What Is Hypotension?

Hypotension, commonly called low blood pressure, is a state in which arterial pressure falls below the range needed to perfuse organs adequately. The National Heart, Lung, and Blood Institute uses a threshold of 90/60 mm Hg as the conventional dividing line, while noting that some healthy people sit below it permanently without symptoms. What matters clinically is not the number alone but whether blood flow to the brain, heart, and kidneys is sufficient, which is why hypotension is defined partly by symptoms such as dizziness, blurred vision, fatigue, and fainting.

Arterial pressure is the product of cardiac output and systemic vascular resistance, so hypotension arises whenever either term falls. Reduced circulating volume from hemorrhage or dehydration, impaired cardiac pumping from infarction or arrhythmia, and inappropriate vasodilation from sepsis or anaphylaxis are the three broad mechanisms. Many common medications, including diuretics, alpha blockers, nitrates, and antidepressants, lower pressure as a side effect. Severe hypotension that compromises tissue oxygen delivery is shock, a medical emergency distinct from the mild chronic low readings that carry no risk.

Regulation and Compensatory Response

Blood pressure is held within limits by a layered control system. Baroreceptors in the carotid sinus and aortic arch sense stretch and adjust sympathetic and parasympathetic outflow within a heartbeat or two, raising heart rate and constricting resistance vessels when pressure falls. Slower loops operate over minutes to days: the renin-angiotensin-aldosterone system and vasopressin release conserve sodium and water and raise vascular tone. Failure at any level produces a characteristic pattern. Autonomic neuropathy from diabetes, Parkinson disease, or amyloidosis blunts the fast reflex, so patients cannot mount the expected tachycardia. Understanding which loop has failed guides both diagnosis and the choice between volume expansion, vasopressors, and pacing.

Orthostatic and Postprandial Hypotension

Orthostatic hypotension is defined as a sustained drop of at least 20 mm Hg systolic or 10 mm Hg diastolic within three minutes of standing. Roughly half a liter of blood pools in the lower body on standing, and if the baroreflex response is inadequate, cerebral perfusion falls transiently. It is a major cause of falls and fracture in older adults and is strongly associated with autonomic dysfunction. A common concern is that treating high blood pressure aggressively will provoke it, but a study reported by NHLBI found that intensive management of hypertension actually lowered the risk of orthostatic hypotension rather than raising it. Related syndromes include postprandial hypotension, driven by splanchnic blood pooling after a meal, and postural orthostatic tachycardia syndrome, which NHLBI has convened a state of the science workshop to address.

Measurement and Predictive Monitoring

Diagnosis depends on measurement technique. Oscillometric cuffs are standard but lose accuracy at low pressures and in irregular rhythms, and tilt table testing is used to reproduce orthostatic symptoms under controlled conditions. In critical care and anesthesia, an indwelling arterial catheter gives beat-to-beat pressure, and continuous noninvasive methods based on volume clamping provide a waveform without arterial puncture. Signal processing on that waveform now supports prediction rather than detection alone: algorithms trained on arterial pressure morphology can flag an impending hypotensive episode several minutes before mean arterial pressure crosses the threshold, an approach examined in a narrative review of the Hypotension Prediction Index and assessed against other machine learning methods in a systematic review and meta-analysis of intraoperative hypotension prediction.

Applications

Work on hypotension has applications in a range of fields, including:

• Perioperative hemodynamic management and closed-loop fluid administration
• Intensive care monitoring and early warning systems for sepsis and shock
• Wearable and ambulatory blood pressure devices for fall risk assessment in older adults
• Autonomic function testing in neurology
• Dialysis machine control, where intradialytic hypotension is a frequent complication
• Aerospace and aviation medicine, including g-induced loss of consciousness research
• Pharmacovigilance and drug safety evaluation