transthoracic pacemaker


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ex·ter·nal pace·mak·er

an artificial cardiac pacemaker of which the electrodes for delivering rhythmic electric stimuli to the heart are placed on the chest wall.

transthoracic pacemaker

[-thôras′ik]
Etymology: L, trans, across; Gk, thorax, chest; L, passus, step; ME, maken
a permanent pacemaker with the pulse generator located in the abdominal wall and the pacing wires attached directly to the epicardium.

pacemaker

(pas'mak?er)
1. Anything that influences the rate and rhythm of occurrence of some activity or process.
2. In cardiology, a specialized cell or group of cells that automatically generates impulses that spread to other regions of the heart. The normal cardiac pacemaker is the sinoatrial node, a group of cells in the right atrium near the entrance of the superior vena cava.
3. A generally accepted term for artificial cardiac pacemaker.
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PACEMAKER: Pacemaker defibrillator beneath the skin

artificial cardiac pacemaker

A device that can trigger mechanical contractions of the heart by emitting periodic electrical discharges. If the device delivers electricity through the chest wall, it is called a transcutaneous pacemaker; if it works via electrodes inserted inside the body, it is called an internal or implantable pacemaker. Pacemakers are used most often to treat patients with symptomatic slow heart rates or long pauses between heart beats (such as patients with third-degree heart block, symptomatic second-degree heart block, bifascicular block with first-degree heart block, carotid sinus hypersensitivity, and tachybrady syndrome) and slow ventricular response rates. Occasionally, though, they also are used for other purposes, such as to capture and override some tachyarrhythmias. See: illustration

All artificial cardiac pacemakers have a pulse generator (a device that gives off an electrical impulse at prescribed intervals), electrical leads (which transmit the impulse to the myocardium), and a battery (usually made of lithium iodide) encased in titanium and implanted surgically in a subcutaneous pocket (usually in the chest). The pacing leads are threaded through a subclavian vein into the right heart, with the primary lead placed in the ventricle, and the second lead (if required) in the atrium. For biventricular pacing, a third lead is placed in the left ventricle. A biventricular pacemaker also may have a built-in cardiovertor-defibrillator. Pacemakers typically have the ability to pace the ventricle, the atrium, or both; to sense electrical discharges coming from cardiac chambers; and to respond to sensed beats. Most pacemakers in the U.S. also are programmable, and many are rate responsive. The features of each pacemaker are identified in its three- to five-letter NASPE/BPG code (developed and revised by the North American Society of Pacing & Electrophysiology and the British Pacing & Electrophysiology Group). The five positions describe the pacer's functions as follows:1. Chambers paced (O=none, A=atrium, V=ventricle, D=dual [atria and ventricles], S=manufacturer's designation for single [atrium or ventricle]; 2. Chambers sensed (O=none, A=atrium, V=ventricle, D=Dual, S=Single; 3. Response to sensed event (O=none, T=triggered, I=inhibited, D=dual [inhibited and triggered]; 4. Rate modulation (O=none, R=rate modulation in response to sensor technology; 5. Multisite pacing (O=none, A=atrium, V=ventricle, D=dual [atrium and ventricle]).Thus a pacemaker with the letter I in position 3 of its code will inhibit firing when it senses an intrinsic beat but will pace the cardiac chamber if no beat is sensed. The letter D in position 5 indicates that both atria and both ventricles are paced, with the left and right chambers stimulated simultaneously to maintain coordination and thus improve cardiac output. When the pacemaker has a rate modulation feature (R in position 4), also known as an adaptive rate mechanism, it works to copy the abilities of a normally functioning heart, such as detecting exercise and triggering pacer rate acceleration to meet the increased metabolic need. Other pacemaker features include hysteresis, which delays unnecessary pacing (which can harm the right ventricle), and allows the patient's intrinsic impulse to provide an atrial kick, which stokes the ventricle and increases cardiac output by 15% to 30% over a beat without atrial kick. Pacers with a “rate-smoothing algorithm” limit heart rate changes to a programmed percentage from one beat to the next, allowing the heart rate to increase or decrease more slowly and providing time for the body to adjust the stroke volume as it would normally in such situations.

Patient care

After pacemaker implantation, follow-up care is provided to ensure that the device is working optimally. A chest x-ray confirms correct placement of the leads and also can identify some complications, such as pneumothorax. Since pacemakers usually are implanted under moderate sedation and analgesia, the patient may still be drowsy on return to the unit from recovery; however, he/she should arouse easily and be able to answer questions and follow commands. Supplemental oxygen is provided (nasal cannula or mask) until the patient is fully awake and his/her oxygen saturation has returned to baseline. Immobilizing the patient's shoulder and arm on the operative side with a sling or ace-wrap for 24 to 48 hr limits movement that could dislodge the tip of the pacing electrode(s) from the endocardial wall, preventing sensing of intrinsic beats or needed pacing. The incision is assessed for bleeding, infection, and incisional discomfort, which is controlled with prescribed analgesia. The patient also is assessed for chest pain, palpitations, dizziness, shortness of breath, hiccuping, and a sensation of pacing in the abdomen, and the cardiologist or surgeon notified if any of these occur. The chest x-ray usually is repeated the following day to document positioning and rule out pneumothorax. Pacer function is checked, and the patient discharged with instructions for activity restrictions and further care. Follow-up care typically includes monitoring the pacemaker's performance, either in the cardiologist's office or by telephonic link-up to ensure, for example, that the pulse generator is triggering a heart rate that is appropriate for the patient's needs, that the leads are working, and that the battery's strength is adequate.

Health care professionals should ascertain the type of pacemaker employed and expectations for its function based on its NASPE/BPG code; monitor the cardiac rate and rhythm for evidence of pacemaker function; assess the patient for evidence of pacemaker failure or noncapture (vertigo, loss of consciousness, hypotension, chest discomfort, dyspnea) and evaluate the patient for effects on cardiac output; teach the patient technique and rationale for monitoring own pulse rate and for care and protection of insertion site; and counsel concerning telephone monitoring check-up, battery replacement, medication regimen, physical activity, and follow-up care. They should encourage the patient to wear or carry medical identification and information indicating the presence and type of pacemaker implanted, along with an electrocardiogram rhythm strip showing pacemaker activity and capture.

CAUTION!

Patients with artificial cardiac pacemakers should avoid MR imaging.

breathing pacemaker

A device that stimulates breathing by delivering electrical pulses to both phrenic nerves from an external radio transmitter to an implanted receiver. It is used in patients with quadriplegia or sleep apnea. See: radiofrequency electrophrenic respiration

DDD pacemaker

A rate-adaptive implanted pacing device. It senses and paces both atrial and ventricular events, triggering the atrioventricular (AV) interval so that AV synchrony is maintained over a wide range of heart rates. This type of pacing has reduced the incidence of pacemaker syndrome. It is the most versatile pacing device used. See: pacing code; pacemaker syndrome

DDI pacemaker

An implanted pacing device that senses both atrial and ventricular events but can inhibit only atrial impulses. This type of pacing is used only when atrioventricular conduction is intact. It may be suitable when frequent atrial tachyarrhythmias cause rapid ventricular rates.

demand pacemaker

An implanted pacemaker that is designed to permit its electrical output to be inhibited by the heart's electrical impulses. This decreases the chances for the pacemaker to induce discomfort or dysrhythmias.

dual-chamber pacemaker

A pacemaker that is also known as an atrioventricular sequential pacemaker because it stimulates both atria and ventricles sequentially.

ectopic pacemaker

Any endogenous cardiac pacemaker other than the sinoatrial node.

fixed-rate pacemaker

A pacemaker that stimulates the heart at a predetermined rate.

internal pacemaker

A cardiac pacemaker placed within the body.

permanent pacemaker

An electronic device for permanent cardiac pacing. The leads are usually inserted transvenously through the subclavian or cephalic veins with leads positioned in the right atrium for atrial pacing or in the right ventricular apex for ventricular pacing. The leads are connected to the pulse generator, which is implanted in a subcutaneous pocket below the clavicle.

programmable pacemaker

An electronic permanent pacemaker in which one or more settings can be changed electronically.

rate-responsive pacemaker

An electronic pacemaker that senses changes in the body's need for adjustment of the cardiac rate as can occur in sleeping, waking, sitting, walking, or running. The device alters cardiac rate by sensing body motion, changes in breathing, or slight changes in blood temperature, which improves the quality of life for active patients. It is also called a rate-adaptive pacemaker.

temporary pacemaker

An electronic device for temporary cardiac pacing (e.g., during cardiac surgery and emergencies). The device consists of an electrode catheter inserted transvenously in the right ventricular apex that receives impulses from an external generator.

transcutaneous pacemaker

An artificial cardiac pacemaker that is located outside the body. The electrodes for delivering the stimulus are located on the chest wall. Synonym: transthoracic pacemaker

transthoracic pacemaker

Transcutaneous pacemaker.

uterine pacemaker

One or more areas in the uterus that stimulate contraction of the myometrium.

wandering pacemaker

A cardiac arrhythmia in which the site of origin of the pacemaker stimulus shifts from one site to another, usually from the atrioventricular node to some other part of the atrium.
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