Mechanism and types of cardioversion and defibrillation
The mechanism of cardioversion and defibrillation involves delivering a strong direct current to the heart in a very short time. This causes all cardiac pacemaker cells to depolarize simultaneously, inactivating any potential reentrant pathways. The pacemaker site with the highest automaticity (usually the sinoatrial node) then resumes control of the heart rhythm.
Cardioversion can be classified into synchronized cardioversion and unsynchronized defibrillation based on whether the R wave is detected during the procedure.
Synchronized cardioversion
The discharge is synchronized with the R wave, occurring approximately 30 ms after the start of the R wave, during the absolute refractory period of the ventricles. This avoids discharging during the vulnerable period of the ventricles (20 - 30 ms before the peak of the T wave), which could induce ventricular tachycardia or fibrillation. Synchronized cardioversion is mainly used for rapid ectopic arrhythmias with an identifiable R wave, such as drug-resistant paroxysmal supraventricular tachycardia, atrial flutter, atrial fibrillation, and ventricular tachycardia. Before cardioversion, ensure the synchronize function on the device is activated.
Unsynchronized defibrillation
It is used when the R wave cannot be distinguished, such as in ventricular fibrillation or flutter. At this point, effective mechanical cardiac contraction has ceased, and there is no cardiac cycle or QRS complex, so discharge should occur immediately without regard to the ventricular vulnerable period.
Indications and contraindications
The main indications for cardioversion and defibrillation include severe, life-threatening malignant arrhythmias and sustained tachyarrhythmias. The general principle is to consider cardioversion or defibrillation for any tachyarrhythmia causing hemodynamic instability unresponsive to medication.
Ventricular arrhythmia
If ventricular tachycardia does not respond quickly to drug treatment or causes severe hemodynamic compromise (consciousness disturbances, severe hypotension, or acute pulmonary edema), synchronized cardioversion should be performed immediately. Treatment should not be delayed by repeatedly using antiarrhythmic drugs. If ventricular tachycardia cannot be successfully converted or recurs frequently after conversion, hypoxia, electrolyte imbalances, and acid-base disturbances should be assessed. Intravenous amiodarone or lidocaine may increase conversion success rate and reduce recurrence after successful cardioversion.
In ventricular fibrillation and flutter, the key to successful resuscitation is timely detection and decisive action. Factors reducing defibrillation success rate include delays, hypoxia, and acidosis. Effective defibrillation should occur within 1 - 3 minutes after ventricular fibrillation onset; the shorter the time, the higher the success rate. For refractory ventricular flutter or fibrillation, intravenous lidocaine or amiodarone may be necessary.
Atrial fibrillation
Synchronized cardioversion is the first-choice treatment for patients with hemodynamic instability or pre-excitation syndrome with rapid ventricular rates. Pre-treatment with amiodarone, ibutilide, or vernakalant can increase conversion success rate. Occasionally, bradycardia may occur after cardioversion, so atropine, isoproterenol, or temporary pacing should be prepared. Anticoagulation before and after cardioversion is required.
Situations unsuitable for emergency cardioversion include:
- Atrial fibrillation with a significantly slow ventricular rate before onset (tachy-brady syndrome)
- Atrial fibrillation due to digitalis toxicity
- Recent arterial embolism or ultrasound-detected atrial thrombus without anticoagulation
Cardioversion may be considered for atrial fibrillation (AF) patients under the following conditions:
- AF history of less than 1 year, with a previous sinus rate not below 60 bpm
- Worsening heart failure or angina after AF difficult to control
- Rapid ventricular rate with poor medication control
- Persistent AF after underlying conditions (hyperthyroidism) controlled
- Persistent AF 3 - 6 months after rheumatic heart disease valve replacement or repair, or 2 - 3 months after congenital heart defect repair
Atrial flutter
Atrial flutter is a tachyarrhythmia that is difficult to control with medication. For patients unresponsive to drugs or experiencing rapid ventricular rates with hemodynamic deterioration (near 1:1 conduction), synchronized direct current cardioversion is recommended, with a success rate of 98% - 100%. It is the preferred treatment for rapid atrial flutter.
Supraventricular tachycardia (SVT)
Most SVT patients do not require cardioversion as the first choice. If other treatments fail to correct SVT and it causes hemodynamic instability due to prolonged episodes, such as hypotension, immediate cardioversion is warranted.
Technical points
The patient should be placed supine on a firm surface (such as a wooden board), with the defibrillator and ECG monitor connected. A lead with a prominent R wave should be chosen for waveform observation, and the defibrillator should be set to synchronized mode.
Common electrode placement is one paddle at the right sternal border in the 2nd and 3rd intercostal space (base of the heart) and the other at the left anterior axillary line in the 5th intercostal space (apex of the heart).
Ensure the paddles are at least 10 cm apart, tightly against the skin, and pressed firmly to maintain low impedance for successful defibrillation.
Before discharge, ensure no one is in contact with the patient, bed, or any connected equipment to avoid electric shock.
Immediately monitor the ECG after cardioversion and closely observe heart rate, rhythm, blood pressure, respiration, and consciousness for 24 hours.
Energy is typically expressed in joules (J), where energy (joules) = power (watts) × time (seconds). The energy level is selected based on arrhythmia type and patient condition.
Table 1 Common energy settings for external transthoracic cardioversion (monophasic waveform)
Complications of cardioversion
Although cardioversion and defibrillation are rapid, safe, and effective treatments for tachyarrhythmias, complications can occur, including:
- Induction of various arrhythmias
- Acute pulmonary edema, hypotension, systemic embolism, and pulmonary embolism
- Myocardial injury (elevated serum cardiac enzymes)
- Skin burns.