POTS and IST: what's the difference, does it matter, and how do you tell?

POTS and inappropriate sinus tachycardia are both characterised by an abnormally fast heart rate. They share many of the same symptoms. They can coexist. And in post-viral presentations, where the underlying autonomic dysfunction doesn’t always respect diagnostic boundaries, the distinction between them can be genuinely difficult to make. This piece covers what each condition is, how to try to tell them apart using the tools available to you, where they overlap, and what the distinction means (and doesn’t mean) for treatment.

This is not medical advice. I’m not a clinician. Always discuss any intervention with your doctor.

What POTS is

Postural Orthostatic Tachycardia Syndrome is defined by the 2015 Heart Rhythm Society Expert Consensus Statement as a sustained increase in heart rate of 30 or more beats per minute within 10 minutes of standing (or an absolute standing heart rate exceeding 120 bpm), in the absence of significant orthostatic hypotension, with symptoms present for at least three months. The key word is postural: the tachycardia is triggered by, or substantially worsened by, being upright.

Patients with POTS typically have a normal or near-normal heart rate when lying down. Their heart rate rises on standing and stays elevated for as long as they remain upright. Common symptoms include lightheadedness, palpitations, breathlessness, fatigue, exercise intolerance, nausea, and cognitive difficulties. Many patients also experience symptoms on exertion that go beyond the postural component, but the defining feature is the orthostatic heart rate response. For more on the diagnostic threshold and what sub-threshold presentations look like, see the 30 bpm threshold article.

POTS is not a single disease. It’s a syndrome with several proposed subtypes, including neuropathic POTS (associated with small fibre neuropathy and impaired peripheral vasoconstriction), hypovolaemic POTS (associated with low blood volume), and hyperadrenergic POTS (associated with excessive sympathetic nervous system activity and elevated norepinephrine levels). These subtypes are not always clearly separable, and many patients have features of more than one.

What IST is

Inappropriate Sinus Tachycardia is defined as a resting heart rate consistently above 100 bpm, or a 24-hour average heart rate above 90 bpm, in the absence of any identifiable cause (such as anaemia, thyroid disease, medication effects, or deconditioning), with associated symptoms. It is a diagnosis of exclusion.

The distinction from POTS is that in IST, the elevated heart rate is not specifically tied to posture. Someone with IST may have a fast heart rate sitting, lying down, and standing. The heart rate may increase further on standing, but the baseline is already elevated regardless of position. Some IST patients also have elevated heart rates during sleep, though this is not universal.

IST shares many symptoms with POTS: palpitations, fatigue, breathlessness, exercise intolerance, lightheadedness, and presyncope. The overlap is extensive enough that IST is frequently misdiagnosed as POTS, and vice versa.

The pathophysiology of IST is poorly understood. Proposed mechanisms include enhanced intrinsic automaticity of the sinus node (the heart’s pacemaker firing too fast on its own), excessive sympathetic tone, reduced parasympathetic tone, or some combination. A key study by Nwazue et al. (2014) used pharmacological autonomic blockade to show that IST patients had significantly higher sympathetic contribution to heart rate compared with POTS patients and healthy controls, but did not have abnormal intrinsic sinus node automaticity. This suggests that for many IST patients, the problem is autonomic rather than a defect in the heart’s electrical system itself.

Where they overlap

The overlap between POTS and IST is substantial and, in some patients, both diagnoses apply simultaneously. A study by Shabtaie et al. (2021) found that among 305 IST patients, approximately 15% also met objective diagnostic criteria for POTS. A further 8% had a clinical POTS diagnosis that could not be confirmed on formal testing.

In post-viral presentations, the overlap is arguably even more common. Post-viral autonomic dysfunction can produce a pattern that includes an elevated resting heart rate (IST-like), an exaggerated postural response (POTS-like), and a disproportionate heart rate response to any stimulus including heat, meals, and minimal exertion. The heart rate may settle to normal range during sleep but be elevated across all waking activities, with an additional postural spike on top. This mixed picture doesn’t fit neatly into either category. The broader context of long COVID, ME/CFS, and dysautonomia is worth understanding if you’re trying to make sense of overlapping diagnoses.

The Cleveland Clinic’s electrophysiology group has described the practical diagnostic difference simply: in POTS, the 24-hour average heart rate will be normal because the patient spends most of their time recumbent or seated, and their heart rate is normal in those positions. In IST, the 24-hour average will be elevated. This is probably the single most useful distinguishing feature.

How to tell them apart: the tests that matter

The Holter monitor (24- or 48-hour ECG)

This is arguably the most important test for distinguishing POTS from IST. A continuous heart rate recording over 24 to 48 hours captures the full picture: sleeping heart rate, waking resting heart rate, heart rate during activity, and crucially, the overall average.

What to look for:

24-hour average heart rate above 90 bpm points toward IST. A normal 24-hour average (typically 70-80 bpm in adults) with episodic spikes during upright activity points more toward POTS.
Sleeping heart rate is informative. A normal sleeping heart rate (broadly 40-60 bpm depending on age and fitness) suggests the autonomic system is functioning normally at rest and the tachycardia is positional or activity-related. An elevated sleeping heart rate suggests more pervasive autonomic dysfunction or intrinsic sinus node overactivity.
Pattern of heart rate throughout the day matters as much as the averages. POTS will show a clear step-change from lying to upright, with a return to normal when recumbent. IST will show a more persistently elevated rate throughout waking hours, often without sharp positional transitions.

An important caveat: “normal” sleeping heart rate should be interpreted against your personal baseline. If your pre-illness resting heart rate was in the low 60s, your sleeping rate was probably in the 40s. A current sleeping rate of 60-65 would look normal on a report but actually represents a significant upward shift for you individually. This kind of personal context is easily missed in clinical interpretation. Tracking your resting heart rate and HRV consistently over time gives you a meaningful baseline to compare against.

The tilt table test

The formal diagnostic test for POTS. You lie flat on a motorised table, baseline readings are taken, and the table is tilted to approximately 70 degrees (near-upright) for up to 45 minutes while heart rate and blood pressure are continuously monitored.

A positive POTS result is a sustained heart rate increase of 30+ bpm within 10 minutes of tilt, without a significant blood pressure drop, with reproduction of symptoms.

The tilt table test doesn’t diagnose IST; it diagnoses or rules out the postural component. A patient with IST alone would be expected to have an elevated heart rate even while lying on the table before tilt, with a further increase on tilt that may or may not meet the 30 bpm POTS threshold.

If you don’t have access to a tilt table test (NHS waiting times for autonomic testing can be very long), the NASA Lean Test is a reasonable home screening tool that captures the postural heart rate response in a structured way. It’s not a substitute for formal testing, but it provides objective data you can bring to appointments.

Repeated home stand tests

Repeated home stand tests over weeks or months are more informative than a single test on a single day. Heart rate responses vary with hydration, sleep quality, time of day, and symptom severity. Recording multiple stand tests and looking at the pattern over time will give you a better picture of whether the postural increase is consistent and significant, or whether the baseline rate is the more concerning feature. The Stand Test Logger provides a structured way to track these over time.

HRV tracking

Heart rate variability is a proxy for autonomic balance, specifically the interplay between sympathetic and parasympathetic nervous system activity. A very low HRV (below 20ms RMSSD on a wrist-based wearable) suggests the autonomic system is stuck in sympathetic overdrive with poor parasympathetic modulation.

Low HRV alone doesn’t distinguish POTS from IST; both can produce it. But tracking HRV over time alongside interventions can help you assess whether something is shifting the underlying autonomic balance, not just the heart rate. The HRV & Resting Heart Rate Tracker is designed for this, and what I’m tracking and why covers the practical side of setting this up.

Catecholamine testing

If hyperadrenergic POTS or a sympathetic overdrive state is suspected, standing and supine plasma norepinephrine levels can be informative. An upright norepinephrine level above 600 pg/ml (and particularly above 1000 pg/ml) suggests a hyperadrenergic state. This is a blood test that can be requested by a GP or specialist, though interpretation requires clinical context.

This test is worth knowing about but isn’t routinely done in most NHS pathways. It can help differentiate between POTS subtypes and may influence medication choices, particularly whether a central sympatholytic is appropriate. See how to prepare for a GP appointment for guidance on how to raise these kinds of tests in a primary care setting.

What the distinction means for treatment

This is where the question of POTS versus IST has practical relevance. Some treatments are well-suited to both conditions; others are better targeted to one or the other.

Treatments that apply to both

Ivabradine works for both POTS and IST by blocking the funny current (If) at the sinoatrial node, reducing the rate at which the heart’s pacemaker fires. It is arguably the most versatile pharmacological option across the POTS-IST spectrum. In the only double-blind, placebo-controlled crossover trial of ivabradine in IST (Cappato et al., 2012), patients reported elimination of over 70% of symptoms, with nearly half achieving complete symptom resolution. In POTS, the evidence is broadly comparable: consistent heart rate reduction and symptom improvement across multiple studies. For more detail, see the ivabradine evidence review.

Ivabradine does not affect blood pressure, autonomic tone, or myocardial contractility. It treats the heart rate without addressing any underlying mechanism. Whether that’s sufficient depends on what’s driving your symptoms.

Fluid and electrolyte optimisation (2.5-3L water daily, increased sodium intake) is a baseline intervention for any form of autonomic tachycardia. It addresses potential hypovolaemia, which exacerbates both POTS and IST. The salt, water, and compression evidence review covers the rationale and practical approach in detail.

Compression garments reduce peripheral blood pooling and are relevant to both conditions, though arguably more directly targeted at the POTS mechanism.

Treatments more relevant to POTS

Fludrocortisone expands blood volume by increasing sodium reabsorption in the kidneys. This is primarily a POTS intervention, targeting the hypovolaemic component. In IST where the tachycardia isn’t primarily driven by positional blood pooling, fludrocortisone is less likely to be helpful.

Midodrine is a peripheral vasoconstrictor that reduces venous pooling on standing. Again, this addresses a POTS-specific mechanism. In pure IST, where the problem isn’t venous pooling, midodrine has limited rationale.

Pyridostigmine enhances parasympathetic (cholinergic) tone throughout the autonomic nervous system. While it reduces orthostatic heart rate in POTS, its more interesting feature is the evidence for improved exercise capacity in the ME/CFS overlap population (Joseph et al., 2022). It works through a different mechanism from ivabradine and can be used alongside it. Pyridostigmine could also theoretically benefit IST patients with reduced parasympathetic tone, but the trial evidence is primarily in POTS populations. For more detail, see the pyridostigmine evidence review.

Recumbent exercise as a rehabilitation strategy is also more directly relevant to POTS, where reconditioning the postural response is part of the management approach, though the post-exertional malaise caveat applies strongly here.

Treatments more relevant to hyperadrenergic presentations (either condition)

If the underlying driver appears to be excessive sympathetic nervous system activity, whether in the context of POTS, IST, or a mixed presentation, a different class of medications becomes relevant: central sympatholytics. These drugs act on the brain to reduce sympathetic outflow at source.

Methyldopa is the oldest of this class. It inhibits the production of norepinephrine and dopamine centrally. It has a longer half-life than clonidine, making it more stable. There’s a clinically useful observation that hyperadrenergic patients tend to tolerate methyldopa well, while other POTS subtypes may get worse on it, which serves as a rough diagnostic signal. Doses for dysautonomia are lower than the traditional antihypertensive doses (typically 125-250mg twice daily). The main downside is sedation and potential worsening of brain fog, which is a significant concern for anyone already dealing with post-viral cognitive symptoms.

Clonidine is an alpha-2 agonist that reduces sympathetic outflow rapidly and potently. A 2025 study by Lin et al. showed that in hyperadrenergic POTS, clonidine reduced sympathetic nerve activity and lowered both average and maximum heart rates, with 80% of patients who tolerated it long-term reporting fewer symptoms at six months. However, its short half-life creates a rebound risk: when the dose wears off, sympathetic activity can snap back above baseline. This is counterproductive in a condition defined by sympathetic overdrive. The clonidine patch (delivering the drug continuously through the skin) partially mitigates this.

Guanfacine is a longer-acting alpha-2 agonist that provides smoother sympathetic suppression without the rebound issues of clonidine. It’s increasingly used for hyperadrenergic POTS, often in the extended-release formulation. Patient reports suggest it can be helpful for both heart rate and associated symptoms such as hot flashes and adrenaline surges. It pairs well with ivabradine (ivabradine for direct rate control, guanfacine for sympathetic modulation), and this combination has been recommended by multiple patient communities and some clinicians.

A note of caution: full central nervous system sympathetic suppression carries risks beyond the target symptoms. Some patients report developing new problems (GI dysfunction, blood pressure instability) that persist after discontinuation. These drugs should be introduced cautiously, at low doses, with monitoring, and ideally under specialist guidance.

Beta-blockers: a note

Beta-blockers (propranolol, bisoprolol) are frequently the first medication tried for both POTS and IST. They reduce heart rate by blocking sympathetic stimulation at the heart. In many patients they are poorly tolerated: they lower blood pressure (problematic in POTS patients who already have normal or low blood pressure), impair the heart rate response to exercise (problematic when exercise tolerance is already severely limited), and cause fatigue. For a detailed comparison of propranolol against other options, including its use in chronic headache, see the amitriptyline, propranolol, and candesartan review.

For IST specifically, the evidence for beta-blockers is weak. The HRS Consensus Statement notes that they are often ineffective, and the comparative evidence now available suggests ivabradine is better tolerated and at least as effective. For POTS, low-dose propranolol (10-20mg) can help with palpitations and tremor in the hyperadrenergic subtype, but higher doses often make things worse.

Does the label matter?

Practically, the distinction between POTS and IST matters most when it changes what you do. If your heart rate is normal when recumbent and rises dramatically on standing, targeting blood volume and peripheral vasoconstriction (fludrocortisone, midodrine, compression) is logical. If your heart rate is elevated regardless of position, those interventions are less likely to help and direct rate control (ivabradine) or sympatholytic therapy becomes more relevant.

In many post-viral presentations, the label matters less than understanding the pattern. A Holter monitor showing your 24-hour average, sleeping rate, and positional variation tells you more than the diagnostic category. Similarly, knowing whether your HRV is severely suppressed tells you something about the autonomic balance that neither label captures. The COMPASS-31 is also worth completing: it screens for autonomic dysfunction across multiple domains and can help frame what you bring to a specialist appointment.

The Nwazue et al. study’s finding that both POTS and IST respond to sympatholytic treatment (because both involve excessive sympathetic activity, just expressed differently) suggests the underlying problem may be more similar than the diagnostic criteria imply. The question for treatment is less “which label do I have?” and more “what’s driving my heart rate up, and what’s the most targeted way to bring it down?”

References

Sheldon RS, Grubb BP, Olshansky B, et al. 2015 Heart Rhythm Society Expert Consensus Statement on the diagnosis and treatment of postural tachycardia syndrome, inappropriate sinus tachycardia, and vasovagal syncope. Heart Rhythm. 2015;12(6):e41-63.

Nwazue VC, Paranjape SY, Black BK, et al. Postural tachycardia syndrome and inappropriate sinus tachycardia: role of autonomic modulation and sinus node automaticity. J Am Heart Assoc. 2014;3(2):e000700.

Shabtaie SA, Witt C, Asirvatham S. Inappropriate sinus tachycardia and postural tachycardia syndrome: distinct entities or overlapping diseases. J Am Coll Cardiol. 2021;77(18 Suppl 1):235.

Cappato R, Castelvecchio S, Ricci C, et al. Clinical efficacy of ivabradine in patients with inappropriate sinus tachycardia: a prospective, randomized, placebo-controlled, double-blind, crossover evaluation. J Am Coll Cardiol. 2012;60(15):1323-1329.

Lin S, Kote A, Andersson T, et al. Using clonidine for neuromodulation in patients with postural orthostatic tachycardia syndrome. Heart Rhythm. 2025;22(9):e781-e790.

Joseph P, Pari R, Miller S, et al. Neurovascular dysregulation and acute exercise intolerance in myalgic encephalomyelitis/chronic fatigue syndrome: a randomized, placebo-controlled trial of pyridostigmine. Chest. 2022;162(5):1116-1126.

Raj SR, Guzman JC, Harvey P, et al. Canadian Cardiovascular Society position statement on postural orthostatic tachycardia syndrome (POTS) and related disorders of chronic orthostatic intolerance. Can J Cardiol. 2020;36(3):357-372.

Ptaszynski P, Kaczmarek K, Ruta J, et al. Metoprolol succinate vs. ivabradine in the treatment of inappropriate sinus tachycardia in patients unresponsive to previous pharmacological therapy. Europace. 2013;15(1):116-121.

Kaczmarek K, et al. Baseline intrinsic heart rate and response to ivabradine treatment in patients with inappropriate sinus tachycardia. Ann Noninvasive Electrocardiol. 2020;25(2):e12709.