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Automatic External Defibrillators (AEDs) have brought patient care into the 21st Century. Originally the idea of putting this kind of "power" into the hands of the BLS Provider, and eventually, even into the hands of the general public, met with quite a bit of resistance. The technology, however, proved itself, even to some of it's staunchest opponents, and today many communities are in the process of "training" their citizenry in the proper use of AEDs. Most, if not all, BLS Squads have undergone the transition and are now equipped with at least one AED. Some areas of industry have also recognized the worth of this technology and have equipped and trained their personnel in the workplace. Of course, all of this is in accord with the recommendations of the International Guidelines 2000 Conference on CPR and Emergency Cardiovascular Care for Public Access or Home-based Defibrillation Programs.
Simply stated, an AED is a device that literally "talks" the provider through a process of evaluating a patient for, attaching the patient to, and activating, the AED therapy.
This device is capable of recognizing two distinct cardiac waveforms. Ventricular Tachycardia and Ventricular Fibrillation.
The first of the two (Ventricular Tachycardia) is a very unstable rhythm. Depending on the rate and the length of time that it has been sustained, it may or may not have a pulse. The cardiac activity still has some sense of organization (note that the "loops" are all basically the same size and shape.)
If there is no pulse associated with this rhythm, then it is considered a life threatening condition and is treated just as if it were Ventricular Fibrillation. (Please remember that in order for Ventricular Tachycardia to be considered an immediate life threat, it must be pulse-less.)
The second of the two (Ventricular Fibrillation) is an
immediate life threat. This wave form does NOT have a pulse associated with it.
There is no organization to this rhythm (note the irregular size and shape of
the "loops.") The "pumping" part of the heart
is quivering like a bag of worms, and it is highly unlikely that this activity
will move any blood. The corrective action for this rhythm is to
"defibrillate" the heart using an electrical
charge.
Let's take a moment an review a normal cardiac rhythm. We should start by looking at the conduction pathways in the heart.
Wow! what can we say? Is that confusing or what?
You see that's the heart that most textbooks put in front of you. That's not our heart. Our heart is different.
Our heart is simpler.
Look!
Its got 4 chambers, just like the other one. It has a sinoatrial node (the blue spot in the upper left corner, no the right corner, no the left.......anatomically "the right corner." On the page, it's the left corner. And it has an atrioventricular node (the yellow spot, near the middle.)
We'll add the rest of the conduction system a little at a time
When the heart "beats" it is the result of a wave of electrical energy that progressively contracts the muscle.
A normal "wave" starts at the sinoatrial node (SA node) and progresses toward the far lower corner of the left ventricle.
The "wave" can "progress" because cardiac cells are "copy-cat" cells. The SA node depolarizes and all the cells touching the SA node are "excited" into depolarizing, which in turn, excites all the cells touching those cells to also depolarize, yatta., yatta, yatta.
Like falling dominoes, all of the cardiac muscle cells are progressively depolarized in the atria, until the depolarization wave reaches the...
atrioventricular node (AV Node.)
The purpose of the AV node is to delay the wave for about a tenth of a second.
And to "channel" the depolarization wave into.....
the left and right bundle branches. See, we've added those, as well as the purkinje (purr-kin-gee) fibers at the distal end of each bundle.
Now, the depolarization wave is conducted down to the bottom of both ventricles where the purkinje fibers innervate thousands of cardiac muscles cells at one time, causing a "copy-cat" reaction that is explosive in intensity and expels the blood from the ventricles into the vascular system.
Oh! There are valves at the "entrance" and "exit" of each of the chambers, that ensure that the blood moves in only one direction when the heart contracts.
Like this!
Okay so theirs is prettier
Hmum.
And so.......... the resulting tracing on a ECG machine would look something like this:
There are basically three "bumps." A little bump, a big tall bump and a medium sized bump. The first bump (the little one) occurs when the atria depolarize. There's a small segment of flat line between the little bump and the big tall bump. This is the time when the depolarization wave is "in" the AV node (the yellow spot from above.) When the "wave" enters the two bundle branches and is very quickly conducted to the purkinje fibers and ultimately to the ventricular muscle mass, the big tall bump occurs. Immediately following the big tall bump is another section of flat line then the medium size bump. During this section of flat line, and during most of the medium sized bump, the heart in incapable of "beating" again. The medium size bump is a graphic representation of the heart "resetting" itself for the next "beat."
So what does all of this prove.
Well, first we want you to understand how the heart's electrical system works. Then, understand that it takes a small amount of time for the cardiac system to reset itself, so that the next "beat" can take place. It is also important to know that the heart has some "built-in" safety mechanisms, that provide a back-up for the SA node if it should stop functioning. You see every cardiac cell is capable of doing what the SA node does, starting the depolarization wave. The SA node starts all normal "beats" because it becomes excited (and depolarizes) faster than any other cardiac cell (usually between 60 and 100 times a minute.) Therefore, "Normal Sinus Rhythm" is a cardiac rate between 60 and 100 that originates from the Sinoatrial Node. The atrial cardiac muscle tissue is also capable of initiating a depolarization wave but it is usually slightly slower than the SA Node, so, that by the time the atrial tissue is getting ready to start the "wave," the SA node already has it going and the atrial tissue is forced to depolarize before it would have without the excitation of the SA node. A part of the atrioventricular node, called the AV Junction is capable of starting a depolarization wave, but its intrinsic rate is about 40 to 60 beats a minute and, again, under normal circumstances the SA node has the AV node running faster than its (AV node) intrinsic rate. Ventricular cardiac muscle tissue is also capable of initiating a wave (rate below 40,) but again the SA node is overriding the ventricular cells.
If, one of those ventricular cells should become irritated.... How, you ask, does one irritate a ventricular cell? Well, if that cell were deprived of oxygen, that would create an unstable cell wall which would lead to a state of irritation. That state of irritation might encourage this normally sloooooww cell to become very excitable. It might even become excited at a rate that is faster than the SA Node. The resulting rhythm is called ventricular tachycardia. One of the two rhythms that our AED can "recognize."
If, a couple of ventricular cells became irritated at or near the same time, and then a couple more ventricular cells became irritated a short time after (but before the depolarization from the first group propagated to the second group,) the resulting "wave" would be occurring in several directions at the same time. Causing a somewhat chaotic depolarization of the ventricles. The chaos would be enhanced as some parts of the ventricles reset faster than other irritated parts and were subsequently depolarized by "left over energy" from one of the last depolarizations. Sound confusing? Well......it is. It is a state of total confusion in the ventricles. The resulting tracing on an ECG machine would be what we have identified (at the top of this page) as Ventricular Fibrillation. The second of the two rhythms that an AED can recognize.
To "fix" these two rhythms (Ventricular Tachycardia without a pulse, and Ventricular Fibrillation) we are going to apply a massive electrical shock to the heart. This will "force" ALL the cardiac cells to depolarize at the same time, and will "recreate" a really BIG TALL bump on an ECG machine. ALL of the cardiac cells are going to go into a short "resting period." The hope is that the SA node will "recover" from this shock before any of the other cells, and that the resulting rhythm will be a pulse producing rhythm if not normal sinus rhythm.
In order to "attach" an AED to a patient, there are several concerns that must be addressed. The safety and well-being of the "operator," as well as others in the immediate area, (with the exception of the dead guy, he's got bigger "eggs to hatch,") MUST be foremost. Consequently, there are several types of environmental conditions that would create a dangerous situation for all those in the immediate area. Environments involving water, in any form (rain, snow, lakes, bathtubs....you get the picture,) may possibly "distribute" some of the energy (and believe me, this is A LOT of energy) to bystanders, the closest of which is the operator of the AED. There is a very good chance that some arcing may occur at the time the AED discharges, and for that reason, this device should not be used in any environment that is considered explosive (especially if you're afraid of heights.) It is paramount that non of the bystanders touch the patient while being defibrillated. Again, some of the energy may be transferred to the bystander. Electricity is conducted, quite nicely, through metal. For that reason, it is probably not advisable to utilize the AED while the patient, operator, and bystanders are all in contact with the same metal surface (like some factory "cat-walks," or public "gangways.") If the patient is found in an unacceptable environment, move the patient to a better location, before using the device. Properly performed Cardiopulmonary Resuscitation will give you the few minutes necessary to "scout out" a different location. Up until just recently, CPR had been the "flagship therapy" for victims of sudden cardiac death in the field, and, in the absence of an AED with a capable operator, this is still the case. However, if AED therapy is available, it is the preferred therapy for these patients. Even if CPR must be withdrawn, or withheld, in order to set-up the machine and properly "connect" the patient. In the algorithm (that's a fancy word for "menu") for AED therapy, CPR is included as part of the therapy and, other than this initial consideration, CPR should never be withheld from a victim of sudden cardiac death. There are, however, several "types" of patients, for whom, AED therapy may not be considered useful. According to the Guidelines 2000 Conference on CPR and Emergency Cardiovascular Care (ECC) defibrillation is a "weight based" therapy and the Automatic External Defibrillator is setup for patients that are over 50 pounds (110 Kg.) and, consequently, would not be used on a patient under 50 pounds. (Some of the newer AEDs employ a newer "form" of energy and MAY be appropriate for patients under 50 pounds. Consult the manufacturer's recommendations for guidance.) According to the 2005 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care, currently there is not enough evidence to recommend for or against the use of AEDs in infants less than one (1) year old This therapy is, as mentioned above, the preferred therapy for patients who have experienced "sudden cardiac death." If a trauma victim, of any age, is found pulse-less, the overwhelming likelihood is that they're state of pulse-less-ness is NOT the result of sudden cardiac death. Therefore it is a generally accepted rule that an AED is not used on trauma patients, where sudden cardiac death is clearly not the cause of pulse-less-ness. If your patient has a valid DNR it would be inappropriate, and in violation of the "Do Not Resuscitate" order to utilize an AED. Although a "Living Will" may contain a DNR, these "Living Wills" are generally very confusing to the layman, should not be "interpreted" in the field, and all APPROPRIATE action should be taken prior to the proper interpretation of the Living Will. Finally, we DO NOT use the AED on any patient that has a pulse. Some argue that the technology protects against "accidental" defibrillation of patients with pulse producing rhythms. This is definitely NOT the case. Ventricular Tachycardia (one of two rhythms recognized by the AED) can, at times produce a pulse. And, as unstable as that rhythm might be, gross defibrillation of it, may only lead to a more unstable condition.
After proper assessment of the patient, and a determination that the patient IS pulse-less, and DOES NOT fall into one of the categories mentioned above, the operator of the AED is encouraged to attach the "hands-off" pads, that are provided with the AED, to the patient. Most, if not all, such pads have icons imprinted on them which show the proper placement. It will help if you remember that the energy is going to travel from one pad to the other pad and that the heart needs to be located between the two pads. There are two sets of rules regarding pad placement. "Anterior/Anterior" placement and "Anterior/Posterior" placement. The former has both pads on the anterior surface of the patient's chest, one just to the left (patient's left) of the sternum (the flat bone in the center of the chest) with the other well to the right of the sternum and partially under the patient's right arm. The latter has one pad on the anterior surface of the chest very slightly to the left of a midline, with the second pad on the posterior surface directly behind the first. If this sounds confusing to you, refer to the icons on the pads, or ask your instructor for a demonstration of proper anterior/anterior and anterior/posterior pad placement. Under most circumstances the anterior/anterior placement will be utilized. But if the patient has an implanted pacemaker, usually placed directly below the left clavicle for right handed patients, then the operator must be certain that the pads are placed no closer than 6 inches to the pacemaker. Not a problem if the pacemaker is on the left side. But if it's on the right side of the thoracic cavity (as it might be for a left-handed patient,) and the 6 inch "buffer zone" can not be maintained, then an anterior/posterior placement of the pads may be necessary.
Once the patient has been properly assessed, and the pads properly attached, the operator is encouraged to turn on the AED. From this point on, voice prompts, will guide the operator through the rest of the procedure, starting with an "analyze" function (usually a button that initiates the analysis) and it is very important not to touch or disturb the patient during this phase because "movement" will disturb the sensitive instrumentation in the AED and possibly lead to an erroneous interpretation. Ultimately, if all the necessary criteria necessary for the machine have been met, the device will prompt the operator to "Shock the patient." And as simple as that might be, the consequences of accidentally shocking a bystander or fellow provider, could be as sever as "creating a second patient." For that reason, just prior to pushing the shock button, the operator must be certain that all bystanders and providers (operator included) are "CLEAR" of the patient or any metal touching the patient (like an ambulance cot.) To achieve this, it is suggested that the operator yell , "CLEAR !" while looking up and down the length of the patient for potential problems. If all is, indeed, CLEAR, push the button. On those occasions when the AED prompts that no shock is advised, perform five (5) cycles of CPR and re-analyze the patient. Repeat this procedure (CPR then analysis) for as many times as local protocol dictate, keeping in mind that your "goals" include getting this patient to the nearest appropriate hospital.
