What are the causes of hypotension?

Causes of Hypotension

Hypotension is caused by multiple pathophysiological mechanisms including cardiovascular dysfunction, autonomic disorders, medication effects, volume depletion, and endocrine abnormalities, all of which can lead to significant morbidity and mortality if not properly identified and managed.

Definition and Classification

Hypotension is generally defined as:

• Systolic blood pressure <90 mmHg
• Diastolic blood pressure <60 mmHg
• Or a significant drop in blood pressure causing symptoms

Types of Hypotension

1. Orthostatic Hypotension

   • Defined as a decrease in systolic BP ≥20 mmHg or diastolic BP ≥10 mmHg within 3 minutes of standing 1
   • Affects approximately 7% of men over 70 years old 2
   • Associated with 64% increase in age-adjusted mortality 2

2. Chronic Hypotension

   • Persistent low blood pressure not related to positional changes
   • Often defined as systolic BP <100 mmHg in non-acute settings 3

3. Acute Hypotension/Shock

   • Sudden, severe drop in blood pressure requiring immediate intervention

Major Causes of Hypotension

1. Medication-Related Causes

• Antihypertensive medications:
  • Beta-blockers 2
  • ACE inhibitors and ARBs 2
  • Calcium channel blockers
  • Diuretics (especially with volume depletion) 2
  • Clonidine and other central α-agonists 2
• Other medications:
  • Nitrates 2
  • Antidepressants (particularly tricyclics)
  • Antipsychotics
  • Opioids
  • Anesthetics

2. Cardiovascular Causes

• Cardiogenic shock:
  • Myocardial infarction with extensive left ventricular damage 2
  • Heart failure (acute or decompensated chronic) 2
  • Arrhythmias (bradycardia, tachyarrhythmias)
  • Valvular dysfunction (especially aortic stenosis, mitral regurgitation) 2
• Pericardial disease:
  • Cardiac tamponade
  • Constrictive pericarditis

3. Volume Depletion

• Hemorrhage/blood loss
• Dehydration from:
  • Vomiting
  • Diarrhea
  • Excessive sweating
  • Inadequate fluid intake
  • Burns
• Third-spacing (fluid shifting into interstitial spaces)

4. Autonomic Dysfunction

• Primary autonomic failure:
  • Pure autonomic failure (Bradbury-Eggleston syndrome) 4
  • Multiple system atrophy (Shy-Drager syndrome) 4
• Secondary autonomic neuropathy:
  • Diabetic autonomic neuropathy 5, 6
  • Amyloidosis
  • Parkinson's disease
  • HIV-related autonomic dysfunction 5
  • Autoimmune autonomic ganglionopathy

5. Endocrine Causes

• Adrenal insufficiency (primary or secondary) 5
• Hypoaldosteronism (primary or secondary) 5
• Pheochromocytoma (paradoxical hypotension) 5
• Carcinoid syndrome 5
• Hypothyroidism (severe)

6. Neurogenic Causes

• Spinal cord injury
• Vasovagal syncope
• Carotid sinus hypersensitivity
• Baroreceptor dysfunction causing blood pressure fluctuations 4

7. Sepsis and Systemic Inflammatory Response

• Septic shock
• Anaphylaxis
• Systemic inflammatory response syndrome

8. Postoperative Hypotension

• Common due to:
  • Anesthetic agents
  • Inadequate fluid administration
  • Ongoing blood loss
  • Inflammatory response to surgery 2

Special Considerations

Orthostatic Hypotension

• Risk factors include:
  • Advanced age
  • Hypertension (paradoxically)
  • Diabetes
  • Parkinson's disease
  • Prolonged bed rest
• Often coexists with supine hypertension, complicating management 1, 7

Perioperative Hypotension

• Causes include:
  • Antihypertensive medications
  • Inadequate IV fluid administration
  • Anesthetic drugs
  • Blood loss
  • Inflammatory response to surgery
  • Arrhythmias
  • Impaired myocardial function 2
• Associated with organ injury, especially acute kidney injury, cardiovascular events, and mortality 2

Diagnostic Approach

Key Clinical Assessments

1. Vital signs pattern:

   • Orthostatic changes (measure BP supine and standing)
   • Heart rate response (tachycardia suggests volume depletion; lack of compensatory tachycardia suggests autonomic dysfunction)
   • Respiratory rate and pattern

2. Volume status assessment:

   • Skin turgor
   • Mucous membrane moisture
   • Jugular venous pressure
   • Peripheral edema

3. Cardiovascular examination:

   • Heart sounds (S3 gallop suggests heart failure)
   • Murmurs (valvular disease)
   • Pericardial rub

4. Medication review:

   • Recent changes in antihypertensive regimen
   • New medications with hypotensive potential

Laboratory and Diagnostic Tests

• Complete blood count (anemia, infection)
• Electrolytes (sodium, potassium abnormalities)
• Renal function (BUN, creatinine)
• Cardiac enzymes if cardiac cause suspected
• Adrenal function tests if endocrine cause suspected
• Echocardiography for suspected cardiac causes 1

Management Considerations

Management should be directed at the underlying cause, but general principles include:

1. Volume resuscitation for hypovolemic states
2. Medication adjustment:
   • Modify or discontinue hypotensive medications 1
   • Consider timing of antihypertensive medications to avoid peak effects during vulnerable periods
3. Positional strategies:
   • Physical counterpressure maneuvers
   • Compression garments for orthostatic hypotension 1
   • Elevating head of bed at night
4. Pharmacologic therapy for persistent orthostatic hypotension:
   • Volume expansion with increased salt and fluid intake 1
   • Fludrocortisone for refractory cases 1, 6
   • Midodrine or droxidopa for neurogenic orthostatic hypotension 6

Key Pitfalls and Caveats

1. Overlooking orthostatic hypotension - Always check for orthostatic changes in blood pressure in elderly patients and those with unexplained falls or syncope

2. Medication timing - The timing of antihypertensive medications can significantly impact the risk of hypotension

3. Paradoxical hypertension-hypotension - Many patients have both hypertension when supine and hypotension when upright, requiring careful balance in management 7

4. Postprandial hypotension - Blood pressure can drop significantly after meals, especially in elderly patients

5. Misdiagnosing shock states - Different types of shock (cardiogenic, hypovolemic, distributive) require different management approaches

6. Overlooking endocrine causes - Adrenal insufficiency and other endocrine disorders can present primarily with hypotension 5