Atrioventricular blocks are a complete or partial, intermittent, or definitive anomaly of the conduction of electrical flow between the atria and ventricles at the AV node. The resulting rhythm is an escape beat (Hisian or ventricular) characterized by an often symptomatic bradycardia.

Sick sinus syndrome is an anomaly of the sinus node inducing conduction dysfunction; defective production of the impulses from the physiological pacemaker, defective intrasinusal conduction, or a conduction disturbance between the sinus node and the atria (SA block). It results in pauses in the P waves and AV dissociation.

These conduction anomalies can be due to degeneration, trauma, or infiltration of the conductive tissue by myocarditis or neoplasia.² Extracardiac causes include systemic diseases with cardiac repercussions, including metabolic or endocrine disorders, vagal hypertonia of structural origin (intracranial mass or neoplasia along the trajectory of the vagal nerve), intoxications, and drug overdoses.

The clinical signs of AV blocks are dependent on the presence of associated cardiac disease and the severity of the block. Commonly displayed symptoms include lethargy, exercise intolerance, and syncope. In some cases, the heart failure induces venous congestion, pulmonary edema, and ascites. The heart rate is slow and fixed.

An electrocardiogram (ECG) can be performed to characterize the grade and severity of the blocks. A grade 1 block corresponds to delayed conduction through the AV node and is characterized by an increased delay between the P and R waves. Grade 2 corresponds to an intermittent conduction failure: the P waves are not all followed by a QRS.

Grade 3 is a complete dissociation: the P waves are never followed by a QRS complex; the latter is altered (usually widened) characteristic of ventricular escape beats.³ The clinical signs of sick sinus syndrome include lethargy, exercise intolerance, and syncope. Signs of heart failure are rare. The ECG shows cardiac pauses that often exceed 10 seconds and which usually resolve with the onset of paroxysmal tachycardia or a ventricular escape complex.³

A pacemaker is indicated when the clinical consequences of the bradycardia are apparent and incapacitating. It is considered for frequencies of less than 90 beats/min in cats and 50 beats/min in dogs.³

For AV blocks, only severe grade 2 blocks and grade 3 blocks are indications for a pacemaker.

A pacemaker is indicated in the treatment of sick sinus syndrome when the sinus pauses are refractory to atropine.

Other indications include the onset of rhythm or conduction anomalies, in particular during the transition between waking and sleep, and during episodes of syncope. A continuous ECG examination (Holter monitoring) over a period of at least 24 hours is required along with a complete clinical workup, including a minimal database (cell counts, clinical chemical panel, and urinalysis) to diagnose possible contraindications (infection, progressive cardiomyopathy).

The transvenous implantation of endocardiac leads for cardiac pacing is a minimally invasive procedure. It is associated with a low mortality rate and an acceptable rate of major complications.^4-7

It requires the use of radioimaging equipment and team that is trained in this type of procedure. The right ventricular apex is the most commonly used pacing site because of the relative ease of transvenous placement.

Major complications are reported in 11% to 37% of cases. The most common major complications include lead displacement, hemorrhage, and infection.^4-7 Implantation of the generator in the neck region, just under the skin, and the need for access via the jugular vein increase the risks of bleeding and infection.⁶

Surgical implantation via intercostal or transdiaphragmatic thoracotomy is indicated when endovascular implantation equipment is not available or when catheterization is contraindicated (chronic vascular lesions, alteration of the jugular vein, cervical injury or infection, increased risk of thrombosis).^8-11 However, the most common indication for epicardial pacemaker placement has been small animal body size.¹² The authors’ experiences have involved dogs with body weights of 9 to 35 kg, and in the author’s opinion, a similar technique is useful in dogs with body weights above approximately 8 kg. Giant breeds may be challenging because the lead applicator may be too short.

The risks of major complications (defined as those requiring repeat surgery) are comparable for surgical and transvenous implantation,¹² which makes surgical implantation a viable alternative. A minimal, transxiphoid approach has also been described for the implantation of an epicardial electrode at the apex of the heart.¹³ It does not allow sufficient access for bipolar implantation.

The thoracoscopic implantation of epicardial electrodes is a minimally invasive procedure, which is as well tolerated as transvenous implantation. In the author’s experience, thoracoscopic implantation is twice as fast as conventional surgery because of the reduced time needed for the surgical approach. This reduces the anesthetic risk when a temporary external pacemaker is not used by decreasing the duration between anesthetic induction and connection of the generator to the lead initiating the cardiac assistance.

The preparation phase should be as short as possible to limit the duration of anesthesia without cardiac assistance. Prophylactic antibiotic therapy is recommended.