What causes hypotension?

Causes of Hypotension

Hypotension results from six major mechanisms: volume depletion (most common), medication effects (especially in older adults), cardiac dysfunction, autonomic failure, vasodilatory states, and endocrine disorders. 1, 2

Volume Depletion and Fluid Loss

Fluid volume depletion is the most common cause of hypotension in clinical practice. 1, 2

• Acute fluid losses from diarrhea, vomiting, fever, or excessive diuresis precipitate hypotension through inadequate circulating volume 1, 2
• Aggressive diuretic therapy, particularly in heart failure patients, leads to volume depletion, electrolyte abnormalities, and acute renal failure 2, 3
• Hemorrhage and acute blood loss reduce circulating volume and oxygen-carrying capacity 4
• Dehydration manifests along a spectrum from compensated tachycardia to decompensated shock 1

Medication-Induced Hypotension

Drug-induced hypotension is particularly prevalent in older adults with polypharmacy and represents a highly reversible cause. 1, 3

Antihypertensive Medications

• ACE inhibitors and ARBs cause hypotension, especially when combined with volume depletion from diuretics 2, 3
• Beta-blockers, particularly those with alpha-blocking properties like carvedilol, typically cause hypotension within 24-48 hours of initiation or dose increase 3
• Calcium channel blockers and centrally acting antihypertensives (clonidine) are non-recommended agents in heart failure due to hypotensive effects 1
• Alpha-blockers cause first-dose hypotension and orthostatic symptoms 5

Other Medication Classes

• Psychotropic medications including phenothiazines, tricyclic antidepressants, and monoamine oxidase inhibitors have significant hypotensive effects 5
• Negative chronotropes and sedatives reduce cardiac output and vascular tone 1
• Dopamine agonists, antianginals, and antiarrhythmics affect cardiovascular hemodynamics 5

Cardiac Causes

Cardiogenic shock is defined by systolic BP <90 mmHg, central filling pressure >20 mmHg, or cardiac index <1.8 L/min/m². 1, 3

• Poor cardiac output from heart failure, myocardial infarction, or cardiomyopathy reduces systemic perfusion 2
• Bradyarrhythmias and tachyarrhythmias impair cardiac filling and output 1
• Mechanical complications of myocardial infarction (papillary muscle rupture, ventricular septal defect, free wall rupture) cause acute hemodynamic collapse 1

Autonomic Dysfunction

Autonomic failure manifests as orthostatic hypotension (≥20 mmHg systolic or ≥10 mmHg diastolic drop within 3 minutes of standing). 1, 2, 3

Primary Autonomic Disorders

• Peripheral autonomic impairment (Bradbury-Eggleston syndrome) causes pure autonomic failure 6
• Central autonomic impairment (Shy-Drager syndrome/multiple system atrophy) involves widespread neurodegeneration 6
• Dopamine-beta-hydroxylase deficiency represents a genetic defect causing norepinephrine absence with dopamine accumulation 6

Secondary Autonomic Dysfunction

• Diabetic autonomic neuropathy impairs cardiovascular reflexes and causes orthostatic hypotension 2, 6
• Reduced baroreflex sensitivity and cardiac vagal function in high-risk patients contribute to blood pressure instability 1
• Baroreceptor dysfunction causes wide blood pressure swings unrelated to posture 6

Vasodilatory and Reflex-Mediated Hypotension

Vasovagal syncope and situational hypotension result from parasympathetic overstimulation with sympathetic withdrawal. 1, 6

• Vasovagal syncope is triggered by emotional upset, pain, or prolonged standing 1
• Situational syncope occurs with specific activities: micturition, coughing, defecation, or carotid sinus pressure 1, 6
• Septic shock (sepsis requiring vasopressors to maintain MAP ≥65 mmHg with lactate >2 mmol/L despite adequate volume resuscitation) causes profound vasodilation 3

Endocrine Causes

Adrenal insufficiency and hypoaldosteronism cause hypotension with characteristic electrolyte abnormalities (hyponatremia and hyperkalemia). 7

Primary Adrenal Disorders

• Primary adrenal failure from autoimmune destruction, hemorrhage, or infection causes combined glucocorticoid and mineralocorticoid deficiency with hyperreninism 7
• Congenital 21-hydroxylase deficiency with salt-wasting presents in infancy with severe hypotension 7
• Isolated aldosterone synthase deficiency is rare in adults 7

Secondary Hypoaldosteronism

• Hyporeninemic hypoaldosteronism occurs with diabetes mellitus, chronic kidney disease, advanced age, and HIV infection 7
• Central adrenal insufficiency (hypopituitarism) causes isolated glucocorticoid deficiency with hyporeninism 7

Other Endocrine Causes

• Pheochromocytoma rarely presents with hypotension, particularly during surgical removal without adequate alpha-blockade preparation 7
• Carcinoid crisis causes flushing with profound hypotension, responsive to subcutaneous somatostatin analogs 7

Perioperative Hypotension

Perioperative hypotension is common after anesthetic induction, especially in older individuals with higher baseline sympathetic drive. 4, 1

• Anesthetic agents impair baroreflex sensitivity, removing key blood pressure control mechanisms 1
• Antihypertensive medications, inadequate intravenous fluid administration, adverse effects of anesthetic drugs, intraoperative and ongoing blood loss, inflammatory response to surgery, arrhythmias, and impaired myocardial function all contribute 4
• Only 54% of patients with postoperative hypotension respond to fluid boluses, suggesting vascular tone or inotropy issues in the remainder 2

Critical Pitfalls

• Relying solely on absolute BP values rather than assessing symptoms and organ perfusion leads to inappropriate management 1, 3
• Failing to identify medication-induced hypotension in older adults with polypharmacy has significant consequences 1, 3
• Overlooking volume depletion as the primary cause delays simple, effective treatment 1, 2
• Inappropriate vasopressor use may correct hypotension but potentially causes multi-organ cellular injury 1