Introduction to Transcutaneous Pacing, by Dr. Mjaye Mazwi.
Hello. My
name is Mjaye Mazwi. I'm a clinical fellow here
at Children's Hospital Boston, and I'm going to speak to you about transcutaneous pacing.
Transcutaneous pacing is something that's used relatively uncommonly in pediatrics.
And so, before presenting this, I want to say, first of all, that you might have to adapt some
of the methods and some of the equipment to what you have present in your environment.
And then secondly, that transcutaneous pacing is not the first clinical method that should be
attempted in the stabilization of patients with bradycardia.
We are first going to discuss the physiology of transcutaneous pacing, how it's done.
Once we've discussed that, we'll talk about the indications for transcutaneous pacing.
We'll move on to look at some of the equipment available in hospitals that is
used to transcutaneously pace people. And then finally, we will simulate
transcutaneously pacing a patient. Introduction.
The physiology of transcutaneous pacing is that you are using direct current to deliver an
electrical charge to the myocardium in an attempt to depolarize enough of the ventricular myocardium
to initiate a systolic contraction. Now the amount of energy that is
experienced by the cells of the ventricular myocardium is a function of two variables.
The first is how much charge you deliver. The higher the charge, the more likely you
are to depolarize the ventricular myocardium once you've crossed that threshold level.
And the second variable is the time for which that charge is delivered.
The longer you deliver the charge for, the more likely it is to cause depolarization.
Transcutaneous pacing makes use of these two, in that when we transcutaneously pace, we have a very
long time during which the charge is delivered.
We call that the pulse width. The pulse width for machines
that allow you to transcutaneously pace is about 20 to 40 milliseconds.
And the reason that that adjustment has been made is that using a longer time of delivery of energy
allows you to deliver a smaller charge with each pacing interval to try to capture the myocardium.
The goal is that you are going to either replace or augment the native electrical activity of
the ventricle and sustain the circulation by means of pads on the patient's chest.
I want to emphasize that the muscle that we are trying to capture when we
transcutaneously pace is the ventricular muscle.
According to the PALS algorithm, in a bradycardic patient that is symptomatically bradycardic,
our initial intervention is CPR and the judicious use of epinephrine and atropine
as appropriate, based upon what you think the etiology of the bradyarrhythmia is.
Transcutaneous pacing should not be your first intervention for bradyarrhythmia in pediatrics.
Equipment.
I'd now like to orient you to some of the equipment that we
use when we transcutaneously pace patients.
A lot of the functionality comes from the actual box, which interacts with the
patient by means of pads or paddles. The pads or paddles are no different
from the pads or paddles that are used when we defibrillate or cardiovert patients.
For an adult-size patient, place the pads over the mid-sternum at the base of the heart and the
mid-axillary line over the apex of the heart. If you have a pediatric-size patient, and/or
the pads come into contact with each other when placing both pads anteriorly,
you should use the anterior-posterior pad placement, where one pad is placed anteriorly
in the mid-sternal position and the other pad is placed posteriorly in the infrascapular position.
Please note that this anterior-posterior placement may lead to ineffective pacing.
During our simulation, I'll leave the pads in front of the patient over the approximate
area of the right ventricular apex and the second pad in the anterior axillary line,
where the approximate posterior base of the left ventricle is.
Pacing will allow electrical energy to pass between our anode and our cathode,
through the mass of ventricular myocardium, which should intervene between the two pads.
There are two sizes of pads. There are pediatric pads,
which are approximately this size, and adult pads, which as you can see are much larger.
The cut-off in deciding when to use pediatric versus adult pads is 10 kilograms.
Patients who are 10 kilograms or less, you should use pediatric pads for.
Patients that are 10 kilograms or more, you should use adult pads for.
The reason that I am emphasizing the use of pads over the use of
paddles for this indication is that pads create a much better contact with the skin surface.
And that is because they have this large, sticky surface,
which conforms to the patient's chest wall or back, and maintains a stable contact.
Paddles, which have a hard surface, can also come in two sizes,
are much less likely to maintain a stable connection with the patient's chest
and are therefore much more likely to lose capture during the process of pacing.
Paddles also require that someone stand by the bedside and maintain
the paddle contact with the patient, which consumes medical personnel that might be better
spent in doing other things to help stabilize the patient or move them to a safer location,
where more definitive therapy can be decided upon.
Looking now at our box, the functions that we are going to use when we pace patients
are built into the normal functionality of these units and are typically clearly marked.
Virtually all devices that allow you to pace will have a certain amount of basic functionality.
So we have a dial on this machine that allows us to select Pacer.
[BEEP] Once we have selected Pacer,-- [BEEPING] --there are typically three other buttons that
we are then able to use to specifically decide upon what we want to use to help pace a patient.
The first is the Pacer Output. When we press the Pacer Output,
it gives us the amplitude or the size of the charge that is to be delivered to the myocardium.
Most pacing units will default to an adult setting, which is about 40 milliamps.
It's reasonable to start with either 20 or 40 milliamps in pediatrics and then adjust from there
as is necessary. To adjust that
on this machine, there are arrow keys that allow you to increase
the amount of delivered electrical energy or decrease it, as may be appropriate.
On this machine next to the Pacer Output button, there is a Pacer Rate button.
When that's highlighted, most machines will default to a standard adult
pacing rate of about 70 beats per minute.
That can be adjusted also by means of the same arrow keys that we just discussed, to a rate
that is more appropriate for pediatrics, such as 100 or 120, depending on the patient's age.
When you have selected settings, the final button is the Start Pacing button.
When that is pressed, you deliver the current that you dialed in as many times a minute as you dialed
in on that second button, to the myocardium. To set up demand ventricular pacing,
first synchronize the pacemaker box with the ECG monitor on the patient.
This can be done by connecting a sync cable from the ECG monitor to the pacemaker.
This will allow the pacemaker to see the patient's QRS complex.
This will allow the pacemaker to synchronize its pacing to the patient's intrinsic heart beat,
thus preventing it from blindly pacing into a more vulnerable period of the heart's repolarization
and causing ventricular arrhythmias. So this is equipment that is in your
hospital and that I recommend that you try to become familiar with
for that rare clinical circumstance in which this might be necessary.
Preparation for Procedure. Let's talk for a minute
about actually selecting the current. What you are trying to do is you're trying
to use as much current as is necessary to capture the ventricular myocardium,
but not any more than you need to. And that's because the application
of electrical charge to the living cells of the ventricular myocardium
ultimately results in ventricular injury. The way that you decide what current to
leave a patient paced at is once you've switched on your machine-- so we'll switch on our machine--
and you have selected pacing, which I have just done, you choose the output that is necessary
to capture the ventricle by continuing to go up on your current or down on your
current until you are able to document capture. And by capture, I mean that the electrical charge
delivered is sufficient to depolarize enough of
the ventricular myocardium to cause a sustained ventricular contraction.
There are two ways of assessing whether or not that has happened.
One, a safer way and a more intuitive way of assessing whether or not you have captured the
myocardium with the amount of energy that you've delivered, is by feeling for the patient's pulse.
Unlike cardioversion and defibrillation, you can safely continue to assess your
patient while you transcutaneously pace. And by continuing to assess and support
your patient, what I mean is you can feel for the pulse; you can continue chest compressions;
and you can safely administer medications without risk of harm to personnel.
And the second is by looking at the little pacing spikes on the QRS complex, either on your machine
or up on your monitor, and making sure that there's evidence of ventricular systole
following the application of that charge. And typically,
that'll appear as a wide QRS complex. We talked earlier about how you can
select a current and you can select a time for which the current is delivered.
With a fairly basic machine like this, you are not able to select the pulse width, or the time.
That's automatically chosen for you. And that's either going to be the 20 or
40 milliseconds that we discussed earlier. What you're able to select, then,
is the delivered charge, how many times a minute you deliver it, and start pacing.
Once you have decided at what current you have captured the myocardium,
and that's that current that either shows up as a wide QRS complex on your monitor, which I will
demonstrate in a minute, or that current that is associated with the presence of a central pulse.
So that's a carotid pulse or femoral pulse or brachial pulse.
You continue to dial that current down until you lose capture.
And the loss of capture is documented either as the loss of evidence of a QRS complex
after your pacing spike on your monitor or the loss of a pulse at the bedside.
Once you have lost capture, that's considered your threshold level.
The last current that you deliver that captured the myocardium is your threshold level.
Typically, to stably pace a patient, you will then select a current that is 10 milliamps
higher than that level at which you lost capture. So assuming that we started at 40 milliamps and we
had good capture and we continued to dial down and we lost capture at 20 milliamps, a safe place to
leave this patient paced would be 30 milliamps, or 10 points higher than our loss of capture.
Case Demonstration.
I'm now going to demonstrate transcutaneous pacing in a patient.
This is a 7-kilogram infant who has presented to our ER and is currently in a
hemodynamically stable sinus rhythm with an acceptable blood pressure of 84 over 39.
Now, I want to emphasize again that the first intervention for bradycardia is,
according to the PALS algorithm, the initiation of CPR if you believe that this is a symptomatic
bradycardia as would be indicated by evidence clinically of poor perfusion, such as
weak pulses, prolonged cap refill time, evidence of decompensated shock, such as a change in
the neurological status, and hypotension when you cycle your blood pressure cuff.
After you have initiated CPR and administered effective medications, if medical management
is unsuccessful, transcutaneous pacing is an additional, temporary option until you figure
out more stable means of capturing the myocardium and supporting the patient's circulation.
We now have a patient with an unstable sinus bradycardia.
It's hemodynamically unstable and it has failed medical management.
So we're going to try to transcutaneously pace this patient.
We start by switching on our machine and selecting the pacer function.
You then select the rate that you want.
This is a 7-kilogram infant, so I'm going to try to pace this baby at about 110 beats a minute.
And then we start with a standard output at 40 milliamps, and we hit Start Pacing.
Currently we are not pacing, so what I'm going to do-- you can see our pacing spikes,
which are not followed by QRS complexes. I'm going to keep augmenting our current until
we can see a QRS complex following our strikes. And there we go at about 60 milliamps.
Each pacing spike is intermittently followed. We still haven't got full capture.
At 65 I appear to have full capture. The little white lines that you are seeing are a
pacing spike, and you can see a peaked QRS complex with that upright R wave following the complex.
Now what I'm going to do, we've established that we have captured the myocardium at a rate of 65.
I'm going to reassess our patient's hemodynamics.
We're now pacing at a rate of about 109. I'm going to feel for a central pulse,
which I'm still having trouble feeling.
And we're going to reassess our blood pressure by cycling our blood pressure cuff.
And there we go.
Now I have a good central pulse. The patient is warming up.
The cap refill has improved, and we have a perfusing blood pressure.
We've established now that we can capture the myocardium at this current.
What I would like to do now that we have stabilized this patient and we think that
we have a good capture of that myocardium, is figure out at what setting we're going to leave
the patient while we figure out a more stable way to support them in the intermediate- to long-term.
That involves going back down on my current to figure out where I lose capture.
So I was only 5 millivolts above where I lose capture.
This is now 6 milliamps-- 60 milliamps, rather. And at 65 milliamps
we start to capture the myocardium. As per the rule that we discussed,
a safe place to leave a patient is 10 milliamps above your threshold.
So if 65 is approximately our threshold, I'm going to go up on our pacer output to 75 milliamps
and leave the patient at this heart rate of 110, and then reassess my perfusion.
We have an improvement in our systolic blood pressure.
We have good, strong central pulses. And we have the consistent appearance of a
QRS complex after our white pacing spike. And that concludes our segment.
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