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12 March, 2012 | Cardiology Equipment, Emergency Equipment, medical equipment

Defibrillator -

Defibrillators – A Measure For Life Saving

The most common form of treatment for any form of life threatening cardiac arrhythmias and pulsless tachycardia is defibrillation, this is the process of giving a measured amount of electrical energy to the patients heart by way of a defibrillator. It is this electrical current that will halt the arrhythmia and thereby enabling the heart to return to a normal rhythm. There are three forms of systems, either implanted, transvenous or external. With most external systems the current is automated , this enables the untrained bystander or layman to utilize them successfully.

Cardiopulmonary resuscitation or the more common term CPR is seen as a temporary treatment that will maintain a minimum flow of blood to the brain. By using a defibrillator the patient will receive a controlled electrical shock that will take the heart from a threatening rhythm such as VF or ventricular fibrillation and restore the normal paced rhythms. The system will retain the voltage and pass it to the patient by way of 2 paddles, or electrodes that are placed on the chest, depending on the system.

Up until the early 1950’s the only way that the heart was shocked , was when the patients chest was open and the electrodes were placed on either side of the exposed heart, this method used an alternating current (AC) of 300 or greater volts. AC current in excess of 1000 volts was applied by using electrode to the closed chest of the patient – this method was introduce late 1950’s.

It was Bernard Lown who in 1959 began researching another technique that charged capacitors up to almost 1000 volts, this having an energy level of between 100-200 joules, thereafter a very heavy exactly measured sinusoidal wave of 5 milliseconds is delivered to the heart by paddle electrode, this is the monophasic wave form.

To understand how monophasic waveforms work we need to know that the current will flow in one direction only from one paddle to the other, this will in fact cause the heart to stop and to restart under its own cognizance. This monophasic method was used for many years, and it thereafter that a biphasic waveform was developed, unlike the monophasic, here the current will flow from one paddle to the other then n the reverse for the second or biphasic.

With modern day research showing that biphasic waveforms have proved to be more effective than the elder monophasic method also when the energy levels are the same it is for this reason that the manufacturers of the external defibrillators are using the biphasic waveform as a standard.

It is clear that for a successful defibrillation to take place that a biphasic waveform lowers the threshold, however the underlying mechanisms are still not fully understood.

Currently the energy levels of defibrillators vary from one to another manufacturer depending on the model. With some systems the energy levels are constant whereas with other devices they are programmable and the exact level can be selected by the user depending on the circumstances at hand. With none or very little comparative data, the manufacturers are unable to make recommendation as to the most effective energy level or biphasic waveform.

Defibrillators – Modern Technology

Modern technology , with the assistance of previous studies based on the implanted device have led to the advent of the Automated External Device (AED) this system has the capacity to analyze the rhythm of the heart and then it will calculate the required charge, thus limiting the clinical skill required and letting emergencies get treated effectively and speedily.

Based on simple computer technology, the above (AED) system has been designed to analyze the heart and then let the user know if a shock is in fact required and if so how much. Being specially designed for the layman with little or no training at all and are somewhat limited in their use for medical practitioners’ who when treating will be required to make a diagnosis and will therefore use a manual or semi automated system.

A problem that is associated with the automatic system is that the ceasing of CPR in order to shock the patient has shown to have a negative effect on the defibrillation process it is here that a professional that is on hand could make the diagnosis and regulate the shock accordingly.

Currently there are two different types of Automated External Devices (AED), the semi and the fully automatic models. The fully automated system will after diagnosing the problem will administer the shock wave automatically, whereas the semi- automatic will after diagnosis will determine is a shock is indeed required and if so the user will need to manually press a button to administer the measure shock. There are additional features depending on the model, where an ECG monitor can be purchased or a manual override.

The electronic circuit of the biphasic defibrillator can be designed around a high voltage-high capacity energy storing condenser. However, contrary to the common monophasic defibrillators it will have two different discharge routes: a system of four high voltage-high current IGBT’s is switched alternatively in two opposite directions – to provide the biphasic discharge wave. (The IGBT’s configuration resembles a four diodes conventional rectifier bridge).