Research Applied to Clinical Practice
| The latest studies in the United
States show that a only very small percentage of the approximately 300,000 annual cardiac
arrests survive (Lombardi, Gallagher, & Gennis, 1994). Because of these
poor outcome rates many world-wide organizations have been promoting and supporting
research on ways to this international problem, via Automatic External Defibrillators
(AED's) in the community.
Transthoracic defibrillators have been around since the 1960's, and the current use of high energy shocks to treat ventricular fibrillation (VF) or ventricular tachycardia (VT), has not changed. This high energy requirement has negatively affected the size of the equipment and battery needs for AED's (Bardy, et al., 1996).
To better understand this issue you need to understand that for over 35 years (in the US) there has been only one type of transthoracic defibrillation, that of a standard dampened sine wave monophasic shock. In plain words, it is an electrical pulse that flows one way between 2 electrode patches (or paddles). Over the many years of study, the theory of impedance and timing of shocks has accumulated in today's standard practice of 25lbs of pressure (if using paddles) with three "stacked shocks". The key has been the sequential raising of the joules (energy) from 200j, to 300j, to a maximum of 360j, then having the subsequent shocks at 360j. In relation to the energy there have been many studies to evaluate the effect of multiple high energy shocks on the heart muscle itself (Weaver, Cobb, Copass, & Hallstrom, 1982, Winkle, et al., 1989, Bardy, et al., 1989, Bardy et al., 1990, Green, et al., 1995, Bardy, et al., 1996). These studies have shown that initially there is significant ST segment changes associated with high energy defibrillation, which can last up to several months (if the patient survives). There has not been many studies evaluating the long term effect of high energy shock on the muscle itself.
The truncated exponential biphasic waveform, in which the polarity is reversed part way through the pulse, has been used in internal pacemakers for over 10 years, and has been used in the former Soviet Union for transthorasic defibrillation for several years (Kerber, et al., 1997). There have been many studies done to prove some of the following points: With the monophasic system there is higher success rate of initial shock conversion from VT or VF than monophasic (85.2% monophasic vs. 97.6% biphasic), The joules are significantly less (200j monophasic, 130 + 20j biphasic) which will affect reserve energy needs, Biphasic is more effective in reversing sustained VF (Jones, et al., 1990, Walcott, et al., 1995, Green, et al., 1995).
Lombardi, G., Gallagher, E.J.,
Gennis, P. (1994). Outcome of out-of-hospital cardiac arrest in New York City: The
pre-hospital arrest survival evaluation (PHASE) study. Journal of the American Medical
Association, 247, 678-683.
"Research Applied to Clinical Practice: Defibrillation &
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