Automated external defibrillators
Automated external defibrillators (AEDs) are a proved method of reducing morbidity and mortality from acute myocardial infarction (heart attack). The most common lethal arrhythmias in adult cardiac arrest can be cured by electricity; early defibrillation is a nationally recognized standard of care. To provide a realistic chance of survival, defibrillation must be available soon after cardiac arrest. An automated external defibrillator (AED) detects and treats cardiac arrest due to the
cardiac arrhythmias, ventricular fibrillation (VF) and ventricular tachycardia (VT). Uncorrected, these arrhythmias rapidly lead to irreversible brain damage and death.
The heart has four chambers with one-way flaps called valves between the upper and lower chambers. The atria are the upper chambers and they receive blood that is being returned to the heart. The right atrium receives blood with little oxygen in it because the blood has already circulated throughout the body delivering oxygen and nutrients. The left atrium fills with newly oxygenated blood returning from the lungs. When the atria pump (contract), they push the blood through valves into the relaxed ventricles. When the ventricles contract, the right ventricle pumps blood into the lungs. The left ventricle pumps blood through the aortic valve to the body, including the heart (through coronary arteries). It is the pressure of the blood reflected on the walls of the arteries which is felt as a pulse. A normal adult healthy heart beats rhythmically at a rate of about 60 to 100 beats per minute when at rest. During strenuous exercise, the heart can increase the amount of blood it pumps up to four times the amount it pumps at rest within only a matter of seconds. The source that drives this mechanical activity is electrical stimulation. This continuous cycle of synchronised contractions is driven by the heart’s electrical system.
Some rhythms which become disrupted (called 'arrhythmias', or 'dysrhythmias') can be fatal and require urgent defibrillation. Defibrillation is achieved by the application of an electrical charge or 'shock' to the heart from a purpose designed device. These devices are associated with heart monitors and are available from the sources mentioned above, and in addition small portable defibrillator devices are now available for more general use by first aiders through the Public Access Defibrillation Program. There are three heart rhythms which are potentially fatal; ventricular tachycardia (VT), ventricular fibrillation (VF), and asystole. It is VF and VT which the automated external defibrillator may successfully treat by causing the arrhythmia to revert to a more normal rhythm. The most common cause of SCA is VF, which is caused by abnormal, erratic and very fast electrical activity in the heart. Due to this 'chaotic' activity the heart cannot pump blood effectively. Defibrillation is the delivery of an electrical shock to the heart, and is the only method of correcting ventricular fibrillation. The automated external defibrillator carries out an analysis of the heart rhythm and advises whether a shock is necessary, but relies on the user to press the appropriate buttons. The way that the AED works is that it monitors the electrical activity of the heart through the electrode pads. The AED accurately classifies the rhythm as 'Shockable' or 'Non-Shockable', and uses voice and/or a display prompts advising you of what action to take. If the AED indicates that a shock is required, follow the prompts and sometimes the shock will re-coordinate the heart’s natural pacemaker, and normal heart function will be restored. Sometimes it may require more than one shock to restore normal heart function, but multiple shocks do not guarantee the heart will restart. However, the sooner that defibrillation is started, the greater the chance of a positive outcome. Some AED's have voice or visual prompts, and some have both. In some cases the AED may have a screen that display the rhythm, and this can be used by operators that have been highly trained in rhythm recognition. Most modern AEDs have the ability to keep records of events electronically. This may be kept in the form of a PC data card that can be used to produce a report.
In ventricular fibrillation the electrical activity of the heart becomes chaotic preventing the ventricle from effectively pumping blood. In ventricular tachycardia the heart beats too fast to effectively pump blood. Frequently, ventricular tachycardia leads to ventricular fibrillation.
Defibrillation, by applying a shock to the entire heart muscle, uniformly clears the heart's electrical system, hopefully allowing it to resynchronise. Unlike regular defibrillators, an automated external defibrillator automatically determines if a shock is indicated, and automatically selects and delivers the appropriate energy level. The user cannot override a "no shock" advisory by an AED. It is called external because the operator applies the electrode pads to the patient's chest, unlike internal defibrillators with electrodes surgically placed inside a patient's body.
Once the pads are attached to the patient, the machine diagnoses the heart rhythm and determines if a shock is needed to treat ventricular tachycardia or ventricular fibrillation. If the machine determines that a shock is necessary, it will charge in preparation to deliver the shock. When charged, the machine instructs the user to ensure no one is touching the patient and then to press a button to actually deliver the shock.
After the shock is delivered, the machine monitors the patient's rhythm to determine if further shocks are necessary. Unlike some more sophisticated defibrillators used by health professionals, if the heart rate is too slow, these devices do not typically pace the heart to make it beat faster.
There are two main types of AEDs on the market today: semi-automatic and fully-automatic. Semi-Automatic AEDs prompt the user to stand clear and then push a shock button to defibrillate. Fully-automatic units sound a 'stand clear' alarm and then deliver the shock automatically without the user having to push the button.