Page created on August 9, 2021. Not updated since.
Cardioversion or electrical conversion refers to giving electrical shock to a patient with an arrhythmia to restore sinus rhythm. It can be done acutely in case of emergencies, or electively in cases where pharmacological treatment of the arrhythmia hasn’t had satisfactory effect.
Cardioversion is, unlike defibrillation, synchronised. This means that the device gives a shock exactly when the cardiac cycle is at the R wave of a QRS complex. This prevents the shock from occurring during the T wave, which could induce the “R on T phenomenon”, which can cause VF.
Cardioversion is indicated for:
- Emergency management of haemodynamically unstable tachyarrhythmias
- Elective management of atrial fibrillation or flutter
Cardioversion may be used for Afib or flutter in cases where the onset is very recent, or if pharmacological treatment has not induced conversion to sinus rhythm. Cardioversion becomes less and less likely to succeed as the duration of the Afib or flutter increases.
Pacemakers (PMs) are used to treat bradyarrhythmias. These devices lie in a subcutaneous “pocket” on the chest and has electrodes going into the heart. The device either continuously paces the heart or paces the heart when it detects that the heart rate is going below a certain threshold, depending on the settings.
The pacemaker can be put into a variety of different modes, each described by a three-letter code describing the function of that mode. The most important indications for pacemakers and the modes used are:
- Sick sinus syndrome (SSS) – AAI mode
- AV block – VDD or DDD mode
- Atrial fibrillation – VVI mode
Not all pacemakers are permanent; temporary pacemakers exist as well. These can pace the heart transcutaneously or transvenously. Temporary pacemakers may be used while the patient is waiting to get a permanent one implanted, or if there is a reversible cause of the pacemaker indication.
Catheter ablation refers to the use of catheters to “burn” (with radiofrequency) or “freeze” (with cryoablation) a part of the heart, with the goal of treating a tachyarrhythmia. The catheter is inserted by the Seldinger technique into a vein (like the femoral vein) and directed toward the heart. Once the catheter is in the heart the electrodes on the tip can be used to detect the area which is responsible for the arrhythmia, after which the same catheter can be used to ablate the area.
Classically (in the 90s), catheter ablation could only be performed in conditions where the culprit was a well-defined anatomical area. Nowadays, modern electrophysiology labs can use modern techniques like 3D mapping to locate and treat arrhythmias with more complex pathomechanisms.
Nowadays, catheter ablation can be used for:
- Most paroxysmal supraventricular tachycardias (AVNRT, AVRT)
- Wolff-Parkinson White syndrome
- Atrial flutter (both typical and atypical)
- Atrial fibrillation
- Monomorphic ventricular tachycardia
- Ventricular fibrillation induced by unifocal premature ventricular beats
For the first two + typical atrial flutter, the cure rate is very high and so catheter ablation is often the first choice compared to pharmacological therapy. Typical atrial flutter originates from a reentry circuit loop in a well-defined anatomical area (the cavotricuspid isthmus), making it an easy target for ablation. Atypical atrial flutter does not occur from a clearly defined area and is more difficult but not impossible to treat.
Atrial fibrillation often originates from or is propagated by myocardium near the pulmonary veins. Pulmonary vein (electrical) isolation, achieved by creating scar tissue around the pulmonary veins with the use of ablation, leads to symptom relief in 60 – 70%.
Implantable cardioverter defibrillators
Implantable cardioverter defibrillators (ICDs) are specialised devices which sense shockable rhythms (ventricular fibrillation, ventricular tachycardia) and immediately and automatically defibrillate the patient. They are used both for primary and for secondary prevention of sudden cardiac death, in patients who are at high risk for it. ICD is indicated for:
- Primary prevention – for patients with symptomatic HF, EF < 35% and a relatively good prognosis (expected to survive for at least 1 year with good functional status)
- Secondary prevention – Patients who have already had VT or VF due to a non-reversible cause
Cardiac resynchronisation therapy
Cardiac resynchronisation therapy (CRT) is used to treat heart failure when there is also a left bundle branch block present. Having a LBBB and heart failure worsens the heart failure because of the desynchronised contraction of the right and the left ventricle. The CRT device is a specialised pacemaker which ensures that the two ventricles contract synchronously.
There exist CRT devices with ICD functionality as well, called CRT-D devices. These are useful because some patients with an indication for CRT also have indication for ICD, and the same patient can’t have two devices.
Unfortunately, not all patients fitted with a CRT device achieve synchronicity. Some patients are just CRT non-responders.