Defibrillators and Cardioverters – By Dr. Gnana Sankaralingam
Defibrillator is a device that gives a high energy shock to the heart of someone who is in a state of cardiac arrest. It is the standard treatment for Ventricular fibrillation (VF) and pulseless Ventricular Tachycardia (VT). It does by completely depolarising myocardium producing momentary stoppage of heart beat (asystole), providing an opportunity for the natrural pacemaker of the heart to resume more coordinated contractile activity. Defibrillation is not indicated if patient is conscious or has recordable pulse and also if the heart has completely stopped (asystole) or gone into a state of pulseless electrical activity. Improperly given shock can cause dangerous arrythmias. Heart which is in asystole (flat line in ECG) cannot be started by defibrillation. It should be first treated by giving cardio-pulmonary resuscitation (CPR) and medications and only if it converts into shockable rhythm (fibrillatory or coarse waves in ECG), cardioversion or defibrillation should be applied.
Defibrillators can be external which could be manual or automated and implantable, depending on the type of device needed. Modern defibrillators are often automated and come with clear voice instructions, making them accessible to the lay persons. Automated external defibrillator (AED) analyses the rhythm of the heart and delivers the shocks as necessary, while for manual ones trained personnel have to assess the rhythm and adjust the shock settings as necessary. Surgically Implanted devices, a combination of cardioverter and defibrillator (ICD), continuously monitor and regularly respond to dangerous arrythmias in patients at risk. Wearable cardioverter – defibrillator is a portable external device worn by patients which monitors the patient throughout the day and can automatically deliver a biphasic shock if VF or VT is sensed. It is indicated for those who do not need implanted ICD immediately. Many of them can also perform the pacemaking function.
Automated external defibrillator (AED) has a micro-processor which analyses heart rhythm of the patient from the ECG signal. All AEDs communicate directions by displaying messages on a screen and voice commands giving step by step directions how to proceed. In fully automated one, machine will automatically charge and deliver the shock. In semi-automated one, operator has to press analysis control button to start and when machine advises that a shock is needed, press the shock control button to deliver it. In both devices, electrical shock is delivered through two adhesive electrode pads placed on the chest one on right border of breast bone below collar bone and other over apex of heart on the left, which act to transmit the rhythm and deliver the shock. Precautions need to be taken before applying AEDs, like avoiding any pacemakers or ICDs implanted, removing medicine patches and drying the chest if it is wet to prevent electric short circuits. Having AEDs on site is essential to ensure that quick action is taken in the event of an urgent necessity.
Cardioverter is a device used to administer shock to bring an abnormal heart rhythm to a normal rhythm. It is an energy storage capacitor-condenser which delivers direct current shock (DC cardioverter). Synchronised electrical cardioversion is an electric shock delivered in synchrony to the cardiac cycle. It uses therapeutic dose of electric current to produce low energy shocks to heart at a specific moment in cardiac cycle (corresponding to R wave of QRS complex), unlike defibrillator which gives shocks at random moment of cardiac cycle. Timing of shock to the R wave prevents the delivery of shock during the vulnerable period (relative refractory period) of the cardiac cycle which could induce ventricular fibrillation. It is an elective procedure unless patient is haemodynamically unstable or unconscious, when the shock is given immediately upon confirmation of the arrythmia. Cardioversion can also be done using anti-arrythmic drugs (pharmacological method).
Electrical cardioversion is usually done as an elective procedure, but can also be done as an emergency procedure. It is necessary if the arrythmia is causing significant symptoms or if it is impacting on the ability of the heart in functioning properly. It is performed under heavy sedation or short acting general anaesthetic. It is contraindicated in patients with clots in the atrium, and those undergoing it must take Warfarin for at least 3 to 4 weeks prior to it. Two electrode pads are placed either both along front of chest or one in front and one at the back and shock is given. Effectiveness of it depends on several important factors including type of rhythm being treated and overall health of the patient. Some may need to have the procedure repeated. Complications are: Stroke which is rare if INR levels are within therapeutic range of 2 to 3; Bradycardia where Intravenous medicines or external pacemaker may be required; Ventricular Tachycardia which is reversed by a further shock. Patients are followed up using anti-arrythmic medications to maintain normal rhythm.
Pharmacological cardioversion by anti-arrythmic drugs is useful for those arrythmias of recent onset. There are various agents that can be effective. Class one – sodium Channel blockers which slow the conduction by blocking sodium channel such as Procainamide and Quinidine; Class two – Beta blockers which inhibit the depolarisation of both Sino-Atrial and Atrio-Ventricular nodes to slow the heart such as Bisoprolol and Metaprolol; Class three – potassium channel blockers which prolong repolarisation by blocking outward potassium current such as Amiodorone and Sotalol; Class four – Calcium channel blockers which inhibit excitability of Sino-Atrial and Atrio-Ventricular nodes to slow down conduction such as Verapamil and Diltiazem. In addition, for Atrial flutter and resistant supraventricular tachycardia, Adenosine could be given to restore sinus rhythm. Manual procedures could be resorted to as first line by performing manouvres to stimulate nerves to increase vagal tone to slow down the rate of heart, such as Valsalva manouvre and Carotid sinus massage.
ICD technology has evolved from the epicardial and transvenous era to extravascular innovation that avoid the complications such as perforating the heart, obstruction to veins, infection and valve damage (Tricuspid regurgitation). ICDs are implanted thorugh a cut in skin below the collar bone similar to pacemakers. Most of them are extra-thoracic (outside the chest) which circumvents the need to enter the vasculature or the heart. In subcutanoeus device (SICD), wires from generator are placed in front of the sternum (breast bone) while in extra-vascular device (EV-ICD), wires from generator are tunneled and placed behind the sternum. SCID needs higher energy thresholds while EV-ICD due to the placement in the subcostal space closer to the heart without sternum as a barrier, needs lower energy requirements, which enables the device to be smaller with long battery life and frequent generator replacement. Both SICD and EV-ICD do not have the capability to deliver pacing of the heart. SICD is the most commonly used of the ICDs despite these shortcomings. ICDs with wire inserted through a vein and attached to the heart, could also function as pacemakers.
Cardiac ablation is minimally invasive procedure used to treat irregular heart rhythm. It involves the use of thin flexible tubes called catheters which are threaded into the heart to deliver energy which could be heat (radio frequency ablation) or cold (cryo-ablation). It creates small scars which block the faulty electrical signals causing the irregular heart beat. It is particularly effective in atrial fibrillation (AF), and also in Atrial flutter and Wolf-Parkinson-White syndrome where it targets specific areas to terminate or modify faulty electrical pathway of the heart responsible for abnormal rhythm. It is recommended for those whose arrythmias do not respond to medicines. There is small risk of damage to the normal pathway of the heart and if it happens, pacemaker will be fitted. It may take 8 to 10 weeks for restoration, but if no improvement another ablation is needed.
