Orthostatic Intolerance Explained: What It Is and Why It Happens

So, in the truest sense of the statement, you can't faint from POTS. And I know that that is jarring for a lot of people to hear. Understanding that one sentence can help you actually understand all of orthostatic intolerance.

So for all of you that are noticing — when I stand up, I feel bad — a lot of times that gets you to a temporary placeholder diagnosis of POTS. But what I want to encourage is actually we take one step back and we just kind of like set POTS over for a moment and we come back to orthostatic intolerance. We're saying like all of this kind of hubs around the idea that I get symptoms when I stand up because I'm having a harder time being able to get blood into my head.

Breaking Orthostatic Intolerance into Buckets

We can divide that orthostatic intolerance label. We kind of put that at the top and we can split that out into buckets, and those can be things that are in the orthostatic hypotension bucket — so where our blood pressure goes down — or in the POTS bucket. And here's what I mean.

So POTS is a very specific diagnosis within the category of orthostatic intolerance. Let's describe a couple words. Orthostatic intolerance. Orthostatic just means like standing up. So I'm laying down, I stand up. Intolerance — I don't do that very well. If you come over and look at the POTS diagnosis, postural orthostatic, right? So I'm standing up again. Tachycardia — my heart rate goes up. Syndrome — and the syndrome part means that when I get that tachycardia when I stand up, I experience these symptoms.

So it's compensation in a way that supports continuing to get blood flow into your body — at least your body, hopefully your brain. Now, where the departure happens is if I'm fainting, then there's a high probability that we're having a moment when we are losing that blood pressure to the brain.

The Orthostatic Hypotension Bucket

And there are a couple ways this happens. This happens through orthostatic hypotension. In the orthostatic hypotension bucket, what we're saying is when you're standing up, the blood pressure is actually dropping. In my experience, a lot of people that have headed down the POTS road are measuring their heart rate really well, but at the expense of measuring the blood pressure — when the blood pressure is actually the key feature because that's what you need in order to be able to move oxygen through the body, especially to the brain.

So if we're going to split orthostatic hypotension, we can break it down really easy. We kind of call traditional orthostatic hypotension when the blood pressure starts to drop within three minutes. So we're seeing like a systolic drop of 20 millimeters of mercury and a diastolic drop of 10 millimeters of mercury, either or. So when we see that blood pressure start to drop really quick in that 3-minute mark — orthostatic hypotension.

We can further kind of classify that where we'll see maybe not in that first three minutes but over time we get a delayed orthostatic hypotension, which just means we get to the same result. It just takes longer. And if we see that there are certain stimuli that will really quickly cause someone to have an event, then we can put that in the next bucket over, which would be vasovagal syncope — where there's some sort of an external stimuli that causes that orthostatic hypotension basically and then causes someone to have a syncopal event or faint.

How POTS Actually Works

So that is kind of like the blood pressure component. But then there's people that will feel all those same symptoms, but they may not faint. They may not pass out, but their blood pressure is actually pretty normal. So then what do you do?

That's where actually POTS comes into the game. Because POTS is basically — my blood pressure is trying to drop, but I got enough in the tank to be able to signal to my heart to come in and save the day. So the heart rate, as this blood pressure is falling, that heart rate starts cranking. And as it goes faster and faster and faster, we're actually pulling that blood pressure back up to normal.

But what that looks like when you take a picture of it, when you snap it real quick, is that you got a normal blood pressure and you have a high heart rate. And that's POTS in a nutshell. We're seeing that the desire for that blood pressure to drop, but it's being fixed or solved or compensated by that heart just cranking away. 30 beats per minute or more if you're an adult, 40 if you're a kid.

And then you say, “OK, well that's really interesting because even though that blood pressure is normal, that actually may not mean that the cerebral perfusion is normal.” And that's kind of wild to think about. It means that up to here, we might be doing okay circulating the blood. But remember, from the heart to the head, we actually have to push it upstream against resistance. We're kayaking upstream. So it takes more work. And we may find that even though someone has a normal blood pressure, they may actually still have that cerebral hypoperfusion which can cause the symptoms that we see in POTS.

The Three POTS Subtypes

We have three main types that have been identified in POTS and they're exactly what you think should happen.

Number one would be a neuropathic type where the nerves that control the blood vessels aren't as strong. So you can imagine if the job to keep blood pressure high is to be able to maintain enough tension in the blood vessel to keep pressure. It's kind of like if you were to put your thumb over the hose and it makes the water shoot out higher. But if you don't have enough strength to hold your thumb over the hose and it collapses, then the pressure is going to be low. And that's what we're seeing. So in neuropathic type, we just can't generate enough pressure in the vessel to maintain blood pressure. So we get the compensation from the heart.

Second type we can think about would be a hyperadrenergic type. We call it hyper POTS. It's exactly what you think should happen if the blood pressure is trying to drop, cerebral perfusion's trying to drop, and we get these compensations that kick in. So as I'm trying to make that heartbeat faster, I'm going to have to signal to the heart more, and I'm going to use norepinephrine as the signaling molecule within the sympathetic nervous system to be able to ask that heart to beat more. As I'm asking it to beat more, but I can't maintain a pressure, it's going to keep that feedback loop wide open. So I'm going to keep flooding the system with that norepinephrine so that the levels are going to be high.

As a consequence, my body temperature might change a little bit. I might feel hot. I might get sweaty. I might feel palpitations or I might feel anxious. They're the things that you should feel if you're feeling like you're being choked out in your head. So it's like a completely normal compensation to this problem we're talking about.

And the third one would be what we call hypovolemic POTS. It's really hard to measure that absolutely. It's hard to know like exactly how much blood volume you have, but you can think about it in terms of — are we accurately sending blood throughout the body in a dynamic way? So is my brain controlling the distribution or the allocation of my blood effectively? And if not, it may cause it to pool in areas and not be able to perfuse in areas.

And so those three subtypes are ones that help us understand this compensation. Whereas that blood pressure wants to go down, it can be kicked up back to normal using compensations with the heart. And those three subtypes help us understand how those compensations may vary if we have different things that are causing the POTS to begin with.

When It's Not POTS or Orthostatic Hypotension

Now the last group is kind of like we're in Goldilocks, right? So, running too hot, that's POTS. Too cold, that's orthostatic hypotension. But there's some people that notice that actually their blood pressure is like pretty well normal and they don't quite meet that heart rate criteria for POTS, but they still feel all of these symptoms that come with either the hypoperfusion symptoms or the hyperadrenergic types of symptoms. So what gives?

These are people that we actually call orthostatic cerebral hypoperfusion syndromes. And in these cases, when we measure the blood flow in the brain, they can have normal blood pressure, semi-normal heart rates, but the perfusion in the brain is not working. Those are really really important ones because those are people that get cast off into no man's land.

But we see in these cases we're looking specifically at the blood vessels in the brain because they operate differently than the blood vessels in the body. And we can have three main types of problems here. We can have problems with autoregulation, which is the reflexes of the blood vessels in the brain. We can have problems with neurovascular coupling — and these are very common especially after we have different types of brain injuries where we're not able to send blood as efficiently to the neurons as we'd like. And then we can have problems with vasomotor reactivity, which is actually the reflex related to carbon dioxide levels that help us shunt blood to the areas that are the most active in the brain.

Why This Changes How We Think About Treatment

So between these things — looking at orthostatic hypotension, looking at POTS, and looking at these variables that are with orthostatic cerebral hypoperfusion — we can kind of get this nice rounded view of orthostatic intolerance as a whole. And then you can understand why you're fainting. If your blood pressure is allowed to go down enough to where we create hypoperfusion, then that's where we may see someone faint. But it means that they probably don't have POTS. But all is not lost because we do have a bucket to put you in.

And we also have ways to start thinking about each one of these problems — it's a different mechanism to solve it in a different way. It begs the question of why are we treating them all generally the same? So someone goes in with POTS symptoms, maybe gets misdiagnosed, maybe doesn't — the protocols are all going to be the same medication-wise, exercise-wise. And hopefully with this, you can see that we wouldn't want to do that.

So one example of that might be if you're someone that has a normal to high blood pressure. This is a common one where we see we have tachycardia, but then also we see the blood pressure is trying to come up, but then we see the cerebral perfusion is actually dropping. A lot of times people will see that blood pressure going up and they'll think we need to bring that pressure down. So they'll use an anti-hypertensive medication and then what we find is that blood pressure is coming down in a way that actually pulls the perfusion down even more. People feel worse.

It's also really common if that blood pressure is being maintained and we put someone on a beta blocker. But remember — what is the purpose of that heart rate going up? It's to keep the blood pressure stable. And so a lot of times what we'll find is people on beta blockers, even though their heart rate comes down, they'll have some symptom relief because they're not having their heart crank away so much. So some of the anxiety might actually go down because that feedback loop is closed. And some of the palpitations might feel a little bit better because the feedback loop is closed. But if we look a little bit closer, for some of these people, by pulling that heart rate down, it actually will pull the blood pressure down or pull the cerebral perfusion down in a way that makes the hypoperfusion symptoms even worse. Makes it harder to be able to go forward.

So these are just examples of how we might just start having a little bit more nuance in how we're thinking of volume expansion. How do we think of compression socks? How do we think of adding salt? How do we think of medications? And how might we use some strategies that are more targeted to the actual problem rather than putting everybody in the same bucket and hoping that some of them are going to feel better and throwing some meds at them and seeing what comes from it.

This is why neuro rehabilitation is the gold standard in this — because it allows us to target the direct mechanism underneath it as the primary treatment rather than simply trying to solve for the symptoms.

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