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Cardiac Arrest Rhythms

Cardiac arrest is the complete loss of cardiac output due to systolic dyfunction. This state will rapidly lead to cessation of breathing, causing cardiorespiratory arrest.

For initial assessment and managment of cardiac arrest, see the article on basic life support. Remember that CPR is the most important aspect of the management, and that it should be started without delay once cardiac arrest has been confirmed.

There are four cardiac rhythms associated with cardiorespiratory arrest. These are divided into the shockable and non-shockable categories.

Shockable Rhythms

Ventricular Fibrillation (VF)

VF is chaotic depolarisation throughout the myocardium and is the most common initial rhythm in cardiac arrest. Its ECG appearance is characterised as a bizarre and irregular appearance with constantly varying amplitude, morphology and rhythm.

Ventricular Fibrillation

Pulseless Ventricular Tachycardia (VT)

VT is rapid ventricular contraction with the impluse arising from a ventricular focus. It can be associated with a cardiac output, hence only pulseless VT can be included as a cardiac arrest rhythm. The importance of a manual pulse check when VT is displayed is therefore emphasised.

Monomorphic VT is a regular broad complex tachycardia. The impulse is generated by a single point of increased automaticity in one of the ventricles, or a ventricular re-entry circuit. The most common cause of monomorphic VT is myocardial scarring secondary to acute myocardial infarction. It is occasionally quite difficult to differentiate between VT and broad-complex supraventricular tachycardia (SVT) with bundle branch block. This is an essential distinction to make in terms of drug therapy, but it is not significant in the context of a cardiac arrest.

Polymorphic VT has beat-to-beat variations in morphology, often a cyclical change in cardiac axis causing a sine wave pattern of amplitude. This is often referred to as torsades de pointes. It is treated similarly to monomorphic VT (with defibrillation), but with additional IV magnesium sulphate in the context of cardiac arrest.

Monomorphic VTPolymorphic VT

Non-Shockable Rhythms


Asystole is a complete lack of electrical activity. It is rarely seen as a totally ‘flat-line’ however, often there will be some wandering of the baseline, possibly even p-wave activity. If asystole is seen, remember to check electrode contact, and that the defibrillator/monitor is set to monitor through the pads.

Pulseless Electrical Activity (PEA)

If a patient in cardiac arrest shows a rhythm that would normally support a cardiac output, their rhythm would be described as PEA. This could be anything from normal sinus rhythm to third degree AV nodal block with left bundle branch block.


Due to the mode by which defibrillation has its effect (below), it is not an appropriate treatment for these two rhythms. The treatment to terminate non-shockable cardiac arrest centres on good quality CPR, good operation of advanced life support protocol and drug therapy to correct reversible causes.


Defibrillation is the treatment of choice for the two shockable rhythms described above. It involves passing an unsynchronised direct current (DC) shock through the myocardium. Defibrillation forces simultaneous depolarisation of the entire myocardium with the intention of allowing recommencement of normal electrical activity. Early defibrillation in cardiac arrest is associated with a significantly improved prognosis.


The following technique is for the commonly utilised Phillips Heartstart XL defibrillator as shown in the image below. Most defibrillators operate in a similar manner however. 


  • Apply the defibrillation electrodes. One should be placed infraclavicularly in the right parasternal area and the second in the mid axillary line over where the V6 12-lead ECG electrode would be positioned


    • Having briefed the team, briefly stop CPR for a rhythm check. This should take no longer than 5 seconds


      • Recommence CPR. If the rhythm is shockable, the remainder of the team should stand clear and CPR should continue whilst the defibrillator is charged


        • Select the energy level, 150 Joules for a biphasic defibrillator (Selector 1)


          • Charge the defibrillator (Button 2)


            • Stop CPR, confirm any oxygen source is at least 1 metre away from the patient and that no-one is in contact with the patient or the patient's surroundings. Clearly announce "Stand clear, shocking."


              • Deliver the defibrillation shock (Button 3)


                • Immediately recommence CPR for 2 minutes. A further rhythm or pulse check is not required


                Note that the cables and black paddles shown docked to the sides of the machine below are now very seldom used in UK clinical practice. They have been almost universally replaced by self adhesive hands free electrodes. If paddles are used, gel pads should be placed over the paddle sites on the patient before the paddles are applied.

                    Phillips Heartstart XL

                    Further Reading


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