"Hey What’s This Funny D...
Highlights
- "Hey what’s this funny donut 🍩 stuck on the crash cart?"
It's a 🧲 magnet! We keep it there because it can be life-saving if applied over a pacemaker or ICD.
A 🧵about magnets, pacemakers & ICDs. Some useful facts that everyone in an ICU ought to know.
#FOAMcc #FOAMed
1/
(View Tweet)
- Pre-tweetorial question:
What happens when you apply a magnet over an ICD?
2 (View Tweet)
- The answer is “all of the above” - we’ll discuss specific scenarios & where each is clinically important.
But before we get to that, we need to review what pacemakers/ICDs do.
3/ (View Tweet)
- Pacemakers electrically stimulate either the atria, ventricle, or both.
Most also sense the patients rhythm. This way they avoid pacing on top of the person’s native beats.
https://t.co/EgN5nxvu2d
4/
(View Tweet)
- Implantable cardioverter defibrillators (ICDs) sense the rhythm & respond to arrhythmias.
When ICDs sense a life-threatening arrhythmia (VT, VF) they can cardiovert/defibrillate or perform anti-tachycardia pacing.
All ICDs also function as pacemakers.
5/
(View Tweet)
- You can usually spot an ICD on CXR because the leads have thick “shock coils" for cardioversion.
(If you are wondering why ICDs have “shock coils" it’s to distribute energy over a larger area to avoid tissue damage. Just like external defib pads!)
https://t.co/ECd7L8qB4T
6/
(View Tweet)
- Pacemakers & ICDs are lifesaving, but occasionally they malfunction (broken lead, trouble sensing, etc).
There’s also certain circumstances (electrosurgery, EOL, etc) where we need to disable certain device functions.
That’s where 🧲 magnets & simple electronics come in!
7/ (View Tweet)
- Before we get to what happens, how does the pacemaker/ICD know that a magnet has been applied?
One of two technologies:
-a Reed switch (older devices)
-a Hall effect sensor (newer devices)
8/
(View Tweet)
- The first - a Reed switch - is a super simple device:
It's just two flexible wires that will close a circuit if a magnetic field is applied.
Doesn't get any simpler than that!
9/ https://t.co/J4Fc2Ri7XP (View Tweet)
- If you look really closely at an older pacemaker you can actually see the Reed switch:
10/
(View Tweet)
- Fun fact: You have almost certainly interacted with a Reed Switch today. You just didn’t know it.
Here are very two common household examples:
11/
(View Tweet)
- More modern pacemakers/ICDs (including all MRI conditional ones) use a slightly more sophisticated device: a Hall Effect sensor.
Hall Effect sensors are more sensitive & output a voltage proportional to the magnetic field strength.
This is more info than just open/close.
12/ https://t.co/d7ApPkZ36k (View Tweet)
- Like a Reed Switch, you’ve almost certainly interacted with a Hall effect sensor today.
They are used to measure the RPMs on rotating parts. For example in a car transmission or an exercise bike . (Yes there’s 1 in a peloton!)
They also provide the compass in smart phones.
13/ https://t.co/N0mUgrcLgD (View Tweet)
- So now that we know how implanted devices sense magnets, what happens if we put a magnet on a pacemaker?
Here’s a great example of the change in ECG 👇
https://t.co/ZnlQW6HrSM
14/
(View Tweet)
- Applying a magnet to a pacemaker puts into an asynchronous mode (e.g. it stops sensing) but continues pacing at a fixed rate.
Specifically, it will put a pacer into AOO/VOO/DOO mode (depending on what leads are present; atrial, ventricular, or both).
15/
(View Tweet)
- Why would we want to do this?
Inappropriate pacing can cause Pacemaker mediated tachycardia (AKA "endless loop tachycardia") wherein the pacer V lead fires, senses a retrograde atrial conduction, & fires again... endlessly.
Applying a magnet interrupts this endless loop!
16/
(View Tweet)
- Another situation occasionally encountered in the OR, is a pacemaker may sense electrocautery pulses and fire inappropriately.
https://t.co/Xwa0DnkrEa
17/
(View Tweet)
- Interestingly, with a 🧲 magnet applied, the pacemakers HR will tell you it's battery status:
For example a BS pacer will beat at 100 bpm if the battery is full🔋(beginning of life = BOL) & 85 if low 🪫(elective replacement = ERT)
See this algorithm👇
https://t.co/F0RynXVMTQ
18/
(View Tweet)
- What about putting a magnet on an ICD?
It does a few more things than putting a magnet on a pacemaker.
The details vary by manufacturer, but in general it will:
- disable all tachy-therapies (pacing, cardioversion, & defibrillation)
- AND begin pacing in asynchronous mode
19/
(View Tweet)
- Why would you ever want to turn off a ICD’s anti-tachycardia function?
Three reasons:
-to stop inappropriate shocks from being delivered
-to prevent the ICD from firing due to electrocautery
-to avoid painful shocks as part of DNR/comfort care
20/ (View Tweet)
- For example a fractured ICD lead can cause improper sensing leading to multiple inappropriate shocks ⚡. Ouch!
Applying a ring 🧲 magnet will temporarily stop the ICD from delivering more unnecessary shocks. (this person will need the lead replaced)
https://t.co/qXPXOtUJt5
21/
(View Tweet)
- Another situation we sometimes see in the ICU is ICDs at the end of life.
Defibrillation by ICD is generally inconsistent with the goals of people who are DNR.
Applying a magnet can help avoid the anguish of futile shocks in a dying person.
See here: https://t.co/1amNJ9Jgji
22/ (View Tweet)
- To summarize, we learned:
- how pacemakers & ICDs respond to magnets
- what happens when a magnet is applied
- specific times that is useful to apply a magnet in the hospital
(bonus: you even learned about ubiquitous magnetic sensors in everyday devices!)
Hope you enjoyed!
23/ (View Tweet)
- Ok a few more bonus 🧲 facts:
Not quite every pacemaker is magnet responsive.
The newest & smallest pacemakers, which are implanted via catheter directly in the ventricular wall, do NOT respond to an external magnet.
https://t.co/3xNv1B0AMJ
24/
(View Tweet)
- Many consumer electronic devices include magnets; rarely these magnets can activate the magnetic sensor in pacemakers or ICDs.
Be mindful of pockets that put these devices over implanted devices!
25/
https://t.co/gMV2zf2H37
(View Tweet)
"Hey What’s This Funny D...
Highlights
- "Hey what’s this funny donut 🍩 stuck on the crash cart?"
It's a 🧲 magnet! We keep it there because it can be life-saving if applied over a pacemaker or ICD.
A 🧵about magnets, pacemakers & ICDs. Some useful facts that everyone in an ICU ought to know.
#FOAMcc #FOAMed
1/
(View Tweet)
- Pre-tweetorial question:
What happens when you apply a magnet over an ICD?
2 (View Tweet)
- The answer is “all of the above” - we’ll discuss specific scenarios & where each is clinically important.
But before we get to that, we need to review what pacemakers/ICDs do.
3/ (View Tweet)
- Pacemakers electrically stimulate either the atria, ventricle, or both.
Most also sense the patients rhythm. This way they avoid pacing on top of the person’s native beats.
https://t.co/EgN5nxvu2d
4/
(View Tweet)
- Implantable cardioverter defibrillators (ICDs) sense the rhythm & respond to arrhythmias.
When ICDs sense a life-threatening arrhythmia (VT, VF) they can cardiovert/defibrillate or perform anti-tachycardia pacing.
All ICDs also function as pacemakers.
5/
(View Tweet)
- You can usually spot an ICD on CXR because the leads have thick “shock coils" for cardioversion.
(If you are wondering why ICDs have “shock coils" it’s to distribute energy over a larger area to avoid tissue damage. Just like external defib pads!)
https://t.co/ECd7L8qB4T
6/
(View Tweet)
- Pacemakers & ICDs are lifesaving, but occasionally they malfunction (broken lead, trouble sensing, etc).
There’s also certain circumstances (electrosurgery, EOL, etc) where we need to disable certain device functions.
That’s where 🧲 magnets & simple electronics come in!
7/ (View Tweet)
- Before we get to what happens, how does the pacemaker/ICD know that a magnet has been applied?
One of two technologies:
-a Reed switch (older devices)
-a Hall effect sensor (newer devices)
8/
(View Tweet)
- The first - a Reed switch - is a super simple device:
It's just two flexible wires that will close a circuit if a magnetic field is applied.
Doesn't get any simpler than that!
9/ https://t.co/J4Fc2Ri7XP (View Tweet)
- If you look really closely at an older pacemaker you can actually see the Reed switch:
10/
(View Tweet)
- Fun fact: You have almost certainly interacted with a Reed Switch today. You just didn’t know it.
Here are very two common household examples:
11/
(View Tweet)
- More modern pacemakers/ICDs (including all MRI conditional ones) use a slightly more sophisticated device: a Hall Effect sensor.
Hall Effect sensors are more sensitive & output a voltage proportional to the magnetic field strength.
This is more info than just open/close.
12/ https://t.co/d7ApPkZ36k (View Tweet)
- Like a Reed Switch, you’ve almost certainly interacted with a Hall effect sensor today.
They are used to measure the RPMs on rotating parts. For example in a car transmission or an exercise bike . (Yes there’s 1 in a peloton!)
They also provide the compass in smart phones.
13/ https://t.co/N0mUgrcLgD (View Tweet)
- So now that we know how implanted devices sense magnets, what happens if we put a magnet on a pacemaker?
Here’s a great example of the change in ECG 👇
https://t.co/ZnlQW6HrSM
14/
(View Tweet)
- Applying a magnet to a pacemaker puts into an asynchronous mode (e.g. it stops sensing) but continues pacing at a fixed rate.
Specifically, it will put a pacer into AOO/VOO/DOO mode (depending on what leads are present; atrial, ventricular, or both).
15/
(View Tweet)
- Why would we want to do this?
Inappropriate pacing can cause Pacemaker mediated tachycardia (AKA "endless loop tachycardia") wherein the pacer V lead fires, senses a retrograde atrial conduction, & fires again... endlessly.
Applying a magnet interrupts this endless loop!
16/
(View Tweet)
- Another situation occasionally encountered in the OR, is a pacemaker may sense electrocautery pulses and fire inappropriately.
https://t.co/Xwa0DnkrEa
17/
(View Tweet)
- Interestingly, with a 🧲 magnet applied, the pacemakers HR will tell you it's battery status:
For example a BS pacer will beat at 100 bpm if the battery is full🔋(beginning of life = BOL) & 85 if low 🪫(elective replacement = ERT)
See this algorithm👇
https://t.co/F0RynXVMTQ
18/
(View Tweet)
- What about putting a magnet on an ICD?
It does a few more things than putting a magnet on a pacemaker.
The details vary by manufacturer, but in general it will:
- disable all tachy-therapies (pacing, cardioversion, & defibrillation)
- AND begin pacing in asynchronous mode
19/
(View Tweet)
- Why would you ever want to turn off a ICD’s anti-tachycardia function?
Three reasons:
-to stop inappropriate shocks from being delivered
-to prevent the ICD from firing due to electrocautery
-to avoid painful shocks as part of DNR/comfort care
20/ (View Tweet)
- For example a fractured ICD lead can cause improper sensing leading to multiple inappropriate shocks ⚡. Ouch!
Applying a ring 🧲 magnet will temporarily stop the ICD from delivering more unnecessary shocks. (this person will need the lead replaced)
https://t.co/qXPXOtUJt5
21/
(View Tweet)
- Another situation we sometimes see in the ICU is ICDs at the end of life.
Defibrillation by ICD is generally inconsistent with the goals of people who are DNR.
Applying a magnet can help avoid the anguish of futile shocks in a dying person.
See here: https://t.co/1amNJ9Jgji
22/ (View Tweet)
- To summarize, we learned:
- how pacemakers & ICDs respond to magnets
- what happens when a magnet is applied
- specific times that is useful to apply a magnet in the hospital
(bonus: you even learned about ubiquitous magnetic sensors in everyday devices!)
Hope you enjoyed!
23/ (View Tweet)
- Ok a few more bonus 🧲 facts:
Not quite every pacemaker is magnet responsive.
The newest & smallest pacemakers, which are implanted via catheter directly in the ventricular wall, do NOT respond to an external magnet.
https://t.co/3xNv1B0AMJ
24/
(View Tweet)
- Many consumer electronic devices include magnets; rarely these magnets can activate the magnetic sensor in pacemakers or ICDs.
Be mindful of pockets that put these devices over implanted devices!
25/
https://t.co/gMV2zf2H37
(View Tweet)