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The pulse on pacemakers
More use signals special-care considerations for small animals


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.

Pre-implantation evaluation

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.


Source: DVM360 MAGAZINE,
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