The pulse on pacemakers
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.Predominant bradyarrhythmias
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.