Effect of pacing current strength on indexes of myocardial activation in humans: influence of chronic infarction and polarity of pacing

Utility of Very High-Output Pacing to Identify VT Circuits in Patients Manifesting Traditionally Inexcitable Scar

BACKGROUND

Entrainment and pace mapping are used to identify critical components (CCs) of ventricular tachycardia (VT) circuits. In patients with dense myocardial scarring, VT circuits may elude capture at standard high pacing outputs (up to 10 mA at a 2-millisecond pulse width).

OBJECTIVES

The purpose of this study was to assess the utility of very high-output pacing (V-HOP, 50 mA at 2 milliseconds) for identifying CCs of VT circuits after standard high pacing output failed to elicit capture in densely scarred myocardial tissue.

METHODS

Our standard VT ablation approach included electroanatomic mapping for substrate characterization and entrainment and/or pace mapping to identify CCs of VT circuits. Patients that required V-HOP to capture sites of interest comprised the study cohort. Ablation endpoints were VT termination and noninducibility.

RESULTS

Twenty-five patients (71 ± 10 years of age, all males) undergoing 26 VT ablations met the inclusion criteria. The mean left ventricular ejection fraction was 30% ± 14%, and 85% had ischemic cardiomyopathy. V-HOP was used to successfully entrain VT in 17 patients, yielding central isthmus sites in 10 and entrance/exit sites in 4. VT terminated with radiofrequency ablation at these sites in 15 patients. In 9 patients, V-HOP identified scar locations with a delayed exit. Acute procedural success was achieved in 24 patients without any adverse events. Over a follow-up period of 16 ± 21 months, 2 patients experienced VT recurrence requiring repeat ablation during which the same location was targeted successfully in 1 patient.

CONCLUSIONS

In VT patients with a dense scar that is traditionally inexcitable, V-HOP can identify CCs of the re-entrant circuit and guide successful ablation.

How to use pace mapping for ventricular tachycardia ablation in post-infarct patients

We aim to describe the technical aspects of pace mapping (PM), as well as the two typical patterns of pacing correlation maps during ventricular tachycardia (VT) ablation. The first main pattern is focal, with a gradual and eccentric decrease of the QRS correlation from the area with the best PM correlation. This focal pattern may be associated with two clinical situations: (1) with some endocardial points showing a good correlation compared to VT morphology: true endocardial exit of VT or endocardial breakthrough of either an intramural or an epicardial circuit; (2) without any endocardial points showing a good correlation compared to VT morphology: the VT may originate from the other ventricle, but the presence of an intramural or an epicardial circuit should be considered in patients with a structural heart disease. The second pattern is the presence of PM points exhibiting a good correlation close to other PM points showing a poor correlation compared to VT morphology: this abrupt change in paced QRS morphology over a short distance indicates divergence of activation wavefronts between these sites and suggests the presence of a slow conduction channel: the VT isthmus.

Subthreshold parameters of cardiac tissue in a bi-layer computer model of heart failure

Current density threshold and liminal area are subthreshold parameters of the cardiac tissue that indicate its susceptibility to external and internal stimulations. Extensive experimental and theoretical research has been conducted to quantify these two parameters in normal conditions for both animal and human models. Here we employed a 2D numerical model of human cardiac tissue to assess these subthreshold parameters under the pathological conditions of heart failure and fibrosis. Stimuli were applied over an area ranging from 0.04 to 1 mm² using various pulse durations. The current density threshold decreased with increasing stimulation area or pulse duration. No significant changes were found in both parameters between control conditions and heart failure in the atrial tissue, while in the ventricular tissue, heart failure resulted in significantly reduced excitability with higher stimulation current magnitudes needed for excitation and larger liminal areas. This results from the specific ionic remodeling in ventricular heart failure that affects both subthreshold active currents such as I(K₁) and connexin 43 conductance. In fibrosis, increased fibroblast to myocyte coupling coefficient had a non-linear influence on current density thresholds, with an initial increase of current magnitude followed by a relaxation phase down to the current magnitude threshold for the control condition with no fibrosis. The results show that subthreshold excitation properties of the myocardium are influenced in a complex, non-linear manner by cardiac pathologies. Such observations may contribute to our understanding of impulse capturing properties, relevant, for example, for the generation of ectopic foci-originated arrhythmias and for the efficient design of cardiac stimulating electrodes.

Impact of changing activation sequence on bipolar electrogram amplitude for voltage mapping of left ventricular infarcts causing ventricular tachycardia

INTRODUCTION

Wavefront direction is a determinant of bipolar electrogram amplitude that could influence identification of low amplitude regions indicating infarction or scar.

METHODS

To assess the importance of activation sequence on electrogram amplitude 11 patients with prior infarction and ventricular tachycardia were studied. At 819 left ventricular sites bipolar electrograms were recorded during atrial pacing and ventricular pacing, followed by unipolar pacing with a stimulus of 10 mA at 2 ms. Sites with a pacing threshold > 10 mA were designated electrically unexcitable scar.

RESULTS

Areas of low voltage (< or =1.5 mV) were present in all patients. Atrial paced and ventricular paced electrogram amplitudes were strongly correlated (r = 0.77; P < 0.0001). Changing the activation sequence (from atrial pacing to ventricular pacing) produced a > 50% change in electrogram amplitude at 28% of sites and a > 100% change at 10% of sites, but only 8% of sites had an electrogram amplitude classified as abnormal (< or =1.5 mV) with one activation sequence and normal (> 1.5 mV) with the other activation sequence. Electrically unexcitable scar (6% of sites) was associated with lower electrogram amplitude but could not be reliably identified based on electrogram amplitude alone for either activation sequence.

CONCLUSION

Voltage maps created with bipolar recordings using these methods should be relatively robust depictions of abnormal ventricular regions despite variable catheter orientation and activation sequences that might be produced by different rhythms.

Identification of the Ventricular Tachycardia Isthmus After Infarction by Pace Mapping

Background— Ventricular tachycardia (VT) isthmuses can be defined by fixed or functional block. During sinus rhythm, pace mapping near the exit of an isthmus should produce a QRS similar to that of VT. Pace mapping at sites proximal to the exit may produce a similar QRS with a longer stimulus-to-QRS interval (S-QRS). The aim of the study was to determine whether a VT isthmus could be identified and followed by pace mapping.

Methods and Results— Left ventricular pace mapping during sinus rhythm was performed at 819 sites in 11 patients with VT late after infarction, and corresponding CARTO maps were reconstructed. An isthmus site was defined by entrainment and/or VT termination by ablation. Pace-mapping data were analyzed from the identified isthmus site and from sites at progressively increasing distances from this initial isthmus site. Sites where pace mapping produced the same QRS with different S-QRS delays were identified to attempt to trace the course of the isthmus. In 11 patients, 13 confluent low-voltage infarct regions were present. In all these regions, parts of VT isthmuses were identified by pace mapping. In 11 of 13 of the identified isthmus parts, the QRS morphology of the pace map matched a VT QRS. In 10 of 11 patients, radiofrequency ablation rendered clinical VTs noninducible. Successful ablation sites were localized within an isthmus identified by pace mapping in all of these 10 patients.

Conclusions— VT isthmuses can be identified and part of their course delineated by pace mapping during sinus rhythm. This method could help target isthmus sites for ablation during stable sinus rhythm.

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.

Basic assessment of paced activation sequence mapping: implications for practical use

Some experiences support the use of atrial paced activation sequence mapping, but there is no systematic study assessing its spatial resolution, reproducibility, and influence of pacing parameters. The aim of this study was to evaluate these issues by using a 24-pole catheter positioned at the atrial aspect of the tricuspid and mitral annuli in 15 patients. Bipolar pacing was performed at two sites (right and left atria), 2 cycle lengths (300 and 500 ms) and two outputs (twice and tenfold the late diastolic threshold voltage for 2-ms pulses). The elapsed time between the atrial activation at the two dipoles adjacent to the pacing dipole (activation time [AT]) was measured during each pacing sequence. Changes in cycle length did not modify the AT. The increase in voltage slightly modified the AT (maximum -2 ms at the RA; 95% CI -3 to -1 ms) due to a greater shortening of the conduction time to the dipole located next to the anode. The 95% limits of the intraobserver and interobserver agreements in the AT measurement were -2 to 3 ms and -3 to 3 ms, respectively. The spatial resolution was studied in ten patients by measuring the AT during pacing from each dipole of a 20-pole catheter with a 1-3-1 mm interelectrode distance. The mean AT change was 10 +/- 4 ms per 6 mm of pacing site displacement (95% CI 8-11 ms, range 2.5-20 ms). In conclusion, paced atrial activation sequence analysis is reproducible, accurate, and relatively independent of pacing parameters.

Relationship of slow conduction detected by pace-mapping to ventricular tachycardia re-entry circuit sites after infarction

OBJECTIVES

This study sought to characterize the relationship of conduction delays detected by pace-mapping, evident as a stimulus to QRS interval (S-QRS) delay >or=40 ms, to ventricular tachycardia (VT) re-entry circuit isthmuses defined by entrainment and ablation.

BACKGROUND

Areas of slow conduction and block in old infarcts cause re-entrant VT.

METHODS

In 12 patients with VT after infarction, pace-mapping was performed at 890 sites. Stimulus to QRS intervals were measured and plotted in three-dimensional reconstructions of the left ventricle. Conduction delay was defined as >or=40 ms and marked delay as >80 ms. The locations of conduction delays were compared to the locations of 14 target areas, defined as the region within a radius of 2 cm of a re-entry circuit isthmus.

RESULTS

Pacing captured at 829 sites; 465 (56%) had no S-QRS delay, 364 (44%) had a delay >or=40 ms, and 127 (15%) had a delay >80 ms. Sites with delays were clustered in 14 discrete regions, 13 of which overlapped target regions. Only 1 of the 14 target regions was not related to an area of S-QRS delay. Sites with marked delays >80 ms were more often in the target (52%) than sites with delays 40 to 80 ms (29%) (p < 0.0001).

CONCLUSIONS

Identification of abnormal conduction during pace-mapping can be used to focus mapping during induced VT to a discrete region of the infarct. Further study is warranted to determine if targeting regions of conduction delay may allow ablation of VT during stable sinus rhythm without mapping during VT.

A prospective multicenter study on the safety of a pacemaker with automatic energy control: influence of the electrical factor on chronic stimulation threshold. PEACE Investigators

The effectiveness and safety of a pacemaker with automatic control of capture was evaluated in 162 patients followed at 27 Spanish centers. The aim of our prospective, multicenter, and randomized trial was to determine the relationship between the voltage output of the pulse generator and the stimulation threshold. We randomized 162 patients (107 men, mean age 75 +/- 12 years). We implanted a ventricular pacemaker model Regency SR+ or SC+ with Pacesetter's low polarization bipolar leads Membrane E 1450. The patients were randomized to receive Autocapture or not; group I (81 patients) Autocapture On, pulse output automatically adjusted and group II (81 patients) Autocapture Off, fixed output parameters (3.9 V, 0.37 ms). We performed a 6-month follow-up measuring stimulation threshold by means of the VARIO test and Autocapture test, evoked response signal, and R wave signal. The mean R wave was 15.77 +/- 3.5 mV at the end of the follow-up for group I, and 14.91 +/- 6.8 mV for group II (P = NS). The measured evoked response at the end of the follow-up was 9.25 mV in Group I and 8.48 mV in Group II (P = NS). The stimulation threshold was not different between groups. The current density created with the voltage and pulse width used in this study (< or = 3.9 V and 0.37 ms) at the tip of this electrode during the maturation process had no influence on the development of the chronic detection and stimulation thresholds.

Permanent overdrive atrial pacing in the chronic management of recurrent postoperative atrial reentrant tachycardia in patients with complex congenital heart disease

This study investigated the value of permanent atrial pacing as an adjunct to the current therapy in the chronic management of recurrent postoperative atrial reentrant tachycardia in patients with complex congenital heart disease. We studied the postpacing clinical course in 18 patients with recurrent atrial reentrant tachycardias unresponsive to conventional therapy who had an implanted atrial pacemaker. The pacemaker was programmed at a lower pacing rate 20% faster than the spontaneous mean daily rate previously determined with 24-hour Holter monitoring. Serial Holter recordings and pacemaker programming sessions were subsequently performed trying to maintain a paced atrial rhythm overdriving the spontaneous rhythm as long as possible. Twenty-four hour Holter monitoring documented a prevalent (> 80%) paced rhythm during the daily hours in all patients during the follow-up; all patients, however, required at least once a variation in programmed mode and pacing rate. Antiarrhythmic medications were discontinued after 6 months if the patient remained arrhythmia free while on pacing. Recurrences of atrial reentrant tachycardia occurred in five patients (29%) during the initial 6 months interval after the pacemaker implantation, while late recurrences occurred in only two patients (11%). One patient died suddenly 10 months after the pacemaker implant. At the end of the follow-up, 15 patients (83%) were arrhythmia-free and only 2 of them were still on antiarrhythmic drugs. We conclude that permanent atrial overdrive pacing can be an important tool in the management of patients with atrial reentrant tachycardia following repair of congenital heart disease.

Effect of high-current stimulation in patients with sustained ventricular tachycardia rendered noninducible by antiarrhythmic drugs

Successful antiarrhythmic drug therapy for sustained ventricular tachycardia (VT) is presumed to be related to effects on myocardium within the re-entrant circuit. To test the hypothesis that prevention of VT induction may be related to effects on myocardium other than that directly involved in the tachycardia circuit, high-current stimulation was used to achieve shorter coupling intervals in 22 patients with sustained uniform VT that was rendered noninducible by antiarrhythmic agents during stimulation at twice threshold. Sustained uniform VT was induced in 10 patients in response to high-current stimulation (group 1), including 4 tachycardias with the same morphology observed in the baseline study. There were no inducible arrhythmias in 12 patients (group 2). Patients were receiving several different antiarrhythmic regimens, but there was no particular drug associated with the induction of VT using high-current stimulation. There was no statistically significant difference between groups 1 and 2 in baseline VT cycle length (247 +/- 41 vs 253 +/- 44 ms), drug-induced increase in effective refractory period (20 +/- 15 vs 16 +/- 7%), QRS duration (25 +/- 10 vs 20 +/- 17%) or maximal current strength delivered (10.9 +/- 5.3 vs 9.3 +/- 4.0 mA). There was no significant difference in local activation with high-current stimulation between groups 1 and 2. In conclusion, sustained uniform VT was induced in 45% (10 of 22) of patients whose arrhythmias were rendered noninducible by antiarrhythmic agents during programmed stimulation at twice threshold.(ABSTRACT TRUNCATED AT 250 WORDS)