;) Oh, and I would try vagal maneuvers ;)
There has been a plethora of articles on management of SVT. ALiEM and REBELEM are two that stick out in recent years to me (references linked within article). Yet, I still speak to people who have not read much of the literature on SVT treatment, and have not considered making changes to their treatment plans. Also, I was recently speaking with Cynthia Griffen (@CMGrffn) about this, and she taught me some stuff too. This will be a quick article on what I feel would be a reasonable approach to the stable SVT patient. Let's get started!
"Oh, and I would try vagal maneuvers..... ;) ;)"
For anyone who has been in an ACLS class, the SVT patient no doubt comes up at some point. For most classes, it's usually rather humorous. Generally, as soon as the student identifies SVT, they know where the scenario is going. They're going to push some adenosine once, then double the dose, then surely the patient will become unstable - CARDIOVERT! Then, without fail, V-Fib. Que CPR.
Instructor: "is there anything ELSE you would do before giving adenosine??"
Instructor: "look at your flow chart"
Student: "oh! I would have tried vagal maneuvers!"
Instructor: "perfect! - they didn't work - continue."
Honestly, if you're doing 'traditional' vagal maneuvers, maybe the idea is a little comical. Why? Because most people don't have much luck with them (or they're doing them wrong). Below is a screen shot from the REVERT trial. I found this through the this REBELEM post. The below screenshot of it is from the standard treatment arm, which included just having the patient blow into a syringe. Their aim was for the patient to hit 40mmHg of pressure for 15 seconds. This is very similar to having the patient 'bear down' like you were probably taught in school.
Standard Arm Figure
The results were not very impressive. Out of 214 patients, only 37 of them converted (17%). 69% of the people in this standard treatment arm ended up receiving adenosine. However, even with these small numbers of conversion, standard vagal maneuvers are still worth a shot. Why? Adenosine is not pleasant to receive, and some SVT sufferers will even avoid treatment because they feel scared of the medication and the way it makes them feel. We want to do everything we can to avoid making the patient uncomfortable. What about the other arm of the trial? What was the non-standard arm?
The non-standard arm of the trial tried something that I found very interesting - they took advantage of how our body reads pressure and influences our hearts conduction system. So... how does our vasculature influence our hearts conduction system?
We have baroreceptors (or you could call them pressoreceptors according to wikipedia) in our neck and aortic arch. These pressure receptors measure the pressure and stretch in the blood vessels and transfer that message to the central nervous system. If the pressure/stretch is high, the receptor tells the sympathetic nervous system (fight/flight/tachycardia) to relax, and the parasympathetic nervous system to ramp up (rest/digest/bradycardia) via the vagus nerve. Stimulating the vagus nerve is out goal in doing vagal maneuvers - the word vagal means "in relation to the vagus nerve." A great example of this is Cushing's reflex in increased intracranial pressure. When the pressure inside vessels becomes elevated, the baroreceptors send a message to the heart to slow down (reflex bradycardia). So how can we use this mechanism to our advantage?
Image retrieved here.
To stimulate the parasympathetic nervous system, we must increase the pressure that the baroreceptors sense. There are at least a couple ways to do this. The first is just having the patient 'bear down' or blow into a syringe. This causes direct pressure via increased intrathoracic pressure from the lungs to directly compress the aortic arch, and no doubt has an aspect of increased afterload to it as well. But, we already know this part. As seen above in the standard treatment arm of the REVERT trail, this didn't work very well. So what can we do next?
Give the patient an intrinsic fluid bolus. Intrinsic literally means something that 'belongs naturally.' How do we do this? We use a couple big bags of blood - they're called the patients legs. In the non-standard treatment arm of the REVERT trial, here is what they did (see the figure below).
Modified Arm Figure
What's the difference? After they had the patient do the standard 'blow in a syringe' method for 15 seconds, that flipped the patient backwards and elevated the legs at least 45 degrees up in the air and kept them there for 15 seconds. What does this accomplish? Let's break this down!
Think about what was going on with the patients blood flow during the initial 'blowing into the syringe' phase. Blood was having a hard time entering the chest cavity because the lungs were so inflated and compressed (decreasing preload). Now that the patient let the pressure in their chest go (stopped blowing into the syringe), blood is able to easily flow into the chest again, and will rush back. This period of time is what this modified valsalva takes advantage of! By raising the legs during this phase, you're going to optimize the preload and cause a bunch of blood to flow to the core, which increases the cardiac output, which increases stimulation of the baroreceptors. What else? On the arterial side of things you're also increasing afterload and direct pressure on the baroreceptors by giving the arteries a wall to climb (against gravity up the legs). So, does this work?
Yes! According to the modified arm of the trial, 43% of patients converted - and it cut the use of adenosine in half. Those are pretty great odds. When Cynthia told me about this trial, she gave me a few tips, which I will share with you now. Thanks Cynthia!
1. Tell the patient what the plan is - it is likely that they have not tried this type of vagal maneuver before. Telling them the specifics of the plan will greatly increase the likelihood of success - and they won't fight you when you flip them backwards.
2. You have to be really good at coaching the patient during the syringe phase. Be your patients cheerleader and encourage them enthusiastically to keep going! "You're doing great! Keep going! You're almost there! I think I see the syringe moving!" Some practitioners will have the patient continue in this phase until the patient feels as though they're going to 'pass out.'
3. Have a person ready to lay the head of the bed down, and people to flip the legs up. Hold here for a minimum of 15 seconds, and maybe longer if the patient will tolerate it. and.. if they're wearing a gown try not expose them to a crowd of people. No one wants to see that.
Okay - Vagal Maneuvers really didn't work
So now you know the best way to optimize success with vagal maneuvers. What if they still do not work? There are more options than just slamming adenosine. Let's look at what our other options are.
When I mentioned ALiEM in the beginning of this article, the blog I was referring to was called "Calcium channel blockers for stable SVT: A first line agent over adenosine?" This article did a great job breaking down the literature on Adenosine vs. CCB as first line treatment for SVT. I highly recommend looking at the charts that are laid out on that page because they are very useful for doing a side-by-side analysis of treatment options.
Here is one of the tables directly from ALiEM that I found very helpful (but be sure to read their whole article, not just look at this one snapshot).
As you can see from the figure above, Cardizem has the highest rate of conversion, but it also took the longest. However, this is strictly time from the start of the infusion to the termination of the SVT. I would argue that there is some time that is not accounted for here. When we consider the route of delivery for adenosine, we should cannulate a larger, more central vein. This can be hard to obtain, and at least for me, is a little more stressful. More than a few times I've watched as my partner (or myself) blows the AC vein and then we stress out because only have one other arm to try. Not to mention that humeral head IO placement seems like a little bit of overkill for a stable SVT patient. When we think about the route of administration for diltiazem, I could infuse this slowly into any fragile 22G in the hand if I had to, there is no fast flush needed. The ease of use, higher conversion rate, and better patient experience (no feelings of adenosine doom) has made me convert (pun intended) to the use of diltiazem infusion over adenosine.
It should also be noted that hypotension was more likely in the CCB arm. Adenosine still has a place in a subset of patients. Diltiazem does have some impact on vasculature since it is a benzothiazepine (vasodilation and negative cardiac actions). Just to refresh:
So how could you use this slow infusion therapy of diltiazem if you chose to? In my opinion, mixing an infusion that can be used for initial delivery and maintenance infusion seems to make the most logistical sense. Mixing 50mg into a 100mL gives a very easy 0.5mg/1mL concentration to work with. I only use 100mL in this instance because it is common for EMS to carry 100mL bags. You could also use a 50mL syringe attached to a 'half-set' or a 50mL bag to make a 1:1 concentration, which would be even easier. I will show how to use both of these concentrations below. To break down the SVT conversion rate to mg and minutes, I thought I would do a timeline to illustrate using the mixture I noted above.
2.5 mg/min using a 50mg in 50mL mixture:
2500 mcg/min with a concentration of 1000 mcg/mL.
2500 (desired dose) x 60 (for mcg/MIN) / 1000 (concentration) = 150 mL / hour.
With a 1:1 mixture, infuse at 150 mL/hour, with a total volume to be infused of 50mL.
With a 0.5:1 mixture (placing 50mg into a 100mL bag), infuse at 300mL/hour with a total volume to be infused on 100mL.
According to Lim et al., here is where people will often convert at using 2.5 mg/min continuous infusion of diltiazem:
Minutes into infusion/total dose delivered
1 min / 2.5mg
2 min / 5 mg
3 min / 7.5mg
4 min / 10mg - 25% of patients will converted here
5 min / 12.5mg - 50% of patients will converted here
6 min / 15 mg
7 min / 17.5 mg - 75% of patients will converted here (at 18.13mg)
8 min / 20 mg
9 min / 22.5 mg
10 min / 25 mg
11 min / 27.5 mg
12 min / 30 mg
13 min / 32.5 mg
14 min / 35 mg
15 min / 37.5mg
16 min / 40. mg
17 min / 42.5mg
18 min / 45 mg
19 min / 47.5mg
20 min / 50 mg
What about after the patient converts out of SVT? Stop the 2.5mg/min infusion, and consider a 2.5mg - 5mg / HOUR dose of diltiazem. Note that generally this maintenance infusion is used for continued rate control for A-Fib or A-Flutter patients after their RVR is controlled, not with SVT patients. However, if the patient has PSVT, you may consider a maintenance infusion. With a 1:1 concentration of diltiazem, simply run the infusion at the same mL/hour as your desired dose of diltiazem / hour. For example, for 2.5mg/hour maintenance, infuse at 2.5mL/hour. For 5mg/hour maintenance, infuse at 5mL/hour. For half the concentration, double the rate. You should use a carrier line for this, since this is a very slow infusion. You might even consider using a lower concentration for maintenance, so that you can run the IV at a faster rate.
I hope you enjoyed this and found some good resources for treatment of these SVT patients!