Understanding the AED mechanics

The AED is a lightweight piece of equipment that has the following general components: a battery, capacitors and circuitry created to examine the heart rate and indicate whether a shock is recommended. The information is sent out to the device by electrode pads used for both observing and administrating shocks. Modern automated external defibrillator manufacturers had the difficult task of making AEDs trustworthy and easy to use, so that any non-medical person can save a life with their devices. The manufacturers had to trim down the size, cost and maintenance of these essential devices.

Automated external defibrillator improvements

Since the early trials of AEDs, significant changes were made to improve the performance and usage of these life-saving devices. The AEDs are now comfortable to handle and can be easily used by anyone who has received a little first aid training.

The first AEDs were very heavy, weighing over 25 pounds. Today these devices are as light as 3-4 pounds, the weight differing from one model to other. In the early 1980s, when the first models of AEDs were tested, the automated external defibrillator consisted of two electrodes that had to be introduced in the mouth of the patient and one electrode that was placed on the chest of the patient. Today, both pads must be placed on the chest: namely one at the cardiac apex and the other at the right sternal border. These pads will monitor and detect the heart rate and indicate if defibrillation is needed. These pads will perform the defibrillation as well.

The latest AEDs have voice prompts that will guide you through the whole process, telling you when you should apply the pads, when to check for pulse, or when the heart has been defibrillated. There are also devices that have visual prompts available for those who have hearing problems or if you are facing difficulties in hearing the guidance or if you are in a noisy, crowded place.

There are three important technical points that are characteristic to AEDs

Rhythm analysis - In the past, the 'primitive' AEDs were limited to act in response to certain heart rhythms, ignoring heart rates lower than 150 electrical complexes per minute. The electrocardiograph wave amplitude (QRS) should have surpassed 0.15 mm in order for the AED to react. Today, there are multiple means of analyzing the cardiac rhythm. The AEDs are able to detect the ECG rhythm establishing the heart rate, no matter how low it is. The automated external defibrillator will also spot the amplitude of the electrocardiographic wave. Besides these traditional methods, the AED of today will evaluate the QRS according to its undulation, morphology, power spectrum density and time. Any unusual level will be reported and the voice prompter of the AED will indicate what action needs to be taken. These complex checks that evaluate the QRS will be made with a frequency varying from 2 to 4 seconds, so that the levels indicated are as precise as possible. Usually, if abnormal figures are reported, and the frequency of any other QRS is double for at least 3 successive checks, then the AED will recommend and prepare to distribute a shock in order to re-instate the normal heart rate of the patient.

Some ventricular fibrillations are so subtle that they need the most precise detection - it is highly important for the rhythm detector to be as accurate as possible. Modern technology has made it possible for automated external defibrillators to make several analyses, so that the shock is delivered only if there is need for one. Today, the possibility of pinpointing ventricular fibrillation is as precise as 76-96%. Also the AEDs of today are able to detect non-ventricular fibrillation rhythms to preciseness next to 100%. In all cases of sudden cardiac arrest, the precision of first aid equipment is decisive. It is important to pinpoint the exact QRS and to detect the existent abnormalities so that the person using the AED is able to administrate the right type of treatment in the shortest time possible.

Time is the most valuable thing when a sudden cardiac arrest occurs. With each minute that the intervention is delayed, there are 10% less chances for the patient to survive.

So after 10 minutes almost nothing can be done, and the result is that only 5-6% of the cases survive after delayed intervention. What appears to be a sudden heart arrest might not be that at all. This is why it is so important that the AED detects the heart rate exactly, if there is one, and indicates if you are dealing with a sudden cardiac arrest or not. It would be very inappropriate and even lethal to administrate shock to someone who does not need it. Fortunately, you may fully trust the AEDs in this matter, as their level of precision is close to maximum.

Biphasic versus monophasic AEDs

There are two types of defibrillation as far as shock delivery is concerned. The shock may be delivered either in only one direction or in two directions at the same time. When the shock is administrated in one single direction, this is called monophasic defibrillation. The shock concentrates on one point only, and the expected result is the resuscitation of the heart.

On the other hand, there is the biphasic defibrillation that splits in two directions when it comes to the shock delivery. One direction will receive half of the intensity of the shock, while the other half of the intensity will go in the opposite direction. Research has been conducted on dogs to see which method of defibrillation is most effective. The biphasic shock delivery results in a smaller conveyance block than the monophasic defibrillation. The biphasic shock also has fewer ST segment changes than the monophasic shock. However, this applies only to dogs. Other tests, conducted on humans this time, have given different results. Surprisingly, the biphasic shocks have the same effect on humans as the monophasic shocks. The monophasic shocks have equal electro physiologic study induced (EPS induced) and pre-hospital ventricular fibrillation (VF) and ventricular tachycardia (VT) to the biphasic shocks.

On humans, the effect of a biphasic waveform of 115 Joules is equivalent to the effect of a monophasic wave of about 200 Joules. Basically, both the mono and the biphasic shocks have about the same strength.

Still, for practical reasons, the majority of the automatic external defibrillators available on the market today use biphasic shocks. This is because the biphasic shock administration consumes less energy when compared to monophasic defibrillation.

The majority of the AED manufacturers have switched to developing defibrillators that employ biphasic waveforms because the lower quantity of energy utilized also means that the batteries that are providing the energy will last for a longer period of time. An added bonus is that the time needed to fully charge the automated external defibrillator will be shorter.

Otherwise, research made on humans has given very small difference between the two alternatives, so neither of the two is better in quality than the other. However, considering the higher availability of defibrillators based on biphasic waveforms, most institutions use these types of AEDs. Furthermore, considering the fact that the AEDs must be always fully charged and ready to perform, the biphasic option is more recommended for all institutions, buildings and public places, because of the lower amount of energy that they require.

AED Polarity

Limited research has been made in the AED electrode polarity field. The only study made on the subject of polarity was completed by Weaver in 1993. He has proven that the chances of survival are the same regardless of polarity, when using a monophasic device.

All in all, the AED is a great ally for the patient when a sudden heart arrest occurs outside a hospital. By introducing AEDs that are precise, lightweight and easy to use and making them available to the general public, automated external defibrillator manufacturers have made it possible for people with no medical training to trust and use these devices without encountering any obstacles.