Safety and Feasibility of Transesophageal Pacing in a Dog

Introduction

The first reported fully implantable pacemaker in a dog was inserted in 1968.¹ A pacemaker delivers battery-supplied electrical stimuli through electrodes to produce an artificially triggered depolarization and heartbeat. The primary indications for artificial pacing in animals include third-degree or complete heart block, sinus bradycardia or sinus arrest (resulting from sick sinus syndrome), high-grade second-degree heart block, and persistent atrial standstill.¹

Since 1968, the equipment and techniques used for cardiac pacing have improved considerably.¹ Initially, pacemaker implantation required thoracotomy and direct application of the pacemaker lead onto the epicardium. Today, percutaneous transvenous leads can be introduced into the heart through a peripheral vein. The lead is then actively placed into the myocardium of the right ventricle to provide pacing.¹ The cost, lifelong special care, and potential complications of venous access, lead placement, or device malfunction must all be taken into consideration by the owners prior to permanent artificial pacing.

Temporary cardiac pacing is a potentially life-saving measure that is used for the treatment of profound bradycardias leading to hemodynamic compromise.² It is also used prior to general anesthesia in hemodynamically unstable patients undergoing permanent pacemaker implantation.² This latter application is done for safety reasons and provides a normal heart rate, a more stable patient, and a more controlled (i.e., less rushed) surgical procedure. Techniques for temporary transvenous and transthoracic pacing have been described.^2–5 Sympathomimetic or parasympatholytic agents may be administered in an attempt to increase the ventricular response rate and reduce the clinical signs associated with severe bradyarrhythmias.⁶ Certain disadvantages, however, accompany each of these options. Temporary transvenous pacing requires considerable skill and can add significantly to the length of the procedure.^2–4 Fluoroscopic guidance for placement of the lead is also often required.^2–4 Temporary transthoracic pacing can cause movement of the patient during the procedure and can be painful.⁵ Pharmacological therapy, such as a continuous-rate infusion of isoproterenol, is sometimes associated with adverse side effects (e.g., arrhythmias).⁶

Temporary esophageal pacing theoretically provides a quicker and easier method of pacing animals with unstable rhythms prior to permanent pacemaker implantation. Temporary esophageal pacing is currently used in infants with heartbeat irregularities, and this preliminary study was undertaken to explore the possibility of transesophageal pacing in dogs within the clinical setting.

Case Report

This study used a 17-kg adult, intact female, healthy, mixedbreed dog from a research colony. The dog was premedicated with acetylpromazine maleate^a (0.05 mg/kg intramuscularly [IM]) and butorphanol^b (0.2 mg/kg IM). Anesthesia was induced with propofol^c (3 mg/kg intravenously [IV] to effect) and maintained with isoflurane^d in oxygen. The dog was placed in left lateral recumbency.

A human transesophageal atrial cardiac pacing system and electrocardiogram (ECG) companion box were used in conjunction with a canine esophageal ECG recording and temperature probe.^e The transesophageal probe was placed within the esophagus and passed, with fluoroscopic guidance, to the level of the stomach [Figure 1]. The esophageal probe consisted of four electrodes corresponding to leads 1 through 4 in the ECG companion box, which could be connected to a routine electrocardiographic monitor or to the cardiac pacing system [Figure 2]. Alligator clamps from a routine electrocardiographic monitor were connected to the ECG companion box for monitoring of the intraesophageal ECG during lead placement. Different bipolar lead configurations were selected within the ECG companion box in order to identify the greatest net deflection and the tallest QRS complex found between two electrodes. The height of the QRS complex was used to indicate the optimal ventricular pacing site based on the presumed relationship between the site of maximal ventricular amplitude and the lowest ventricular pacing threshold. The two initial electrodes on the esophageal probe for pacing were chosen based on the corresponding lead configuration. The ECG leads were then removed from the pacing circuit and placed externally on the dog to allow for surface ECG monitoring during pacing.

The cardiac pacing system was modified from human settings to be capable of delivering up to 80 milliamps (mA), with a pulse duration of 20 milliseconds (ms). The cardiac stimulator was connected to the ECG companion box and the transesophageal pacing probe. Initial pacing attempts used electrodes 1 and 2, with 30 mA and a pulse duration of 2 ms. The current delivered was gradually increased to 80 mA and 20 ms while monitoring the surface ECG for evidence of successful ventricular pacing. This procedure was repeated in all electrode configurations (electrodes 1 and 3, electrodes 1 and 4, electrodes 2 and 4, etc.).

A third electrode was added to the pacing circuit using a small connector with alligator clamps on either end. This connector was used to add a third electrode into the circuit. Multiple three-electrode combinations were then tried (electrodes 1, 2, and 3; electrodes 1, 2, and 4; electrodes 2, 3, and 4; etc.). Energy stimulation was gradually increased from 30 mA and 2 ms to 80 mA and 20 ms while monitoring the surface ECG for evidence of successful ventricular pacing.

A chest patch containing an electrode^f was placed over the left thorax at the site of the apex beat, and it was added to the pacing circuit [Figure 3]. Pacing was attempted using a combination of two esophageal electrodes and the chest patch. Energy stimulation was again increased from 30 mA and 2 ms up to 80 mA and 20 ms. Multiple combinations of esophageal electrode configurations and the chest patch were used.

Pacing with transesophageal electrodes 1 and 3 and transesophageal pacing with the chest patch in the circuit allowed for consistent atrial pacing. Consistent pacing of the ventricles was not achieved with any of the abovedescribed methods.

Prior to pacing, intraesophageal ECGs were compared with different lead configurations. The greatest net deflection and tallest QRS complexes were seen with electrodes 1 and 3, which had an R wave height of 3.7 mV. The difference in net deflection was secondary to the electrode placement within the thorax. As seen on fluoroscopy, electrode 1 was just cranial and electrode 3 was just caudal to the heart [Figure 1]. As the ventricles depolarized, the net electrical signal traveled from the sinoatrial node in the atrium, down the ventricles, away from electrode 1, and toward electrode 3. This allowed for a positive net deflection on the ECG for this configuration. Utilizing other lead configurations reduced the net QRS deflection on the surface ECG [Figures 4A, 4B].

Pacing via esophageal electrodes 1 and 2 (30 mA, 2 ms) caused diaphragmatic stimulation and consistent atrial pacing [Figure 5]. Other lead configurations also caused diaphragmatic stimulation but only achieved inconsistent atrial pacing. Adding the chest patch into the two electrode circuits (80 mA, 2 ms) also caused diaphragmatic stimulation and consistent atrial pacing without stimulating ventricular pacing [Figure 6].

After approximately 30 minutes of intermittent pacing, the dog showed signs of pain (i.e., muscle shaking/ movement) that required progressively higher levels of anesthesia to control. Following recovery from anesthesia, the dog was monitored for 10 days after the study. No clinical signs of esophageal discomfort or dysphagia were noted during the 10-day monitoring period following pacing.

Discussion

Temporary cardiac pacing can be used in animals with hemodynamically unstable bradyarrhythmias, such as thirddegree atrioventricular block and sick sinus syndrome.⁷ Prolonged periods of bradycardia may increase the risk of ventricular fibrillation from myocardial hypoxia, so it is reasonable to perform temporary pacing with heart rates <40 beats per minute or during unstable escape rhythms.⁷ Temporary transvenous pacing is a procedure that requires considerable training and skill to accomplish.^2–4 Additionally, transvenous methods usually require fluoroscopic guidance for insertion of leads into the right ventricle for adequate pacing. The procedure can add considerable time to permanent pacemaker implantation.^2–4 Additionally, a previous retrospective study indicated temporary pacing may not prevent fatal arrhythmias.⁷ This previous study suggested that greater pacemaker implantation experience and shorter anesthesia time may be more important than temporary cardiac pacing for preventing complications in these animals.⁷

For this reason, transthoracic pacing was investigated in dogs with bradyarrhythmias undergoing general anesthesia for either permanent pacemaker implantation or other surgical procedures.⁵ While transthoracic pacing was effective and useful as a temporary means of pacing the ventricles, it was not without complications (e.g., mild to severe muscle movement in all dogs [n=42], with two requiring neuromuscular blockade to allow the procedure to continue).⁵ Additionally, transthoracic pacing was thought to be painful and frequently required a deeper plane of anesthesia.⁵

The study reported here evaluated a method of temporary transesophageal ventricular pacing. No prior studies have specifically examined temporary transesophageal pacing as a method of temporary ventricular pacing in the dog. One previous study of transesophageal defibrillation had good success at pacing canine ventricles with a transesophageal probe and a chest patch using 38 mAand a pulse duration of 2.5 ms.⁸ The consistency of ventricular pacing was not indicated in this latter research study.⁸

The cardiac pacing system used in the present study is manufactured for transesophageal pacing of the atria in humans. This system was modified to increase the likelihood of successful pacing of the ventricles in the dog. The esophageal electrode system had multiple lead configurations, and the electrodes also had a larger contact surface area. In the dog studied, atrial pacing was consistent at 30 mA and 2 ms using two electrodes of the esophageal probe within the circuit. Adding the chest patch also allowed for consistent atrial pacing with 80 mA and 20 ms, but it failed to achieve ventricular pacing. The chest patch did not provide increased pacing control even with stimulation of a larger area of the myocardium. The technique used in the current study was unable to achieve ventricular pacing through the esophagus, even at higher energy and pulse durations. All esophageal electrode configurations of the transesophageal probe elicited considerable movement of the dog from diaphragmatic and phrenic nerve stimulation, regardless of pulse energy and duration.

This inability to achieve esophageal ventricular pacing may be related to canine anatomical factors. The close proximity of the esophagus to the atria allows for esophageal atrial pacing in the dog; however, the thoracic cavity conformation increases the distance from the esophagus to the ventricles (as compared to the human). In the human, the esophagus extends along the cardiac base-to-apex axis and has more esophageal cardiac contact; in the dog, the esophagus is perpendicular to this axis.^9,10 For these same anatomical reasons, ventricular imaging by transesophageal echocardiography is very rewarding in humans.^9,10

The esophageal probe used in this study was unable to pace the ventricles because of the physical placement of the electrodes within the esophagus and thorax. In future studies, a stylette might be used to curve the flexible esophageal probe ventrally within the stomach, which would bring the two most distal leads in closer proximity to the ventricles. Gastroesophageal electrodes have been found to be safe and effective for atrial and ventricular pacing in humans.^9,10 Further study is indicated to explore gastroesophageal electrodes as an alternative for temporary transvenous cardiac pacing.

In the current study, transesophageal pacing was only attempted with the dog in left lateral recumbency, because this is the position needed for permanent pacemaker implantation. The external jugular vein is almost always used for permanent pacemaker implantation, and the right jugular vein is preferred because of an occasional persistent left cranial vena cava.¹¹ Previous transesophageal pacing systems achieved consistent atrial pacing of dogs in dorsal recumbency only.¹² This previous study indicated dogs in left lateral recumbency had only high output and unstable atrial pacing, and stable atrial pacing was only achieved in dorsal recumbency when a chest patch (20 mA, 13 ms) was used.¹² The study also concluded that esophageal dilatation may occur during anesthesia and lateral recumbency, thereby causing poor electrical contact and poor pacing. However, in the present study, esophageal dilatation did not occur, and atrial pacing occurred consistently in lateral recumbency.

Pacing of the atria using the transesophageal probe and chest patch was successful in this dog; however, atrial pacing was not the purpose of the study. Temporary transvenous pacing is most often needed in severely hemodynamically unstable dogs prior to permanent pacemaker implantation because of their high-grade or complete atrioventricular block. These animals do benefit from atrial pacing because of the lack of conduction to the ventricles through the atrioventricular node. Transesophageal atrial pacing may provide temporary benefit, however, for dogs presented as emergencies with sick sinus syndrome or severe sinus bradycardia.

This model of transesophageal ventricular pacing requires further study. This study indicated that transesophageal pacing required general anesthesia; increasing depths of anesthesia were required to control muscle spasms. Should this model be refined for the clinical setting, the need for general anesthesia would be a disadvantage in hemodynamically unstable dogs when compared to temporary transvenous pacing, which can be accomplished with sedation (prior to general anesthesia and permanent pacemaker implantation). This disadvantage also exists for temporary transthoracic pacing.⁵

Conclusion

In this study, transesophageal atrial pacing was successful in a dog; however, transesophageal ventricular pacing was not achieved. Because of the conformation of the canine esophagus and heart, currently available transesophageal probes cannot likely be used to pace the ventricles unless intragastric electrodes produce a better outcome. Diaphragmatic stimulation from transesophageal pacing caused considerable movement in this dog. Because of the large variations in canine chest anatomy, further studies are necessary to explore transesophageal pacing as a substitute for transvenous or transthoracic ventricular pacing.