A Novel Approach for Transcoronary Pacing in a Porcine Model

Abstract: Background. Transcoronary pacing for the treatment of bradycardias during percutaneous coronary intervention (PCI) is a useful technique in interventional cardiology. The standard technique is unipolar pacing with the guidewire in the coronary artery against a cutaneous patch electrode. We developed a novel approach for transcoronary pacing by using intravascular electrodes in different positions in the aorta in a porcine model. Methods and Results. Unipolar transcoronary pacing was applied in 8 pigs under general anesthesia using a standard floppy guidewire in a coronary artery as the cathode with additional insulation of the guidewire by a monorail angioplasty balloon. Intravascular electrodes positioned in the aorta thoracalis and the aorta abdominalis served as indifferent anodes. The efficacy of transcoronary pacing with intravascular anodal electrodes was assessed by measurement of threshold and impedance data and the magnitude of the epicardial electrogram in comparison to unipolar transvenous pacing using the same indifferent anodal electrodes. Transcoronary pacing with the guidewire-balloon combination using indifferent intravascular electrodes was effective in all cases. Transcoronary pacing thresholds obtained against the indifferent coil electrodes in the aorta thoracalis (0.8 ± 0.5 V) and in the aorta abdominalis (0.8 ± 0.5 V) were similar to those obtained with unipolar transvenous pacing (0.7 ± 0.3 V and 0.6 ± 0.2 V, respectively), whereas the tip-electrode in the aorta thoracalis serving as indifferent anode produced significantly higher pacing thresholds (guidewire, 2.8 ± 2.6 V; transvenous lead, 1.5 ± 0.8 V). The lower pacing threshold of the coil-electrodes was associated with significantly lower impedance values (aorta thoracalis, 285 ± 63 ohm; aorta abdominalis, 294 ± 61 ohm) as compared to the tip-electrode in the aorta thoracalis (718 ± 254 ohm). The amplitude of the epicardial electrogram acquired by the intracoronary guidewire was without significant differences between the indifferent electrodes. Conclusions. Transcoronary pacing in the animal model using a standard guidewire with balloon insulation and intravascular indifferent electrodes is depending on the optimal configuration of the anodal electrode. The use of intravascular coil electrodes with a sufficient surface area can produce 100% capture at thresholds comparable to transvenous pacing. Therefore, technical integration of these coil electrodes into the access sheath or the guiding catheter with respect to handling these tools in daily clinical practice in the catheterization laboratory could further facilitate the transcoronary pacing approach.

J INVASIVE CARDIOL 2012;24(9):451-455

Key words: temporary transcoronary pacing, intravascular electrodes, PCI

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Transcoronary pacing for the treatment of sudden, unexpected bradycardias during coronary interventions is an attractive alternative to the installation of temporary transvenous cardiac pacing.^1-3 Using this approach, severe complications of inserting a temporary transvenous lead⁴ in case of unheralded bradycardia or high-grade heart block during a coronary intervention (perforation with tamponade,⁵ ventricular arrhythmias,^6-9 and local vascular complications at the insertion site¹⁰) can be avoided. Furthermore, while performing coronary interventions via radial approach, there is no suitable venous access available in the sterile area for rapid insertion of a transvenous pacing lead.

The standard technique for transcoronary pacing is a unipolar pacing configuration with the guidewire in the coronary artery serving as the different cathode against an indifferent cutaneous patch electrode.^1,2

With an optimal position of the cutaneous patch electrode and the use of a balloon catheter for further insulation of the guidewire, our group could demonstrate comparable low pacing thresholds as obtained by standard transvenous pacing.¹¹

The purpose of the present study was to further examine a novel approach for transcoronary pacing by using intravascular electrodes in the aorta, thus avoiding the need of an additional cutaneous patch electrode. Particularly, the effects of different intravascular electrodes on pacing threshold, impedance data, and magnitude of the epicardial electrogram recording (R-wave) — depending on the surface area and the position of these electrodes in the aorta — have been  evaluated in detail.

Methods

Transcoronary pacing was examined in 8 adult pigs in an animal catheterization laboratory as described previously.^11,12 The study was conducted according to the regulations of the local animal authorities.

Each animal was placed on the table of a monoplane catheter laboratory in supine position. After induction of general anesthesia and mechanical ventilation, two neck vessels (carotid artery and jugular vein) and a femoral artery were chosen for vascular access sites: a 6 Fr sheath in the carotid artery for coronary access and transcoronary pacing, another 6 Fr sheath in the internal jugular vein for transvenous pacing and a 9 Fr sheath in the femoral artery for placement of a dual-coil ICD-lead (both coil-electrodes and the tip-electrode serving as indifferent intravascular electrodes).

To avoid thrombus formation during coronary angiography and pacing,¹³ anti-coagulation was established with unfractionated heparin. Angiography of both coronary arteries was performed using standard 6 Fr Judkins guiding catheters as used for coronary interventions in human.

Pacing technique. Coronary pacing was performed in a unipolar setting with the guidewire in the coronary vessel serving as the cathode in any case. Therefore, the tip of the coronary guidewire (Hi-Torque Floppy-II; Boston Scientific) was carefully advanced into a distal branch of the target coronary artery. Outside the body, the stiff end of the guidewire (without coating) was connected to the cathode of an external pulse generator (model 3105, Guidant Corporation; maximum output 10 V at 2.5 ms impulse duration) by a sterile alligator clamp (Figure 1). According to the results of our previously published data,¹¹ electrical insulation of the guidewire was given by the guiding catheter in addition to a standard monorail angioplasty balloon (Boston Scientific) advanced over the guidewire until the tip of the balloon reached the beginning of the radiopaque end of the guidewire.

For testing of the novel concept of intravascular electrodes serving as indifferent electrodes, a commercially available dual coil lead (Endotac Reliance G, Boston Scientific; distal anchor with passive fixing) as used for implantable defibrillators (ICD) was inserted via the arterial sheath (A. femoralis) into the aorta. The surfaces of the coil electrodes were 450 mm² for the distal and 660 mm² for the proximal one, whereas the surface of the distal tip electrode was only 2 mm². Of note, this dual coil lead was positioned in the aorta without active fixing.

The ICD-lead was positioned in the aorta so that the proximal coil was in the aorta abdominalis and the distal coil including the tip electrode in the aorta thoracalis (Figure 2). The coil in the aorta thoracalis (CAT) should simulate an indifferent electrode at the guiding catheter, whereas the coil in the aorta abdominalis (CAA) was supposed to be equivalent regarding the electrical properties to an indifferent electrode at the access sheath. This position of the coil electrodes was verified fluoroscopically. The corresponding plugs for the distal and the proximal coil as far as the tip of the electrode were connected to the anode of the pulse generator by another set of alligator clamps.

A temporary pacing electrode was placed via the jugular vein sheath into the apex of the right ventricle (Figure 2). For unipolar transvenous pacing, the tip electrode of the temporary lead was connected to the cathode of the external pacemaker, whereas the coil-electrodes as far as the tip-electrode of the ICD-lead served as indifferent anodes (Figure 1).

Pacing protocol. The guidewire was advanced through the guiding catheter into the first coronary target vessel. A small, standard, Monorail PCI balloon was advanced over the wire until the beginning of the radiopaque end of the guidewire was reached. The back of the guidewire (outside the body) was connected to the cathode of the external pacemaker. The anodal electrode of the pacemaker was connected consecutively with the different intravascular electrodes: the proximal and the distal coil and the tip of the dual-coil lead.

Before transcoronary pacing was started, the magnitude of the epicardial electrogram — the R-wave — was measured via the guidewire located in a distal branch of the coronary vessel.

Transcoronary pacing was then initiated at a rate of 10-20 beats/min faster than the intrinsic heart rate. Pacing was started at the maximum device output (10 V/2.5 ms). Output voltage was subsequently reduced until the pacing threshold was reached. Pacing was continued above pacing threshold for several seconds for measuring the pacing impedance. This procedure was performed against all indifferent anodal electrodes.

Following completion of the transcoronary pacing protocol, the guidewire and balloon were removed and inspected for adherence of thrombotic material.

These transcoronary pacing procedures were repeated with the guidewire positioned in all three coronary arteries, and with the transvenous pacing lead in the apex of the right ventricle.

Statistical analysis. All descriptive analyses describe mean and standard deviations of the data. Pearson correlation coefficients with P-values were calculated to describe linear association between the measurement of pacing thresholds, impedances, and R-waves. Mixed models with fixed effects were fitted to describe repeated measurements at three anodal electrodes and three vessels in each pig. A common correlation among the observations from a single pig was assumed corresponding to the compound-symmetry structure. To describe the influence of fixed effects and their interactions, an ANOVA F-type statistic was used. Calculations were made with Predictive Analysis Software (SPSS, Inc).

Results

Coronary artery disease or coronary anomalies were not found in any of the study animals.

Transcoronary pacing

Pacing thresholds. Transcoronary pacing with the guidewire insulated by an angioplasty balloon was effective in all coronary vessels against all indifferent electrodes, demonstrating a 100% pacing efficacy. The unipolar transcoronary pacing thresholds for the specific indifferent electrodes are summarized in Table 1.

Within the indifferent intravascular anodal electrodes, the pacing thresholds of the coil-electrodes in the aorta were 0.8 ± 0.5 V for both positions — coil aorta thoracalis (CAT) and coil aorta abdominalis (CAA), whereas the threshold using the tip-electrode positioned in the aorta thoracalis (tip aorta thoracalis; TAT) was 2.0 ± 1.3 V. Significant differences between the three different electrodes (CAT and CAA vs TAT) were shown (P<.001).

Pacing impedances. The pacing impedances using the tip of the ICD-lead (718 ± 254 ohm) as anode were significantly higher as compared to the pacing impedances yielded by the coil-electrodes at 285 ± 63 ohm with the coil in the aorta thoracalis and 294 ± 61 ohm with the coil in the aorta abdominalis in use (Table 2). However, within the two aortal coil-electrodes, there was no difference between the pacing impedances. Significant differences between the three different electrodes (CAT and CAA vs TAT) were shown (P<.001).

Epicardial R-wave. The magnitude of the epicardial signal — the R-wave — varied between 8.6 ± 6.6 mV (tip electrode of the ICD lead in the aorta thoracalis) and 10.2 ± 8.3 mV (coil in the aorta abdominalis; Table 3) without significant differences between all three indifferent electrodes (P=.43).

Transvenous pacing

Pacing thresholds. Temporary transvenous pacing was effective in all cases in a unipolar setting against all anodal electrode configurations.

Comparable to the transcoronary pacing approach, the coil-electrodes generated the lowest pacing thresholds (0.6 ± 0.2 V coil aorta abdominalis and 0.7 ± 0.3 V coil aorta thoracalis), whereas the tip-electrode in use produced a higher pacing threshold (1.5 ± 0.8 V; Figure 3). Significant differences between the three different electrodes (CAT and CAA vs TAT) were shown (P=.001).

Pacing impedances. The unipolar pacing impedance of the tip electrode of the ICD-lead (867 ± 329 ohm) as the anode was higher as compared to the pacing impedances yielded by the coil-electrodes (ranging from 313 ± 39 ohm to 349 ± 73 ohm; Table 4). Significant differences between the three different electrodes (CAT and CAA vs TAT) were shown (P<.001).

Endocardial R-wave. The magnitude of the endocardial signal measured in a unipolar setting against the different anodal electrodes ranged from 4.8 ± 1.5 mV (tip electrode of the ICD lead in the aorta thoracalis) to 7.0 ± 2.4 mV (coil in the aorta abdominalis; Table 3) with significant differences between the three indifferent electrodes (P=.045).

Complications. No complications of transcoronary pacing could be observed. There was no thrombotic material adherent to the guidewires after removal, nor were intracoronary thrombi documented. Skeletal muscle contractions at highest output levels disappeared promptly after reduction of pacing amplitude.

Discussion

To our knowledge, this is the first application of intravascular coil electrodes as indifferent electrodes for transcoronary pacing in an animal model replacing the skin patch electrodes.

The effect of different positions of skin patch electrodes serving as indifferent anodal electrodes in transcoronary pacing was evaluated in our previously published animal study.¹¹ The skin patch electrodes were placed in the groin of the animal and at the anterior and posterior chest wall. Only the patch on the posterior side of the chest wall yielded a 100% pacing efficacy using the bare guidewire with significantly reduced pacing thresholds as compared to both other patch positions. Additional insulation of the guidewire by an angioplasty balloon markedly reduced pacing threshold as compared to the bare guidewire. Transcoronary pacing with additional insulation of the guidewire by an angioplasty balloon was effective in all coronary vessels and against all patch positions.¹¹ However, the lowest pacing thresholds were yielded again with the posterior patch in use. Because of the lower pacing thresholds with the guidewire-balloon combination compared to the bare guidewire, the guidewire-balloon combination as the cathode and a posterior chest wall skin patch electrode became our recommended set-up.

In the present study, a novel concept for transcoronary pacing was investigated. Adapting to the unipolar pacing set-up in implantable cardiac pacemakers (with the surface of the subcutaneous device serving as indifferent anodal electrode), the use of intracorporal indifferent electrodes should avoid the transcutaneous electrical resistance14 and thereby reduce the pacing thresholds.

The idea to use an ICD-lead as indifferent electrode was derived from the pacing configuration of cardiac resynchronization therapy devices (CRT devices). CRT is usually performed by an epicardial pacing of the left ventricle via the coronary sinus — quite similar to the transcoronary pacing approach — in combination with endocardial right ventricular pacing by means of a bipolar pacemaker lead. In case of CRT-D (CRT including ICD therapy), mostly dual-coil ICD-leads are used, including a distal and proximal coil electrode. Depending on the manufacturer, these leads have distally an additional tip-electrode or a tip-ring-electrode combination. Modern CRT devices have multiple programmable pacing configurations including pacing from a typically lateral side branch of the coronary sinus against the distal coil of the ICD-lead positioned in the right ventricle. However, this pacing configuration holds the option of anodal pacing leading to false threshold data.

To avoid this potentially conflicting problem of anodal pacing in our animal model, the ICD-lead was inserted into the aorta. The distal coil and the tip electrode have had the same position in the aorta thoracalis but different surfaces (450 mm² vs 2 mm²), whereas the proximal coil was positioned in the aorta abdominalis.

The results of our measurements demonstrate low pacing thresholds for the two coil electrodes irrespective of their position in the aorta, whereas pacing against the tip-electrode with the very small surface but identical position as the distal coil-electrode yielded significantly higher pacing thresholds (Figure 3). The impedances showed a similar behavior: the pacing impedances using the coil-electrodes were significantly lower as compared to the tip-electrode.

The position of the indifferent electrode within the aorta was — in opposite to the skin patch electrodes — without influence on the pacing efficacy, as long as there is a sufficient surface area available serving as indifferent electrode. A very small surface of the indifferent electrode — simulated by the tip-electrode of the ICD-lead — will increase the pacing impedance and consecutively increase the pacing threshold.

The magnitude of the epicardial electrogram did not vary between the indifferent electrodes used despite significant differences in pacing threshold and pacing impedance, so that high amplitude of the R-wave is not a useful predictor for a low pacing threshold.

Interestingly, the electrical parameters of the two coil electrodes in combination with the guidewire balloon setting for the cathode were comparable to the impedance and threshold data obtained by the optimal patch position at the posterior chest wall.¹¹

Transcoronary pacing: potential benefits of an indifferent intravascular electrode. Transcoronary pacing using a skin patch at the posterior chest wall produced a pacing efficacy 100% in the pig model,¹¹ highlighting the importance of an optimal localisation of the skin patch electrode. In the present study, similar low pacing thresholds and a 100% pacing efficacy could be obtained by intravascular coil-electrodes irrespectively from the position of the coil in the aorta. An electrode with a sufficient surface could be technically implemented into either the guiding catheter or the access sheath. As far as the position of the intravascular indifferent electrode with a sufficient surface (according to our data, at least 450 mm²) has no impact on pacing threshold and pacing efficacy, the implementation into the access sheath could become the favorable approach. With a “pacing sheath” in use, the time to apply transcoronary pacing with the guidewire placed in the target vessel could be further reduced especially in case of emergency. If a pacemaker will be prepared and connected with two sterile alligator-clip cables, the time required for establishing transcoronary pacing is reduced to some seconds: just attaching one sterile alligator clip to the sheath and one to the distal extracorporeal part of the guidewire will immediately enable sufficient and reliable pacing in case of unheralded bradycardias.

Study limitations. These data from our small animal studies cannot be transferred into humans without proof of principle regarding the optimal pacing setup — especially the new concept of an intravascular coil electrode — during PCI in man.

Conclusion

Transcoronary pacing using a guidewire-balloon combination and an intravascular indifferent electrode with sufficient surface is as effective as transvenous pacing using an additional RV-lead.

If a suitable intravascular electrode could be implemented in the access sheath or the guiding catheter this may reduce the time for installation of temporary pacing in unheralded bradycardias during PCI: the operator has only to connect the “pacing sheath” or the “pacing guiding catheter” with an integrated indifferent electrode and the external end of the guidewire to an external pacemaker. This can be done within seconds those avoiding hemodynamically relevant bradycardias. During the time of the percutaneous coronary intervention, transcoronary stimulation again seems to be a safe method. A transvenous approach seems to be unavoidable only in cases of a prolonged necessity of pacing beyond PCI.    

To the best of our knowledge, there is no evidence for a risk of damage to the coronary artery caused by temporary transcoronary pacing. None of the patients from our clinical study on transcoronary pacing³ who underwent re-angiography for clinical reasons showed abnormalities in the formerly stimulated coronary segments, which could have been judged as pacing-induced (data not published).

According to the low pacing thresholds (at about 1 V) achieved by the optimal transcoronary pacing setup in the investigated animal model, we expect comparable conditions in daily clinical practice, so that skeletal muscle twitching seems to be unlikely.

If these animal data can be reproduced in humans, transcoronary pacing will become the method of choice for treatment of bradycardias during coronary interventions.

Acknowledgment. The authors wish to thank the members of the IMTM GmbH Rottmersleben for their support in performing this study.

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From the ¹Department of Medicine I, Carl von Basedow-Klinikum, Merseburg, Germany, ²Institute of Medical Epidemiology, Biostatistics and Informatics, Mainz, Germany, ³Department of Medicine III, Martin-Luther-University Halle-Wittenberg, University Clinics Halle(Saale), Germany, and ⁴IMTM GmbH, Rottmersleben, Germany.
Funding: This study was supported by a restricted grant from Boston Scientific Corporation (BSCI, Gießen, Germany). None of the authors has any affiliations with this company. Data analysis and interpretation were performed completely independent from this company.
Disclosure: The authors have completed and returned the ICMJE Form for Disclosure of Potential Conflicts of Interest. The authors report no conflicts of interest regarding the content herein.
Manuscript submitted February 27, 2012, provisional acceptance given April 2, 2012, final version accepted April 23, 2012.
Address for correspondence: Konstantin M. Heinroth, MD, Department of Medicine III, Martin-Luther-University of Halle-Wittenberg, Ernst-Grube-Straße 40, 06097 Halle, Germany. Email: konstantin.heinroth@medizin.uni-halle.de