Automatic And Implantable Defibrillators

In 1933, William Kouwenhoven observed in dogs that closed-chest electrical shocks delivered within 30 s of inducing ventricular fibrillation (VF) were 98 percent effective in terminating the dysrhythmia. After 2 min of VF, the rate of resuscitation fell to 27 percent. He reported similar results in human subjects. 14 Modern research indicates that the likelihood for successful resuscitation decreases roughly 10 percent/min after the onset of VF. Thus, the goal of emergency cardiac care is to deliver defibrillation as quickly as possible.

Two recent technological developments have led to the more rapid application of defibrillation. Automatic external defibrillators allow first responders to rapidly institute defibrillation. Even laypersons, such as family members or bystanders, can learn to use these devices. In addition, implanted defibrillators allow patients with frequent malignant ventricular dysrhythmias to carry their own defibrillators with them at all times. Emergency physicians need to be familiar with these devices and the special considerations associated with their use.

Automatic External Defibrillators

Automatic external defibrillators (AEDs) have relatively simple controls and can be used by minimally trained providers to initiate defibrillation. At the patient's right sternal border and cardiac apex, the operator places electrode pads, which are used both for monitoring and defibrillating. Once attached, the AED analyzes the cardiac rhythm and initiates its treatment algorithm. A fully automatic device will deliver a countershock once ventricular tachycardia or VF has been sensed. The AED gives an audible announcement that defibrillation will commence, and the only way the operator can prevent discharge is to turn off the device. A semiautomatic AED analyzes the rhythm and then advises whether a shock is indicated. The operator must press the control button in order to initiate defibrillation; the operator may also override the AED and administer a countershock even if the device has not sensed a shockable rhythm.

AEDs shock patients in VF several times sequentially until an organized cardiac rhythm results or until the maximum number of shocks allowed by the programmed algorithm is reached. Many devices also provide a record of rhythms and events during their use, which allows the emergency physician to subsequently reconstruct the sequence of events during resuscitation.

An AED may be placed on an unstable patient in anticipation of subsequent deterioration, but the device should not be activated until or unless the patient is pulseless. Since motion artifacts may confuse the rhythm-analysis circuitry, the AED should not be in the sensing mode during CPR, during transport, or if the patient has a seizure. Unlike transcutaneous pacemakers, AEDs can deliver a debilitating shock to the operator or other personnel, and the same precautions regarding contact with the patient during defibrillation should be followed with AEDs as with standard defibrillators. The failure of an AED to restore a perfusing rhythm is a poor prognostic sign often associated with long arrest times or arrest rhythms other than VF. When an AED fails to resuscitate a patient in arrest, the cardiac rhythm should be identified and treated. If the rhythm is refractory VF, drug therapy should be instituted while continuing further defibrillation attempts.

AEDs are most effective in tiered emergency medical services systems where AED-equipped first responders reach the patient rapidly and are backed up by the later arrival of paramedics with full advanced life-support capabilities.15 There is ongoing interest in making AEDs available for widespread use by nonmedical personnel and the lay public.1 17

Implantable Cardioverter-Defibrillators

The first human placement of an implantable cardioverter-defibrillator (ICD) took place in 1980 at the Johns Hopkins Hospital. 18 Since that time it has become the treatment of choice for sudden cardiac death, reducing mortality from about 30 to 45 percent/year to less than 2 percent/year. 19 This remarkable efficacy, coupled with the failure (and potentially proarrhythmic effects) of pharmacologic therapy and the increasing sophistication and miniaturization of the devices, has led to an explosion in ICD use. Through 1994 there had been over 50,000 ICDs implanted worldwide; in 1995 there were more than 20,000 implantations in the United States alone.

An ICD consists of a pulse generator, a lead system with both sensing and shocking electrodes, circuitry to analyze the cardiac rhythm and trigger defibrillation, and a power supply. There are currently three generations of ICDs. In each successive generation, the devices have become smaller, more sophisticated, more reliable, and easier to implant. Second-generation ICDs are still quite common. These devices generally were placed by thoracotomy or sternotomy, and defibrillation occurred through electrodes positioned inside or outside the pericardium. Rate-sensing electrodes were placed epicardially or transvenously ( Fig 18-6). The sensing algorithms for second-generation ICDs were relatively unsophisticated; they reliably detected ventricular tachycardia and VF but would cause the devices to inappropriately shock supraventricular rhythms as well. Roughly one-third of patients with second-generation ICDs receive at least one shock triggered by a supraventricular tachycardia during the working life of the device.

FIG. 18-6. Typical second generation ICD electrode arrangements. A. Spring-patch pathway. B. Patch-patch pathway. (From Chapman PD, Veseth-Rogers JL, Duquette SE: The implantable defibrillator and the emergency physician. Ann Emerg Med 18:579, 1989. Used by permission.)

Third-generation ICDs have a volume of about 60 mL, or roughly one-quarter that of second-generation devices. The sensing-pacing-defibrillation electrodes are placed transvenously, and the device itself is generally implanted subcutaneously in the subpectoral region or in an abdominal pocket. A subcutaneous patch electrode may be used to help lower the defibrillation threshold. On a chest radiograph, at first glance these devices can easily be mistaken for conventional pacemakers. Careful examination of the electrodes will reveal their true nature ( Fig 18-7). Newer ICDs are better at discriminating supraventricular tachycardia and are capable of a variety of responses to ventricular tachycardia and VF. Most are programmed to follow a tiered approach to ventricular dysrhythmias: antitachycardia pacing, low-energy cardioversion, and finally defibrillation. Depending on the frequency of discharge and whether the pacemaker function is used, the latest ICDs have a projected life span of about 8 years.

FIG. 18-7. Chest radiograph of a patient with a nonthoracotomy implanted third generation defibrillator. The ICD is in the left subpectoral area. The electrodes (arrows) have been placed transvenously. The proximal spring electrode is positioned at the junction of the superior vena cava and the right atrium. The distal electrode is in the right ventricle; only a portion of its lead is visible.

EMERGENCY DEPARTMENT EVALUATION AND THERAPY ICDs are remarkably effective in preventing sudden cardiac death. The most common cause of death in patients with ICDs is congestive heart failure, which should be managed in standard fashion in the emergency department. However, the most common reason an ICD patient comes to the emergency department is to be evaluated for the appropriateness of a previously delivered shock. Causes of inappropriate shock delivery are summarized in Table 18-3. In evaluating a patient complaining of one or more ICD shocks, it is important to determine the number of shocks delivered, the activity of the patient at the time, and any prodromal symptoms or postshock trauma. Recent changes in antiarrhythmic drug dosage should be noted. The physical examination should focus on the vital signs, the cardiovascular status, the generator pocket, and evidence of incidental trauma. The patient should be monitored during the evaluation. An ECG should be obtained and interpreted with the knowledge that ST-segment elevations or depressions due exclusively to the shock will resolve within 15 min. A chest radiograph may reveal electrode migration, displacement, or fracture. Drug levels of antiarrhythmics should be determined and electrolyte disturbances explored.


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TABLE 18-3 Potential Causes of Inappropriate ICD Shock Delivery

Since admission criteria are often institution- and cardiologist-specific, consultation with the patient's cardiologist is essential. General admission guidelines include all unstable patients, patients with two or more shocks in a 1-week period, the presence of correctable causes of dysrhythmia (e.g., electrolyte imbalances, drug toxicity, or ischemia), any sign of infection, mechanical disruption of the ICD or leads, and patients who need additional cardiologic investigation for possible malfunction of the device.

For an ICD patient in cardiac arrest, normal basic and advanced resuscitation measures are indicated. If defibrillation is necessary, the operator should avoid placing either paddle directly over the ICD. The presence of epicardial patch electrodes may shield the myocardium from the countershock and may therefore necessitate repositioning of the paddles. CPR may be performed in the usual fashion. If the ICD should discharge during CPR, the provider may perceive a small electrical shock, but it is neither uncomfortable nor dangerous.

Occasionally it may become necessary to temporarily deactivate the ICD, as in the case of inappropriate shock for a stable rhythm. Second-generation devices may be deactivated by placing a donut-shaped magnet over the right upper quadrant ( Fig 18-8) of the pulse generator for 30 s until the intermittent beeping ceases and a solid tone is heard. The magnet is then removed. If this does not succeed, then deactivation is attempted by placing the magnet over the opposite corner of the pulse generator (some are surgically positioned upside down). The response of third-generation devices to a magnet can be complex, but generally defibrillation is deactivated only when the magnet is present. This requires taping the magnet to the skin overlying the ICD. Defibrillation can be reenabled by removing the magnet. Some third-generation devices are programmed so that they cannot be deactivated by a magnetic field. All ICDs should be evaluated by a cardiologist after exposure to a magnet.

FIG. 18-8. Correct magnet placement to deactivate second generation ICD. (From Chapman PD, Veseth-Rogers JL, Duquette SE: The implantable defibrillator and the emergency physician. Ann Emerg Med 18:579, 1989. Used by permission.)


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