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 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 to try and 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 momentarily stun the heart and thus allowing a normal sinus rhythm to kick in via the hearts normal electricity centre i.e. the sinoatrial node.

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

At the end of the 18th century Prevost and Batelli, two physiologists 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 1950's 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 1980's the biphasic waveform was discovered. This provided a shock at lower levels of energy which were just as efficacious as monophasic shocks.

Also see - 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 1 cycle every 10 milliseconds.
• They are associated with less 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% where as with biphasic shocks this increases to 90%.²
• However, this efficacy of biphasic defibrillators over monophasic defibrillators has not been consistently reported. The TIMBER trial (Transthoracic incremental monophasic versus biphasic defibrillation by emergency responders) failed to detect any differences in survival using either systems nor in the likelihood of terminating VF.³

Types of defibrillators

1. Automated external defibrillators (AED)⁴
   • These are useful as its use does not require special medical training
   • They are found in public places e.g. offices, airport, train stations, shopping centres
   • They analyse the heart rhythm and then charge and deliver a shock if appropriate
   • However, they can not be overridden manually and can take 10-20 seconds to determine arrhythmias
2. Semi-automated AED
   • This is similar to AED but can be overridden and usually has 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 no mess with gels
• Paddles require at least 25lbs of pressure which is not needed with adhesive electrodes
• Adhesive electrodes also allow good ECG trace with out 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 CPR algorithm recommends single shocks started at and repeated at 360J¹
• Biphasic - the CPR algorithm recommends shocks initially of 150-200J and subsequent shocks of 150-360J¹

The BIPHASIC trial in 2007 compared lower fixed (150,150, 150J) and gradually increasing energy (200,300, 360J) 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 1970's - although research had been going on for almost a decade prior to this.
ICDs can

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

They continuously monitor the patients 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 trial (AVID) 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 randomized trials (The Canadian Implantable Defibrillator trial and Cardiac arrest study Hamburg) 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%.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 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 under way.

Uses

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

• Cardioversion is used for rhythm control
• Not all cardioversion is successful and at 1 year 50% redevelop atrial fibrillation¹¹
• 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 3 weeks before and at least 4 weeks after¹¹
• Some centres use transoesophageal echocardiogram during the procedure to look for thrombus - although a few patients still develop TED despite negative results¹²
• Sotalol or amiodarone can be given for at least 4 weeks prior to cardioversion in patients who have had a previous failure to cardiovert or early recurrence of atrial fibrillation¹¹
• 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
• Majority of cardioversions are elective procedures; but some are performed when patients are acutely unwell with the tachycardia e.g. chest pain, breathlessness
• Turn on machine and attach adhesive electrodes
• Choose energy level
• Get a clearly visible trace on the monitor e.g. using lead II
• Hit "synch" button - usually a blip or dot appears on the monitor marking each QRS complex
• Monophasic - begin with 100 J for atrial fibrillation and 50J for atrial tachycardia or ventricular tachycardia (and escalate if necessary; up to 300J)
• Biphasic - begin with 50J and escalate as follows if necessary - 100, 150J
• Charge
• Ensure all clear around the bed
• Discharge or shock - there may be a 1-2 second delay as the machine ensures synchronization
• 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