Defibrillation and Cardioversion

• Defibrillation - is the treatment for immediately life-threatening arrhythmias with which the patient does not have a pulse, i.e. ventricular fibrillation (VF) or pulseless ventricular tachycardia (VT).
• Cardioversion - is any process that aims to convert an arrhythmia back to sinus rhythm. Electrical cardioversion is used when the patient has a pulse but is either unstable, or chemical cardioversion has failed or is unlikely to be successful. These scenarios may be associated with chest pain, pulmonary oedema, syncope or hypotension. It is also used in less urgent cases, e.g. atrial fibrillation (AF) to try to revert the rhythm back to sinus. This article only deals with electrical cardioversion.

The aim in both is to deliver electrical energy to the heart to stun the heart momentarily and thus allow a normal sinus rhythm to kick in via the heart's normal electricity centre, i.e. the sinoatrial node.

1. Defibrillation
2. Implantable cardiac defibrillators (ICDs)
3. Cardioversion

At the end of the 18th century two physiologists, Prévost and Batelli, performed shock experiments on the hearts of dogs. They applied electrical shocks and discovered that small shocks put the dogs' hearts in to VF and this was successfully reversed with a larger shock. It was first used in humans by Claude Beck, a cardiothoracic surgeon - on a 14 year-old boy undergoing cardiothoracic surgery for congenital heart disease. Electrodes were placed across the open heart. Closed chest defibrillation was not discovered until the 1950s in Russia. But it was not until 1959 that Bernard Lown designed the modern-day monophasic defibrillator. This is based on the charging of capacitors and then delivering of a shock by paddles over a few milliseconds. In the 1980s the biphasic waveform was discovered. This provided a shock at lower levels of energy which were just as efficacious as monophasic shocks.

Also see separate article Adult Cardiopulmonary Arrest, dealing with adult advanced life support¹

Differences between monophasic and biphasic systems

• In monophasic systems, the current travels only in one direction - from one paddle to the other.
• In biphasic systems, the current travels towards the positive paddle and then reverses and goes back; this occurs several times.²
• Biphasic shocks deliver one cycle every 10 milliseconds.
• They are associated with fewer burns and less myocardial damage.
• With monophasic shocks, the rate of first shock success in cardiac arrests due to a shockable rhythm is only 60%, whereas with biphasic shocks, this increases to 90%.²
• However, this efficacy of biphasic defibrillators over monophasic defibrillators has not been consistently reported. The Transthoracic Incremental Monophasic Versus Biphasic Defibrillation by Emergency Responders (TIMBER) trial failed to detect any differences in survival using either system. Also the same study failed to find any differences between the defibrillators in terminating VF.³

Types of defibrillators

1. Automated external defibrillators (AEDs)⁴
   • These are useful, as their use does not require special medical training
   • They are found in public places, e.g. offices, airports, train stations, shopping centres
   • They analyse the heart rhythm and then charge and deliver a shock if appropriate
   • However, they cannot be overridden manually and can take 10-20 seconds to determine arrhythmias
2. Semi-automated AEDs
   • These are similar to AEDs but can be overridden and usually have an ECG display
   • They tend to be used by paramedics
   • They also have the ability to pace
3. Standard defibrillators with monitor - may be monophasic or biphasic
4. Transvenous or implanted

Paddles versus adhesive patches

• Paddles were originally used but their use is being superseded by adhesive patches
• Adhesive patches are placed most commonly anterio-apically - anterior patch goes under the right clavicle and the apical patch is placed at the apex
• Adhesive electrodes are better, as they stick to the chest wall, so there is no mess with gels
• Paddles require at least 25 lbs of pressure, which is not needed with adhesive electrodes
• Adhesive electrodes also allow good ECG trace without interference
• They are also safer, as no operator required - although, before discharging a shock, it is important to ensure everyone is clear of the patient

Energy levels for defibrillation (usually written on machine)

• Monophasic - the cardiopulmonary resuscitation (CPR) algorithm recommends single shocks started at and repeated at 360 J¹
• Biphasic - the CPR algorithm recommends shocks initially of 150-200 J and subsequent shocks of 150-360 J¹

The Biphasic Trial in 2007 compared lower fixed (150, 150, 150 J) and gradually increasing energy (200, 300, 360 J) shocks for out-of-hospital cardiac arrests.⁵ Escalating energy shocks were associated with more frequent conversion and termination of VF as opposed to low-level fixed shocks. This applied to patients who remained in VF after the first shock.

ICDs were discovered in the 1970s, although research had been going on for almost a decade prior to this.
ICDs can

• Sense atrial and ventricular signals
• Detect and thus classify sensed signals
• Provide therapy to terminate VF/VT
• Pace and/or perform cardiac resynchronisation

They continuously monitor the patient's heart rhythm and then deliver a shock if there is an abnormal rhythm, usually VF or VT. They monitor and record the heart rhythm throughout an arrhythmia.

Sudden cardiac death (SCD) occurs in patients with cardiac conditions, including QT prolongation and left ventricular dysfunction. Mortality from SCD is highest in New York Heart Association (NYHA) class II onwards, i.e. those with well-compensated heart failure are at risk.⁶

Primary prevention of SCD

The antiarrhythmics versus ICD (AVID) trial reported that survival was greater for patients inserted with an ICD after VF, VT with syncope or sustained VT with a low ejection fraction, compared with drug treatments (mostly amiodarone).This trial along with two other randomised trials - the Canadian Implantable Defibrillator Study (CIDS) and the Cardiac Arrest Study Hamburg (CASH) - revealed an overall 15-23% reduction in mortality in patients with an ICD.⁶ Similar conclusions have been reached in trials looking at the benefits of ICD in ischaemic cardiomyopathy (with an ejection fraction <35%).⁶

Problems with ICDs

• Firing continuously and inappropriately - occurs in up to 25% of patients.⁸
• This is a medical emergency, as it may lead to another life-threatening arrhythmia. The battery may also run out and shocks usually cause a lot of discomfort to patients.
• Patients who receive more than one shock or are unwell need to be evaluated as for ischaemic heart disease, as they may have had an ischaemic event or ongoing arrhythmias. Placing a magnet over the ICD causes it to stop functioning - only advisable in a hospital setting with cardiac monitoring.
• Other problems can occur during the placement of an ICD, e.g. pneumothorax, infection and cardiac tamponade.

Practical issues surrounding ICDs

• They are usually placed in the left infraclavicular region and are palpable (rarely they are located in the abdomen or right infraclavicular regions)
• Patients are followed up every 1-6 months which includes interrogation and testing of the ICD device
• ICDs treat arrhythmias but do not prevent them from occurring

Cost-effectiveness

There has been much concern that ICD therapy is not cost-effective. Each unit costs tens of thousands of pounds. Further work in this area is currently underway.

Uses

• Decompensated rapid AF with a rapid ventricular response, e.g. hypotensive patient, not responding to medical therapy⁹
• VT with a pulse
• Supraventricular tachycardias including AF; not acutely urgent¹⁰

In cardioversion the shock has to be properly timed, so that it does not occur during the vulnerable period, i.e. during the T wave. If this occurs then VT can be triggered.

Atrial fibrillation

• Cardioversion is used for rhythm control
• Not all cardioversion is successful and, at one year, 50% redevelop AF¹¹
• Medical treatments and cardioversion are of similar efficacy (unless permanent AF)
• Cardioversion of AF is associated with increased risk of thromboembolic disease (TED); thus, anticoagulation is required for at least three weeks before and at least four weeks afterwards¹¹
• Some centres use transoesophageal echocardiogram during the procedure in order to look for thrombus, although a few patients still develop TED despite negative results¹²
• Sotalol or amiodarone can be given for at least four weeks prior to cardioversion in patients who have had a previous failure to cardiovert or early recurrence of AF¹¹
• Others advocate the use of medications such as sotalol and amiodarone to maintain sinus rhythm after cardioversion¹³

How to cardiovert

• Cardioversions are performed under general anaesthesia or sedation
• The majority of cardioversions are elective procedures; however, some are performed when patients are acutely unwell with tachycardia, e.g. chest pain, breathlessness
• Turn on the machine and attach adhesive electrodes (efficacy may be better with anterior-posterior electrodes)¹⁴
• Choose the energy level
• Get a clearly visible trace on the monitor, e.g. using lead II
• Hit the 'synch' button - usually a blip or dot appears on the monitor, marking each QRS complex
• Higher starting energy is associated with better success and fewer shocks¹⁴
• Monophasic - begin with 300 - 360 J for AF and lower for atrial tachycardia or VT (and escalate if necessary; up to 300 J)¹⁴
• Biphasic - begin with 200 J¹⁴
• Charge
• Ensure all is clear around the bed
• Discharge or shock - there may be a 1 to 2-second delay as the machine ensures synchronisation
• Check rhythm after the shock - if sinus rhythm then stop; if not, then you may need to deliver another shock at higher energy levels
• Look for burns afterwards and obtain a 12-lead ECG

1. Adult advanced life support guidelines, Resuscitation Council UK (2005)
2. Adgey AA, Spence MS, Walsh SJ; Theory and practice of defibrillation: (2) defibrillation for ventricular fibrillation. Heart. 2005 Jan;91(1):118
3. Kudenchuk PJ, Cobb LA, Copass MK, et al; Transthoracic incremental monophasic versus biphasic defibrillation by emergency responders (TIMBER): a randomized comparison of monophasic with biphasic waveform ascending energy defibrillation for the resuscitation of out Circulation. 2006 Nov 7;114(19):2010 [abstract]
4. Liddle R, Davies CS, Colquhoun M, et al; ABC of resuscitation. The automated external defibrillator. BMJ. 2003 Nov 22;327(7425):1216
5. Stiell IG, Walker RG, Nesbitt LP, et al; BIPHASIC Trial: a randomized comparison of fixed lower versus escalating higher energy levels for defibrillation in out Circulation. 2007 Mar 27;115(12):1511 [abstract]
6. Cesario DA, Dec GW; Implantable cardioverter J Am Coll Cardiol. 2006 Apr 18;47(8):1507 [abstract]
7. Arrhythmia - implantable cardioverter defibrillators, NICE Technology Appraisal (January 2006)
8. Kadish A, Mehra M; Heart failure devices: implantable cardioverter Circulation. 2005 Jun 21;111(24):3327
9. Jahangiri M, Weir G, Mandal K, et al; Current strategies in the management of atrial fibrillation. Ann Thorac Surg. 2006 Jul;82(1):357 [abstract]
10. Hebbar AK, Hueston WJ; Management of common arrhythmias: Part I. Supraventricular arrhythmias. Am Fam Physician. 2002 Jun 15;65(12):2479 [abstract]
11. Sulke N, Sayers F, Lip GY; Rhythm control and cardioversion. Heart. 2007 Jan;93(1):29
12. Adgey AA, Walsh SJ; Theory and practice of defibrillation: (1) Atrial fibrillation and DC conversion. Heart. 2004 Dec;90(12):1493
13. McNamara RL, Tamariz LJ, Segal JB, et al; Management of atrial fibrillation: review of the evidence for the role of pharmacologic therapy, electrical cardioversion, and echocardiography. Ann Intern Med. 2003 Dec 16;139(12):1018 [abstract]
14. Reiffel JA; Cardioversion for atrial fibrillation: treatment options and advances. Pacing Clin Electrophysiol. 2009 Aug;32(8):1073-84. [abstract]

• Atrial Fibrillation: The management of atrial fibrillation, NICE Clinical Guideline (Jun 2006)