ECG Pointers: Pacemakers, Magnets, and Hiding Under the Charge Nurse’s Desk

Authors: Lloyd Tannenbaum, MD (EM Attending Physician, Geisinger Wyoming Valley, PA) // Reviewer: Brit Long, MD (@long_brit)

Hello and welcome back to ECG Pointers, a series designed to make you more confident in your ECG interpretations.  This week, we feature a post from Dr. Tannenbaum’s ECG Teaching Cases, a free ECG resource. Please check it out. Without further ado, let’s look at some ECGs!

“Hey Doc, come quick, they’re pulling a guy out of a car in the parking lot.”  That sounds bad.  You get up and hustle up front to see several nurses and techs pulling a limp, grey body from a car.  Your team is ready to spring into action.  They get him out of the car and onto a stretcher.  Then suddenly, he takes a deep breath and is flailing.  His color is coming back.  He’s confused and agitated.  His wife comes running over to calm him down and says, “Calm down!  It’s just like the other times! The doctors here can help you!”  You rush him back to the resus bay to get more history.

Now that it’s a little calmer, you get a little bit more history.  Your patient is a 57-year-old male with a past history of sarcoidosis, hypothyroidism, and a permanent pacemaker placed due to complete heart block last year.  As you’re talking to him, he tells you that he’s been having “intense passing out” episodes over the past two weeks that come out of nowhere.  He just drops and it’s really freaking him out.  Today he fell flat on his face.  His wife finally had enough and made him come to the ER to get checked out.  He passed out again on the way in, but came to right as your team was pulling him out of his car.  Your tech gets him hooked up to the ECG machine and obtains the following ECG:

Rate: 54 beat per minute (9 beats in a 10 second strip)

Rhythm: oh boy.  Well, there are pacer spikes that mostly march out along the bottom of the ECG, but it seems like some aren’t producing QRS complexes.  That can’t be good, right?

Axis: Left axis

Intervals: Well.  The PR interval is sometimes there if the atrial lead fires and conducts a P wave, which, it seems, only happens on the first and last beat of the ECG.  The QRS complexes actually look surprisingly narrow, despite it being a paced rhythm.  I think in V3, it gives the most convincing wide QRS complex out of all of them, so I think I’m leaning towards a widened QRS.  Remember pacemakers rely on muscle to muscle spread of electrical activity, so we expect a wide QRS.  The QTc also looks maybe a little prolonged.

Morphology: Well, it’s not normal, that’s for sure.  We’re not obviously hitting Sgarbossa’s critera, so I wouldn’t activate a STEMI alert, but it sure doesn’t look good.

Final read:  We have a patient with repeated episodes of syncope with a pacemaker that may not be functioning correctly.

Well, this is when it starts to get interesting.  As you were pondering over the ECG, you notice the patient’s color starts to fade and he gets a vacant stare in his eyes.

His wife looks at you and says, “It’s happening again!”

“SIR!” You yell at him, “Are you ok?”  One of your partners who was back in the resus bay working on a different patient heard you yell and comes over to check on you.  The patient has no pulse so you start CPR.  You tell your partner that you’re worried about a malfunctioning pacemaker and he looks at you and says, “I’ll be right back!”  You see him … dive under the charge nurse’s desk?  Are you seeing this right?  You have no time to worry about what your partner is doing or try and figure out why he’s hiding under a desk, your patient is crashing!  At this point, the whole team has arrived and you’re 1 round of ACLS in.  Let’s take a short detour to talk about pacemaker malfunctions before we figure out what happened to this patient.

Of course, no discussion of pacemakers is complete without this (abbreviated) chart to explain the pacemaker letters:

Ok, you caught me.  There are really 5 letters in a pacemaker’s setting.  From the ED’s point of view, we really only use the first 3.  So, I’m going to just focus on them.  However, if you want the full chart and a great review on pacemaker malfunctions, see Pacemaker Malfunction–Review of Permanent Pacemakers and Malfunctions Encountered in the Emergency Department – ScienceDirect

Let’s dive in. 

The first letter tells you which chamber is paced (if the pacemaker is trying to pace a heart, where will the electricity go?)

The second letter tells you which chamber is sensed (If the pacemaker is looking to see if there is any native heart activity, which chamber or chambers is it looking for activity in?)

The third letter tells you what happens if senses activity (will the pacemaker “block itself” or prevent itself from firing if it senses native heart activity?  Or without a beat within a certain amount of time, will that “trigger” the pacemaker to fire? Or does it do both?)

Ok, so let’s practice:

Round 1: Dave is a 57 yo M with a DDD pacemaker.  What does that mean?  Well, the first D tells you that both the atria and the ventricles are paced, the second D tells you that the pacemaker is sensing (or looking for native activity) from both the atria and the ventricles.  Finally, the last D tells you that the pacemaker will be inhibited by native activity or triggered by a lack of native activity after a certain amount of time.

Round 2: Dave is a 57 yo M with an AAI pacemaker.  What does that mean?  Well, Dave’s pacemaker is only pacing the atria and it is only looking for atrial activity.  IF there is atrial activity, the pacemaker is inhibited and doesn’t fire. If the atria don’t fire within a set amount of time, the pacemaker will fire.

Ok, now that we know the basics of pacemakers, what do we do when they malfunction? (Besides hide under the charge nurse’s desk)

There are three major types of pacemaker malfunctions to keep in mind:

Failure to Pace

Failure to Sense

Failure to Capture

Let’s start with failure to pace.  Failure to pace means that the pacemaker is not generating a stimulus to pace as would be expected.  This can be caused by lead fracture or Oversensing.  Oversensing is where the pacemaker is interpreting something as native electrical activity, except the pacemaker is wrong.  In residency, the example our chiefs banged into our heads was a painter who’s painting a house and every time he paints large brush strokes, his pacemaker interprets his pec firing as his heart firing.  This would lead to his pacemaker “oversensing” the pec activity as cardiac activity and not firing.  This would probably lead to the painter passing out (which, you could argue would make his pec stop firing and his pacemaker start firing again)

Next up, failure to sense: This is when a pacemaker is firing when it shouldn’t be.  There is native cardiac activity, but the pacemaker ignores it and fires anyway.  This can be caused by lead dislodgement or poor myocardial contact. This puts the patient at risk for the R on T phenomenon!

Finally, failure to capture.  One of the most interesting malfunctions.  The pacemaker recognizes that it’s time to signal the heart to contract, and it does so, but the heart doesn’t respond.  You’ll see pacer spikes on the ECG, but the heart doesn’t contract.  This is commonly called … bad. It can be due to a bunch of different reasons, potentially lead migration, low output, battery failure, or scaring at the site of the leads that prevents the electricity from causing myocardial contraction.

So, what do you do with a malfunctioning pacemaker?   You can’t really “turn it off and turn it on again” can you?  Maybe?  Enter the magnet.  What happens when you put a magnet on a pacemaker?  Does it “turn it off?”  Well, no.  If it did, we’d have a real big problem.  Putting a magnet on a pacemaker turns it into VOO mode (or DOO or AOO mode depending on where the leads are), or put more simply, the pacemaker will fire at a set rate (determined by the manufacturer) between 70 – 100 beats per minute.  The pacemaker is not sensing for any kind of activity, so it will fire whether there is native activity or not.

Hey, wait a sec!  Did you guys forget there’s a patient coding?  I guess we should check in on him.  You’re running the code, when suddenly your partner comes back looking triumphant!  He holds up the pacemaker magnet that was stuck under the charge nurse’s desk and put’s it on your patient’s chest. With another dramatic gasp, the patient wakes up and now he is pacing at a rate of 85.  EP and the pacemaker rep are standing next to you ready to reprogram the pacemaker and increase the output.  After reprogramming, the ECG now looks like this (and the patient is alive and well):

Rate: 65

Rhythm: Dual chamber paced (as signified by the pacer spikes before the P waves and QRS complexes

Axis: normal axis

Intervals: Normal PR interval, widened QRS complex

Morphology: appropriate for a paced rhythm

Final read: Dual chamber paced rhythm at a rate of 65

Case wrap up: This was a very interesting case and a very challenging case to decipher.  The patient’s pacemaker had only been placed one year ago and should have still be working fine.  The current working diagnosis from EP is that there the pacemaker leads induced some kind of myocardial scar or sclerosis due to his cardiac sarcoidosis, thus needing more output from the pacemaker to obtain capture.  But why did the magnet work?  In this case, the magnet reverted the pacemaker to DOO mode, but it also performs a threshold test, and if it fails, the pacemaker automatically increases the output to 5V, which was enough to re-obtain capture!  Please note that an output increase is not the case with all pacemakers. 

Ok, let’s wrap it up with a summary:

• Pacemaker failure can usually be broken down into:
  1. Failure to Pace – the pacemaker doesn’t fire when it should
  2. Failure to Sense – the pacemaker fires when it shouldn’t
  3. Failure to Capture – the pacemaker fires, but the heart doesn’t contract
• Magnets revert the pacemaker to DOO/VOO/AOO and will sometimes boost the output
  1. Not all pacemakers boost output when a magnet is applied
• No matter if you’re an inter or a PGY-25, sometimes you just need to take a minute and hide under a desk

For some extra bonus science, here’s the patient’s old EKG from when he presented needing a pacemaker:

Rate: 30

Rhythm: Not normal sinus

Axis: Left axis deviation

Intervals: Look closely at the PR intervals for this one.  They’re not consistent.  You can march out the P waves and the QRS complexes.  Narrow QRS complexes

Morphology: no ST elevation concerning for STEMI

Final read:  I had to do a double take on this one.  At first it looks like a 3:1 AV block, but the PR interval is changing from the beats that appear to be conducted.  This is more consistent with a 3^rd degree AV block (or complete heart block).

For more amazing ECGs, please check out Dr. Tannenbaum’s ECG Teaching Cases