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Defibrillation and Cardioversion 9/1/04
Hi all. Another one! As usual, please remember that this is not an “official” reference in any way at all – it’s what a preceptor would teach to a new orientee at the shop-floor level. Please let me know when you find mistakes – I’ll fix them right away. Thanks!
1- What is fibrillation?
1-1: What is atrial fibrillation?
1-2: What is ventricular fibrillation?
2- What is de-fibrillation?
3- What is cardioversion?
4- What is a defibrillator?
4-1: the monitor
4-2: the capacitor
4-3: the numbered buttons 1,2,3; output dials
4-4: the paddles and the pads
5- How do defibrillators work?
5-1: What is depolarization?
5-2: What does electricity have to do with it?
5-3: What is a joule?
5-4: What is monophasic defibrillation?
5-5: What is biphasic defibrillation?
5-6: What is “transthoracic impedance”?
6- How do I cardiovert someone?
6-1: Cardioverting a-fib.
6-2: Cardioverting VT-with-a-pressure.
7- How do I defibrillate someone?
7-1: Defibrillating VT
7-2: Defibrillating VF
8- What bad things do I have to watch for during cardioversion or defibrillation?
8-1: Using synchronization correctly.
8-2: Keeping the process orderly.
8-3: Clearing the bed.
8-4: Using contact gel properly – contact burns.
9- What things should I do after the cardioversion/defibrillation?
1- What is fibrillation?
Fibrillation is an arrhythmia that affects either the atria as a pair, or the ventricles as a pair, producing “a-fib”, or “v-fib”, respectively. (Come to think of it, if a person is in VF, do their atria fibrillate as well? Does it matter?) Most cardiac rhythms are organized – they’re regular in some way, producing some sort of regular (as opposed to disorganized), rhythmic motion of the chambers, hopefully producing a blood pressure. In fibrillation, the cardiac tissue of the chambers involved wiggles about like (classic phrase) “a bag of worms”. Does a chamber wiggling like a bag of worms pump any blood, produce a cardiac output, eject any fraction of its contents? No, it does not!
As I always try to point out, all the waves that you see on EKG strips actually represent some kind of physical motion of one or the other set of cardiac chambers, and the trick is to try to visualize what those chambers are doing in any given rhythm situation. Let’s see if a quick review of some strips helps the visualization process. Can I have the first slide please?
Here we are: look familiar? Sinus rhythm. Organized, rhythmic, producing stable contraction of the chambers – first the atria, then the ventricles. So - visualizing on the mental screen, that’s what I see: nice orderly motion, first above, then below.
Okay so far? Right – next slide, please. OK: atrial flutter. Still organized: the atria are contracting rapidly, sure, at about 300 bpm, and the ventricles are responding to every third or fourth impulse, slowly enough that the ventricular chambers have time to fill up nicely between beats, fast enough to probably maintain a good blood pressure. So I visualize the atria clipping along, with the ventricles contracting every third or fourth time.
This one? Well – is it organized? Actually it is: see the pattern of doubles? It’s a little easier to figure out by looking at the lower part of the strip – this is a sinus rhythm, and after every sinus beat comes a PAC, followed by a compensatory pause. So yes, still organized. “Regularly irregular”.
How about this one? Yup, VT. Ugly, scary, but still organized, regular – the chambers (which ones?) are moving in a steady manner. On your mental screen you should see the ventricular walls contracting very rapidly – do they have time to fill? Should we shock this rhythm? It depends…
1-1: What is atrial fibrillation?
How about this one? Not organized? Should we shock this rhythm? A-fib for sure can be a shockable rhythm, but look at the QRS rate – in the 70’s. What would have to be happening to make this a shockable situation? What do you visualize here? Atria: bag of worms. Ventricles – occasional, but normally conducted QRS’s. Are they too slow or too fast to make a blood pressure? How do you tell?
1-2: What is ventricular fibrillation?
Here’s an ugly one - you probably recognize this one right off. Doesn’t look organized to me! What rhythm is this? Visualize the ventricles – everybody see the worms? What should we do?
2- What is de-fibrillation?
So: all set on organized, and not organized? The treatment for nasty arrhythmias is often electical, right? The point is: one type of treatment: cardioversion - is for the organized kind of rhythm, and the other is, uh…for the other kind! Defibrillation is for disorganized rhythms.
What you want to do is to send a fixed amount of electrical energy along the normal conduction path of the heart: along the Lead II pathway. Can I have the next slide please?
(What’s this one? Oh yeah...)
Audience, this is the foxglove plant, the one that digitalis comes from. (The Chief Review Editor likes flowers…)
Here’s a diagram of the normal lead II: the positive electrode is down near the apex of the ventricles, the negative one is at the atrial end. Everybody remembers that the normal direction that the cardiac impulse takes is from the SA node at the northwest corner, up near Oregon, down and towards the southeast in Florida, where the positive electrode lives? And that the signal moves along the pathway as the cells depolarize, in sequence, along that pathway?
Negative electrode goes here (Oregon)
(here) (Ground electrode goes here…)
(here)
Positive electrode here (Florida)
www.arrhythmia.org/ general/whatis/
The idea is that applying an electrical impulse - of a specified amount of power - along the conduction pathway should depolarize all the rhythm-generating-and-transmitting cells at once. Bam! (Emeril? That you?) Now, hopefully, one of the normal, built-in, “intrinsic” pacemakers will take over – and in fact, often enough, they do! Remember: Defibrillation is the method to use in disorganized rhythms like VF, as in “Go defibrillate that disorganized rhythm, you doofus!” (Got to watch your language in those emergency situations.)
2- What is cardioversion?
This is the other one. Cardioversion is the electrical maneuver that you use for organized rhythms.The difference between cardioversion and defibrillation is pretty simple – the secret lies in timing the shock correctly. If you’re treating an organized rhythm, and if the electricity you give the patient happens to arrive during the “vulnerable” period of the T-wave, then the unhappy “R-on-T” thing may occur. Everybody remember what that is?- an ectopic beat, (or a jolt of external electricity – in either case a stimulating electrical signal) landing in the conduction system during the vulnerable period can trigger VF. Bad!
So – how to prevent this ugly thing? The defibrillator/cardioverter has the ability to track the QRS’s, and to stick a visible marker on each one. This lets the machine figure out when it’s safe to deliver the jolt. All too well do I remember seeing a patient once, cardioverted out of an a-fib with the machine in defib mode – that’s to say, not synchronized (forgot to push the “Sync” button, did ya?) – and who immediately went into VF, which responded to a defib shock. Fortunately.
The three situations where I’ve seen cardioversions occur are:
- Decompensated, rapid atrial fibrillation (afib with “RVR”: rapid ventricular response”) – this means that the ventricular response rate to the a-fib is so fast that the chambers can’t fill with blood between the beats. No filling, no blood pressure. Generally if a patient is in rapid a-fib and still has a blood pressure, the team will try meds first: verapamil, beta-blockers, etc.
- VT-with-a-pressure (as opposed to VT- with-no-pressure; what ACLS calls “pulseless VT”). More on how to actually do a cardioversion below.
- SVTs: narrow-complex rhythms, which are usually really bloody fast – up in the 200’s. Can your ventricles fill and empty properly at that rate? Mine can’t, mate!
All organized rhythms. “Organized” is of course pretty relative: AF is a sort of “just-about-organized-enough” rhythm.
4- What is a defibrillator?
The goal here is to try to understand what the machine is trying to do. Let’s say your patient pops into a nasty rhythm – not handling it very well, not making much of a blood pressure; and you want to deliver electricity – what do you need?
4-1: The monitor.
First: you need to be able to see what’s going on. This is of course one of the reasons why our patients are monitored at the bedside: so you can see what rhythm your patient is in. Defibrillators are built to travel– so they have a monitor screen built in.
Second: suppose you want to cardiovert instead of defibrillate – in other words, deliver a timed shock rather than a blind one. The machine is going to need to see the rhythm to do this, and you need to be able to make sure it’s seeing the right thing. This is a useful concept: lots of the devices in the unit are trying to “see” the patient in one way or the other – your job is to make sure they do, and that you learn how to interpret what they’re trying to tell you.
Anyhow, the machine needs to see the patient. You’re either going to have to put sensing wires on the patient that go back into the defibrillator (which may not have time for), or use the paddles as sensors. Our machines have a paddle-monitor mode called “quick look” – the procedure is to gel up the paddles, make sure the monitor is in paddle mode (the word ‘paddle’ appears on our monitors’ screens). Hold the paddles firmly against the skin in the defib position (with gel!) and get a good look at the patient’s rhythm. I bet that the newer defib pads do the same thing. I should know this…
4-2: The capacitor.
Unless you have a really long electric cord, you’ll need a battery to run any transportable medical device. Rechargeable batteries are why all these devices are so flippin’ heavy, and the defibrillator is no exception, so we keep ours on rolling carts that we can whip up and down the unit.
The battery stores electricity, but only if the machine is plugged in when you’re not using it. You do not want to arrive on the scene with a dead defibrillator! The battery feeds electricity into a capacitor – this I think of as a black box that holds whatever amount of electricity you choose, for a fixed amount of time. The capacitor fills up with electricity when you push the button that selects the charge you want to give. Our machines charge up with sounds that let you know what’s happening: once you push the button, you hear a rising whine as the charge collects - that turns into a steady, high-pitched whistle when the machine is ready to discharge. Loud, but effective. Practice this.
4-3: Numbered buttons: 1,2,3: output dials
Here the goal is to try to keep things very simple: there are three things you need to do when operating the defibrillator, and the people who make these machines are trying to help you do them when you may be, let us say, a bit distracted by the situation. I’m going by the machines in our institution here – make sure you know what to do with your own, although the three moves are probably the same:
1: Button number one (actually on our machines it’s a dial, but it has a big number one next to it) turns the machine on, and sets the machine with the joules (the amount of electrical energy) you want to give.
2: Button number two charges the capacitor to the level you picked. At this point we hear the rising tones that tell you that the capacitor is charging up, and then the steady tone that says that it’s ready to go.
3: Button number three lives on the paddles (there are actually two “number three” buttons, so you don’t squeeze just one by mistake and fire the device before you’re ready) and discharges them.
4-4: The paddles and the pads:
I hear that (rather like myself), paddles are considered “old-tech” – nowadays the thing to do is to slap on sticky defibrillation pads that hook up to the machine – the same ones as external pacing pads – then stand back, charge and discharge the machine from a few feet away. I have seen this done, but most times in our unit we make one quick defib move or two – there’s much less defibrillating going on since clotbusters came along and fewer people complete their MI’s.
In the MICU, I think that the pads are more for the elective cardioversion kind of maneuver rather than the emergent defibrillation thing. The critical point is that you really want to just jump in there and shock that rhythm – you don’t want to futz around with the pad packaging, the wires, changing the cable connectors so the pads are hooked up instead of the paddles…get the job done quickly. If the paddles are hooked up and ready, use them – don’t waste time; you can hook up the pads later and leave them on the patient for use if the problem happens again.
5- How do defibrillators work?
5-1: What is depolarization?
(I have to stick this in: my son pointed out a while ago that when a white bear is captured, and taken from his iceberg to the zoo, he becomes “de-polarized”. Excellent!)
Here’s how I understand it. Cardiac pacing and conduction cells work by a sort of magic ion pump dance: the concentrated ions on the outside of the cells all flow inwards at once, then outwards again. Swoosh, swoosh. The charges around the cell reverse as the ions flow in, or out, and the polarity flips: the cells are de-polarized – then re-polarized. Is that clever engineering, or what?
In the process of the depolarization dance along the cardiac conduction pathway, a measurable electrical energy is generated: (P-wave for atria, QRS for ventricles – remember?) Then re-polarized. (T-wave.) The conduction cells do this dance in sequence, along the conduction route from the SA node to the AV node, along through the bundle of His, (hey - where’s Her bundle?) and on downwards through the bundle branches into the contractile tissues in the ventricles.