What causes hypotension?

Causes of Hypotension

Hypotension can result from multiple etiologies including dehydration, medications, autonomic dysfunction, cardiac disorders, and endocrine abnormalities, with treatment directed at the underlying cause. 1

Definition and Clinical Significance

• Hypotension is defined by symptoms occurring at systolic and diastolic blood pressures below 90 and 60 mmHg respectively, though there is no universal threshold unlike hypertension 2
• Clinically significant hypotension should be assessed based on symptoms and organ perfusion rather than absolute blood pressure values alone 1
• Severe hypotension (systolic blood pressure <80 mmHg) or hypotension causing major symptoms requires immediate attention and potential adjustment of medical treatment 1

Major Causes of Hypotension

Dehydration and Volume Depletion

• Fluid volume depletion from acute causes (diarrhea, vomiting, fever, excessive diuresis) is a common cause of hypotension 1
• Dehydration manifests along a spectrum from tachycardia to shock, depending on whether compensation is adequate 1
• Orthostatic tolerance worsens with dehydration and is exacerbated by heat stress, which promotes vasodilation 1

Medication-Induced Hypotension

• Drug-induced hypotension is particularly prevalent in older adults with multiple comorbidities and polypharmacy 1, 3
• Common culprit medications include:
  • Antihypertensives: diuretics, vasodilators, venodilators 1
  • Negative chronotropes (beta-blockers) and sedatives 1
  • Psychiatric medications: phenothiazines, tricyclic antidepressants, MAO inhibitors 3
  • Cardiovascular drugs: dopamine agonists, antianginals, antiarrhythmics 3
  • Non-recommended agents for heart failure patients: calcium channel blockers, centrally acting antihypertensives, alpha-blockers 1

Orthostatic Hypotension

• Defined as a drop of ≥20 mmHg in systolic BP and/or ≥10 mmHg in diastolic BP within 3 minutes of standing 1
• Associated with autonomic dysfunction, which increases with age 4
• Common in neurodegenerative diseases, autoimmune conditions, diabetes, hypertension, heart failure, and kidney failure 4
• Can be classified as:
  • Primary (neurogenic): due to autonomic nervous system disorders 5
  • Secondary: medication effects, volume depletion, or other medical conditions 1

Cardiac Causes

• Cardiogenic shock: characterized by systolic pressure <90 mmHg, central filling pressure >20 mmHg, or cardiac index <1.8 L/min/m² 1
• Usually associated with extensive left ventricular damage but can occur with other cardiac conditions 1
• Other cardiac causes include:
  • Bradyarrhythmias (especially in inferior myocardial infarction) 1
  • Tachyarrhythmias causing reduced cardiac output 1
  • Mechanical complications of myocardial infarction (ventricular septal defect, mitral regurgitation) 1

Endocrine Causes

• Adrenal insufficiency (primary or secondary) can cause acute or chronic hypotension 2
• Isolated hypoaldosteronism (primary or secondary) presents with hypotension, hyponatremia, and hyperkalemia 2
• Pheochromocytoma can paradoxically cause hypotension, especially during surgical removal without proper preparation 2
• Carcinoid syndrome can present with flushing and hypotension during carcinoid crisis 2
• Diabetic autonomic neuropathy leading to orthostatic hypotension 2, 4

Autonomic Dysfunction

• Peripheral autonomic impairment (Bradbury-Eggleston syndrome) 5
• Central autonomic impairment (Shy-Drager syndrome) 5
• Genetic defects such as dopamine-beta-hydroxylase deficiency 5
• Baroreceptor dysfunction causing wide blood pressure swings unrelated to posture 5
• Reduced baroreflex sensitivity and cardiac vagal function in high-risk patients 1

Situational and Reflex-Mediated Hypotension

• Vasovagal syncope triggered by specific scenarios (emotional upset, pain) 1
• Situational syncope associated with specific activities:
  • Micturition (post-micturition syncope)
  • Coughing (cough syncope)
  • Defecation (defecation syncope)
  • Straining or arising from squatting 1

Perioperative Hypotension

• Common after anesthetic induction, especially in older individuals with higher baseline sympathetic drive 1
• Anesthetic agents impair baroreflex sensitivity, removing key blood pressure control mechanisms 1
• Postoperative hypotension is often unrecognized and may be more harmful than intraoperative hypotension due to longer duration 1

Special Considerations

Hypotension in Heart Failure

• Low blood pressure in heart failure patients requires careful assessment of organ perfusion rather than relying solely on BP values 1
• In patients with heart failure and reduced ejection fraction (HFrEF), low BP may reflect advanced disease stage 1
• Guideline-directed medical therapy should be initiated at low doses in hypotensive HFrEF patients, starting with medications that have minimal BP-lowering effects 1

Hypotension in Critical Illness

• Cardiogenic shock in myocardial infarction requires exclusion of other causes like hypovolemia, vasovagal reactions, electrolyte disturbances, or arrhythmias 1
• In acute settings, rapid assessment of volume status and cardiac function is essential 1
• Nerve agent intoxication can cause severe hypotension through parasympathetic overstimulation after an initial hypertensive phase 1

Pitfalls and Caveats

• Relying solely on absolute BP values rather than assessing symptoms and organ perfusion can lead to inappropriate management 1
• Failing to identify medication-induced hypotension, especially in older adults with polypharmacy 1, 3
• Overlooking chronic constitutional hypotension as a cause of significant morbidity 6
• Missing orthostatic hypotension due to inadequate testing (measurements should be taken both supine and standing) 1
• Inappropriate use of vasopressors may correct hypotension but potentially at the expense of multi-organ cellular injury 1
• Discontinuing essential medications without considering alternative approaches or lower dosing strategies 1