Optimizing of Transcoronary Pacing in a Porcine Model

ABSTRACT: Background. Transcoronary pacing for the treatment of bradycardia during percutaneous coronary intervention (PCI) is not well established, but may be a useful technique in interventional cardiology. We developed a porcine model to examine the feasibility and efficacy of transcoronary pacing during PCI. Methods and Results. Eight pigs under general anesthesia underwent unipolar transcoronary pacing with a standard floppy guidewire in a coronary artery (as the cathode) and a skin patch electrode (as the anode). We examined the effect of skin-patch position (groin vs. anterior or posterior chest wall), the presence of an angioplasty balloon on the guidewire and also which coronary artery was “wired” on the efficacy of pacing as assessed by capture and threshold data. Pacing with the bare guidewire and a maximum output of 10 V was successful in 54% of the animals with a groin patch and the anterior chest-wall patch, but in 100% with a posterior chest-wall patch. The pacing thresholds were 8.3 ± 2.2 V, 7.6 ± 2.8 V and 3.4 ± 2.4 V with the patch in these sites, respectively. With an angioplasty balloon over the guidewire, pacing efficacy increased to 100% (irrespective of the target vessel or patch location) and pacing thresholds were significantly reduced (p Methods The study examined coronary pacing in 8 pigs and was conducted within the regulations of local animal-based research standards and with appropriate consent from the relevant local authorities. Eight adult pigs (3 male, 5 female) were studied. In each animal, general anesthesia was induced and maintained with isoflurane, nitrous oxide and ketamine (adapted to body weight) and mechanical ventilation was instituted before the animal was placed in the supine position. The neck vessels were chosen for vascular access since they are larger than the femoral vessels in this species. A 6 French (Fr) sheath was placed in the carotid artery (for coronary access and pacing) and a 7 Fr sheath in the internal jugular vein (for transvenous pacing). Animals received unfractionated heparin (Fa. Braun, Melsungen, Germany) at 70 units per kilo at the beginning of the procedure. Coronary angiography was performed with iopamidol 370 (Solutrast, Fa. Altana, Konstanz, Germany) at body temperature. Angiography of both coronary arteries was performed using standard 6 Fr Judkins guide catheters (Cordis Corp., Miami Lakes, Florida) before the pacing protocol to exclude coronary artery disease or coronary anomalies. Pacing technique. For coronary pacing, the tip of the coronary guidewire was advanced into a distal branch, either of the main vessel itself, or into a small side branch. To avoid damage to the coronary vessels, only floppy-tip guidewires (Boston Scientific Corp., Natick, Massachusetts) were used. A sterile alligator clamp was used to connect the back of the guidewire (the uncoated stiff end) to the cathode of an external pulse generator (model 3105, Guidant Corp., Santa Clara, California), with a maximum output of 10 V at a maximum pulse width of 2.5 milliseconds (ms). The guiding catheter offered some electrical insulation of the guidewire from surrounding organs and tissues. To examine the effect of any additional insulation by balloon catheters, a standard monorail angioplasty balloon (Boston Scientific) was advanced over the guidewire until the tip of the balloon reached the beginning of the radiopaque end of the guidewire. The anode of the pulse generator was connected to a self-adhesive skin electrode with a surface area of about 100 cm2. We developed a standard protocol using three skin electrodes, with one placed in the groin, on the posterior and the anterior chest wall (Figure 1). The skin electrodes were connected with a junction box. Each animal acted as its own control. To allow a comparison of the efficacy of transcoronary pacing with the current “gold standard” of temporary pacing, a transvenous electrode was placed via the jugular vein sheath into the apex. Transvenous pacing was performed using standard bipolar settings with the distal electrode set as the cathode. Additionally, transvenous pacing was performed with the ventricular lead in a unipolar setup using the different skin patches as the anode to demonstrate unipolar transvenous pacing employing skin patches. Pacing protocol. The guidewire was manipulated through the guiding catheter into the first target vessel. The back of the guidewire (outside the body) was connected to the cathode of the external pacemaker using a simple metallic crocodile clip. The anode electrode of the pacemaker was connected consecutively to a skin patch in the groin, on the posterior chest wall and, finally, the anterior chest wall. Transcoronary pacing was initiated at a rate of 20 beats per minute (bpm) faster than the sinus rate. Pacing commenced at the maximum device output of 10 V with a 2.5 ms impulse duration and with a subsequent reduction in output voltage until the pacing threshold was reached. Pacing was continued above pacing threshold for several seconds to measure the pacing impedance. This procedure was performed with the anode patch in all three positions. While fixing the position of the guidewire in the target vessel, a small standard 1.5 x 20 mm Maverick Monorail PCI balloon (Boston Scientific) was advanced over the wire to the mid-point of the target vessel. All measurements were repeated with the angioplasty balloon in this position. Following completion of the transcoronary pacing protocol, the guidewire and balloon were removed and inspected for adherence of thrombotic material. The transcoronary pacing procedures were repeated with the guidewire in all three coronary arteries. Finally, the same measurements (R wave, impedance and pacing threshold) were performed with a standard temporary pacing electrode in the apex of the RV using a bipolar setting and again using a unipolar setting consecutively against all three skin patch locations. Animal characteristics. Coronary pacing was tested in 8 pigs (body weight 29.3 ± 2.7 kg; length 104 ± 2 cm; mean BMI 26.9). All pigs were kept under identical conditions for 1 month before the beginning of the study. Statistical analysis. Data are presented as mean ± standard deviation. Statistical analysis was performed using multiple analysis of variance, Kruskall-Wallis test and Fisher’s Exact test to compare more than two sets of data. A p-value of Results None of the pigs had coronary artery disease or coronary abnormalities. The guidewire was successfully manipulated into all three coronary arteries without complications. In all 8 pigs the pacing protocol was completed successfully. Coronary pacing with the “bare” guidewire. Coronary pacing (maximum output at 10 V, impulse duration 2.5 ms) against the posterior patch was effective in all three vessels in all pigs. The mean pacing efficacy of the anterior patch and the patch in the groin was 54% — less than that of transcoronary pacing using the posterior chest wall electrode. Pacing efficacy also varied according to the coronary vessel attempted. When used together with the anterior patch and the patch in the groin, pacing efficacy was 75% in the right circumflex artery (RCX), 50% in the left anterior descending artery (LAD) and in 37.5% in the right coronary artery (RCA). Pacing thresholds for the different sites of coronary pacing are detailed in Table 1. If no effective pacing was achieved in a vessel against a specific patch, the pacing threshold was set to 10 V at 2.5 ms. Mean pacing threshold against the posterior patch was significantly lower, at 3.4 ± 2.4 V compared to the anterior patch (7.6 ± 2.8 V) and the groin patch (8.3 ± 2.2 V; p Discussion To the best of our knowledge, this is the first application of a large-animal PCI-based model to systematically investigate the technique and efficacy of transcoronary pacing including the effects of unipolar skin-patch position and angioplasty-balloon use on the guidewire. Transcoronary pacing thresholds are comparable to transvenous pacing. Pacing efficacy of transcoronary pacing in our previous study in patients undergoing PCI15 was 85.7% using a unipolar setup and a cutaneous patch in the groin. Using the same patch position in our pig model, the pacing efficacy was as low as 54%. Using a skin patch at the posterior chest wall improved efficacy in the pig model to 100%. These data support the simple approach of transcoronary pacing using the guidewire and a cutaneous patch electrode, provided the skin patch is suitably located. These results are in contrast to those of Mixton et al,4 who found unacceptably high impedance (and hence thresholds) for transcoronary pacing using skin electrodes, although no information about skin-electrode location is provided in their paper. We speculate that the variation in results in these studies may be due to differences in skin-patch position. As a result of high pacing thresholds using a skin electrode, these authors used a steel monofilament suture (as used by cardiac surgeons to fix epicardial electrodes) as an indifferent electrode. Some of the disadvantages of this approach include an increase in material costs and potential bleeding complications. Our group found that transcoronary pacing thresholds in humans undergoing PCI were 6.6 ± 2.3 V at a 2.5 ms impulse duration while using a patch in the groin and a guidewire without balloon insulation. In the present animal study, the pacing threshold obtained with the same set-up was modestly higher, at 8.3 ± 2.2 V. Despite finding higher thresholds using the pig model, use of the optimal patch position (posterior chest-wall patch) and additional guidewire insulation by an angioplasty balloon (as is usually the case during PCI) generated acceptable pacing thresholds that were comparable to bipolar transvenous pacing. Our data illustrate the importance of the skin-patch position. Pacing thresholds were highly dependent on skin- patch position, varying from 1.6 V (patch in the groin) to 0.8 V (posterior chest-wall patch). However, using an appropriately positioned patch, we were able to demonstrate the feasibility of unipolar pacing against a cutaneous patch without the need for sewn surgical cutaneous electrodes. Transcoronary pacing — A reliable pacing concept for treatment of unexpected bradycardias in PCI procedures. The results of our animal study demonstrate that transcoronary pacing with coated guidewires is easy to implement and achieves reliable pacing with acceptable thresholds similar to those of the transvenous pacing technique without any identifiable complications. With an optimal pacing set-up, there were no significant differences in pacing thresholds between the current “gold standard” (bipolar transvenous pacing) and the unipolar transcoronary pacing with a standard PCI guidewire (with balloon insulation) against an indifferent patch electrode placed in the posterior chest wall position (Figure 3). There remain no methodological concerns over the transcoronary pacing concept, particularly given that we achieved 100% pacing efficacy with our model without additional costs or risks. The optimal set-up for unipolar transcoronary pacing. The optimal setup for transcoronary pacing in the pig was a combination of an angioplasty guidewire in the coronary artery (with an angioplasty balloon on the wire) and a cutaneous skin patch on the posterior chest wall. We had anticipated that the guiding catheter would provide enough insulation along a sufficient length of the guidewire to allow acceptable pacing thresholds. In fact, our data show that guiding catheter insulation alone is insufficient to allow 100% pacing reliability due to high pacing thresholds. Application of a regular monorail angioplasty balloon to the guidewire insulates, in combination with the guiding catheter, all but the distal (intracoronary) tip of the guidewire and reduces pacing thresholds sufficiently to generate 100% pacing efficacy using a posterior chest-wall skin patch. Given this 100% efficacy with a monorail angioplasty balloon, we did not test “over-the-wire” balloons for further improvement of guidewire insulation. Study limitations. These data from a small animal study cannot be applied to humans without proof of principle regarding the optimal pacing set-up during PCI in humans. To investigate this, we recommend conducting a multicenter study of the influence of patch position and guidewire insulation by standard angioplasty balloons on the safety and feasibility of transcoronary pacing in humans. Conclusion Transcoronary pacing using the optimal set-up of a posterior-patch position and an angioplasty balloon over the guidewire achieves the same threshold and pacing efficacy as transvenous pacing. Consequently, transcoronary pacing may become the method of choice for standby pacing in interventional cardiology. Provided its clinical utility in humans can be shown, the technique may prove quicker and less expensive to implement, poses less risk to the patient than transvenous pacing, and may supersede prophylactic installation of transvenous pacing. If transcoronary pacing has a role in the immediate management of unheralded bradycardia during PCI, where the bradycardia is sustained, transcoronary pacing may provide a bridge to the implementation of transvenous pacing. This may apply, particularly given the increasing use of radial access, which limits the operator’s immediate access to the the femoral vein, meaning that transcoronary pacing (via existing radial artery access) may provide a method of immediate pacing. Thus, the need for emergency transvenous pacing while the patient undergoes chest compressions can be avoided. How long transcoronary pacing can be implemented remains to be determined. Previous data suggesting a time limitation of transcoronary pacing (due to intracoronary thrombus development) was conducted with a protocol of minimal antiplatelet therapy14 and may not apply in today’s clinical scenario. In any case, transcoronary pacing has been performed safely for up to 30 minutes without complications.4

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