What causes hypotension?

Causes of Low Blood Pressure (Hypotension)

Low blood pressure (hypotension) is primarily caused by dehydration, medications, and autonomic dysfunction, with treatment focused on addressing the underlying cause and maintaining adequate organ perfusion.

Definition of Hypotension

• Hypotension is generally defined as:
  • Systolic blood pressure below 90 mmHg
  • Diastolic blood pressure below 60 mmHg
  • Or a significant drop from baseline (≥20 mmHg systolic or ≥10 mmHg diastolic within 3 minutes of standing) 1

Major Causes of Hypotension

1. Volume Depletion

• Dehydration: Inadequate fluid intake, excessive fluid loss (vomiting, diarrhea, fever, excessive sweating) 2
• Blood loss: Trauma, gastrointestinal bleeding, surgical blood loss 2
• Salt depletion: Excessive diuretic use, low sodium intake in susceptible individuals 2

2. Medication-Related Causes

• Antihypertensive medications:
  • ACE inhibitors/ARBs
  • Beta-blockers
  • Calcium channel blockers
  • Diuretics 2
• Other medications:
  • Antidepressants (especially tricyclics)
  • Antipsychotics
  • Opioids
  • Sedatives
  • Vasodilators 2

3. Cardiovascular Causes

• Poor cardiac output: Heart failure, arrhythmias, valvular disease 2
• Vasodilation: Sepsis, anaphylaxis, neurogenic shock 2
• Structural heart disease: Aortic stenosis, hypertrophic cardiomyopathy 2

4. Neurogenic/Autonomic Causes

• Primary autonomic failure: Pure autonomic failure (Bradbury-Eggleston syndrome) 3
• Secondary autonomic dysfunction:
  • Diabetic autonomic neuropathy
  • Parkinson's disease
  • Multiple system atrophy (Shy-Drager syndrome)
  • Amyloidosis 4, 1
• Baroreceptor dysfunction: Causing wide swings in blood pressure 3

5. Endocrine Causes

• Adrenal insufficiency: Primary (Addison's disease) or secondary (pituitary dysfunction) 4
• Hypoaldosteronism: Primary or secondary 4
• Pheochromocytoma: Rare cause, especially during surgical removal 4
• Diabetic dysautonomia: Autonomic neuropathy affecting blood pressure regulation 4, 1

6. Situational/Positional Causes

• Orthostatic hypotension: Blood pressure drop when changing position 1, 5
• Postprandial hypotension: Blood pressure drop after meals 2, 6
• Vasovagal syncope: Triggered by specific stimuli (emotional stress, pain, prolonged standing) 2

7. Other Causes

• Advanced age: Decreased baroreceptor sensitivity and autonomic function 2, 6
• Prolonged bed rest: Deconditioning of cardiovascular system 7
• Pregnancy: Physiologic vasodilation 1
• Heat exposure: Vasodilation and volume depletion 2

Special Considerations

Orthostatic Hypotension

• Defined as a drop in blood pressure (≥20 mmHg systolic or ≥10 mmHg diastolic) within 3 minutes of standing 1, 5
• Common symptoms: dizziness, lightheadedness, blurred vision, weakness, fatigue 1
• Risk factors: advanced age, hypertension, diabetes, Parkinson's disease 8

Medication-Induced Hypotension

• Particularly common in older adults with multiple comorbidities and polypharmacy 2
• Discontinuation or dose reduction of offending medications often improves symptoms 2
• Careful monitoring needed when adjusting medications due to potential worsening of underlying conditions 2

Hypotension in Heart Failure

• May indicate poor cardiac output or excessive medication effect 2
• Requires careful balance of medications to avoid both hypotension and congestion 2
• Target lowest effective doses of evidence-based therapies 2

Diagnostic Approach

• Measure blood pressure in different positions (supine, sitting, standing) 1
• Review medication list thoroughly for potential causes 2
• Assess volume status (skin turgor, mucous membranes, jugular venous pressure) 2
• Consider specialized testing for suspected autonomic dysfunction (tilt-table testing) 1
• Evaluate for endocrine causes with appropriate laboratory testing when suspected 4

Management Principles

• Treat the underlying cause when identified 2, 1
• Volume repletion for dehydration (oral or IV fluids with sodium) 2
• Medication adjustment - reduce or discontinue offending agents when possible 2
• Physical counterpressure maneuvers for orthostatic symptoms (leg crossing, muscle tensing) 8
• Pharmacologic therapy for persistent neurogenic orthostatic hypotension:
  • Midodrine (alpha-1 agonist) for refractory cases 7, 5
  • Fludrocortisone for volume expansion 8, 5

Prevention Strategies

• Adequate hydration and salt intake (if not contraindicated) 2
• Slow position changes (sitting before standing) 1
• Compression garments for orthostatic hypotension 8
• Elevation of head of bed at night (for supine hypertension with orthostatic hypotension) 7
• Regular physical activity to improve cardiovascular tone 1

Remember that chronic hypotension can lead to inadequate organ perfusion and should be addressed promptly, especially when symptomatic or in high-risk populations such as the elderly.