• Diagnostic criteria for postural orthostatic tachycardia syndrome
  All of the following criteria must be met:

• Sustained heart rate increase of ≥ 30 beats/min (or ≥ 40 beats/min if patient is aged 12–19 yr) within 10 minutes of upright posture.

• Absence of significant orthostatic hypotension (magnitude of blood pressure drop ≥ 20/10 mm Hg).

• Very frequent symptoms of orthostatic intolerance that are worse while upright, with rapid improvement upon return to a supine position. Symptoms vary between individuals, but often include lightheadedness, palpitations, tremulousness, generalized weakness, blurred vision and fatigue.

• Symptom duration ≥ 3 months.

• Absence of other conditions that could explain sinus tachycardia (Box 3).

The orthostatic tachycardia must occur in the absence of classical orthostatic hypotension, but transient initial orthostatic hypotension10 does not preclude a diagnosis of POTS.5 The patient’s heart rate should rise by at least 30 beats/min (or ≥ 40 beats/min if patient is aged 12–19 yr) in at least 2 measurements taken at least 1 minute apart (Box 2). The Canadian Cardiovascular Society statement5 set a minimum supine heart rate of 60 beats/min to prevent the diagnosis of POTS being made in a patient with a low resting heart rate that increases to a normal level on standing.

It is physiologically normal for orthostatic tachycardia to vary slightly from day to day and for diurnal variability to exist such that greater orthostatic tachycardia occurs in the morning than later in the day.11 If a clinician has a high suspicion of POTS, but a patient does not meet the criterion for orthostatic tachycardia at their initial evaluation, reassessment at a later date is prudent, preferably in the morning.