Getting back to basics with permanent pacemakers, part I
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.
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.
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.
Because the pacemaker’s purpose is to maintain CO, a single-chamber ventricular pacemaker needs to keep the patient’s heart rate higher to achieve the desired CO. For example, the patient may need a sinus rhythm at a rate of 70 bpm to maintain a CO of 6 liters/ minute; a single-chamber ventricular pacemaker would need to maintain a rate of 80 bpm to achieve this same CO. In this case, letting the heart’s intrinsic sinus rhythm be in control is in the patient’s best interest. Hysteresis is used to delay the pacemaker from taking over until the sinus rhythm drops below the hysteresis rate of 70 bpm, rather than the pacemaker’s programmed rate of 80 bpm.
Because hysteresis appears as a longer-than-normal delay on the ECG strip (see Recognizing Hysteresis), it may be misinterpreted as failure to fire or inappropriate sensing. The major difference between these malfunctions and hysteresis is that the only time pacemaker discharge is delayed is when a pacemakertriggered QRS follows an intrinsic QRS. This is because the pacemaker is trying to let the natural sinus rhythm keep control. The interval between any two paced beats should always be the same and should equal the pacemaker’s programmed rate.
If search hysteresis is activated, the pacemaker will delay pacing at set intervals. If the patient has an intrinsic impulse during this time, the pacemaker is inhibited and regular hysteresis programming is activated. If the patient doesn’t have an intrinsic impulse, the pacemaker will resume pacing at the higher pacemaker rate.
By checking occasionally for a natural rhythm that would produce a higher CO, the pacemaker can extend its battery life and maximize the patient’s CO. Because unnecessary right ventricular pacing also may harm the heart, search hysteresis is designed to reduce unnecessary pacing. However, the search hysteresis function complicates identifying pacemaker malfunctions, so the rhythm strip or 12-lead ECG tracing must be evaluated carefully.
When the heart suddenly changes its mind
Normally, heart rates change over time in response to the body’s metabolic demands. If the heart rate declines over time, the heart can adjust stroke volume to maintain adequate CO. But after a large, sudden drop in heart rate, stroke volume can’t increase quickly enough, causing a dramatic fall in CO and signs and symptoms such as hypotension, chest pain, syncope, and nausea.
To address this problem, some pacemakers have a rate-smoothing algorithm to give the heart enough time to adjust stroke volume. Rate smoothing limits changes in heart rate to a programmed percentage from one beat to the next. This lets the heart slowly speed up or slow down, giving the body time to adjust stroke volume. The pacemaker converts the rate into a time interval between beats. For example, pacing the heart at 60 bpm requires one beat every 1,000 milliseconds.
Suppose a patient is in normal sinus rhythm at 100 bpm. If the patient’s heart suddenly stops, the pacemaker will pick up at its programmed lower rate; for example, 60 bpm. But if the patients heart rate drops from 100 bpm to 60 bpm, the heart and body have no chance to make adjustments to preserve CO.
If the patient’s pacemaker is programmed with a rate-smoothing algorithm at 5%, the pacemaker will let the R-R interval lengthen by just 30 milliseconds. If the patient doesn’t have a natural beat within 630 milliseconds, the pacemaker discharges, and the new rate will be about 95 bpm.
The waiting time for the next cycle is based on the previous R-R interval (in this case, 630 milliseconds). Again, the pacemaker lets the interval increase by 5%, or about 662 milliseconds. Now the heart rate is about 91 bpm. This gradual slowing continues until the pacemaker reaches its programmed rate of 60 bpm or the patients natural rate exceeds the pacemaker rate.
Increasing the heart rate works in similar fashion, with the pacemaker shortening the R-R interval by 5% at each step until the desired heart rate (or maximum pacemaker rate) is reached.
Recognizing this on an ECG can be tricky; rate smoothing may make it appear that the pacemaker is discharging irregularly. The key is a lengthening or shortening R-R interval on several consecutive beats.
Bernstein, A., et al.: “The Revised NASPE/BPG Generic Code for Antibradycardic, Adaptive-Rate, and Multisite Pacing. North American Society of Pacing and Electrophysiology/British Pacing and Electrophysiology Group,” Pacing and Clinical Electrophysiology. 25(2):260-264, February 2002.
Hummel, J., et al.: Pocket Guide for Cardiac Electrophysiology. Philadelphia, Pa., WB. Saunders Co., 1999.
By Henry B. Geiter, Jr., RN, CCRN
Henry B. Geiter, Jr., is a staff nurse at Bayfront Medical Center in St. Petersburg, Fla.; a critical care transport nurse for Sunstar-EMS in Clearwater, Fla; an adjunct instructor at St. Petersburg (Fla.) College; and owner of http://www.nurse411.com, a nurse resource Web site.
Copyright Springhouse Corporation Oct 2004
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