Getting back to basics with permanent pacemakers, part I

Nursing, Oct 2004 by Geiter, Henry B Jr

Learn about new functions, new codes, and patient care.

Today's pacemakers have more functions than ever, making rhythm interpretation challenging. In this article, I'll review the basics of pacemakers, including their functions, and discuss what you'll need to know when caring for a patient who has one. In a future article, I'll describe pacemaker complications.

Pacer basics

Permanent pacemakers stimulate depolarization, causing myocardial cells to contract. Often prescribed for patients with symptomatic bradycardia, pacemakers have many indications, including chronic atrial fibrillation and slow ventricular response rate.

The pulse generator, usually consisting of lithium batteries and electrical microcircuitry encased in titanium, is implanted surgically in a subcutaneous pocket, usually in the patient's chest. The pacing leads-flexible, insulated wires with one or two electrodes at each tip-are threaded through a subclavian vein into the heart under fluoroscopy. One lead is placed in the right ventricle; a second lead, if used, is placed in the right atrium.

The pacemaker (or pacing) rate is programmed in milliseconds. To convert a heart rate from beats per minute (bpm) to milliseconds, divide 60,000 by the heart rate. (For example, a heart rate of 70 bpm equals 857 milliseconds.) To convert a rate in milliseconds to bpm, divide 60,000 by the millisecond rate.

The atrioventricular (AV) interval, measured in milliseconds, corresponds to the PR interval on an electrocardiogram (ECG). Pacemakers use this parameter when determining how long to wait, after the atria are stimulated, before stimulating the ventricles. (More on this later.)

When you care for a patient with a permanent pacemaker, you'll need some basic information, such as the type and mode of pacemaker. Other information, such as date of implantation, frequency of use, and programming changes are helpful, but most patients don't have this information.

The pacer code

Modern pacemakers can be programmed noninvasively and provide information via telemetry. A pacemaker's features are listed in its three- to five-letter code, developed by the North American Society of Pacing and Electrophysiology and the British Pacing and Electrophysiology Group. The code, known as the NASPE/BPG generic code, has undergone several revisions as pacemakers acquired more functions. (See Reading Pacemaker Codes for a description of the most recent revision.)

Each of the five letters or positions describes a function: Positions I and II describe the chamber or chambers paced and sensed, respectively. Position III describes the pacemaker's action when it senses intrinsic, spontaneous cardiac depolarizations. For example, a pacemaker with an I (inhibited) designation in Position III will inhibit firing when it senses an intrinsic beat, but will pace the cardiac chamber if no beat is sensed.

Position IV indicates the presence or absence of rate modulation, which I'll describe in more detail shortly. Position V, which in the past designated the pacemaker's antitachycardia and shock functions, has been revised to designate the location or absence of pacemaker multisite pacing; for example, biatrial or biventricular pacing, in which left and right chambers are stimulated together to maintain coordination and improve cardiac output. (Antitachycardia and shock functions are now described by a different code.)

Staying on the beat

A pacemakers rate modulation feature (also known as adaptive rate mechanism) attempts to replicate the ability of the normal functioning heart. For example, during exercise, when a patient's metabolic needs increase, a complex network of nerves, sensors, and hormones increase heart rate. The electronic pacemaker tries to accomplish this same task by using piezoelectric crystal sensors that detect states of exercise and trigger accelerations in pacing rate. This immediate stimulus allows heart rate to respond as soon as the need begins to increase-as long as the patient is stimulating the chest muscles.

Some pacemakers have two types of sensors-piezoelectric crystals and a sensor that rapidly responds to changes in minute ventilation or QT interval. The relationship between the QT interval and heart rate is fairly predictable, with the QT rate decreasing as heart rate increases. Studies have shown that two sensors provide better rate response to exercise than one sensor.

Pacemaker functions

Hysteresis, from the Greek for "to lag behind," means a delay of effect behind the cause. In pacemakers, this means delaying pacing to maximize patient benefit. Let's look at when this feature would be used.

One problem with a single-chamber ventricular pacemaker is the loss of atrial kick, resulting in a 15% to 30% drop in cardiac output (CO). For example, if you compare the CO of a patient with ventricular pacing at 80 bpm to his CO at his own natural sinus rhythm of 80 bpm, you'd probably find that the CO is significantly higher with sinus rhythm because of the proper coordination between atrial and ventricular contractions. Even with a heart rate of 75 bpm, his CO would be higher than provided by a single-chamber ventricular pacemaker set at 80 bpm because what's lost in heart rate is made up in stroke volume.