eMedicine Specialties > Emergency Medicine > Implantable Devices
External Pacemakers
Updated: Feb 5, 2008
Overview
Transcutaneous external cardiac pacing (TEP) is a 200-year-old concept made marginally practical by Zoll's work in the early 1950s. It was not until the 1980s, however, that technological improvements made TEPs less cumbersome and better tolerated by patients. Studies using these devices demonstrated that external pacing has a role as a temporizing measure in patients with symptomatic bradycardia and a pulse but is of little benefit in pulseless situations.
Use of a TEP may also be of benefit for overdrive pacing in treatment of certain tachycardias. Current American Heart Association Advanced Cardiac Life Support (AHA ACLS) guidelines recommend TEP as a temporizing measure for symptomatic bradycardia and as a consideration for treatment of asystole.
History
Early history
In 1791, Galvani reported that electrical current applied across the heart of a dead frog resulted in myocardial contraction. Building on this principal, Duchenne (1872) successfully resuscitated a drowning victim, attaching an electrode to a leg while rhythmically tapping the precordium with another electrode. Gould (1929) also reported successful resuscitation of a patient in cardiac arrest by using a self-designed transthoracic pacemaker. Hyman was the first to coin the term artificial cardiac pacemaker. In 1932, he published the design of an external pulse generator for use in animals.
In 1952, Zoll reported the successful use of subcutaneous (simultaneous precordial and transesophageal) needle electrodes in pacing 2 patients with ventricular standstill secondary to Morgagni-Stokes-Adams syndrome. He later reported the development and successful use of the first true transcutaneous pacemaker and monitor. This device used a pair of 3-cm metal electrodes secured to the chest wall and delivered 2-millisecond (msec), 120-volt, alternating current impulses.
Development of modern transvenous pacemakers
The introduction of permanent implantable transvenous pacemakers by Chardack, Furman, Senning, Elmqvist, Thevenet, Lillehei, and others in the late 1950s limited the use of the more painful and cumbersome external models.
In 1981, Zoll patented and introduced a transcutaneous external pacemaker with a longer pulse duration (40 msec) and a larger electrode surface area (80 cm2). This reduced the current required for capture as well as patient discomfort. Modern devices have also incorporated improved electronics that allow filtration of artifact from contraction of skeletal muscle, thereby permitting simultaneous monitoring of cardiac rhythm through the pacer pads. These important electronic changes, coupled with the design of devices that are easily portable and simple to use, paved the way for renewed interest in TEPs. In 1982, the FDA approved use of the Zoll TEP for patients with heart rates less than 40 beats per minute and asystole. The current AHA ACLS guidelines include use of TEPs for symptomatic bradycardias.
TEP Electrophysiology
Pulse duration
An important feature of modern TEPs is the use of a long electrical pulse duration. Early TEPs used short (1-2 msec) impulses, more closely resembling the action potential duration of skeletal muscle rather than cardiac muscle (20-40 msec). This preferentially stimulated skeletal muscle with resultant patient discomfort. Zoll found that increasing the duration from 1 msec to 4 msec caused a 3-fold reduction in threshold (the current required for stimulation). Increasing the pulse duration from 4 msec to 40 msec further halves the threshold, but longer durations produce no additional advantage. Current TEPs deliver 40-msec pulses (Zoll) or 20-msec pulses (all others).
Electrodes
Another feature of modern TEPs is the use of large electrodes. As a general rule, pain is directly related to the amount of current delivered and inversely related to the skin surface area over which it is delivered. Thus, pain is minimized by using electrodes with large surface areas. Pain levels typically plateau once the electrode surface area exceeds 10 cm2. Most commercially available electrodes are 80-100 cm2.
Current
Clinical Use of Transcutaneous Pacing
Indications
Transcutaneous pacing is indicated as a temporizing measure for treatment of symptomatic bradycardias, including sinus bradycardias and atrioventricular (AV) nodal blocks. Pads may also be applied prophylactically to patients with these rhythms who are maintaining a stable blood pressure.
Transcutaneous pacing has been used successfully for overdrive pacing of tachyarrhythmias.
Transcutaneous pacing is not considered beneficial in the treatment of asystole.
Patient information
Prior to beginning pacing, the patient should be informed of the reason for pacing, the discomfort that may be experienced, and the option of providing analgesia or sedation if needed.
Pad application
Pacemaker pads, often labeled "front/back" or "anterior/posterior" are applied over the cardiac apex and just medial to the left scapula. It may be necessary to clip or shave excessive body hair to ensure good contact of the electrodes.
Pacemaker operation
For most patients, the heart rate should be set to 80. The current should initially be set to zero milliamperes (mA). The pacemaker unit is then turned on, and the current is increased in 10-mA increments until capture is achieved. In the unconscious patient, beginning with a high current output (200 mA) to rapidly achieve capture and then decreasing the output to the level needed to maintain capture is acceptable.
Synchronous/asynchronous modes
In the asynchronous or fixed-rate mode, the TEP delivers an electrical stimulus at preset intervals, independent of intrinsic cardiac activity. In theory, this could induce arrhythmias if stimulation occurs during the vulnerable period of the cardiac cycle. Synchronous pacing is a demand mode in which the pacer fires only when no complex is sensed for a predetermined amount of time. Pacing generally should be started in the synchronous mode.
Confirmation of capture
Electrical capture is documented electrocardiographically by the presence of a widened QRS followed by an ST segment and T wave. Clinically, these waveforms should correlate with palpable carotid pulses. In the hypotensive patient, bedside ultrasonography may be used to confirm cardiac contraction correlating with electrical activity.
ECG monitoring
The ability of the pacemaker devise to monitor the cardiac electrical activity and document electrical capture varies between models. Blanking protection changes the high output pacing stimulus to a smaller ECG waveform, making cardiac electrical activity easier to visualize. If blanking protection is not present, a separate ECG monitor is needed to determine capture.
Minimizing discomfort
Pain from modern TEPs is related primarily to contraction of skeletal muscle. This can be minimized by proper anterior pad placement, specifically, just medial to rather than directly over the left pectoral muscle, and by using the minimum current necessary to ensure electrical capture. Sedation with a short-acting agent, such as midazolam, an intravenous analgesia with an opiate, or both should be considered if the clinical situation permits their use.
Complications
Few complications are associated with use of TEPs. Pain is the most common side effect and as noted above may be minimized by proper pad placement, use of the lowest effective current, and judicious administration of sedatives and analgesics. Coughing and hiccups may occur secondary to stimulation of the diaphragm and thoracic muscles. Skin burns have been reported with prolonged use.
Although not technically a complication of TEPs, failure to capture is not the desired clinical outcome. Potential causes include improper pad placement (directly over the sternum, scapula, or thoracic spine), poor skin contact (excessive hair, wet skin, or pad loosely applied), inadequate current output, and faulty or improperly set-up equipment. Anatomic impediments to current delivery may include fluid (pericardial effusion) or air (pneumothorax, chronic obstructive pulmonary disease [COPD]).
Hemodynamics
Varghese reported that external pacing simultaneously stimulated all 4 heart chambers in dogs. Madsen, however, echocardiographically demonstrated in humans that atrial stimulation was retrograde without opening the mitral valve.
Studies have shown no difference in hemodynamics between transcutaneous pacemakers and transvenous pacemakers, using comparable rates in complete heart block and cardiac arrest.
The atrial-pacing threshold in humans is generally much higher than that for the ventricles; thus, current needed to stimulate all 4 chambers is not tolerated, even by patients who are sedated. This results in loss of the "atrial kick" and a reduction in cardiac output. Talit studied healthy volunteers and found, via Doppler measurements, that both stroke volume and cardiac output were reduced even when pacing at a rate 15-30% higher than the sinus baseline. Thus, external pacing may need higher pulse rate settings than expected in patients with symptomatic bradycardia to ensure that an adequate blood pressure is achieved.
Special Considerations - Cardiopulmonary Resuscitation
Cardiopulmonary resuscitation (CPR) can be performed with the TEP pads in place. The low current delivered and the insulation of the flexible TEP pads result in no electrical hazard to the person performing CPR. However, turning the unit off during CPR is advisable.
In TEPs without an intrinsic defibrillator, separate leads need to be applied. The external pacemaker should be turned off or turned to monitoring mode when performing defibrillation or cardioversion on a patient. Defibrillator paddles should be placed at least 2-3 cm away from TEP stimulation pads to prevent arcing of current. Pacing pads should be placed in the anterior/posterior position.
External Pacemaker Uses/Studies
Asystole
Most hospital and prehospital studies report no long-term survivors from asystole when using external pacing. Small studies reported a 4% and 8% survival rate when TEP was initiated early in cardiac arrest. Survival rates ranging from 7-100% have been reported, albeit in studies with few subjects using TEPs early in bradyasystolic arrest. Many of these studies do not describe what other ACLS modalities were used. No pediatric patients in asystole have been reported to survive with external pacing.
Unstable/symptomatic bradycardias
Hemodynamically unstable bradycardias have 50-100% survival-to-discharge rates reported in prehospital and hospital settings. Two neonates with AV block who survived with the assistance of immediate external pacing have been reported.
Tachyarrhythmias
Single and multiple beat pacing stimulation have been described as a useful treatment of tachycardias. The objective is to place a ventricular extrasystole during the vulnerable period of the cardiac cycle. More than 150 cases of successful "overdrive pacing" (stimulating at a rate greater than the tachycardia) for tachycardias using TEPs have been noted. Overall termination rates for ventricular tachycardia reportedly have been between 57% and 95%; however, acceleration occurred in 4-26% of the reported attempts. Fisher reported termination in 57% and acceleration in 0.5% using single beat capture compared with 94% termination and 3.6% acceleration in 3.6% using multiple beat rapid burst attempts.1
Safety
No enzymatic, electrocardiographic, or microscopic evidence of myocardial damage has been found after pacing (dogs and humans) for as long as 60 minutes. Zoll reported that stimulation of the anoxic dog heart during the vulnerable period did not produce ventricular fibrillation (VF) or ventricular tachycardia (VT) unless one was using current that was 10 times higher than pacing threshold.
Manufacturer Web Sites
For further information on external pacemakers, visit the following Web sites: Zoll Medical, Physio-Control, and Philips Medical.
For information about external pacemaker testing devices, visit the following Web sites: Fluke Biomedical and Netech.
Keywords
transcutaneous external cardiac pacing, TEP, symptomatic bradycardia, asystole, permanent implantable transvenous pacemaker, transcutaneous external pacemaker
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References
Fisher JD, Kim SG, Matos JA, Ostrow E. Comparative effectiveness of pacing techniques for termination of well-tolerated sustained ventricular tachycardia. Pacing Clin Electrophysiol. Sep 1983;6(5 Pt 1):915-22. [Medline].
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Bocka JJ. External transcutaneous pacemakers. Ann Emerg Med. Dec 1989;18(12):1280-6. [Medline].
Eitel DR, Guzzardi LJ, Stein SE, Drawbaugh RE, Hess DR, Walton SL. Noninvasive transcutaneous cardiac pacing in prehospital cardiac arrest. Ann Emerg Med. May 1987;16(5):531-4. [Medline].
Ettin D, Cook T. Using ultrasound to determine external pacer capture. J Emerg Med. Nov-Dec 1999;17(6):1007-9. [Medline].
Ettin D, Cook T. Using ultrasound to determine external pacer capture. J Emerg Med. Nov-Dec 1999;17(6):1007-9. [Medline].
Grubb BP, Samoil D, Temesy-Armos P, Hahn H, Elliott L. The use of external, noninvasive pacing for the termination of supraventricular tachycardia in the emergency department setting. Ann Emerg Med. Apr 1993;22(4):714-7. [Medline].
Jaggarao NS, Heber M, Grainger R, Vincent R, Chamberlain DA, Aronson AL. Use of an automated external defibrillator-pacemaker by ambulance staff. Lancet. Jul 10 1982;2(8289):73-5. [Medline].
Olson CM, Jastremski MS, Smith RW, Tyndall GJ, Montgomery GF, Daye MC. External cardiac pacing for out-of-hospital bradyasystolic arrest. Am J Emerg Med. Mar 1985;3(2):129-31. [Medline].
Paris PM, Stewart RD, Kaplan RM, Whipkey R. Transcutaneous pacing for bradyasystolic cardiac arrests in prehospital care. Ann Emerg Med. Apr 1985;14(4):320-3. [Medline].
Syverud SA, Dalsey WC, Hedges JR. Transcutaneous and transvenous cardiac pacing for early bradyasystolic cardiac arrest. Ann Emerg Med. Feb 1986;15(2):121-4. [Medline].
Vukov LF, Johnson DQ. External transcutaneous pacemakers in interhospital transport of cardiac patients. Ann Emerg Med. Jul 1989;18(7):738-40. [Medline].
Vukov LF, White RD. External transcutaneous pacemakers in prehospital cardiac arrest. Ann Emerg Med. May 1988;17(5):554-5. [Medline].
White JM, Nowak RM, Martin GB, Best R, Carden DL, Tomlanovich MC. Immediate emergency department external cardiac pacing for prehospital bradyasystolic arrest. Ann Emerg Med. Apr 1985;14(4):298-302. [Medline].
Zoll PM, Linenthal AJ, Norman LR. Treatment of Stokes-Adams disease by external electric stimulation of the heart. Circulation. Apr 1954;9(4):482-93. [Medline].
Further Reading
Keywords
transcutaneous external cardiac pacing, TEP, symptomatic bradycardia, asystole, permanent implantable transvenous pacemaker, transcutaneous external pacemaker
