Defibrillators and Defibrillation
In the Encyclopedia and Dictionary of Medicine, Nursing and Allied Health, a defibrillator is defined as an apparatus used to produce defibrillation by the application of brief electroshock to the heart directly or through electrodes placed on the chest wall.
The definitions of defibrillation in the same publication are 1) termination of atrial or ventricular fibrillation, usually by electric shock and 2) separation of tissue fibers by blunt dissection. By the correct use of the defibrillator the second definition can be avoided.
AC defibrillators were developed by Paul M. Zoll, MD, in the mid 1950s but not widely marketed. In August 1962, Bernard Lown, MD, published an article in the Journal of Cardiology titled Comparison of Alternating Current With Direct Current Electroshock Across the Closed Chest. Approximately 1 year later, DC defibrillators were introduced to the market and rapidly gained clinical acceptance. The first one did have a defect in that the paddles were tied to ground, and some clinicians as well as the patients got a jolt. From the mid 1960s to about 2000, the most common of the waveforms was the Edmark waveform. Other waveforms had little commercial acceptance during that period. Please review the current/time waveform in the defibrillator service manuals that you have, remembering that the load in ohms will change both the amplitude and width of the waveform.
Some of the early units had built-in external pacers, which were very painful, as 150 V were delivered to an ECG electrode on the chest. At this time electrodes were either plates, needles, or Welch cups, and were reusable. Basically, the pacing current was delivered to too small an area.
Until the early 1970s defibrillators did not have built-in monitors. In the mid 1970s, controversy erupted over how much energy was stored by the defibrillator and how much was delivered. On some units the meters indicated stored energy while others indicated delivered energy. The delivered energy was about 80% of the stored energy, which contributed to the confusion. The clinicians pushed for standardization, which was formalized in the early 1980s. Also part of that controversy was whether the terms watts, watt seconds, or joules (J) should be used to describe the energy delivered. joules finally won out.
Another controversy was over how much power was needed to defibrillate a person. Some researchers stated that 500 J to 600 J would be needed to defibrillate a person weighing over 200 lbs. After some animal studies, the researchers found that the second definition of defibrillation mentioned above occurred in the heart when that much power was delivered. Clinical changes in how fibrillation was detected and responded to proved to be more effective, caused less damage to the patient, and was much less costly. Rapid detection and several quick shots progressing from 200 J to 320 J became the clinical standards.
From the mid 1960s to the mid 1980s it was not unusual for a defibrillator either to automatically recharge after discharge or to not automatically dump the charge after a few seconds or a minute. That feature caused some serious pain to unsuspecting people who brushed against the paddles. I speak from experience on this one.
The defibrillator is a very simple device, basically a power source, an LCR circuit, and some switches (relays). For the most part it is very reliable and lasts for years.
Most defibrillators came with various sizes and shapes of paddles. Burns were very common in the early days, as current densities were not fully understood and some paddles had 90º corners. Fires were reported, as the conductive gel would run and create a current path to whatever was on the beds. At the time of introduction, most monitors did not have the protective circuits in the input, so a defibrillation discharge would wipe out the monitor. The first protective circuits on the inputs worked, but it could take as long as 30 seconds for the trace on the scope or chart recorder to return to baseline.
A problem that is still occurring with the paddles is that bumps can develop at the contact point where the self-testing is done. If not removed during preventive maintenance inspections, these bumps can cause burns since more current will move through the bump than through the rest of the paddle.
Many hospitals and emergency medical services have gone to the hands-off method of defibrillation. Electrodes, typically 7 x 12 cm in size, are applied to the patient and connected to the defibrillator via a special cable. All the controls are on the defibrillator, rather than on the paddles. From a cost standpoint it is important that the hospital and the emergency medical service groups are using the same technology, as changing the electrodes can be costly and time consuming when seconds count.
The latest change in defibrillators and defibrillation has been the introduction of biphasic waveforms and automatic external discharge (AED) units. Most AED units are biphasic. The AED unit analyzes the patients cardiac rhythm, and, if fibrillation is present, the unit will deliver a shock, analyze again, and, if needed, provide another shock. This procedure is repeated as needed until the patient is converted or the unit turned off.
The biphasic waveform (see the waveform in the defibrillator manual that you have) has both positive and negative energy, longer pulses, and lower voltages and currents. The biphasic waveform is becoming the waveform of choice in many institutions. Also note that the biphasic waveform is not affected as much by patient load as is the Edmark waveform.
The difference between defibrillation and cardioversion is another area of misunderstanding. Cardiovesion is a procedure wherein an electric shock is delivered to a patient at a precise interval, after an R wave is detected, to convert atrial fibrillationsometimes called flutteratrial tachycardia, and some ventricular arrthymias into normal rhythms. If there is no R wave the cardioversion cannot be done.
1) To cardiovert a patient with a trial fibrillation what must be present?
2) The Edmark waveform is not affected by the patient load?
3) The output of a defibrillator is measured in _______.
4) Lown is also famous for _____________.
Answers: 1-d; 2-b; 3-d; 4-b
David Harrington, PhD, is director of staff development and training at Technology in Medicine, Holliston, Mass.
Contributing to this article was Robert Freeman, a TiM BMET assigned to Quincy Medical Center in Quincy, Mass.