Irish cardiologist Prof Frank Pantridge revolutionised emergency medicine and saved countless lives when he invented the portable defibrillator almost five decades ago. Since the mid-1950s, it was accepted that most cardiac arrests resulted from ventricular fibrillation – a fast, irregular heartbeat caused by abnormal firing of electrical signals in the ventricles of the heart – and could be corrected by delivering an electric shock to the heart. Many hospitals equipped themselves with mains defibrillators, but the Co Down doctor realised that, as the majority of coronary deaths occurred outside the hospital, it would make more sense to take the defibrillator to the patient.

Prof Pantridge produced the first ‘portable’ defibrillator in 1965, weighing in at a hefty 70 kilos and operating from two car batteries in the back of an old ambulance. Today, its high-tech descendants – automated external defibrillators (AEDs) – are as light as one kilo and located throughout communities all over the world, in schools, music venues, at sporting events and shopping centres, even in the home. Every day, numerous lives are being rescued by the prompt delivery of defibrillation, proving that a shock to the system can save your life.

According to the Irish Heart Foundation (IHF), approximately 5,000 people per annum, or 14 people per day, die from cardiac arrest in this country. Three-quarters of these are individuals who are going about their daily business and up to 80% of such incidents occur in the presence of family and friends. 

The terrible reality is that an out-of-hospital cardiac arrest has the worst possible outcome for survival, primarily because resuscitation is often delayed or absent. Resuscitation for a cardiac arrest is time-critical. The Pre-Hospital Emergency Care Council (PHECC) in Ireland estimates that the chances of recovery for those who have suffered a cardiac arrest are reduced by approximately 10% for every minute that passes. 

If a defibrillator is used within three minutes of going into cardiac arrest, however, the patient will have a 70% chance of surviving. Without defibrillation and CPR, only 2% of out-of-hospital cardiac arrest victims survive. 

“The two shockable cardiac arrest rhythms are ventricular fibrillation and pulseless ventricular tachycardia. Rapid defibrillation is essential in these two instances and takes priority over any other treatment or care,” explains Colm Patton, manager of the state-of-the-art interventional cardiology suite (ICS/cardiac cath lab) in the Bon Secours Hospital, Glasnevin, Dublin. “There is no doubt that early defibrillation, whether in hospital or in the community, saves lives. The quicker a cardiac arrest victim can be reached by an AED and trained user, the greater their chance of survival.”

Colm says that the number of AEDs available in public places has risen in recent years, particularly in highly populated locations such as Croke Park, the O2, major shopping centres, airports and many sporting clubs around the country. An increasing number of GP surgeries also have AEDs and staff members are trained in their use. 

“Ideally, all areas of the country would have access to an AED and trained user within the shortest possible time. This would include some of the more remote parts of the country, which may not necessarily be heavily populated but which are often popular destinations for tourists,” he suggests.

How defibrillators work

Defibrillation is based on the knowledge that contraction of the heart, and the resulting circulation, is under the control of the electrical conduction system of the heart. In a normal, healthy heart, each beat begins with an electrical signal from the sinoatrial node (SAN) – a bundle of neurons located in the upper part of the right atrium of the heart and commonly referred to as the ‘heart’s pacemaker’. The electrical impulse from the SAN triggers a sequence of electrical events in the heart to control the orderly sequence of muscle contractions that pump the blood out of the heart. 

A problem with any part of the heart’s electrical system can cause arrhythmias, during which the heart can beat too fast, too slow, or with an irregular rhythm. A defibrillator delivers a therapeutic dose of electrical energy to reset the electrical state of the heart so that it can again beat to a rhythm controlled by its own natural pacemaker cells. Defibrillators can be external, wearable or surgically implanted, depending on the type of device used, but all operate on the same principle. If the monitoring circuit of the defibrillator detects a ventricular arrhythmia, a capacitor is charged with an appropriate level of voltage, either by an operator or automatically. When the shock is discharged – automatically or manually – a current is delivered directly to the heart to interrupt the arrhythmia and restore normal conduction of electrical impulses through the heart.

External defibrillators deliver the current via two electrode pads placed on the bare chest at specific positions. The electric current must pass through the heart to have any effect. The European Resuscitation Council Guidelines for Resuscitation 2010 advise the operator to place one AED pad to the right of the sternum, below the clavicle, and the other in the left mid-axillary line (approximately where the V6 electrocardiogram [ECG] electrode is positioned). It is important that this pad is placed sufficiently laterally and is clear of any breast tissue.

Defibrillators in hospital

Patients who have a cardiac arrest in hospital should, if it is indicated, be defibrillated as quickly as possible – ideally within three minutes. The number and location of external defibrillators in Irish hospitals varies, although most hospital wards and other clinical areas have access to defibrillators with both manual and automatic modes. Manual external defibrillators are used in conjunction with (or more often have inbuilt) ECG readers, which the healthcare provider uses to diagnose a cardiac condition. Automatic defibrillators enable first responders, including staff without ECG interpretation skills, to defibrillate the patient while awaiting the arrival of the cardiac arrest team who can then select and use the manual mode.

At Bon Secours, Glasnevin, a total of 15 defibrillators are located strategically throughout the hospital. “We have between 200 to 220 patients at any one time. Every patient area has easy access to a defibrillator while the cardiac and more acute areas have dedicated defibrillators. We feel this is a sufficient number,” Colm Patton tells Modern Medicine.

With regard to training, he points out that all healthcare staff at the hospital who have daily interactions with patients receive training in electrical defibrillation using AEDs. In addition to this, healthcare staff working in cardiac or potentially more acute areas receive advanced cardiac life support (ACLS) training. The training is recertified every two years.

Healthcare assistants and porters at the Glasnevin hospital are also trained in the use of AEDs, with a view to providing defibrillation to the victim of a cardiac arrest as quickly as possible. This increases the victim’s chance of survival whether they are an inpatient, a staff member or a visitor in the car park. Colm adds that training is based on the American Heart Association’s (AHA) guidelines and protocols.

Defibrillators in the community

All AEDs analyse the victim’s ECG rhythm and determine the need for a shock. The semi-automatic AED indicates the need for a shock, which is delivered by the operator by pressing a button, while the fully automatic AED administers the shock without the need for intervention by the operator. 

These devices are extremely useful in the community setting because they are simple to use and can be operated by anyone if the instructions are followed. Simplicity of operation is a key feature: controls are kept to a minimum, while voice and visual prompts guide rescuers. Some AEDs combine guidance for defibrillation with guidance for the delivery of optimal chest compressions. Once the machine is turned on, the rescuer will be prompted to apply the two electrode pads provided with the AED to the victim’s bare chest. Then the AED will begin to monitor the victim’s heart rhythm. If a ‘shockable’ rhythm is detected, the machine will charge itself and instruct the rescuer to press the shock button and/or stand clear of the victim. The AED will not allow a shock to be delivered unless it recognises ventricular fibrillation.

A joint initiative between PHECC, the HSE National Ambulance Service and the IHF was launched last year to compile Ireland’s first national defibrillator register. This will assist in reducing deaths from cardiac arrest by informing the public of the exact location of the defibrillator in their area. The register pilot has commenced in the Midlands region, as there is already a register in existence there.

“Survival rates following cardiac arrest have improved from 1-5% in the past few years, however, having a register of defibrillators will further improve these survival rates. Many communities and companies may have a defibrillator available for use that local people don’t know about. The defibrillator register aims to rectify this,” a PHECC representative stated.

Community-based doctors are also being recruited to manage life-threatening emergencies though the Medical Emergency Responders: Integration and Training (MERIT) project run by UCD’s Centre for Emergency Medical Science. 

Future advances

Defibrillator technology is advancing rapidly. AED interaction with the rescuer through voice prompts is now established and future technology may enable more specific instructions to be given by voice prompt. The evolving ability of defibrillators to assess the rhythm while CPR is in progress is an important advance and enables rescuers to assess the rhythm without interrupting external chest compressions. In the future, waveform analysis may also enable the defibrillator to calculate the optimal time at which to give a shock.