination averaging technique. The thin lead had a lower resistance (77.1 +/- 27.4 omega vs 88.9 +/- 30.3 omega, P < 0.001) than did the thick lead. There was no significant difference in stored energy DFTs (9.9 +/- 2.5 vs 10.3 +/- 2.7, P = 0.098 2-sided, P = 0.049 1- sided). This was in spite of the fact that the long thin lead had a portion of its RV coil extending above the tricuspid valve and, thus, not contributing efficiently to the ventricular gradients in the small dog heart. We conclude that a long thin right ventricular electrode and a standard short thick electrode had equivalent defibrillation performance. This preliminary result should be confirmed in clinical studies as it could lead to significantly thinner transvenous lead systems.