Cardioversion and Defibrillation

Chapter 53 Cardioversion and Defibrillation



Steven G. Cole, DVM, DACVECC, DACVIM (Cardiology)



KEY POINTS



• Defibrillation and cardioversion involve the application of an electrical shock to terminate a cardiac arrhythmia.

• Electrical defibrillation is a key component of advanced life support and is the only effective treatment for ventricular fibrillation or pulseless ventricular tachycardia in patients with cardiopulmonary arrest. Chemical defibrillation is not a viable alternative.

• Synchronized cardioversion is used for treatment of unstable or drug-refractory supraventricular or ventricular tachyarrhythmias and delivers the electrical impulse at a specific portion of the cardiac cycle to avoid the vulnerable period during the T wave.

• Defibrillator use poses some risk to staff and should be done by trained personnel following a standard protocol.


INTRODUCTION


Electrical defibrillation and cardioversion are advanced techniques that involve the application of an electrical shock to terminate a cardiac arrhythmia. Both of these techniques use current generated from a defibrillator to cause simultaneous global depolarization of the myocardium, thereby interrupting rhythms that depend on organized or disorganized reentrant mechanisms for their maintenance.1 Although defibrillation and cardioversion are very similar, they differ somewhat in their application.


Specifically, defibrillation refers to an unsynchronized impulse delivered in an effort to abolish ventricular fibrillation or pulseless ventricular tachycardia and is a key component of advanced life support and cardiopulmonary resuscitation (CPR) (see Chapter 4, Cardiopulmonary Resuscitation). In comparison, synchronized cardioversion refers to an impulse that is delivered during a specific portion of the cardiac cycle to terminate a symptomatic supraventricular or ventricular arrhythmia.


It should be noted that defibrillation and cardioversion are used to treat symptomatic and potentially life-threatening arrhythmias. In cardiopulmonary arrest associated with a rhythm of ventricular fibrillation or pulseless ventricular tachycardia, the use of a defibrillator is unquestioned and is likely the only method that may restore spontaneous circulation. For other arrhythmias, including sustained, symptomatic supraventricular or ventricular tachycardia, the role of electrical therapy is not clearly defined for veterinary patients in an emergency or critical care setting. Certainly cardiac arrhythmias are common in critically ill dogs and cats and are associated with primary cardiac disease as well as with a variety of noncardiac conditions. However, most of these arrhythmias, including isolated single ventricular premature complexes or accelerated idioventricular rhythms, do not require specific therapy. When the arrhythmia requires management, drug therapy is the appropriate therapeutic choice for nearly all patients. Only in rare situations is the emergency use of synchronized cardioversion indicated. In all cases, patients that are successfully defibrillated or cardioverted should be monitored intensively and treated aggressively to prevent deterioration of their postshock rhythm.



EQUIPMENT


Many newer defibrillators have the capability to perform both synchronized cardioversion and defibrillation. Defibrillators are available from many manufacturers in both portable and stand-alone configurations. Ideally, the defibrillator is placed on a crash cart in a centrally located arrest station along with supplies for vascular access, airway management, and resuscitation drugs. Although both defibrillation and cardioversion may be performed with standard paddles, several accessories make the procedures easier to perform in veterinary patients. First, an attachment called a posterior paddle (Color Plate 53-1), which is a flat paddle that can be placed under the patient, makes it possible to cardiovert or defibrillate a patient in lateral recumbency. This minimizes the awkwardness and increased danger to staff that arises from defibrillating a patient in dorsal recumbency. Second, many defibrillators can be used with adhesive patches instead of paddles. These patches come in both pediatric and adult sizes for use on both small and large patients and provide similar benefits to a posterior paddle.



Monophasic Versus Biphasic Waveforms


Two types of defibrillators are available: monophasic and biphasic. These differ by the shape of the waveform that is generated by the defibrillator. Monophasic defibrillators generate a single positive current, and biphasic defibrillators generate a positive followed by a negative current (Figure 53-1). The bidirectional energy flow produced by biphasic defibrillators allows lower energy settings to be employed, and there is evidence suggesting that this technology allows for more efficient defibrillation with less myocardial damage.2 Although biphasic defibrillators are becoming more common in human medicine and recommendations for energy settings have been incorporated into human CPR guidelines,3 monophasic defibrillators are much more common in veterinary hospitals. Accordingly, guidelines for energy settings in small animal patients are based on defibrillators with monophasic waveforms.4


image

Figure 53-1 Monophasic (left) versus biphasic (right) defibrillator waveforms.



SAFETY


Electrical defibrillators can be among the most effective, as well as the most dangerous, pieces of equipment in a veterinary hospital. Inadvertent delivery of current to staff can result in painful electrical shocks, burns, and potentially serious cardiac arrhythmias. The risk of injury to staff increases when the animal is in dorsal recumbency, because it is more likely that the patient’s limbs will touch nearby personnel. It is also important to recognize that metal surfaces may conduct current, and it is recommended that nonconducting rubber or foam pads be placed on metal tables near the crash cart. Additionally, poor contact between the paddles and the chest may result in arcing of current across the surface of the patient. This can result in fire, especially when alcohol has been placed on the fur. As a result, only contact gel should be used to apply electrocardiogram (ECG) electrodes when defibrillation or cardioversion may be necessary. Because of these potential hazards, it is vital that staff members are educated in the use of the defibrillator and that protocols for cardioversion and defibrillation are followed carefully.

Related posts:

  1. Deteriorating Mental Status
  2. Hyperthermia and Fever
  3. Ventilator-Associated Lung Injury
  4. Allergic Airway Disease in Dogs and Cats and Feline Bronchopulmonary Disease

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Sep 10, 2016 | Posted by in SMALL ANIMAL | Comments Off on Cardioversion and Defibrillation

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