Question 6

College Answer

Cardioversion is the delivery of energy that is synchronised with the QRS complex in an attempt to revert an abnormal rhythm.  Defibrillation is the non-synchronised (ie. random) delivery of energy and is used in unstable rhythms (eg. pulseless VT or VF).

The potential benefits (correction of the underlying rhythm) need to be balanced against the potential risks, especially in the critically ill, and should not be undertaken lightly.

Success rates vary with the characteristics of the underlying rhythm (highest in SVT and atrial flutter, and lower in AF [inversely related to left atrial size, duration of AF, and precipitating cause still being present {eg. hyperadrenegic state secondary to sepsis}]), and the energy delivered (often deliver 150 to 200J biphasic, lower with atrial flutter). Potential risks include:

•    Failure    of    cardioversion    (insufficient    energy    delivered,    technical     factors, misdiagnosis of rhythm [eg. sinus tachycardia!])

•    Requirement for some degree of sedation and analgesia; potential for awareness

•    Hypotension

•    Myocardial damage (ST changes and myocardial dysfunction usually short term; minimal elevation in troponins)

•    Arrhythmias (eg. SVT, non-sustained VT, rarely more malignant [more likely if digoxin toxic and hypokalaemic])

•    Conduction  abnormalities  (bradycardia,  and  heart  block  common;  occasionally needing temporary pacing)

•    Embolisation (especially if AF present > 48 hrs and not anticoagulated; strategy may include TOE)

•    Damage to permanent pacemaker (if not careful with electrode placement)

•    Others include pulmonary oedema, skin burns and risk of sparking/fire.

Discussion

This is a weird question. Sometimes, one cannot help but cardiovert somebody. For instance, the most recent guidelines from the ARC recommend that synchronised cardioversion be first-line therapy for any hemodynamically unstable tachyarrhythmia. One may as well ask the candidates to critically evaluate micturition; to be sure alternatives exist but it is really the well-established standard.

Anyway, this question could be interpreted slightly differently.

Taking into account the inevitability of cardioversion in certain situations, one could critically evaluate the risks and benefits of it with a focus on the semi-elective patient, in whom pharmacological cardioversion is an option.

Rationale for the use of direct current to convert cardiac rhythm

Cardioversion is the use of a short ( 200msec) discharge of direct current which is synchronised with the QRS complexes, so as to convert an abnormal rhythm to sinus rhythm. It has not always been direct current (Claude Beck's 1947 model defibrillator used AC straight from the wall outlet, and generally only Soviet defibrillators were biphasic DC until the 1960s). Ultimately, direct current was found to be safer: a larger amount of energy could be delivered in a short period of time. The mechanism remains incompletely understood. Various groups have suggested various explanations. Direct current travels around the cells as well as through them;  the effect is to change the transmembrane electrical potentials. One might expect all the cells to depolarise because all of the voltage-gated ion channels suddenly open, but the effect does not seem uniform: some cells depolarise and others hyperpolarise. In any case, this disrupts the normal propagation of action potentials. In this manner, DC current depolarises a sufficiently large amount of cardiac tissue, putting it into a refractory period and preventing the propagation of a reentrant current (which then dies away).

Advantages of DC cardioversion over chemical cardioversion

• Electrical cardioversion is immediately effective (when it is effective)
• It may be life-saving in a haemodynamically unstable arrhythmia
• Automated devices ensure synchronisation so that  R-on-T phenomena should not occur
• There are relatively few long-term side effects associated with it (i.e. unlike long-term amiodarone it won't give you pulmonary fibrosis) 

Disadvantages of electrical cardioversion in comparison to antiarrhythmic drugs

• It is not a long-term strategy: if the underlying pathology has not been fixed, reversion to sinus rhythm will not be sustained.
• There is the risk of arterial thromboembolism, although this is probably much the same risk as with pharmacological cardioversion, or with spontaneous reversion to sinus rhythm for that matter
• It requires a sedated patient.
• It will result in a raised troponin, which may obscure the presence of genuine myocardial infarction.
• It requires the accurate diagnosis of rhythm

Accepted applications of electrical cardioversion

• Atrial flutter (good chance of success)
• Supraventricular tachycardia (good chance of success)
• VT
• Atrial fibrillation (poor chance of success, especially if the AF has been going on for a very long time).

Potential complications of electrical cardioversion

• Ventricular fibrillation may develop due to lack of synchronization.
• Sudden restoration of sinus rhythm can dislodge intracardiac thrombi.
• Transient left bundle branch block may develop. In fact any sort of conduction block may develop, including complete heart block.
• Transient left ventricular systolic dysfunction may develop. In fact you could cause myocardial damage.
• There may be skin burns due to incorrect use of the equipment.
• If the patient has a pacemaker, you may damage it with the direct current. 
• If the patient has digoxin toxicity, one may induce VF in such a patient.

Mayr, Andreas, et al. "Effectiveness of direct-current cardioversion for treatment of supraventricular tachyarrhythmias, in particular atrial fibrillation, in surgical intensive care patients*." Critical care medicine 31.2 (2003): 401-405.

Trappe, Hans-Joachim, Bodo Brandts, and Peter Weismueller. "Arrhythmias in the intensive care patient." Current opinion in critical care 9.5 (2003): 345-355.