POTS

Postural Orthostatic Tachycardia Syndrome (POTS) is a form of dysautonomia — a malfunction of the autonomic nervous system’s regulation of heart rate and blood pressure in response to positional changes.

The Defining Feature

On standing, the heart rate increases by ≥30 bpm (or to ≥120 bpm absolute) within 10 minutes, without a significant drop in blood pressure. The tachycardia is a compensatory response — the body is trying to maintain perfusion against gravity, but the normal mechanisms for doing so aren’t working properly.

In children and adolescents, the threshold is ≥40 bpm, because younger people normally have a slightly larger postural heart rate response.

What’s Actually Happening

When you stand, gravity pulls blood toward your legs. Normally, the autonomic nervous system responds in milliseconds:

1. Baroreceptors in the carotid sinus and aortic arch detect the slight drop in blood pressure
2. The sympathetic nervous system increases heart rate and constricts peripheral blood vessels
3. Skeletal muscle contractions in the legs compress veins, pushing blood back toward the heart
4. Blood volume is redistributed efficiently — you barely notice

In POTS, one or more of these mechanisms fails partially:

Neuropathic POTS: Damage to small peripheral nerve fibers impairs sympathetic vasoconstriction in the legs. Blood pools. The heart compensates by beating faster. This type connects to Small Fiber Neuropathy and may be caused by chronic mast cell mediator exposure damaging nerve fibers over time.

Hyperadrenergic POTS: The sympathetic nervous system overreacts. Norepinephrine levels are elevated, producing excessive tachycardia plus blood pressure instability (sometimes hypertension on standing rather than hypotension). This type often co-occurs with MCAS because mast cell mediator release can directly stimulate the sympathetic nervous system.

Hypovolemic POTS: Low circulating blood volume means there’s less blood to redistribute. The heart must work harder to maintain perfusion. This can be caused by chronic mast cell mediator-driven fluid shifts (Histamine increases vascular permeability, allowing fluid to leak from blood into tissues).

Many patients have features of more than one type.

The Mast Cell Connection

Several Mast Cell Mediators directly produce POTS-like hemodynamics:

• Histamine → vasodilation → pooling → compensatory tachycardia
• PGD2 → vasodilation
• Heparin → may affect blood volume and coagulation
• Cytokines → chronic inflammation may damage autonomic nerves
• Leukotrienes → vascular permeability, bronchoconstriction (compounds the cardiovascular stress)

Additionally, Degranulation itself can trigger acute POTS-like episodes — sudden mast cell activation produces a bolus of vasodilating mediators, causing a rapid drop in vascular resistance and a compensatory tachycardia spike.

See The Trifecta — MCAS, POTS, and EDS for how this connects to the broader pattern.

Symptoms Beyond Tachycardia

POTS is more than a fast heart rate. The autonomic dysfunction affects multiple systems:

• Lightheadedness, presyncope, or frank syncope on standing
• Exercise intolerance (cardiovascular system can’t upregulate properly)
• Blood pooling in legs (visible as purple/mottled discoloration)
• Temperature dysregulation (sweating abnormalities)
• GI dysmotility (the autonomic nervous system also runs the gut)
• Brain fog (reduced cerebral perfusion, especially upright)
• Fatigue
• Sleep fragmentation — adrenaline surges during positional changes at night (see Sleep and Histamine)

Diagnosis

• Tilt table test: Patient is strapped to a table that moves from supine to upright while heart rate and blood pressure are monitored
• Active standing test: Simpler — lie down for 5-10 minutes, then stand still for 10 minutes while monitoring heart rate. More accessible and can be done at home as a screening tool
• Norepinephrine levels: Elevated standing norepinephrine suggests hyperadrenergic type

The anxiety misdiagnosis

POTS tachycardia feels like anxiety because the sympathetic nervous system activation is real — the adrenaline is actually elevated. But the cause is cardiovascular dysregulation, not a psychological state. Patients are frequently told they have anxiety disorder. Beta-blockers prescribed “for anxiety” may actually help because they’re treating the tachycardia. The mechanism matters because the treatment strategy differs substantially.