ADVERTISEMENT
Shocking Success
We often have a bit of fear in the backs of our minds when we deliver large currents of energy into people in cardiac arrest. What exactly are we doing? From the surface, defibrillation seems straightforward—even a lay person with minimal training can operate an AED. But as advanced ALS providers, we must have better understandings of what we're doing when we defibrillate, the use and placement of pads and paddles and the differences between monophasic and biphasic defibrillation.
What Is Defibrillation?
Up to 38% of the time when a heart goes into cardiac arrest, it goes into a rhythm of ventricular fibrillation. The treatment for this condition is a larger dose of energy to overpower this fibrillation and, hopefully, allow the normal pacemaker cells in the heart to restart and begin rhythmic and organized contraction of the cardiac muscle. The electricity to do this is delivered in joules. One joule is equivalent to one watt of power for one second. One joule is power enough to lift an apple approximately 10 feet off the ground, and 30 joules will weld wire to steel.
Defibrillation research has generally been linked to patients in ventricular fibrillation or pulseless ventricular tachycardia. Neither of these patterns allows for organized function of myocardial muscle, and cardiac perfusion and blood flow to the body are therefore interrupted when they occur. In fibrillation, the disorganized rhythm has no pattern of regular polarization and depolarization. When the energy (joules) is delivered to the patient, the concept is to have it pass through the chest wall, enter the upper right corner of the right atrium (near the sinoatrial node) and progress through the heart's normal conduction system, ending at the bottom left corner of the left ventricle. This massive delivery of energy causes all the cardiac cells to depolarize at once. After the cells and heart recover, the heart's normal conductive system may "reset" and begin firing with one pacemaker site, resulting in an organized EKG rhythm.
In 2005, the American Heart Association (AHA) updated its recommendations for defibrillation coupled with CPR. In the past, a person in cardiac arrest and v-fib or pulseless v-tach was immediately defibrillated. If their rhythm did not change, up to three total defibrillations could be delivered, then CPR initiated.
Now, following additional research, the use of three "stacked shocks" has been discontinued in favor of more effective CPR. Current Advanced Cardiac Life Support thus teaches two different practices:
- In the case of a witnessed cardiac arrest, with v-fib or pulseless v-tach on the monitor, personnel should immediately defibrillate once, then begin CPR.
- In the case of an unwitnessed cardiac arrest, personnel should immediately begin CPR for five cycles, then check an EKG rhythm, defibrillating once if v-fib or pulseless v-tach is on the monitor, and then resume CPR.
Why the change? The AHA clearly states that basic CPR alone is unlikely to eliminate v-fib, and that electrical defibrillation is required. Additionally, for every minute between collapse and defibrillation without CPR, survival rates decrease 7%–10%. With bystander CPR, the decrease is only 3%–4% per minute. In recent animal studies, interrupting chest compressions for rescue breathing and EKG analysis is associated with reduced survival rates (recall that the AHA has now defined survival as discharge from hospital neurologically intact). In 2002 and '03, research showed that compression interruption was also linked to a decreased likelihood of converting v-fib to a perfusing EKG rhythm. Finally, data in 2005 showed that in CPR by healthcare providers, in-hospital and out, chest compressions were performed only 51%–76% of the total CPR time.
During cardiac arrest, myocardial cells rapidly lose their stores of oxygen and energy. Rapid and effective chest compressions can help restore them, essentially prepping the cells to begin working again. Since v-fib is the most commonly seen EKG rhythm in cardiac arrest, healthcare providers can defibrillate in the case of a witnessed arrest, as these stores are likely to not be fully depleted. In the case of an unwitnessed arrest and an unknown status of these myocardial stores, effective CPR may help restore the cells' supplies. When the cells are nourished, these studies indicate, they will be more likely to respond to defibrillation attempts and convert to another rhythm.
Pads/Paddles and Their Placement
The electrical impulse of external defibrillation is delivered through paddles or multifunction electrodes (MFE), more commonly known as pads. Whichever is used, the same initial steps should be followed. First, the patient's chest must be bare. Use caution with necklaces or chains, as they can connect the pads or paddles and cause an arc of electricity. Second, ensure the chest is dry—use a towel to remove water or sweat. If the patient is in water, they should be moved to a dry location before defibrillation (snow is OK). Finally, if there's significant chest hair, the patient should have the area of pad/paddle placement shaved or "waxed": Apply a spare set of MFE pads to the chest hair and rapidly rip them off, taking the hair with them. Then apply a new set of pads.
The AHA indicates three possible locations for pads and paddles. The first and most widely used is the sternal-apical placement, where the left pad/paddle is placed on the upper right chest wall and the right one is placed on the left lateral side of the patient's chest, even with their left breast. The second option is the biaxial position, where the pads/paddles are placed directly opposite each other on the right and left lateral chest walls, even with the breasts. The third option is the anterior-posterior placement, where the left pad/paddle is placed on the upper right chest wall and the right one is placed on the left side of the upper back.
Do not place pads or paddles directly over any medication patches on the skin (many are clear, so look carefully). Remove any patches and wipe the skin clean before applying. Additionally, observe and palpate for implanted medical devices (pacemaker, implanted defibrillator, etc.) near the planned site of placement. Adjust positioning of the pad/paddle so there's at least one inch between it and any implanted device. Contrary to urban legend, these devices will not explode, but rather malfunction and/or absorb some of the energy destined for the heart, decreasing the likelihood of successful defibrillation.
Defibrillation using paddles requires the use of gel or gel pads to improve contact between the paddles and the chest. Do not use any other type of gel or cream. Using other gels (such as Vaseline or Surgilube) or no gel will cause burns to the patient and decrease the effective amount of energy transferred. Overzealous use of gel can cause the points of contact to ooze into one big puddle, creating an arc and burns to the patient. In addition, when using paddles, the provider must provide sufficient pressure on the paddles to overcome resistance of the chest wall. This is done by pressing downward with the paddles to approximately 25 pounds of pressure. (This is more than most people realize. Try lying down and carefully placing a 25-pound dumbbell or bag of dog food on your chest to get a better idea.)
Most defibrillators are now equipped with MFE pads, as recommended by the AHA, and this changes some of the methods of use. Services that use different models of defibrillators and/or AEDs may find that the cable connections vary widely. Check your equipment before you need it to make sure everything's compatible. Pads are pre-gelled, but check the expiration date, as the gel may dry out if it's too old (this will obviously create burns). When applying pads, carefully smoothe them out to provide for uniform adhesion. Bubbles indicate air pockets, which may heat up during defibrillation and cause burns (and may actually cause the pad to blow off the patient). Pads provide the advantage of not having to be held in place and pressed down (their increased surface area compensates for chest wall resistance).
One downfall of pads is the distance of the provider from the patient during defibrillation. When a provider puts paddles on the patient, most people know to step back, helping to ensure the patient is clear before delivering a shock. When using pads, overzealous providers may forget to clear the patient properly before delivering the shock and potentially injure fellow caregivers. (It has happened to this author.)
Biphasic vs. Monophasic
Research indicates a difference in defibrillation outcomes based on the characteristics of the electrical impulse provided. Monophasic defibrillation has been the mainstay for many years. Monophasic electrical impulses deliver the specified amount of electricity from one pad directly to the other pad. With the cardiac muscle placed between the pads, the electricity instantly depolarizes the myocardium. Monophasic defibrillators have an average rate of successful conversion of ventricular fibrillation of 60%–80%.
Biphasic defibrillation is delivered from one pad to the other, then back to the original pad, basically defibrillating in two directions. There are two variations, based on the shape of the waveform. These waveforms differ by impulse acceleration and length of time, and are known as biphasic truncated exponential (BTE) waveforms and rectilinear biphasic waveforms. Although the waveforms have not been studied extensively against each other, biphasic has an overall rate of successful conversion of v-fib greater than 90%.
Current energy settings recommended by the AHA depend largely on the type of defibrillator (mono- or biphasic) and, if biphasic, which waveform is used. The organization recommends that monophasic defibrillation be performed on adults at 360j for all shocks. In biphasic defibrillation, the AHA recommends the lowest possible setting that has shown success, in the range of 120j–200j. Accordingly, each manufacturer and device will set the initial and subsequent joule settings specific to each monitor, generally 120j, 150j and 200j. The AHA further states that any shocks after the first should be equal to or greater than the last, up to 200j. Studies have shown that 120j with a biphasic defibrillator is equivalent to 360j with a monophasic one, but yields significantly less tissue and cell damage.
Conclusion
Defibrillation is the definitive treatment for lethal arrhythmias, and rapid application and delivery can help ensure greater survival from cardiac arrest. EMS providers should review and understand the specific machines they use for patient care, and know and use the electrical settings recommended by the manufacturer. With correct application and the use of CPR to enhance cardiac cells' response to energy delivery, defibrillation is a key asset in the management of the cardiac arrest victim.
Bibliography
Abella BS, Alvarado JP, Myklebust H, Edelson DP, Barry A, O'Hearn N, Vanden Hoek TL, Becker LB. Quality of cardiopulmonary resuscitation during in-hospital cardiac arrest. JAMA 293: 305–10, 2005.
American Heart Association. Statistical Fact Sheet: Miscellaneous 2007 Update. 2007.
American Heart Association. Part 5: Pediatric electrical therapies: Automated external defibrillators, defibrillators, defibrillation, cardioversion, and pacing. Circ 112: 35–46, 2005.
Efimon IR. History of fibrillation and defibrillation. Washington University, July 12, 2004.
Eftestol T, Sunde K, Steen PA. Effects of interrupting precordial compressions on the calculated probability of defibrillation success during out-of-hospital cardiac arrest. Circ 105: 2,270–73, 2002.
Van Alem AP, Chapman FW, Lank P, Hart AA, Koster RW. A prospective randomized and blinded comparison of first shock success of monophasic and biphasic waveforms in out-of-hospital cardiac arrest. Resusc 58: 17–24, 2003.
Wik L, Kramer-Johansen J. Myklebust H, Sorebo H, Svensson L, Fellows B, Steen PA. Quality of cardiopulmonary resuscitation during in-hospital cardiac arrest. JAMA 293: 299–304, 2005.
ZOLL Medical Corp. E-Series Users Manual: Defibrillation, 2006.
Marc A. Minkler, NREMT-P, CCEMT-P, is a paramedic/firefighter with the Portland (ME) Fire Department and has been a student of EMS for over 19 years. He is the author of several internationally published EMS instructor programs. Reach him at pfd225@roadrunner.com.
