Orthostatic intolerance describes conditions where symptoms develop upon moving to an upright position and are relieved by lying down. This occurs because the body struggles to maintain stable blood flow to the brain against the pull of gravity when standing. Orthostatic Hypotension (OH) and Postural Orthostatic Tachycardia Syndrome (POTS) are the two most common conditions under this umbrella. While both cause debilitating symptoms like dizziness and lightheadedness when upright, they are distinct disorders with different defining physiological markers and underlying mechanisms.
Understanding Orthostatic Hypotension
Orthostatic Hypotension (OH) is defined by a significant drop in blood pressure when a person moves from a lying or sitting position to standing. This pressure drop quickly reduces blood flow (perfusion) to the brain, causing characteristic symptoms. The formal diagnostic criteria require a sustained reduction of at least 20 mmHg in systolic blood pressure or 10 mmHg in diastolic blood pressure within three minutes of standing.
The underlying cause is a failure of the body’s autonomic nervous system to adequately compensate for the gravitational blood shift. When a healthy person stands, the body reflexively constricts blood vessels in the lower extremities (vasoconstriction) and slightly increases heart rate to prevent blood from pooling. In OH, this compensatory mechanism is impaired, often due to issues with autonomic nerves or hypovolemia (low blood volume). Insufficient vasoconstriction allows blood to pool, reducing the volume returning to the heart and consequently dropping blood pressure.
Understanding Postural Orthostatic Tachycardia Syndrome
Postural Orthostatic Tachycardia Syndrome (POTS) is characterized by an excessive increase in heart rate upon standing, without a significant blood pressure drop. The heart rate spike is the body’s attempt to compensate for the reduction in blood return to the heart caused by gravity.
The key diagnostic criteria for POTS is a sustained increase in heart rate of 30 beats per minute (bpm) or more within 10 minutes of standing for adults. For adolescents aged 12 to 19, a sustained increase of 40 bpm is required. This tachycardia must occur without the simultaneous drop in blood pressure that defines OH. Mechanisms behind POTS are complex, often involving low blood volume, neuropathic issues leading to excessive blood pooling, or an overactive sympathetic nervous system.
Key Differences and Relationship
The fundamental difference between OH and POTS lies in the body’s primary physiological response to standing. OH is defined by a failure to maintain blood pressure, marked by a significant blood pressure drop. POTS is defined by a failure to regulate heart rate, marked by an excessive heart rate spike while blood pressure remains stable.
Both conditions are forms of dysautonomia, involving a dysfunction of the autonomic nervous system. While they share symptoms like dizziness and lightheadedness, symptom profiles can differ. OH symptoms relate directly to poor cerebral perfusion and resolve quickly upon lying down. POTS can involve more widespread chronic issues, including brain fog, fatigue, and gastrointestinal problems.
Clinically, a diagnosis of classic OH typically excludes a diagnosis of POTS, although patients may experience features of both. The distinction rests on which measurable parameter—blood pressure or heart rate—crosses the specified diagnostic threshold. This separation is important because the underlying mechanisms and treatment strategies are different.
Diagnosis and Management Approaches
The definitive tool used to distinguish between these two conditions is the Tilt Table Test. This test systematically monitors blood pressure and heart rate as the patient is tilted upright, allowing clinicians to observe sustained changes. The results determine if the patient meets the criteria for the blood pressure drop of OH or the heart rate increase of POTS.
Management strategies are tailored to the specific physiological problem identified. For OH, treatment focuses on stabilizing blood pressure and correcting the underlying cause. Non-pharmacological approaches include increasing salt and fluid intake, wearing compression garments to reduce blood pooling, and performing physical counter-maneuvers like leg crossing. Medications may be used to increase blood volume or promote vasoconstriction.
POTS management centers on increasing fluid and sodium intake to expand blood volume, often targeting 2-3 liters of fluid and 5-10 grams of salt daily. Specialized exercise programs, often starting with recumbent activities like rowing or swimming, are encouraged to improve physical stamina. Medications for POTS often aim to control the excessive heart rate, such as low-dose beta-blockers, or to enhance volume retention.