On The Sideline - QB QT Confusion
QB QT Confusion
You're sitting on the sideline of a high school football game for an event standby. You've never actually been called out on the field for a player, which you're happy about because the audience watching you work, would be rather stressful... However, today is not a lucky day for anyone. After a quick and stressful scramble the QB throws the ball 50 yards downfield - touchdown! The QB stumbles, takes a knee, and falls over. Exhaustion? Relief? His teammates gather around him and notice that he's completely out of it. Staff rushes the field and quickly determines that the patient is pulseless. You enter the field from the sidelines prepared to work an arrest in front of the crowd.
The most important treatment? Maintain chest compression fraction with solid BLS and defibrillate. You remember that magnesium sulfate is indicated for polymorphic ventricular tachycardia / torsades de pointes, which you administer through the IO that was established. After a couple rounds of defibrillation, the patient regains a pulse.
You of course take an ECG to see if you can determine the cause of the arrest. You obtain the following 12 lead:
So, what's next?
If you're like me, Long QT Syndrome (LQTS) was mentioned in school, but that was about it. We now have a major suspicion that this patient suffered TDP due to his QT interval, but where do we go from here?
I was once told...
"Isoproterenol is the treatment of choice, but you can use Atropine because everyone has that."
Is that correct? Should you speed the heart up? or slow it down? This is where the confusion comes from - there are different types of LQTS which require treatments that are essentially opposite of each other. Wait, what? Yeah - you can have a patient with a prolonged QT interval, and depending on what caused it, they require treatment that will probably cause the other type to code again... which totally sucks. Let's dissect the QT interval first, then talk about the types of LQTS and learn how to try not to screw up the treatments.
The QT Interval
The 'doc in the box' actually isn't too bad at measuring your QT interval, so you should see it on your ECG parameters. However, if you want a quick way to eyeball it, just check to see if the T wave extends past the half-way point between R waves.
If you're looking for something more exact, these are the measurements you're looking for.
Remember this? You were likely taught with a normal cardiomyocyte action potential looked like, and how it produces a normal ventricular complex. ARP = absolute refractory period. RRP = relative refractory period.
We're going to zoom in on phase 3 (potassium out). When Potassium is being pumped out, this is our relative refractory period. You might recall that the RRP is when badness can occur if the heart is stimulated during this period. The badness ensues because some myocytes are able to be stimulated, while others are not. This causes an uneven depolarization, and potentially a fatal arrhythmia. In the most common forms of congenital LQTS (we'll talk about acquired later), there is something wrong with phase 3 - what is it?
The ion channels that handle the outflow of potassium during phase 3 are called your IKR and IKS.
I = Ion
K = Potassium
R = Rapid
S = Sustained
The rapid channel (IKR) moves potassium out quickly as we turn from the plateau phase to the initial downslope of the action potential. Then, the sustained channel (IKS) maintains the outflow of potassium until repolarization is complete. If one of these channels breaks, it's a lot like a broken car - it just doesn't move as fast. Depending on the type of congenital LQTS you have, one of these channels is slower than it should be (resulting in a long QT internal, and a strange looking T wave).
Long QT 1 is a mutation of the sustained potassium channel - IKS. It's also the most common congenital abnormality, accounting for about 50% of cases. Notice in the picture below how the T wave gets off to a great start (rapid channel works fine) but then it just keeps extending (broken IKS).
Long QT 2 is a mutation of the rapid potassium channel - IKR. LQT2 is the second most common congenital abnormality, accounting for about 35% of cases. Notice in the picture below how the T wave gets off to a slow start (rapid channel is broken) but then it speeds up and tries to finish (working IKS).
Alright, now for one that is a little strange. In LQT3, there is nothing wrong with the potassium channels - there is an addition of sodium inflow. This LQTS account for about 5-10% of cases, and presents quite differently. This additional sodium channel prolongs the plateau phase, which results in a prolonged ST segment (moving the T wave back).
I'm going to start with congenital LQTS, and then we'll look into acquired LQTS afterwards.
Graph from: Circulation 2001; 103:89.
As you can see, the triggers are exercise, distressing emotion / surprise, and rest/sleep.
LQT1 is strongly associated with exercise
LQT2 is strongly associated with both emotion/surprise and rest/sleep
LQT3 is strongly associated with rest/sleep (and minor association with the other two causes)
With all of this variety in triggers, how can you tell which one the patient has, and if the long QT interval is acquired or cognitional? One way is by looking at the morphology of the T wave itself. You may also get a clue if the patient is unlikely to have an acquired cause (toxicological?). Also, acquired cases of LQTS are exacerbated by BRADYcardia, while congenital cases are exacerbated by TACHYcardia. Just to make that clear:
Congenital = Exacerbated by TACHYcardia.
Acquired = Exacerbated by BRADYcardia.
Going back to our quarterback - he's young, unlikely to take any medications that would prolong his QT interval, and also unlikely to have a severe toxicological exposure that would prolong his QT interval. He also was participating in exercise at the time of arrest, and has a T wave morphology that matches LQT1 (genetic mutation of the IKS channel). So, do you think preventing him from entering back into an arrhythmia would involve speeding his heart up with something like atropine, or lowering his heart rate with a beta blocker? If we suspect a congenital cause, he would receive a beta blocker.
Sympathetic tone is so strongly associated with fatal arrhythmias in congenital LQTS that these patients may even have a cardiothoracic sympathectomy. This is a procedure in which sympathetic nerves are disconnected from the heart, so as to stop the fight or flight messages from reaching the heart. Not surprisingly, they're also prescribed beta blockers.
Acquired triggers are different. The word "acquired" obviously means that that obtained it from somewhere. This prolonged QT interval is either from a medication, some other substance they ingested, something that they absorbed, or something they lost (like magnesium and/or potassium loss from diarrhea). Again, the patient with an acquired LQTS is exacerbated by bradycardia - why? The lower the heart rate gets, the more isoelectric line you're dealing with (longer T-P segment). The longer the T-P segment is, the more refractory time you have. The more refractory time you have, likelihood of an ectopic beat occurring increases. To make that word vomit a bit more simple, the bigger the space is in-between complexes, the more time there is for something to go wrong (like a PVC that initiates an arrhythmia).
By the way - I would list all the medications, toxins, and conditions that can cause LQTS, but there are so many that you should just use a reference such as UpToDate (which is where most of this information is compiled due to its complexity and the breadth of the literature).
Let's sum up treatment for the two different types of LQTS.
If their LQTS is congenital, your goal is to limit sympathetic tone to the heart. This means administering a beta blocker and keeping the patient calm (which might mean sedation and analgesia if indicated).
If they're acquired, your goal is to limit bradycardia, which means speeding the heart up. This may include atropine, an epinephrine infusion, or transthoracic pacing at higher rates (overdrive pacing at ~100 BPM). PVC treatment would warrant a sodium channel blocker like lidocaine (because you need to avoid amiodarone which prolongs the QT directly).
Jake Good made this info graph - thanks man!
Knowing if you patient has acquired or congenital LQTS is tough. It's not something we see often, and knowing that treatments are essentially completely opposite depending on which your patient has is stressful. However, I hope this blog (and podcast to follow) help differentiate between the two and fill in the knowledge gap that is often left in our brains after school. Jake Good put it best when we recorded this podcast that will be released after this blog: "If your patient presents in a PVTACH or TDP, resuscitate first, differentiate later." The emergent treatment will be similar, but preventing them from re-arresting is going to make all the difference.