You are in: eMedicine Specialties > Emergency Medicine > IMPLANTABLE DEVICES External PacemakersArticle Last Updated: Feb 5, 2008AUTHOR AND EDITOR INFORMATIONSection 1 of 11
  Author: Joseph J Bocka, MD, Attending Physician, Emergency Medical Service/Liaison for several squads; Director of Shelby Emergency Department, Assistant Director, Department of Emergency Medicine, Med Central Health System (Mansfield and Shelby, Ohio) Joseph J Bocka is a member of the following medical societies: American Academy of Emergency Medicine, American College of Emergency Physicians, American Medical Association, National Association of EMS Physicians, and Phi Beta Kappa Editors: Robert M McNamara, MD, FAAEM, Professor of Emergency Medicine, Temple University; Chief, Department of Internal Medicine, Section of Emergency Medicine, Temple University Hospital; Francisco Talavera, PharmD, PhD, Senior Pharmacy Editor, eMedicine; A Antoine Kazzi, MD, Chief of Service, Department of Emergency Medicine, Medical Director of the Emergency Unit, American University of Beirut; John D Halamka, MD, MS, Associate Professor of Medicine, Harvard Medical School, Beth Israel Deaconess Medical Center; Chief Information Officer, CareGroup Healthcare System and Harvard Medical School; Attending Physician, Division of Emergency Medicine, Beth Israel Deaconess Medical Center; Steven C Dronen, MD, FAAEM, Director of Emergency Services, Director of Chest Pain Center, Department of Emergency Medicine, Ft Sanders Sevier Medical Center Author and Editor Disclosure Synonyms and related keywords: transcutaneous external cardiac pacing, TEP, symptomatic bradycardia, asystole, permanent implantable transvenous pacemaker, transcutaneous external pacemaker OVERVIEWSection 2 of 11
  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. HISTORYSection 3 of 11
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 ELECTROPHYSIOLOGYSection 4 of 11
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). 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. CLINICAL USE OF TRANSCUTANEOUS PACINGSection 5 of 11
Indications 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 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. HEMODYNAMICSSection 6 of 11
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 RESUSCITATIONSection 7 of 11
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/STUDIESSection 8 of 11
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 SAFETYSection 9 of 11
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 SITESSection 10 of 11
For further information on external pacemakers, visit the following Web sites: Zoll Medical, Physio-Control, and Philips Medical. REFERENCESSection 11 of 11
Article Last Updated: Feb 5, 2008 |
