Cardioversion and Defibrillation
Avani T. Mehta
Paulo J. Oliveira
I. GENERAL PRINCIPLES
A. Basic concepts.
1. Electric countershock.
a. Delivering electrical energy to depolarize the myocardium to terminate a tachyarrhythmia.
b. Definitions.
i. Cardioversion delivers a synchronized shock, coinciding with the QRS complex on the electrocardiogram (ECG).
(a) If countershocked during the “vulnerable” period, late ventricular systole marked by T wave on the ECG, there is risk of inducing ventricular fibrillation (VF).
(b) Primarily used for emergency and elective treatment of various tachyarrhythmias.
ii. Defibrillation delivers an unsynchronized shock to terminate VF and pulseless ventricular tachycardia (VT).
(a) Because VF/pulseless VT is immediately life threatening and there is no well-defined QRS complex, an unsynchronized electrical countershock is delivered.
(b) Defibrillation success is defined as termination of VF for at least 5 seconds following the shock, not by restoration of a perfusing rhythm.
B. Physiology of arrhythmias requiring cardioversion or defibrillation.
1. Reentry.
a. Arrhythmias involving reentrant circuits (electrical activation over a closed conduction pathway) can be terminated with electric countershock.
i. Examples include atrial fibrillation (AFib), atrial flutter, atrioventricular (AV) nodal reentrant tachycardia, and most VT and VF.
b. Cardioversion and defibrillation disrupt reentry by depolarizing at least a threshold quantity of excitable tissue.
2. Increased automaticity.
a. Arrhythmias involving increased impulse formation do not respond to electric countershock.
b. Examples of unresponsive arrhythmias involving triggered activity are sinus tachycardia, focal atrial tachycardia, and some types of VT.
C. Mechanism of action.
1. Effective countershock silences an adequate portion of the myocardium through depolarization, while the remaining myocardium cannot perpetuate the arrhythmia.
2. Factors affecting successful shock include energy level, type of shock waveform, transthoracic impedance, and myocardial refractory state.
3. Subthreshold shocks may extinguish fibrillatory wavefronts, but often new wavefronts will form causing perpetuation of the fibrillation.
II. INDICATIONS
A. Urgent.
1. Hemodynamic instability.
2. Acute respiratory distress, congestive heart failure, and angina.
3. Important to recognize sinus tachycardia from rhythms that benefit from cardioversion or defibrillation.
B. Elective.
1. Absence of acute symptoms and signs.
2. Weigh risks and benefits.
III. PRECAUTIONS
A. Digitalis toxicity and electrolyte imbalance can increase the risk of inducing VT and VF.
B. Severe conduction disease (i.e., sick sinus syndrome) increases the risk of developing significant bradyarrhythmia after cardioversion.
C. Risk of thromboembolism in patients with AFib.
IV. PROCEDURE
A. Technical considerations.
1. Waveform types.
a. Monophasic.
i. Unipolar, delivers current in one direction.
ii. Standard in older defibrillators.
iii. Requires higher energy levels to terminate arrhythmia.
b. Biphasic.
i. Bipolar, delivers current in two directions with polarity reversal during the return phase.
ii. Standard on most defibrillators.
iii. Fewer shocks and lower total energy to terminate, with equal (perhaps superior) efficacy and improved safety profile.
2. Electrodes.
a. Handheld paddles.
i. Larger paddle size decreases transthoracic resistance, increasing energy delivery.
ii. Pressure applied to chest with paddles decreases impedance and potentially improves efficacy of countershock.
iii. Current recommendations favor use of gel pads to decrease the risk of arcing and skin burns.
b. Self-adhesive pads.
i. More common as convenient and easy to use.
ii. Advantages: equally effective, no gel required, and minimizes risk to staff as less contact with bed and patient during delivery of shock.
iii. May allow for temporary external pacing depending on model.
c. Anatomic placement.
