The hallmark sign of POTS is a measured increase in heart rate by at least 30 beats per minute within 10 minutes of assuming an upright position. For people aged between 12 and 19, the minimum increase for diagnosis is 40 beats per minute. This symptom is known as orthostatic (upright) tachycardia (fast heart rate). It occurs without any coinciding drop in blood pressure, as that would indicate orthostatic hypotension. It should be noted, however, that certain medications to treat POTS may cause orthostatic hypotension. It is accompanied by other features of orthostatic intolerance—symptoms which develop in an upright position and are relieved by reclining. These orthostatic symptoms include palpitations, light-headedness, chest discomfort, shortness of breath, nausea, weakness or "heaviness" in the lower legs, blurred vision and cognitive difficulties. Symptoms may be exacerbated with prolonged sitting, prolonged standing, alcohol, heat, exercise, or eating a large meal.

In up to one third of people with POTS, fainting occurs in response to postural changes or exercise. Migraine-like headaches are common, sometimes with symptoms worsening in an upright position (orthostatic headache). Some people with POTS develop acrocyanosis, or blotchy, red/blue skin upon standing, especially over the feet (indicative of blood pooling). 48% of people with POTS report chronic fatigue and 32% report sleep disturbances. Others exhibit only the cardinal symptom of orthostatic tachycardia.

POTS can co-occur in all types of Ehlers–Danlos syndrome (EDS), a hereditary connective tissue disorder marked by loose hypermobile joints prone to subluxations and dislocations, skin that exhibits moderate or greater laxity, easy bruising, and many other symptoms. A trifecta of POTS, EDS, and Mast Cell Activation Syndrome (MCAS) is becoming increasingly more common, with a genetic marker common among all three conditions. POTS is also often accompanied by vasovagal syncope, with a 25% overlap being reported. There is significant overlap between POTS and chronic fatigue syndrome, with evidence of POTS in 25–50% of CFS cases. Fatigue and reduced exercise tolerance are prominent symptoms of both conditions, and dysautonomia may underlie both conditions.

Postural orthostatic tachycardia syndrome (POTS) is a condition in which a change from lying to standing causes an abnormally large increase in heart rate. This occurs with symptoms that may include lightheadedness, trouble thinking, blurry vision, or weakness. Other commonly associated conditions include irritable bowel disease, insomnia, chronic headaches, Ehlers-Danlos syndrome, fibromyalgia.

The cause of POTS is poorly understood. Often it begins after a viral infection, surgery, or pregnancy. Risk factors include a family history of the condition. Diagnosis in adults is based on an increase in heart rate of more than 30 beats per minute within ten minutes of standing up which is accompanied by symptoms. Low blood pressure with standing, however, does not occur. Other conditions which can cause similar symptoms, such as prolonged bedrest, dehydration, hyperthyroidism, anemia, and certain medications, must not be present.

Treatment may include avoiding factors that bring on symptoms, increasing dietary salt and water, compression stockings, exercise, cognitive behavioral therapy, and medications. Medications used may include beta blockers, pyridostigmine, midodrine, or fludrocortisone. More than 50% of people whose condition was triggered by a viral infection get better within five years. About 90% get better with treatment. It is estimated that 0.5 to 3 million people are affected in the United States. The average age of onset is 20 years old and it occurs more often in females.

Orthostatic hypotension is characterised by symptoms that occur after standing (from lying or sitting), particularly when this is done rapidly. Many report lightheadedness (a feeling that one might be about to faint), sometimes severe. Generalized weakness or tiredness may also occur. Some also report difficulty concentrating, blurred vision, tremulousness, vertigo, anxiety, palpitations (awareness of the heartbeat), feeling sweaty or clammy, and sometimes nausea. A person may look pale.

The tilt table test is an evaluative clinical test to help identify postural hypotension, a common cause of presyncope or syncope. A tilt angle of 60 and 70 degrees is optimal and maintains a high degree of specificity. A positive sign with the tilt table test must be taken in context of patient history, with consideration of pertinent clinical findings before coming to a conclusion.

Orthostatic hypotension, also known as postural hypotension, occurs when a person's blood pressure falls when suddenly standing up from a lying or sitting position. It is defined as a fall in systolic blood pressure of at least 20 mm Hg or diastolic blood pressure of at least 10 mm Hg when a person assumes a standing position. It occurs predominantly by delayed constriction of the lower body blood vessels, which is normally required to maintain an adequate blood pressure when changing position to standing. As a result, blood pools in the blood vessels of the legs for a longer period and less is returned to the heart, thereby leading to a reduced cardiac output. Mild orthostatic hypotension is common and can occur briefly in anyone, although it is prevalent in particular among the elderly and those with known low blood pressure. Severe drops in blood pressure can lead to fainting, with a possibility of injury.

There are numerous possible causes for orthostatic hypotension, such as certain medications (e.g. alpha blockers), autonomic neuropathy, decreased blood volume, and age-related blood vessel stiffness.

Apart from addressing the underlying cause, orthostatic hypotension may be treated with a recommendation to increase salt and water intake (to increase the blood volume), wearing compression stockings, and sometimes medication (fludrocortisone, midodrine or others).

Presyncope is a state of lightheadedness, muscular weakness, blurred vision, and feeling faint (as opposed to a syncope, which is actually fainting). Presyncope is most often cardiovascular in cause. In many people, lightheadedness is a symptom of orthostatic hypotension. Orthostatic hypotension occurs when blood pressure drops significantly when the patient stands from a supine (horizontal) or seatted position. If loss of consciousness occurs in this situation, it is termed syncope.

Presyncope is frequently reported in people with autonomic dysfunctions such as the postural orthostatic tachycardia syndrome (POTS).

The primary symptoms of hypotension are lightheadedness or dizziness.

If the blood pressure is sufficiently low, fainting may occur.

Low blood pressure is sometimes associated with certain symptoms, many of which are related to causes rather than effects of hypotension:

- chest pain

- shortness of breath

- irregular heartbeat

- fever higher than 38.3 °C (101 °F)

- headache

- stiff neck

- severe upper back pain

- cough with sputum

- Prolonged diarrhea or vomiting

- dyspepsia (indigestion)

- dysuria (painful urination)

- adverse effect of medications

- acute, life-threatening allergic reaction

- seizures

- loss of consciousness

- profound fatigue

- temporary blurring or loss of vision

- Black tarry stools

Patients who suffer from acute OI usually manifest the disorder by a temporary loss of consciousness and posture, with rapid recovery (simple faints, or syncope), as well as remaining conscious during their loss of posture. This is different from a syncope caused by cardiac problems because there are known triggers for the fainting spell (standing, heat, emotion) and identifiable prodromal symptoms (nausea, blurred vision, headache). As Dr. Julian M. Stewart, an expert in OI from New York Medical College states, "Many syncopal patients have no intercurrent illness; between faints, they are well."

Symptoms:

- Altered vision (blurred vision, "white outs"/gray outs, black outs, double vision)

- Anxiety

- Exercise intolerance

- Fatigue

- Headache

- Heart palpitations, as the heart races to compensate for the falling blood pressure

- Hyperpnea or sensation of difficulty breathing or swallowing (see also hyperventilation syndrome)

- Lightheadedness

- Sweating

- Tremulousness

- Weakness

A classic manifestation of acute OI is a soldier who faints after standing rigidly at attention for an extended period of time.

Patients with chronic orthostatic intolerance have symptoms on most or all days. Their symptoms may include most of the symptoms of acute OI, plus:

- Nausea

- Neurocognitive deficits, such as attention problems

- Pallor

- Sensitivity to heat

- Sleep problems

- Other vasomotor symptoms.

Symptoms reported by patients vary in frequency and severity.

Symptoms associated with IST include:

- Frequent or sustained palpitations

- Dyspnea (shortness of breath) and palpitations on exertion

- Pre-syncope (feeling as if about to faint)

- Fatigue (physical)

- Dizziness

- Exercise intolerance

- Occasional paresthesia and cramping

- Symptoms associated with autonomic nervous system disturbance, including GI disturbance

Orthostatic hypotension, also called "postural hypotension", is a common form of low blood pressure. It occurs after a change in body position, typically when a person stands up from either a seated or lying position. It is usually transient and represents a delay in the normal compensatory ability of the autonomic nervous system. It is commonly seen in hypovolemia and as a result of various medications. In addition to blood pressure-lowering medications, many psychiatric medications, in particular antidepressants, can have this side effect. Simple blood pressure and heart rate measurements while lying, seated, and standing (with a two-minute delay in between each position change) can confirm the presence of orthostatic hypotension. Orthostatic hypotension is indicated if there is a drop in 20 mmHg of systolic pressure (and a 10 mmHg drop in diastolic pressure in some facilities) and a 20 beats per minute increase in heart rate.

Vasovagal syncope is a form of dysautonomia characterized by an inappropriate drop in blood pressure while in the upright position. Vasovagal syncope occurs as a result of increased activity of the vagus nerve, the mainstay of the parasympathetic nervous system .

Another, but rarer form, is postprandial hypotension, a drastic decline in blood pressure that occurs 30 to 75 minutes after eating substantial meals. When a great deal of blood is diverted to the intestines (a kind of "splanchnic blood pooling") to facilitate digestion and absorption, the body must increase cardiac output and peripheral vasoconstriction to maintain enough blood pressure to perfuse vital organs, such as the brain. Postprandial hypotension is believed to be caused by the autonomic nervous system not compensating appropriately, because of aging or a specific disorder.

Hypotension is a feature of Flammer syndrome which is characterized by cold hands and feet and predisposes to normal tension glaucoma.

The body has several feedback mechanisms to maintain adequate blood flow and blood pressure. If blood pressure decreases, the heart beats faster in an attempt to raise it. This is called reflex tachycardia. This can happen in response to a decrease in blood volume (through dehydration or bleeding), or an unexpected change in blood flow. The most common cause of the latter is orthostatic hypotension (also called postural hypotension). Fever, hyperventilation, diarrhea and severe infections can also cause tachycardia, primarily due to increase in metabolic demands.

An increase in sympathetic nervous system stimulation causes the heart rate to increase, both by the direct action of sympathetic nerve fibers on the heart and by causing the endocrine system to release hormones such as epinephrine (adrenaline), which have a similar effect. Increased sympathetic stimulation is usually due to physical or psychological stress. This is the basis for the so-called fight-or-flight response, but such stimulation can also be induced by stimulants such as ephedrine, amphetamines or cocaine. Certain endocrine disorders such as pheochromocytoma can also cause epinephrine release and can result in tachycardia independent of nervous system stimulation. Hyperthyroidism can also cause tachycardia. The upper limit of normal rate for sinus tachycardia is thought to be 220 bpm minus age.

Usually in women with no heart problems, this syndrome is characterized by normal resting heart rate but exaggerated postural sinus tachycardia with or without orthostatic hypotension.

An electrocardiogram (ECG) is used to classify the type of tachycardia. They may be classified into narrow and wide complex based on the QRS complex. Presented order of most to least common, they are:

- Narrow complex

- Sinus tachycardia, which originates from the sino-atrial (SA) node, near the base of the superior vena cava

- Atrial fibrillation

- Atrial flutter

- AV nodal reentrant tachycardia

- Accessory pathway mediated tachycardia

- Atrial tachycardia

- Multifocal atrial tachycardia

- Junctional tachycardia

- Wide complex

- Ventricular tachycardia, any tachycardia that originates in the ventricles

- Any narrow complex tachycardia combined with a problem with the conduction system of the heart, often termed "supraventricular tachycardia with aberrancy"

- A narrow complex tachycardia with an accessory conduction pathway, often termed "supraventricular tachycardia with pre-excitation" (e.g. Wolff–Parkinson–White syndrome)

- Pacemaker-tracked or pacemaker-mediated tachycardia

Tachycardias may be classified as either narrow complex tachycardias (supraventricular tachycardias) or wide complex tachycardias. Narrow and wide refer to the width of the QRS complex on the ECG. Narrow complex tachycardias tend to originate in the atria, while wide complex tachycardias tend to originate in the ventricles. Tachycardias can be further classified as either regular or irregular.

No formal diagnostic criteria exist. A diagnosis of Inappropriate sinus tachycardia is primarily one of exclusion and the following may be observed:

- Exclusion of all other causes of sinus tachycardia

- Common forms of supraventricular tachycardia (SVT) must be excluded

- Normal P wave morphology

- A resting sinus tachycardia is usually (but not always) present

- Nocturnal dip in heart rate

- Inappropriate heart rate response on exertion

- Mean heart rate in 24hrs >95 bpm

- Symptoms are documented to be due to tachycardia

- Hypotension is occasionally observed

- Syncope (fainting) is occasionally reported

Sinus tachycardia (also colloquially known as sinus tach or sinus tachy) is a sinus rhythm with an elevated rate of impulses, defined as a rate greater than 100 beats/min (bpm) in an average adult. The normal resting heart rate in the average male adult ranges from 60–100 bpm and women 60-90bpm. Note that the normal heart rate varies with age, with infants having normal heart rate of 110–150 bpm, in contrast to the elderly, who have slower normals.

Symptoms of Da Costa's syndrome include fatigue upon exertion, shortness of breath, palpitations, sweating, and chest pain. Physical examination reveals no physical abnormalities causing the symptoms.

Da Costa's syndrome is generally considered a physical manifestation of an anxiety disorder.

A degenerative disease of the autonomic nervous system, symptoms include dizziness and fainting (caused by orthostatic hypotension), visual disturbances and neck pain. Chest pain, fatigue and sexual dysfunction are less common symptoms that may also occur. Symptoms are worse when standing; sometimes one may relieve symptoms by lying down.

The most common symptom is dizziness or syncope which often occurs during exercise or as a response to emotional stress.

On an EKG, Junctional Tachycardia exhibits the following classic criteria:

- P-Waves: The p-wave may be inverted in leads II, III and V or not visible

- Narrow QRS complexes (which is consistent with non-ventricular rhythms)

It can coexist with other superventricular tachycardias due to the disassociation between the SA node and the AV node. Junctional Tachycardia can appear similar to atrioventricular nodal reentrant tachycardia.

One form is junctional ectopic tachycardia.

In 1925, Bradbury and Eggleston first characterized three patients seemingly with a common syndrome, with what they described as "the occurrence of syncopal attacks after or during exertion or even after standing erect for some minutes. Other features in the three patients are a slow, unchanging pulse rate, incapacity to perspire, a lowered basal metabolism and signs of slight and indefinite changes in the nervous system. Each of these patients felt much worse during the heat of summer." Further research identified multiple causes for these syndromic findings, now grouped as primary autonomic disorders (also called primary dysautonomia), including Pure Autonomic Failure, Multiple System Atrophy, and Parkinson's. The primary differentiating characteristic of Pure autonomic failure is decreased circulation and synthesis of norepinephrine, and dysfunction localized peripherally. It is relevant to note that progression to central nervous system neurodegeneration can also occur.

CPVT typically start manifesting during the first or second decade of life. The majority of events occur during childhood with more than 60% of affected individuals having their first episode of syncope or cardiac arrest by age 12-20.

The symptoms of Dysautonomia, which are numerous and vary widely for each individual, are due to inefficient or unbalanced efferent signals sent via both systems. The primary symptoms in individuals with dysautonomia include

It can be associated with digitalis toxicity. It may be also be due to onset of acute coronary syndrome, heart failure, conduction system diseases with enhanced automaticity, or administration of theophylline.