The number that hides in the standing

Most people with POTS can tell you the moment things go wrong. You stand up to refill the kettle, and the room tips slightly. Your heart starts up like it has somewhere to be. There's a grey shimmer at the edges of your vision, a wave of heat, the strange effort of staying upright for something that should take no effort at all.

The trouble is that by the time you've described all of this to a doctor, you're sitting down. And sitting down, your heart rate looks fine. This is the central frustration of postural orthostatic tachycardia syndrome: the evidence only appears in the transition, and the transition is over in seconds.

The good news is that the most diagnostic piece of information about POTS is also one of the easiest to capture at home. You don't need a tilt table or a lab. You need a clock, somewhere to lie down, and a way to read your pulse.

What actually happens when you stand

Gravity is relentless and uninterested in your comfort. The instant you stand, it pulls blood downward — into the veins of your legs and the large reservoir of vessels around your gut, the splanchnic circulation. In the first moments of standing, somewhere in the range of 500 to 1,000 milliliters of blood shifts toward the lower half of your body.

In a body with smooth autonomic control, this is invisible. Sensors in your arteries called baroreceptors notice the drop in pressure, the veins in your legs squeeze to push blood back up, and your heart rate ticks up just enough to hold everything steady. You never feel it.

In POTS, the bottom of that loop doesn't hold. Blood pools where gravity put it, less returns to the heart, and each beat pumps a smaller volume. To keep blood pressure from collapsing, the sympathetic nervous system compensates the only way it easily can — by driving the heart faster. And faster. The tachycardia isn't the disease; it's the rescue effort. That's why the defining feature of POTS is a heart rate that climbs steeply on standing while blood pressure stays roughly normal.

The threshold that defines it

Clinical consensus puts a specific number on this. POTS is marked by a sustained heart rate increase of at least 30 beats per minute within ten minutes of standing — or at least 40 beats per minute for adolescents between roughly 12 and 19 — in the absence of a significant drop in blood pressure, with symptoms persisting for at least three months.

Two words in there matter more than the rest: sustained and within ten minutes. A brief spike when you first pop up is normal for almost everyone. What distinguishes POTS is that the elevated rate doesn't settle. It holds, or keeps creeping, across those ten minutes. A reading taken thirty seconds after standing tells you almost nothing. The pattern over ten minutes tells you almost everything.

The 10-minute stand test

This is sometimes called a poor man's tilt table, or after the protocol it resembles, the NASA Lean Test. You can do a reasonable version of it at home, and doing it the same way each time is what makes it worth anything.

Lie down and rest quietly for about ten minutes. Don't skip this — your body needs to reach a true resting baseline, not the leftover state from whatever you were doing before. At the end, take your resting heart rate while still lying down.

Then stand up normally and stay standing, as still as is safe, leaning lightly against a wall if you need the security. Take your heart rate at standing, then again at intervals — say one, three, five, and ten minutes. Note any symptoms as they arrive: the lightheadedness, the palpitations, the visible mottling or purpling of your lower legs as blood pools, the cognitive fog that POTS patients describe so consistently it has its own shorthand.

What you're looking for is the shape of the curve. How far above your lying baseline does your heart rate go, and does it stay there? A single peak number is a data point. The trajectory across ten minutes is the actual finding.

A word of caution: if you feel genuinely faint, sit or lie down. The test is information, not a test of endurance, and there is nothing to prove by staying upright until you fall.

Why one reading lies, and a log tells the truth

Here is the part that catches people off guard. POTS is not stable from day to day. The same person can run a 35-beat jump one morning and a 70-beat jump the next, and both are real.

That variability isn't noise to be averaged away — it's the signal. POTS responds powerfully to context, and the context is often legible once you write it down beside the numbers.

Heat dilates your blood vessels and makes pooling worse, which is why a hot shower or a warm afternoon can flatten you. Meals, especially carbohydrate-heavy ones, divert a surge of blood to the digestive tract; many people feel markedly worse in the hour after eating. Mornings are frequently the hardest, because a night of lying down and losing fluid leaves you mildly volume-depleted before the day even starts. The menstrual cycle moves symptoms for many. So does dehydration, poor sleep, a passing illness, or a stretch of necessary bed rest that quietly deconditions the body and tightens the whole spiral.

None of these connections are visible in a single reading. They only surface across weeks, when you can lay your standing numbers next to what you ate, how you slept, where you were in your cycle, how hot the room was. That is the difference between knowing you have bad days and knowing which days are bad and why. The first is a complaint. The second is something you and a clinician can actually work with — and increasingly, the data that supports treatment decisions about fluids, salt, compression, and exercise.

What the pattern lets you do

Once the pattern is in front of you, the abstract advice for POTS becomes specific to you. If your numbers crater in the hour after lunch, you learn to keep meals smaller and sit afterward. If heat is your trigger, you stop scheduling anything important after a hot shower. If your worst readings cluster on under-hydrated mornings, the standard guidance to increase fluids and salt stops being generic and starts being a lever you can see working when the next morning's curve sits lower than the last.

You also build something a fifteen-minute appointment can't produce on its own: a record. Walking into a clinic with weeks of standing heart rates, symptoms, and context turns a vague conversation into a real one. It can shorten the long, exhausting road many people travel before POTS is recognized at all.

Bringing the measurements together

This is the quiet work Stable is built for. It's a POTS tracker designed around exactly this practice — logging your orthostatic heart rate, your symptoms, and the daily context around them, so the pattern hiding inside the variability has somewhere to become visible. Instead of a scatter of readings in a notes app, you get the shape over time: the triggers that repeat, the interventions that move the curve, the record worth bringing to your doctor.

You don't need an app to do the ten-minute stand test, and you should try it regardless — it's genuinely useful on its own. But if you want the pattern to assemble itself instead of doing the bookkeeping by hand, that's what Stable is here to hold. You can find it at https://stable.lumenlabs.works.