Know the Defibrillators ..

Cardiac Defibrillators - Restoring the Rhythm of Life

with courtesy from dr.sanjeevsood and www.medicalbuyer.co.in

"Many defibrillation attempts have failed due to poor technique and faulty devices." Group Capt (Dr) Sanjeev Sood, Hospital and Health Systems Administrator, Air Force Hospital, Chandigarh Based on the simple principle, external defibrillation involves ,conduction of current along the long axis of the heart when electrodes are placed on the chest upon discharge of a capacitor, which has been charged to a voltage level of about 4000 volts, generating an electric pulse of about 50 to 10 milliseconds. Before the patient is treated, the capacitor is charged to a voltage level corresponding to the desired energy level. At the time of the defibrillation discharge, the high-voltage switch, which normally separates the capacitor charge from the patient, is closed. Automatic Features From a power supply standpoint, defibrillators can be categorized into three different groups. Battery-operated defibrillators are most commonly used as mobile units, for patient transport within the hospital or in ambulances or in public places like airports. A great number of unsuccessful defibrillation attempts have been caused by failure to keep the batteries charged. Another common type of defibrillator is the mains-operated defibrillator with integral batteries, which are trickle charged as long as they are connected to mains supply. When used as portable units, these defibrillators cannot be discharged as many times as ones with replaceable batteries. Wholly mains-operated defibrillators are intended for stationary usage only. From a functional viewpoint, defibrillators can be categorized according to the number of automatic features they possess. With manual defibrillators, the user must assess the need for defibrillation and trigger the discharge by pressing the power switches on the electrode handles. Semi-automatic defibrillators are furnished with an advisory feature, consisting of software-integrated logic that analyses the ECG signal and indicates when defibrillation should be carried out. The semi-automatic defibrillators are available with or without automatic capacitor charging. Completely automatic defibrillators will trigger the discharge without user intervention as soon as the logic has deemed that treatment is necessary. These simple-to-use units are based on computer technology, which is designed to analyze the heart rhythm itself, and then advise the user whether a shock is required. Indications The most common indications for defibrillation are ventricular fibrillation, often a complication leading to a heart attack. Atrial fibrillation, atrial flutter, and ventricular tachycardia may also be treated with defibrillation. In these cases, the electric defibrillation shock must be synchronized with the cardiac cycle, so that shock is avoided during the T wave, that is, during the "vulnerable period" of the ventricular re-polarization. Synchronization is achieved by obtaining the ECG signal and triggering the discharge during the appropriate phase of the cardiac cycle. This type of treatment is called synchronized cardioversion. However, modern defibrillators automatically discharge at the phase of cardiac cycle. Pitfalls Many defibrillation attempts have failed due to poor technique and faulty devices. Defibrillators must therefore be checked frequently by qualified clinical engineers and the nursing staff. Cables must be intact, batteries kept charged, and accessories easily accessible. The person carrying out the treatment must be proficient in the proper technique. There have been cases where improperly trained paramedics have actually defibrillated patients who were still conscious. Requirements A cardiac defibrillator should be reliable (rescue-ready when needed), simple in design (almost anyone can use the AED), reassuring (AED knows when to and when not to deliver the shock so users do not have to worry about "doing it right"), comprehensive (AED text screen is helpful in noisy and chaotic environments), light weight, and have a long life battery. The regular availability of adhesive pads should be ensured since fresh pads are required before each DC shock.