Q: Please review cardiac pacemaker technology in dogs and cats.
A: H. Edward Durham, at the 2006 American College of Veterinary Internal Medicine Forum in Louisville, Ky., gave a lecture on "Cardiac Pacemakers in Small Animals." Here are some relevant points:
Cardiac pacemaker implantation is increasingly available for treatment of symptomatic bradyarrhythmia in small animals. Although veterinary cardiologists perform most pacemaker implantations, veterinarians in private practice increasingly manage dogs and cats with a pacemaker for follow-up care. Awareness of appropriate care for these patients can alleviate potential problems with the pacemaker system.
The most-common indications for cardiac pacing in veterinary medicine are symptomatic bradycardias. The predominant bradyarrhythmias that necessitate pacing are third-degree atrioventricular block (AVB), second-degree atrioventricular block, sick-sinus syndrome (SSS) and persistent atrial standstill.
Third-degree AVB is a complete disassociation of atrial to ventricular impulses. It is characterized clinically by profound bradycardia, weakness and/or syncope. Electrocardiographically, third-degree AVB appears as non-conducted P waves with a junctional or ventricular escape rhythm. The patient's hemodynamic status is maintained by the ventricular escape rhythm. Heart rates between 30 and 60 beats per minute are not uncommon in dogs with third-degree AVB, although escape rates can be faster in cats (100-110 bpm).
Sick-sinus syndrome is an arrest of impulses from the sinoatrial node, causing asystole long enough to cause syncope, and is typically nonresponsive to multiple atropine administrations or related drugs. The asystole can be interrupted by another sinus impulse or an escape impulse that restarts the heart, and the patient recovers from the syncope. This condition generally is not considered an emergency.
High-grade, second-degree AVB usually is not an emergency, but is an arrhythmia that should be promptly addressed. It is defined as a normal to prolonged P to R interval with many non-conducted P waves. High-grade, second-degree AVB can progress to third-degree AVB and is, therefore, an indication for potential permanent cardiac pacemaker implantation.
Persistent atrial standstill is rare and associated with atrioventricular muscle disease and a lack of atrial contractions. This differs from hyperkalemia-associated atrial standstill.
The pacemaker system
The pacemaker system consists of the pulse generator and pacing lead. The pulse generator delivers the electrical impulse through an electrode in the pacing lead to stimulate (depolarize) the heart. Either the ventricle alone (single-chamber pacing) or the ventricle and the atrium (dual-chamber pacing) can be paced. Dual-chamber pacing has become more common. The pulse generator is powered by a lithium-iodide battery with a life expectancy of eight to 10 years.
The pulse generators used in veterinary medicine are usually human units that have exceeded their shelf life for human implantation. These pulse generators have four to six years of battery life when acquired for veterinary use.
Both unipolar and bipolar generators are available. A bipolar system uses a lead with two electrodes at the tip. Current is delivered through one electrode and returns to the generator via the other electrode and the pacing lead. Unipolar systems use the patient's body as the return conduction system. The bipolar system is preferred because it is less likely to cause muscle fasciculations as the impulse returns to the pulse generator.
There are also endocardial and epicardial lead placement systems for cardiac pacing. Epicardial pacing systems consist of a generator with a pacing lead that is attached to the outer surface (epicardium) of the left ventricle. This system is typically implanted using an abdominal incision with a transdiaphragmatic approach to the pericardium or by a lateral thoracotomy. The lead is literally screwed into the epicardium with a corkscrew-type tip on the lead. The lead is then brought through the diaphragm and attached to the generator, which is in the abdomen or in the subcutaneous tissue outside of the abdominal cavity.
Epicardial pacing is most commonly used in very small patients when vascular access is a problem, when there is another reason for a celiotomy or when training or equipment for transvenous pacing is lacking. Epicardial leads are typically unipolar.
Endocardial transvenous pacing is less invasive than epicardial pacing and potentially can be implanted in a shorter period of time. Endocardial leads can be either unipolar or bipolar. In the endocardial transvenous pacing system, the pacing lead is passed via the jugular vein (transvenous) through the right atrium into the apex of the right ventricle. Either jugular may be used. This procedure must be done with fluoroscopic guidance to ensure proper placement of the lead.
Once in place, the lead is either actively screwed into the endocardium with a corkscrew-like tip or is held in place passively by grappling hook-type projections (tines) that snag the trabeculae of the right ventricle. These systems hold the lead tip in place until scar tissue surrounds the lead tip, permanently securing it. The jugular vein is ligated distal to the lead insertion point and the lead is secured with the proximal portion of the vessel. (Ligation of the jugular vein causes no problems, because collateral vascularization takes over venous return.) The remaining proximal portion of the lead is then tunneled under the superficial muscles of the neck and connected to the pulse generator. The lead is secured to the pulse generator with setscrews.
These tiny screws are essential because they establish electrical contact and secure the attachment between the lead and the generator.
The pulse generator is sutured into a pocket under the superficial muscles on the dorsolateral aspect of the neck. Lead placement can be done under heavy sedation, local anesthetic and neuromuscular blockade, thereby reducing the risk of general anesthesia for patients until the cardiac rhythm can be controlled.
The endocardial lead can be attached to a temporary pulse generator while the generator pocket is dissected and the permanent system connected. Currently, the endocardial pacing system is most commonly used due to equipment availability, ease of placement, lower morbidity and a decreased anesthetic complication rate compared to epicardial pacing.
The pacemaker batteries, though relatively low-voltage (about 3 volts), are capable of producing sufficient energy (measured in volts known as amplitude) to depolarize the heart. This voltage is delivered as a pulse of brief duration. Current needs to be controlled so that sufficient energy is delivered to depolarize the heart and battery strength is not wasted.
A programmer in control
The pacemaker discharge rate (i.e., the heart rate) also should be controlled. These and other pacemaker settings can be manipulated with a programmer that controls the pulse generator non-invasively with a magnetic field guided by a computer. A programmer is available for all pacemakers, although they are not interchangeable. Each manufacturer of pacing systems also makes programmers for their systems.
Modern programmers work with a variety of models from one manufacturer. Parameters that are programmable vary with the model of pulse generator, but nearly all pacemakers allow control of the pulse (heart) rate, voltage amplitude, pulse width and level of sensing native heartbeats. Newer generators are highly programmable, allowing for programming of sleep modes, variable rates, automatic threshold tests, arrhythmia recording and other parameters.
Pacing modes are identified by a four-letter code. The first letter denotes the chamber paced, the second letter is the chamber sensed, the third letter indicates the pulse generator function when a native beat is sensed, and the final letter indicates if a pacemaker increases pace rate in response to activity. The abbreviations are V (ventricle), A (atria), O (nothing), I (inhibit) and R (rate responsive).
Most systems are programmed to fire only when the patient's rate drops below a preset value (known as VVI, or demand mode). In VVI mode, the designation indicates that the ventricle is paced, the ventricle sensed and the pulse generator is inhibited from firing when a native beat is detected. This inhibition keeps the pacemaker from firing at inappropriate times. Other pacing modes, including dual-chamber pacing to achieve a more physiologic contraction, are now available. Pacing modes and variables can be changed non-invasively using the programmer.
In the case of emergency pacing, the patient evaluation may be abbreviated. The suspected diagnosis is confirmed with an ECG. Other diagnostic tests, such as radiographs, are performed as indicated to determine the anesthetic risk and identify concurrent disease. The presence of concurrent heart disease that could alter the long-term prognosis is also evaluated by echocardiogram. In the situation of a non-emergency pacing, time permits a full work-up, including serum chemistry profile, complete blood count, urinalysis, thoracic radiographs, ECG, echocardiogram and non-invasive blood pressure.
In some cases, a 24-hour ambulatory ECG (Holter monitor) may be required to confirm the diagnosis. These data are used to rule out other causes of the arrhythmia that may obviate the need for cardiac pacing and indicate prognosis.
In the face of severe dilated cardiomyopathy, for example, the long-term benefits of pacemaker therapy are diminished. Venipuncture should not be performed using either jugular vein prior to referring a patient for pacemaker implantation. A hematoma around the jugular may preclude using that vein for the pacing lead. The lateral saphenous vein is preferred for blood collection so the cephalic veins can be preserved for intravenous catheter placement.
Patients with signs of congestive heart failure should be stabilized appropriately with medications such as furosemide, nitroglycerin and possibly an ACE inhibitor with or without temporary pacing.
In some cases, a constant-rate infusion of isoproterenol or dopamine may be administered to increase the heart rate of escape rhythms and augment cardiac contractility. Lidocaine, digoxin, beta-blockers and/or calcium channel-blockers are contraindicated because these drugs can worsen the bradycardia or suppress the ventricular escape beats that are sustaining the patient.
Patients with severe bradycardia or escape activity that changes rate erratically may require temporary transvenous pacing with a special catheter to reduce the risk of sudden death from anesthesia.
A combined anesthetic protocol of heavy intravenous sedation, neuromuscular blockade, intubation and ventilation, and local anesthesia can be used to gain vascular access and place a pacing lead for this function. An external temporary pulse generator is then attached, using sterile connectors. This allows control of the heart rate during the procedure and allows safe administration of inhalation anesthesia.
An alternative to temporary catheter pacing is transthoracic pacing, in which two wide electrodes are adhered to each side of the shaved thorax of the patient, through which just enough current is applied to depolarize the heart with an external pacing system.
Transvenous endocardial pacing is the primary method of pacing today. The surgical site is small and post-operative pain is less than with epicardial pacing. Post-implantation care involves analgesia, sedation and continuous electrocardiographic monitoring. Antibiotics are given peri-operatively and post-operatively. Opiate sedation/analgesia also keep the patient quiet after surgery to avoid dislodging the lead, thereby necessitating surgical repositioning. A sterile bandage is placed around the neck to protect the surgical site and reduce seroma formation. Radiographs are taken in the right lateral and dorsoventral views the next day to confirm lead placement.
Avoid positioning the patient on its back during the first two weeks after implantation because this may dislodge the lead. Neck leads or choke-chain collars should never be used on a dog with a transvenous pacemaker system; they should always wear a harness.
An ECG is used to verify pacemaker function and rate. Pulse generator parameters are programmed to a base setting and the patient is released from the hospital one to three days after implantation.
At discharge, the owner is instructed how to take the patient's heart rate and asked to check it daily until the next visit. If the heart rate goes below the pacing rate, the owner is instructed to call the hospital immediately.
Owners are asked to return for suture removal 10 days to 14 days from discharge. At this visit an ECG and thoracic radiographs are performed. The pulse generator also is evaluated with the programmer. As the endocardium develops a reaction around the pacemaker lead tip where it contacts the heart, increased resistance to the pacing impulse can develop.
The minimum current for the pulse generator needed to capture the ventricle, causing it to depolarize and contract is determined. To verify capture, a continuous ECG is performed while programming the pacemaker, with the current gradually reduced until capture is lost. When this threshold is determined, the pacemaker is programmed at a sufficiently high output to assure capture in the event that resistance increases. Typically, an output setting of two times the threshold is programmed to allow a sufficient margin of safety.
A similar follow-up visit is scheduled about one month after implantation and again at three months, at which time radiographs and pulse-generator evaluation are performed. Unless complications arise, the owner is asked to see a local veterinarian for biannual radiographs and to visit the cardiologist annually for pacemaker system evaluation. At all follow-up visits, an ECG, echocardiogram and pacemaker programming are repeated. Other tests may be added as indicated.
Complications of pacemakers can range from mild to serious. The most common are seroma or hematoma around the pulse generator, dislodgment of the pacing lead from the heart, congestive heart failure from concurrent heart disease or subsequent myocardial failure, tissue reaction sufficient to prevent pacing capture (known as exit block) and malfunctions of the pacing system.
Infections occur infrequently, but can be months after the implantation, from hematogenous spread. System malfunctions include lead fracture or dislodgement, battery depletion or damage to the pulse generator circuitry from electrical impulses (i.e., electrocautery or defibrillation).
One should never attempt venipuncture in the jugular vein that carries the pacing lead. Striking the lead with a hypodermic needle could damage the insulation around it, or the actual metallic core of the lead, resulting in immediate malfunction of the system and potential life-threatening complications.
Patients with pacemakers have a distinctive electrocardiogram pattern that shows a pacing spike and typically demonstrate wide QRS complexes. Knowing the pacemaker rate setting can alert the veterinarian to depletion of battery life in a pulse generator. Pulse generators will automatically reduce their rate when the battery weakens, indicating it is time to replace the pacemaker.
Dr. Hoskins is owner of DocuTech Services. He is a diplomate of the American College of Veterinary Internal Medicine with specialities in small animal pediatrics. He can be reached at (225) 955-3252, fax: (214) 242-2200 or e-mail: [email protected]