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 Electrogram Sensing

Le recueil de ľéléctrogramme intracardiaque donne ľinformation nécessaire aux systèmes de stimulation à la demande. Les troubles du recueil surviennent dans près de 50% de stimulateurs unipolaires, ceux de ľoreillette sont encore plus fréquents. Le recueil par électrode orthogonale ne demande pas de contact avec le myocarde et on obtient de hauts potentiels ďorigine auriciilaire avec rejection totale des pontentiels ventriculaires. Ce système permet la sequence ďactivation aussi bien que la vérification de la capture.
Intracardiac electrogram sensing is the afferent limb of demand pacing systems. Under- and over-sensing at the ventricular level has been demonstrated in almost 50% of implanted unipolar pulse generators, and atrial eJectrogram sensing problems are more common. A unique dedicated sensor employing noncontacting and circumferentially placed orthogonal electrodes was tested at both the atrial and ventricular levels. Electrograms demonstrated voltages similar to those recorded from contacting electrodes, but with complete far-field rejection of signals from the opposite chamber. Orthogonal electrograms allowed for activation sequencing and capture verification as well.     

A New Orthogonal Lead for P Synchronous Pacing

P synchronous pacing has long been identified as advantageous for patients with atrioveniricular conduction defects and intact sinus node function. Prior endocavitury systems have been infrequently employed, because of unreliable P wave sensing from standard ring electrodes in the atrium or the requirement for a second atriaJ sensing lead. A single endocardial lead employing a unipolar ventricular stimulating electrode and an orthogonal P wave sensing design was developed and tested in 22 patients undergoing electrophysiologic study or pacemaker implantation. Thirteen centimeters from the stimulating tip of a standard permanent pacing lead, three or four electrodes with a surface area of one millimeter squared, equidistant from the tip, were placed circumferentially about the catheter. With the catheter tip normally placed in the right ventricular apex, atrial sensing eJectrodes were positioned in the mid-high lateral right atrium, adjacent to, but not affixed to, the right atrial wall. Bipolar orthogonal leads X and Y were obtained. In 22 patients, during sinus rhythm, atrial electrogram voltages in the X axis of 2.47 plus or minus 1.6 millivolts and 2.32 plus or minus 1.6 millivolts in the Y axis were recorded. QRS voltages of 0.078 millivolts and 0.073 millivolts, respectively, allowed dramatic ability to discriminate P from QRS complexes (P/QRS equals 32/1). There was no change in QRS voltages recorded during spontaneous premature ventricular contractions, bipolar or unipolar ventricular pacing. A single catheter designed for P synchronous pacing empJoying circumferentially placed atrial sensing electrodes has demonstrated unique atrial sensing voltages with excellent QRS signal rejection. (PACE, Vol. 4, November-December, 1981)     

Electrophysiological properties of a new permanent endocardial lead for uni- and bipolar pacing

Unipolar and bipolar electrode systems were compared for electrogram amplitudes and slew rates, signal source impedance, and myocardial stimulation threshold and resistance in 15 consecutive patients who received a new endocardial electrode (Cordis 325-161). The bipolar electrograms showed the highest amplitude in nine of the patients (60%). The unipolar and bipolar electrograms were equal in four patients (26.7%), whereas the unipolar electrograms were highest in only two patients (13.3%). The difference in mean amplitude between bipolar (11.1 mV) and unipolar (10.1 mV) electrograms was statistically significant (p 0.05). Mean slew rates were almost equal (1.7 versus 1.6 V/s; p greater than 0.1). The bipolar electrode system always gave somewhat higher signal source impedance than the unipolar system (p 0.001). The current threshold was significantly lower during bipolar pacing (0.59 mA) in constant current pacing mode, than during unipolar pacing (0.65 mA) (p less than 0.05). No significant differences were found during constant voltage pacing. Stimulation resistance was highest in the bipolar electrode system (p less than 0.001). We conclude that the bipolar electrode system is as good as, or better than, the unipolar system both for ventricular sensing and for pacing.     

Electrogram Signals Recorded from Acute and Chronic Pacemaker Implantation Sites in Pacemaker Patients

Electrogram signals recorded from typical pacemaker implantation sites may be useful for a variety of pacemaker system functions including pacemaker follow-up, atrial and ventricular sensing (event detection), and triggered electrogram storage. We quantified the electrical characteristics of pacemaker pocket electrograms using a subcutaneous electrode array (SEA) in a population of 48 patients undergoing initial or replacement pacemaker implantation. SEA recorded intrinsic R wave amplitudes measured peak to peak averaged 118 μV and 65 μV for the two recorded SEA electrograms and were significantly different (P < 0.001); paced R wave amplitudes averaged 180 μV and 110 μV. P wave amplitudes averaged 39 μV and 26 μV. No statistically significant difference in amplitudes were observed between acute versus chronic pacemaker pocket or indication for pacing (A V block, sick sinus syndrome). Signal to noise ratios, using R wave amplitude as signal, were lower in the SEA electrogram on average (11 dB) compared to the intracardiac electrogram (27 dB), but sufficient for diagnostic assessment. R wave/P wave ratios for SEA signals were lower than surface and intracardiac values 3.1 and 2.7 compared to a range of 6.2–9.8, indicating a relative enhancement of P waves to R waves in SEA signals. In summary, SEA electrograms are of sufficient amplitude and signal quality (signal to noise ratio) to hold promise for future implantable device features such as electrogram telemetry, enhanced sensing, and diagnostic data storage.     

Automatic Capture Verification by Charge-Neutral Sensing

Automatic capture verification can prolong pulse generator longevity and increase patient safety. However, the detection of evoked response following pacing is complicated due to afterpotentials caused by polarization of electrodes. This study describes a new capture verification scheme, which neutralizes the charges between the pacing electrodes. The hypothesis of the charge-neutral sensing is that the afterpotentials in the ring and the tip are opposite in polarity when pacing in a bipolar mode between ring and tip. Summing the unipolar signals sensed at the tip and the ring should effectively cancel the afterpotentials. This scheme was implemented in an external computer based system and tested during pacemaker implant/replacement on 23 patients during VVI pacing (17 acutely implanted leads and 6 chronic leads). Surface ECG was recorded to provide a marker for capture and noncapture. The pacing voltage was gradually decreased until a noncapture beat was noted. To avoid fusion beats, the pacing rate was programmed ˜50% higher than the intrinsic rate. The evoked response was high pass filtered and the integral average was calculated for both capture and noncapture beats. The system signal to noise ratio (SNR) was expressed as ratio of the minimum integral average of all capture beats to the maximum integral average of all noncapture beats. The system SNR was 8.6 ± 1.3 (mean ± S.E.M; range 1.5–22.8), indicating that the charge-neutral sensing method has, on average, a ninefold safety margin in providing capture verification. Further, evaluation is needed to fully assess this feature in patients with chronic leads.     

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.     

Orthogonal Atrial Appendage Sensing

The characteristics of electrograms derived from a solid platinum-iridium pacing catheter tip in contact with the right atrial appendage are compared to those derived from a Target-tip electrode. Both are then compared to electrograms from two noncontacting orthogonal electrodes positioned more proximally within the atriai appendage. Wave form morphology and spectral energy distribution were determined for the three sets of electrograms. It is concluded that orthogonal electrodes placed within the atrial appendage may offer enhanced atrial sensing required by more sophisticated pacemakers.     

Evaluation of Fusion Beat Detection with a New Ventricular Automatic Capture Algorithm in ICDs

This study evaluated a newly developed automatic capture verification scheme for implantable cardioverter defibrillators (ICDs) regarding discrimination of capture, fusion, and noncapture beats, with an emphasis on fusion detection. The algorithm uses evoked response detection based on a sensing vector from right ventricular shocking coil to Can. Patients undergoing ICD implant or replacement were enrolled in this study. An external system was used for pacing and data acquisition. To provoke ventricular fusion beats, VVI patients were paced close to the rate of their intrinsic rhythm and DDD patients were paced close at their intrinsic PR interval. Surface ECG and wideband filtered intracardiac electrograms were recorded for off-line analysis. Each paced beat was independently classified visually by surface ECG and by the automatic detection algorithm. The algorithm performance was then evaluated by comparing the classification results. Twenty-seven patients (22 males, 5 females; 63.8 ± 12.5 years) were analyzed. Device and lead demographics were: 18 DDD/9 VVI; 16 dedicated bipolar, 11 integrated bipolar leads; 18 acute, 9 chronic (3.7 ± 2.0 years) leads. In total, 2064 beats were analyzed, including 1,477 fusion beats and 587 capture beats. Fusion detection sensitivity and specificity were 99.5% and 99.0%, respectively. Seven true-fusion beats (0.5%) were classified as capture and 6 capture beats (1.0%) were identified as fusions. Capture or fusion beats were never detected as non-capture beats. It is concluded that the algorithm was effective in detecting fusion beats. It could potentially be used in ICD applications that need accurate fusion detection.     

Clinical Utility of Telemetered Intracardiac Electrograms in Diagnosing a Design Dependent Lead Malfunction

Inhibition of pacemakers due to false signals from malfunctioning pacing leads has been previously reported. Three cases are reported in which inappropriate pauses were observed shortly following implantation. In all three cases, active fixation leads with an electronically active screw and ring tip electrode were used. All leads were manufactured by Oscor Medical, St. Petersburg, Florida. In case one, models PY-61 (ventricle) and PY-51 (atrium) were used. In case two, models PY-61 were used in the ventricle and the atrium. In case three, models PY-52 were used both in the atrium and ventricle. Thresholds at the time of implantation were acceptable. In cases one and two, inappropriate pauses were noted following paced ventricular beats. In case three, inappropriate inhibition of the ventricular output was noted intermittently after some of the paced atrial beats. Spurious signals were identified as the cause of the apparent oversensing problem by using electrocardiograms with annotated telemetry information and noninvasively telemetered intracardiac electrograms. The amplitude of the spurious signals varied between 2 and 14 mV. In all cases, the problems resolved with time. A mechanism for the generation of false transients is proposed and the value of telemetered intracardiac electrograms is discussed.     

Sensing Characteristics of Unipolar and Bipolar Orthogonal Floating Atrial Electrodes: Morphology and Spectral Analysis

We investigated wave morphology and spectral energy distributions of signals picked up by floating atrial unipolar and bipolar orthogonal sensing electrodes. Our data show that atrial P and QRS waves from unipolar floating electrodes are comparable. On the other hand, atrial P and QRS waves from bipolar orthogonal floating electrodes are significantly different. Even at high and mid right atrial locations, QRS waves are either absent or much smaller in amplitude and lower in frequency content than P waves. We conclude that the bipolar orthogonal floating atrial electrode is superior to the unipolar one for sensing due to its P to QRS wave discriminating power, which makes complex input filters or algorithms unnecessary. Our data support the idea that physiologic pacing with a VDD or VAT pacemaker is possible using a single pass lead.     

Initial Experience with 1.5-mm2 High Impedance, Steroid-luting Pacing Electrodes

In this human study, 21 atrial and 62 ventricular 1.5-mm² unipolar steroid-eluting pacing electrodes were implanted in 64 patients. Pacing thresholds, lead impedance, and sensing measurements were measured via pacemaker telemetry within 24 hours postimplont, and at 1, 2, 3, 4, 6, 12. 24. and 52 weeks. Acute pacing impedances measured via a pacing systems analyzer were 1,039 ± 292 (atrial) and 1,268 ± 313 ohms (ventricular). A10%-15% decline in the mean telemetered atrial and ventricular pacing impedances was observed at 1 week, but thereafter remained stable. Acute pacing thresholds at 0.5 ms were 0.5 ± 0.3 V (atrial) and 0.4 ± 0.1 V (ventricular). Filtered P and B wave amplitudes were 3.7 ± 2.3 mV and 14.9 ± 5.9 mV, respectively. In 21 patients, no complications related to the atrial electrode were observed. Of 62 patients with ventricular electrodes, 4 patients (6%) experienced complications and required surgical intervention. On these, causative factors included micro-dislodgment (l patient), and perforation (l patient). Sudden unexplained exit block occurred late (> 6 weeks) in two patients. In the remainder of patients, pacing thresholds and sensed electrogram amplitudes remained stable throughout the 52-week follow-up period. Conclusions: The- present study validates that smaller surface (i.e., 1.5 mm²) steroid- eluting electrode designs offer excellent pacing and sensing performance with significantly higher pacing impedances. Although questions remain as to the cause of late exit block in two patients in this series, this relatively small surface electrode design offers promise toward achieving greater pacing efficiency and a theoretical 13%-16% (minimum) enhancement in permanent pacemaker longevity.     

Chronic Ventricular Electrograms: Do Steroid-Eluting Leads differ from Conventional Leads?

SCHUCHERT, A., ET AL.: Chronic Ventricular Electrograms: Do Steroid-Eluting Leads differ from Conventional Leads? The aim of steroid-eluting leads is to reduce chronic pacing thresholds. Whether steroid-eluting leads also modify acute and chronic R wave amplitudes as well as R wave sensing of the pulse generator was investigated in 31 patients with a unipolar ventricular pacemaker. Four different leads were implanted: Two steroid-eluting leads with different electrode surface areas (8 mm² and 5.8 mm²) and two conventional leads (Target Tip, Elgiloy lead). At implantation filtered R wave amplitudes, peak-to-peak values, and slew rates were measured by a pacing system analyzer. One year after implantation R wave amplitudes were directly determined from intracardiac electrograms and compared to the peak-to-peak data at implantation. Additionally, R wave inhibition was evaluated at a sensitivity setting of 5 mV. There were no differences among the four leads in respect to any of the parameters studied at implantation. At follow-up, no differences in R wave amplitudes were found leading to an appropriate sensing in all leads. Steroid-eluting leads did not differ from conventional leads and a smaller electrode surface area of 5.8 mm² had no influence on ventricular electrogram. Together with a pacemaker with an input impedance of 37 kohms R wave sensing was a correct setting of 5 mV.     

Single Lead Atrial Synchronous Ventricular Pacing: A Dream Come True

Single lead, atrial synchronous pacing systems were developed in the late 1970s. Clinical experience has demonstrated the need to position the "floating" atrial electrode in the mid-to-high right atrium and the need for a specially designed pulse generator (with very high atrial sensitivity) to provide a high quality and amplitude atrial electrogram for consistent sensing. A 12-year experience with different electrode configurations, from the first unipolar designed in 1980 to the most recent atrial bipolar electrodes, has confirmed the validity of the original concept and the long-term reliability of the single lead atrial synchronous pacing system, which can reliably produce long-term atrial sensing and ventricular stimulation in the presence of normal sinoatrial function.     

Morphologic differences of the endocardial electrogram in beats of sinus and ventricular origin

The lack of accurate arrhythmia detection and identification is one of the major obstacles to improvement in the efficacy of antitachycardia devices. We evaluated a method for detection of beats of ventricular origin compared to sinus rhythm based on the morphology of the endocardial electrogram. In order to compare mechanically induced ventricular beats to normal sinus beats, endocardial electrograms from a standard pacing electrode were recorded from eight open-chested dogs. Time and frequency domain features analyzed included peak-to-peak amplitude (AMP), maximal slew rate (dV/dT), and frequency content (-3 dB downpoint). Quantitative morphologic comparison of the waveforms was performed using standard correlation and by the absolute area of difference between the waveform and a sinus beat template. The AMP and dV/dT for a group of ventricular beats did not differ significantly from beats of sinus origin. In the unipolar configuration -3 dB for ventricular beats was significantly different from sinus beats (p = .01), but overlap occurred in three of eight cases. Conversely, using either method of assessment of morphological differences, all ventricular beats could be identified without overlapping the values for normal beats. We concluded that morphologic analysis of the endocardial electrogram by such methods may be a highly accurate means of distinguishing between beats of sinus and ventricular origins. This technique may also be applicable to the problem of automatic rhythm identification by implanted devices.     

Changes in Morphology of the Paced QRS Complex Related to Atrial Contraction

A patient with 2:1 AV block underwent temporary ventricuJar pacing. AU the paced stimuli resuited in ventricular capture, but a marked variability in morphology of the paced QRS complexes occurred. Two different types of paced QRS complex (labeled A and B) were recognized. Type B complexes were manifest only when the pacing stimulus was preceded hy a sinus P wave within a time interval ranging from 0.15 to 0.52 sec. The P wave-induced changes in morphology of the paced QRS complexes were interpreted as due to displacement of the pacing ventricular lead caused by atrial systole.     

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.     

Muscle noise effects on atrial evoked response sensing: implications on atrial auto-threshold and auto-capture determination

Background: Automatic pacing capture verification algorithms that are based on detection of local evoked response signals are susceptible to interference from myopotential noise. This is especially true in the atrium where the lower signal amplitude and signal‐to‐noise ratio make pacing capture verification more challenging. The aim of this study is to evaluate the impact of myopotential noise induced by various maneuvers on the atrial evoked response (AER) signal. Methods: Data were collected from 18 patients (7 M/11 F, 77.6 ± 8.8 years), implanted with a dual chamber rate response (DDDR) pacemaker and acute and chronic right atrial leads and right ventricular leads, at three regular follow‐up visits scheduled at 1, 3, and 6 months after pacemaker implantation or replacement procedures. During each study visit, manual atrial pacing threshold tests during unipolar pacing in the DDD mode were conducted with two evoked response sensing configurations (RA_Ring→ Ind and RA_Ring→ V_Tip) while patients performed six different maneuvers. Noise estimates were calculated for each maneuver. Results: The greatest noise estimates in both sensing configurations were measured during the hand‐pressing exercise (0.649 ± 0.342 mV in the RA_Ring→ Ind vector, 0.309 ± 0.223 mV in the RA_Ring→ V_Tip vector). No significant differences in overall noise estimates were observed between the follow‐up visits. Of the beats that exhibited noise estimates greater than a sensing floor of 0.3 mV, up to 22% of cardiac beats had a signal‐to‐noise ratio less than 2 in the RA_Ring→ Ind configuration. Conclusions: Results indicate that myopotential noise generated by performing maneuvers has a demonstrable impact on AER sensing. Therefore, noise mitigation processes are necessary in atrial automatic pacing capture verification algorithms based on evoked response signals to identify and appropriately manage noise. (PACE 2011; 34:460–466)     

Differential Bipolar Sensing of a Dual Chamber Pacemaker

Differential bipolar sensing was evaluated in 10 consecutive patients with symptomatic heart block managed with dual chamber pacing. During pacemaker implantation atrial and ventricular electrograms were recorded using unipolar (UP) and differential bipolar (DBP) sensing amplifiers. The mean peak-to-peak amplitudes of the UP and DBP atrial electrograms were 3.3 +/- 1.2 mV and 4.2 +/- 1.2 mV, respectively. The difference was statistically significant (p less than 0.05). The mean peak-to-peak amplitudes of the ventricular electrograms were, respectively, 6.8 +/- 1.5 mV and 7.5 +/- 1.4 mV (p less than 0.01). Within 6 weeks after pacemaker implantation, patients visited the outpatient clinic. Isometric exercise tests were performed during UP and DBP sensing of the pacing system. Myopotential sensing in the ventricle occurred in nine patients during UP sensing and in none of the patients during DBP sensing (p less than 0.01) at a sensitivity setting of 0.5 mV. In addition, chest wall stimulation was performed to assess the effects of far-field signals on the ventricular sensing circuit of the pulse generator. Chest wall stimuli inhibited ventricular output during UP sensing in all 10 patients, whereas during DBP sensing inhibition of the ventricular channel occurred in three patients and then only at high output (greater than 8 V) settings. The susceptibility of the pacing system to crosstalk was also determined. However, neither during UP sensing nor during DBP sensing could cross-stimulation or cross-inhibition be demonstrated. In conclusion, DBP sensing is superior to UP sensing in terms of myopotential and far-field sensing.(ABSTRACT TRUNCATED AT 250 WORDS)     

Ablation lesion size correlates with pacing threshold: a physiological basis for use of pacing to assess ablation lesions

The virtual electrode model predicts that pacing stimulus strength should reflect proximity of the pacing electrode to excitable myocardium, allowing pacing threshold to assess radiofrequency (RF) ablation lesions and unexcitable scar. The purpose of this study is to correlate RF lesion size with pacing threshold and electrogram (EG) amplitude change at the ablation site. In four swine (32-58 kg, 20 ventricular RF lesions were created using a 4-mm tip electrode catheters under fluoroscopic and electroanatomic guidance. Unipolar pacing threshold and bipolar and unipolar EG amplitude were measured before and after ablation and compared with lesion size measured in the fixed, serially sectioned tissue. Lesion diameter ranged from 6.4 to 19 mm and volume ranged from 29 to 1920 mm3. Ablation increased the pacing threshold by 320%, from 0.9 +/- 0.3 to 3.6 +/- 2.6 mA, P < 0.001. The change in pacing threshold correlated with lesion volume R = 0.88, P < 0.001). Linear regression predicts that lesion volume (mm3) = 160 X rise in pacing threshold + 13. Ablation reduced peak to peak bipolar EG amplitude by 56%, from 2.5 +/- 2.0 mV to 1.1 +/- 0.6 mV (P = 0.005). Unipolar EG amplitude diminished by only 22% from 4.0 +/- 1.6 to 3.2 +/- 0.9 mV postablation (P = 0.005). The correlations of lesion volume with change in either bipolar R = 0.14, P = 0.6) or unipolar R = 0.18, P = 0.6) EG amplitude were poor. Pacing threshold correlates with RF ablation lesion size, consistent with the virtual electrode model. In normal myocardium, change in pacing threshold is likely to be a better marker of lesion size than electrogram amplitude.