Origin of electrical activation within the right atrial and left ventricular walls: differentiation by electrogram characteristics using the noncontact mapping system

Clinical data using the noncontact mapping system (Ensite 3000) suggest that characteristics of the reconstructed unipolar electrograms may predict the origin of electrical activation within the atrial and ventricular walls (endocardial vs myocardial vs epicardial origin). Experimental data are lacking. In ten open-chest pigs (mean body weight 62 kg) cardiac pacing was performed at a cycle length of 600 ms with a pulse width of 2 ms and twice diastolic threshold from the endo-, the myo-, and the epicardium, respectively. Pacing was undertaken at three right atrial and three left ventricular sites, and cardiac activation was recorded with the Ensite system. Reconstructed unipolar electrograms at the location of earliest endocardial activation assessed by color coded isopotential maps were analyzed systematically for differences in morphology. The positive predictive value of atrial electrograms exhibiting an initial R wave during pacing for a subendocardial origin (i.e., myocardial or epicardial) was 0.96. The negative predictive value was 0.48. Electrograms generated during myocardial pacing exhibited increased maximal negative voltage and maximal dV/dt (-3 +/- 1.8 mV, -798 +/- 860 mV/ms, respectively) than the electrograms obtained during endocardial (-2 +/- 1 mV, -377 +/- 251 mV/ms, respectively) and epicardial pacing (-2.1 +/- 0.7 mV, -440 +/- 401 mV/ms, respectively, P<0.01 for both parameters). During pacing at the left ventricular wall, occurrence of an initial R wave did not differ significantly between electrograms reconstructed during endocardial and subendocardial pacing. All other characteristics of the unipolar ventricular electrograms analyzed, except latency, did not differ significantly when compared to stimulation depth. Morphological characteristics of unipolar electrograms generated by the noncontact mapping system during pacing of the atrium allowed for discrimination of an endocardial versus a subendocardial origin of activation. At the ventricular level, characteristics of unipolar electrograms did not predict the origin of cardiac activation in this experimental setting.     

The Sock Electrode Array: A Tool for Determining Global Epicardial Activation during Unstable Arrhythmias

The conventional technique for mapping the sequence of epicardial activation uses a hand-held electrode moved over the heart to record from a number of epicardial sites one at a time, and requires 5–15 minutes to record from 50 or more sites distributed over the entire ventricular epicardium. This method is inadequate for arrhythmias that are transient or vary from beat to beat. To overcome these limitations the "sock electrode array," a contour-fitting sock containing 26 or 52 electrodes, has been developed. The nylon mesh sock is pulled over the heart and permits simultaneous recording of potentials from electrodes distributed over the entire ventricular epicardium. The electro-grams are recorded and converted to digital form for computer generation of isochronous maps. Maps of the epicardial activation sequence derived from the sock electrode were compared to those obtained by the hand-held electrode in six normal dogs during sinus rhythm and ventricular pacing. The sequence of local activation times acquired by both methods showed similar areas of early and late activation and comparable isochronous maps. The hand-held electrode technique required 10–15 minutes for data acquisition and another 15–30 minutes for analysis. The sock electrode array allowed electrograms from 26 epicardial electrodes to be recorded simultaneously during one cardiac cycle and computer generated isochronous maps could be displayed within 10 minutes. This method allows rapid recording and analysis of epicardial electrical phenomena and should meet the time constraints imposed during the intraoperative study of ventricular tachyarrhythmias in patients.     

Effects of simultaneous insertion of 66 plunge needle electrodes on myocardial activation, function, and structure

Transmural recordings using plunge needle electrodes are useful in mapping ventricular tachyarrhythmia, but they interfere with activation sequences or damage the myocardium. This study evaluated the effects of insertion of 66 transmural needles on myocardial activation, structure, and function. Epicardial maps were performed at thoracotomy using a 40-electrode plaque in five mongrel dogs. Sixty-six transmural plunge needles were introduced into the anterior aspect of the septum and left ventricle. Transmural maps of unipolar electrograms were recorded every 15 minutes via 124 electrodes over a 2-hour period. Epicardial maps were repeated after the needles were removed. All recordings were performed during sinus rhythm and ventricular pacing at 300- and 200-ms cycle lengths. Gated heart pool studies were performed preoperatively and 2 weeks after thoracotomy. Programmed ventricular stimulation was performed 2 weeks after thoracotomy. In total, 15,996 electrograms were analyzed. Maximum negative dV/dt of each electrogram and the activation time at each electrode did not change significantly over the 2 hours of needle insertion. After removal of the needles, epicardial maps were unchanged compared to before needle insertion. Mean left ventricular ejection fraction 2 weeks after needle insertion was 59% versus 58% before needle insertion (P=0.9). No dogs had inducible ventricular tachycardia. Histology showed contraction bands of 0.8-mm diameter adjacent to the needle tracks but no scarring. Insertion of 66 closely spaced plunge needles did not distort epicardial or transmural maps. Multiple needles did not result in myocardial scarring, left ventricular dysfunction, or predispose to ventricular tachycardia.     

A Gastroesophageal Electrode for Atrial and Ventricular Pacing

Temporary transvenous cardiac pacing requires technical expertise and access to fluoroscopy. We have developed a gastroesophageal electrode capable of atrial and ventricular pacing. The flexible polythene gastroesophageal electrode is passed into the stomach under light sedation. Five ring electrodes, now positioned in the lower esophagus, are used for atrial pacing. A point source (cathode) on the distal tip of the electrode, now positioned in the gastric fundus. is used for ventricular pacing. Two configurations of atrial and ventricular pacing were compared: unipolar and bipolar. During unipolar ventricular pacing the indifferent electrode (anode) was a high impedance chest pad. For bipolar ventricular pacing the indifferent electrode was a ring electrode placed 2 cm proximal to the tip. Unipolar atrial pacing was performed with 1 of 5 proximal ring electrodes acting as cathode ("cathodic") or as anode ("anodic") in conjunction with a chest pad. Bipolar atrial pacing was performed using combinations of 2 of 5 ring electrodes. Atrial capture was obtained in all 55 subjects attempted. When all electrode combinations were compared, atrial capture was significantly more frequent using the bipolar approach (153/210 bipolar, 65/210 unipolar; t = 7.37, P < 0.001). For unipolar atrial pacing, cathodic stimulation (from esophagus) was more successful than anodic stimulation (cathodic 62/105, anodic 20/105; t = 5.81, P < 0.001). In 43 subjects attempted unipolar ventricular pacing resulted in a higher frequency of capture than the bipolar approach (unipolar 41/43 (95.3%), bipolar 19/43 (44.2%); P < 0.001). In conclusion, atrial pacing was optimal using pairs of ring electrodes ("bipolar") while ventricular pacing was optimal using the distal electrode tip (cathode) in conjunction with a chest pad electrode ("unipolar"). This gastroesophageal electrode may be useful in the emergency management of acute bradyarrhythmias and for elective electrophysiological studies.     

Effects of proximal ventricular septal pacing on hemodynamics and ventricular activation

Recently the use of alternate site pacing to improve cardiac function in patients with bradyarrhythmias has increased. In the present study, hemodynamics of right ventricular septal pacing were studied in seven dogs. A bipolar screw-in lead and endocardial lead were placed in the proximal right ventricular septum and right ventricular apex, respectively. The right ventricle was paced from each site. A conductance catheter and Millar catheter were inserted into the left ventricle to determine the left ventricular pressure and the pressure-volume loop. Cardiac output was measured using the thermodilution method. In five of the seven dogs, ventricular activation was documented by isochronal epicardial activation mapping during each pacing mode. Mean arterial pressure and cardiac output during septal pacing were significantly higher than during apical pacing (110 +/- 17 mmHg vs 100 +/- 18 mmHg; 1.00 +/- 0.39 L/min vs 0.89 +/- 0.33 L/min). The positive dp/dt during septal pacing was significantly higher than during apical pacing (2137 +/- 535 mmHg/s vs 1911 +/- 404 mmHg/s). End-systolic elastance during septal pacing was significantly higher compared to apical pacing (13.1 +/- 0.3 mmHg/mL vs 8.9 +/- 4.0 mmHg/mL). The ventricular activation time during septal pacing was significantly shorter than during apical pacing. The epicardial maps generated during septal pacing were similar to those from atrial pacing. We conclude that hemodynamics and interventricular conduction are less disturbed by proximal right ventricular septal pacing than apical pacing in dogs with normal hearts.     

Distinct Activation Patterns of Idioventricular Rhythms and Sympathetically-Induced Ventricular Tachycardias in Dogs with Atrioventricular Block

To investigate mechanisms of ventricular impulse formation in response to sympathetic stimulation in the healthy canine heart in situ, we compared the patterns of ventricular activation during the idioventricular rhythms arising after complete atrioventricular (AV) block and ventricular tachycardias induced by RSG or LSG stimulation. Isochronal maps were generated by computer from 116-127 unipolar electrograms recorded from the entire ventricular epicardium in 15 open chest, anesthetized dogs. In eight of these, bipolar electrograms were recorded with plunge electrodes from 11 selected endocardial sites located below epicardial breakthrough areas. Intracardiac recordings from the His-Purkinje system were made with electrode catheters. After electrograms were recorded during sinus rhythm, complete AV block was induced by injecting formaldehyde into the AV node and idioventricular rhythms occurred spontaneously at a rate of 37 +/- 12 beats/min (mean +/- SD, n = 25). During idioventricular rhythms, endocardial activation preceded the earliest epicardial breakthrough, which occurred in either the right anterior paraseptal region, antero-apical left ventricle, or postero-apical left ventricle. These sites were consistent with a focal origin in the subendocardial His-Purkinje system. Total epicardial activation times lasted for 47 +/- 13 msec (n = 40). Idioventricular rhythms were suppressed by overdrive pacing (intermittent trains of ten beats with decremental cycle length from 500 to 200 msec) or by intravenous calcium infusion (to plasma levels of 10.1-15.2 mM). Right or left stellate ganglion stimulation increased idioventricular rhythm rates (to 52 +/- 13 beats/min, n = 28) and also induced, in all preparations, ventricular tachycardias that had significantly faster rates (189 +/- 55 beats/min, n = 27, P less than 0.005). Ventricular fibrillation was induced after brief runs of ventricular tachycardia in five of the preparations. During ventricular tachycardias, epicardial activation occurred on the right ventricular outflow tract or the postero-lateral wall of the left ventricle, and preceded endocardial activation in 50% of cases. Total epicardial activation times (103 +/- 29 beats/min) were significantly longer than during idioventricular rhythms (P less than 0.005). Ventricular tachycardias displayed overdrive excitation at critical pacing cycle lengths (360-280 msec) and were not suppressed by calcium infusion. Thus, differential mechanisms of impulse formation with distinct localizations can be elicited from healthy ventricular myocardium.     

Body Surface Laplacian Mapping of Cardiac Excitation in Intact Pigs

Relating body surface electrocardiographic signals to regional myocardial events has been a major effort in cardiac electrophysiology. Convenlional electrocardiographic means do not provide sufficient spatial resolution to resolve distributed cardiac electrical activity. The purpose of this investigation was to evaluate and study the validity of a new technique—body surface Lapiacian mapping—in a well-controlled experimental setting, and to test the hypothesis that the body surface Laplacian map (BSLM) can resolve normal and abnormal ventricular depolarization patterns and localize the initial site of ventricular depolarization with high spatial resolution. In this study. BSLMs were constructed from direct measurements of the surface Laplacian of the body surface potentials using an array of 64 concentric bipolar Laplacian electrodes. BSLMs were compared to body surface potential maps (BSPMs) during normal and ectopic ventricular activation in intact anesthetized pigs. The BSLM displayed highly localized images of cardiac electrical activity, indicating its ability to resolve myacardiai events. The BSLM in pigs identified the pacemaking focus overlying the known location of ihe epicardial pacing electrode, and imaged the activation sequence associated with exogenous ventricular pacing. In contrast, in all cases the BSPM revealed a diffuse distribution of activity over the chest. The present results suggest that the BSLM provides sufficient spatial resolution to relate body surface recordings to regional myocardial events and is able to detect ventricular depolarization patterns with greater resolution than the conventional BSPM.     

Isochronal difference mapping: an approach for mapping dynamic changes during reentrant ventricular tachycardia

During clinical electrophysiological study for treatment of reentrant ventricular tachycardia, activation maps constructed from the acquired electrophysiological data can be difficult to interpret when the reentrant circuit is changing from one cardiac cycle to the next. Reduction of complexity would be beneficial but has been difficult. A new technical method termed isochronal difference mapping (IDM) was devised to reduce complexity and enhance distinctive conduction patterns present in the data. Electrograms were acquired from 196 sites using a canine model of a reentrant ventricular tachycardia circuit with a figure eight conduction pattern occurring in the epicardial border zone. Activation maps were constructed for all cardiac cycles during episodes of tachycardia in five experiments. IDM maps were then created, which are subtractive comparisons of the activation maps from two different cardiac cycles during a given tachycardia episode. In each map the electrical activation occurring for only one or for both of the cardiac cycles was separately highlighted in distinct spatial areas of the border zone during an isochronal interval. Based on the mappings, areas of conduction velocity change, regions of breakthrough of the wavefront across functional lines of block, regions with coherent activation, and regions with irregular activation became readily apparent. IDM maps showed that when cycle length prolonged due to deceleration of conduction within the reentrant circuit isthmus, conduction velocity increased elsewhere in the circuit. IDM accentuates cycle-to-cycle differences in multi-channel electrophysiological data and can be used to reduce complexity and enhance distinctive conduction patterns.     

Inadvertent Atrial Location of a Sutureless Epicardial Pacemaker Electrode

A 70-year-old woman had an epicardial pacing system implanted after transvenous right ventricular endocardial pacing resulted in cardiac perforation. At the time of revision of the epicardial system for pacing failure, it was noted that one of the bipolar epicardial electrodes was pacing atrial tissue. The differential diagnosis of this unusual finding is discussed and recommendation urged for systematic analysis of pacing system function at the time of implant, in order to avoid inadvertent electrode plpcement.     

Ventricular Cross Stimulation Using a Pacing System Analyzer

Cross stimulation refers to the situation in which a pacing lead positioned in one cardiac chambers results in stimulation of another chamber. We report three cases of ventricular cross stimulation observed at the time of pacing system implantation. The cross stimulation occurred through bipolar atrial and ventricular leads connected in bipolar configuration to a dual chamber pacing systems analyzer. Analysis of events during cross stimulation indicated that the current pathway was occurring within the pacing system analyzer itself, due to low resistance internal connections between the ring electrodes of the atrial and ventricular pacing systems analyzer channels. Cross stimulation within the pacing system analyzer can be eliminated by making the ventricular channel unipolar. Awareness of the phenomenon reported here should assist in the avoidance of the mistaken diagnosis of pacing lead migration and malposition during pacing system implant procedures.     

The effect of anodal stimulation on V-V timing at varying V-V intervals

We studied the effect of anodal capture at the ring electrode of the right ventricular (RV) lead on interventricular (V-V) timing during biventricular (BiV) pacing, in which left ventricular (LV) pacing was preceding RV pacing. The V-V interval was programmed from 80 to 4 ms (LV first) in the LV unipolar (LV tip--generator can) followed by the LV tip-RV ring pacing configuration. In the LV unipolar configuration, V-V programming leads to a continuous change in morphology of the QRS complex according to a change in collision of both activation fronts. When using the LV tip-RV ring configuration with anodal capture at the RV ring electrode no change in QRS morphology was recorded varying the V-V interval from 80 to 60 and 40 ms. However, at V-V intervals between 20 and 4 ms a change in morphology of the QRS complex was recorded, which was due to additional cathodal stimulation of the RV tip electrode during RV stimulation.     

Latency during left ventricular pacing from the lateral cardiac veins: a cause of ineffectual biventricular pacing

We report three patients with cardiomyopathy and pronounced stimulus to QRS latency during left ventricular (LV) pacing from an epicardial cardiac vein. Delayed LV activation during simultaneous biventricular pacing produced an electrocardiographic pattern dominated by right ventricular stimulation. Hemodynamic parameters improved immediately after advancing LV stimulation (in one patient) or pacing the LV only (in two patients) coupled with dramatic improvement of heart failure symptoms.     

Improved Chronic Epicardial Pacing in Children: Steroid Contribution to Porous Platinized Electrodes

Although new "low threshold" epicardial electrodes combine steroid with a porous, platinized-platinum surface, the actual contribution of steroid elution has not been established. We evaluated this new electrode surface design with and without steroid in 13 children, ages 1-22 years. Both electrodes are unipolar and of similar surface area. The Medtronic Model 4951-P is a barb design for epimyocardial insertion without steroid while the Model 10295A is a steroid eluting, epicardial disk-shaped design. Both electrodes were implanted for atrial and ventricular pacing. At implant, sensed P and R waves, and pacing impedances were comparable between both electrodes. There were no significant differences between initial measured pulse width or calculated energy thresholds for the first 2 months following implant. Strength-duration curves for both electrodes at 1 month were comparable to implant values. After 2 months, the threshold of the nonsteroid electrode peaked and stabilized at a significantly higher (P less than 0.05) level than the more constant steroid eluting electrode. This difference continued for the first year following implant. We conclude that the new porous, platinized-platinum electrode design intrinsically limits initial electrode-tissue interface reactivity in children and improves epicardial pacing with low chronic threshold values. Steroid elution augments these intrinsic qualities by maintaining fibrous capsule stability with more constant low thresholds over time.     

Orthogonal ventricular electrogram sensing

Inappropriate demand pacing is most commonly due to improper ventricular electrogram sensing. Filters and programmable sensitivities improve electrogram sensing of conducted beats, but paced electrograms cannot be sensed by conventional unipolar or bipolar systems. A permanent pacing lead with a standard tip electrode and three orthogonal 0.8 mm2 sensing electrodes located circumferentially 2 cm proximal to the pacing tip was tested in 22 patients. The tip electrode was placed in the right ventricular apex in standard pacing position. Orthogonal electrodes were not in contact with ventricular myocardium. Orthogonal ventricular electrograms from 54 electrode pairs were compared with unipolar tip electrograms during conducted rhythms and paced beats. Tip ventricular electrograms averaged 12.8 mV with 3.04 mVT waves. Orthogonally recorded ventricular electrograms during conducted beats averaged 8.86 mV with T waves of 1.57 mV. During pacing, tip ventricular electrograms were obscured by the stimulus artifact and repolarization events. Orthogonal ventricular electrograms, however, demonstrated small discrete stimuli of 1.99 mV followed by discrete ventricular electrograms of 9.19 mV and T waves of 1.9 mV. Orthogonal ventricular electrograms compared favorably with contacting tip electrograms during conducted beats and provided a redundant sensing capability. During pacing, orthogonal ventricular electrograms allowed the capability for capture verification. A new pacing catheter allows for improved ventricular electrogram sensing and capture verification.     

Orthogonal Ventricular Electrogram Sensing

Inappropriate demand pacing is most commonly due to improper ventricular electrogram sensing. Filters and programmable sensitivities improve eleclrogram sensing of conducted beats, but paced electrograms cannot be sensed by conventional unipolar or bipolar systems. A permanent pacing lead with a standard tip electrode and three orthogonal 0.8 mm² sensing electrodes located circumferentially 2 cm proximal to the pacing tip was tested in 22 patients. The tip electrode was placed in the right ventricular apex in standard pacing position. Orthogonal electrodes were not in contact with ventricular myocardium. Orthogonal ventricular eleclrograms from 54 electrode pairs were compared with unipolar tip electrograms during conducted rhythms and paced beats. Tip ventricular electrograms averaged 12.8 mV with 3.04 mVT waves. Orthogonally recorded ventricular electrograms during conducted beals averaged 8.86 mV with T waves of 1.57 mV. During pacing, tip ventricular electrograms were obscured by the stimulus artifact and repolarization events. Orthogonal ventricular electrograms, however, demonstrated small discrete stimuli of 1.99 mV followed by discrete ventricuJar electrograms of 9.19 mV and T waves of 1.9 mV. Orthogonal ventricular electrograms compared favorably with contacting tip electrograms during conducted beats and provided a redundant sensing capability. During pacing, orthogonal ventricular electrograms allowed the capability for capture verification. A new pacing catheter allows for improved ventricuJar electrogram sensing and capture verification.     

Electrical characteristics of a split cathodal pacing configuration

Several electrical configurations can be used for biventricular pacing to achieve cardiac resynchronization. Commercially approved biventricular pacing systems stimulate the RV with an endocardial lead and the LV with a unipolar lead positioned in the cardiac venous circulation using the tip electrodes of both leads linked as a common cathode. The distribution of current with this parallel circuit, split cathodal configuration is dependent on the separate impedances of the two leads. A total of 19 patients with left bundle branch block and congestive heart failure underwent implantation of a cardiac venous lead and standard bipolar right atrial and RV pacing leads. Stimulation thresholds and impedances were measured for the RV and LV in five electrical configurations: (1) unipolar LV from the cardiac venous lead; (2) bipolar LV using the tip electrode in the cardiac vein as the cathode and the ring electrode of the RV lead as the anode; (3) bipolar RV from the RV lead; (4) unipolar split cathodal stimulation of the cardiac venous and RV leads; and (5) bipolar split cathodal stimulation of the cardiac venous and RV leads. Repeat measurements of RV and LV thresholds were made from the pulse generator at 1-year follow-up. The LV stimulation threshold increased from 0.7 +/- 0.5 V in the unipolar configuration to 1.0 +/- 0.8 V in the unipolar split cathodal configuration (P = 0.01) and from 1.0 +/- 0.7 V in the bipolar configuration to 1.3 +/- 0.9 V in the bipolar split cathodal configuration (P < 0.001). The RV stimulation threshold increased from 0.3 +/- 0.2 V in the bipolar configuration to 0.5 +/- 0.2 V in the bipolar split cathodal configuration (P = 0.005). The bipolar impedance measured 874 +/- 299 Omega for the coronary venous lead, 705 +/- 152 for the RV lead, 442 +/- 87 in the split unipolar cathodal configuration, and 516 +/- 64 in the bipolar split cathodal configuration. At 1-year follow-up, the LV stimulation threshold was 1.8 +/- 1.6 in the unipolar split cathodal configuration and 2.4 +/- 1.6 in the bipolar split cathodal configuration (P = 0.003). The RV stimulation threshold at 1 year was 0.7 +/- 0.3 in the unipolar split cathodal configuration and 0.8 +/- 0.3 in the bipolar split cathodal configuration (P = 0.02). The split cathodal configuration significantly increases the apparent stimulation threshold for both the LV and the RV as compared with individual stimulation of either chamber alone. Programming to the bipolar split cathodal configuration further increases the apparent stimulation threshold. These observations support the development of pacing systems with separate LV and RV output circuits for resynchronization therapy.     

Experimental Measures of Ventricular Activation and Synchrony

Background: A widened QRS complex as a primary indication for cardiac resynchronization therapy (CRT) for heart failure patients has been reported to be an inconsistent indicator for dyssynchronous ventricular activation. The purpose of this study was to conduct a detailed experimental investigation of total ventricular activation time (TVAT), determine how to measure it accurately, and compare it to the commonly used measure of QRS width. In addition, we investigated a measure of electrical synchrony and determined its relationship to the duration of ventricular activation. Methods: Unipolar electrograms (EGs) were recorded from the myocardial volume using plunge needle electrodes, from the epicardial surface using “sock” electrode arrays, and from the surface of an electrolytic torso‐shaped tank. EGs were analyzed to determine a root mean square (RMS)‐based measure of ventricular activation and electrical ventricular synchrony. Results: The RMS‐based technique provided an accurate means of measuring TVAT from unipolar EGs recorded from the heart, the entire tank surface, or the precordial leads. In normal canine hearts, a quantification of ventricular electrical synchrony (VES) for normal ventricular activation showed that the ventricles activate, on average, within 3 ms of each other with the left typically activating first. Conclusion: Conclusions from this study are: (1) ventricular activation was reflected accurately by the RMS width obtained from direct cardiac measurements and from precordial leads on the tank surface and (2) VES was not strongly correlated with TVAT.     

A New Low Threshold Platinized Epicardial Pacing Electrode: Comparative Evaluation in Immature Canines

A prospective comparison of pacing and sensing capabilities between the conventional Medtronic Model 4951 platinum-iridium epicardial pacing electrode and a new modified "platinized" version of the same electrode was performed in immature canines to determine if the new electrode design improves pacing in the immature myocardium. The conventional electrode was modified by electroplating platinum black particles onto the surface to increase the effective or true microscopic surface area, yet essentially maintain the same overall geometric electrode size. Both epicardial electrodes were inserted into the right ventricular myocardium with the lead pad sutured to the epicardium, and externalized to the scruff in five puppies (age 3 months). An additional left ventricular lead was implanted to permit chronic pacing following epicardially-induced atrioventricular block. Acute and chronic sensing and pacing capabilities of each externalized electrode were performed at implant and weekly up to 4 months. Histologic examination of each electrode implant site was performed at the end of the study period. At implant, both electrodes exhibited comparable values for sensed R waves, lead impedances, and pacing thresholds. During the study, the platinized electrode exhibited lower pacing thresholds. Analysis of all postimplant data demonstrated this threshold difference to be significantly lower (P less than .01) for the platinized version. Lead impedance and sensing capabilities remained comparable between the two designs. Histologic study demonstrated less fibrotic infiltration at the platinized electrode site. This preliminary evaluation indicates that for the duration of the postimplant study period, the platinized epicardial electrode design was associated with significantly lower thresholds and less fibrosis as a function of time compared to the conventional smooth electrode surface design. The new platinized electrode limits exit block in the developing immature myocardium and permits safe pacing at lower pulse widths and voltages to increase battery life.     

"In-Line" Bipolar, Steroid-Eluting, High Impedance, Epimyocardial Pacing Lead

Recent advances in electrode surface designs have eliminated traditional threshold differences between endo- and epicardial pacing leads. Since the epicardial approach offers the potential of direct left ventricular pacing and the transvenous approach may not be feasible or warranted in all instances, more advanced leads are being designed to optimize epicardial pacing capabilities. This study was conducted to evaluate a bipolar epimyocardial lead. Six immature canines (age 3 months) were instrumented. The lead (Medtronic mode! 10389) is a single-pass, "in-line" bipolar electrode with low current drain and high impedance, with an intramyocardial steroid-eluting cathode and nonsteroid epicardial anode. Twelve ventricular leads were implanted (two per animal) and the animals followed for 6 months with weekly analysis of pacing and sensing capabilities. Results at explant were compared with implant values and showed no significant differences between sensed R waves or in R wave slew rates in unipolar or bipolar modes. Explant lead impedances remained high in both modes: bipolar, 1658 ± 331; and unipolar, 1327 ± 308 Ω (P < 0.05). Chronic voltage (V) threshold at 0.5 ms showed no significant change from implant values during the study: unipolar, 0.3 ± 0.06 versus 1.0 ± 0.8; and bipolar, 0.4 ± 0.06 versus 1.6 ± 1.2. Histologic review showed negligible fibrous reaction at the electrode-tissue interface. This study introduces a high impedance, low threshold, "in-line" bipolar pacing lead design capable of stable chronic pacing with implant facilitated by a single suture technique.     

Detection of Atrial Activation By Intraventricular Electrogram Morphology Analysis: A Study to Determine the Feasibility of P Wave Synchronous Pacing From a Standard Ventricular Lead

The detection of atrial activation from a standard ventricular pacing lead with standard ventricular electrodes would provide patients with VVI and VVIR pacing systems atrial rate response and atrial synchrony. In addition to potentially increasing cardiac output appropriately in these patients at rest and during moderate exercise, P wave sensing with such a device could help reduce pacemaker syndrome. In this study, unipolar signals from distal and proximal intraventricular electrodes were recorded from the right ventricular apex in 20 patients. Unipolar eJectrograms from 16 patients were recorded using temporary electrophysiology catheters and in four patients using permanent pacemaker leads. Approximately 3 minutes of data per patient were acquired and analyzed. After selection of a P wave template, the difference in baseline normalized area between the template and signal was calculated on a point-by-point basis. The percent of atrial depolarizations correctly detected was determined for each patient and lead configuration at the optimal threshold. Far-field P wave accuracy was better at the proximal electrode (74 ± 25%) than at the distal electrode (57 ± 34%) (P < 0.025). At the proximal electrode, 15/20 (75%) patients had > 70% accuracy and 11/20 (55%) patients had > 80% accuracy. At the distal electrode, 10/21 (48%) patients had > 70% accuracy and 7/21 (33%) patients had > 80% accuracy. In conclusion, far-field detection of atrial activation at the ventricular proximal electrode appears possible with sufficient accuracy to provide periods of atrial rate response and synchrony in patients with a single standard lead.