What are the causes and treatments for hypotension?

Causes of Dangerously Low Blood Pressure

Dangerously low blood pressure requiring immediate intervention occurs from four primary mechanisms: hypovolemia (volume depletion), vasodilation (loss of vascular tone), cardiac pump failure (low cardiac output), and bradycardia or arrhythmias. 1

Life-Threatening Causes Requiring Immediate Recognition

Hypovolemic Shock

• Blood loss from trauma, gastrointestinal bleeding, or surgical hemorrhage represents the most common cause of acute hypotension requiring emergent treatment 2
• Volume depletion from severe dehydration, excessive diuretic use, or third-spacing of fluids (burns, pancreatitis) 2, 1
• Approximately 50% of hypotensive patients are NOT hypovolemic, making reflexive fluid administration dangerous without proper assessment 2, 1

Cardiogenic Causes

• Acute myocardial infarction with pump failure, particularly involving large areas of myocardium 2, 3
• Acute pulmonary edema from left ventricular failure 2
• Cardiac arrhythmias including severe bradycardia or tachyarrhythmias that compromise cardiac output 2, 1
• Mechanical complications of MI including papillary muscle rupture, ventricular septal defect, or free wall rupture 3

Distributive Shock

• Septic shock from overwhelming infection causing profound vasodilation 4
• Anaphylaxis with massive histamine release and vascular collapse 1
• Neurogenic shock from spinal cord injury causing loss of sympathetic tone 1

Endocrine Emergencies

• Acute adrenal crisis (Addisonian crisis) from primary or secondary adrenal insufficiency, which is rare but life-threatening 2, 5
• Severe hypothyroidism (myxedema coma) 5
• Pheochromocytoma crisis during surgical removal without adequate alpha-blockade preparation 5

Obstructive Causes

• Aortic dissection causing acute blood loss or cardiac tamponade 2
• Massive pulmonary embolism obstructing right ventricular outflow 1
• Tension pneumothorax compressing the heart and great vessels 1

Perioperative and Acute Care Settings

Intraoperative Hypotension

• Anesthetic agents causing vasodilation and myocardial depression, particularly with cyclopropane or halogenated hydrocarbons 4
• Spinal or epidural anesthesia causing sympathetic blockade 6
• Acute blood loss during surgery requiring damage control resuscitation 2

Post-Anesthesia Care Unit

• Residual anesthetic effects combined with inadequate volume resuscitation 2
• Unrecognized bleeding in the immediate postoperative period 2
• Cardiac dysfunction from perioperative myocardial injury 2

Chronic Conditions Predisposing to Dangerous Hypotension

Autonomic Dysfunction

• Diabetic autonomic neuropathy causing orthostatic hypotension and impaired cardiovascular reflexes 5, 7
• Pure autonomic failure and multiple system atrophy 7
• Parkinson's disease with associated autonomic dysfunction 7

Medication-Induced

• Excessive antihypertensive therapy, particularly in elderly patients with aggressive blood pressure targets 2, 7
• MAO inhibitors potentiating the effects of vasopressors and causing unpredictable responses 4
• Tricyclic antidepressants potentiating cardiovascular effects of adrenergic agents 4
• Diuretic overuse causing volume depletion 7

Cardiac Conditions

• Advanced heart failure with reduced ejection fraction where baseline systolic pressure <100 mmHg predicts dangerous drops 2
• Acute decompensated heart failure requiring careful balance between perfusion and congestion 2, 3

Special Clinical Scenarios

Acute Ischemic Stroke

• Persistent arterial hypotension is rare in acute stroke but when present suggests aortic dissection, volume depletion, or myocardial ischemia 2
• Hypotension in stroke requires immediate investigation as it compromises cerebral perfusion pressure 2

Traumatic Brain Injury

• Mean arterial pressure must be maintained ≥80 mmHg to ensure adequate cerebral perfusion, making permissive hypotension absolutely contraindicated 1
• Any hypotension in severe TBI is dangerous and requires immediate correction 1

Trauma Without Brain Injury

• Permissive hypotension (systolic 80-90 mmHg) is actually the target during uncontrolled hemorrhage until surgical control is achieved 2, 1
• Aggressive fluid resuscitation before hemorrhage control increases mortality 1

Critical Metabolic Derangements

Hypoglycemia

• Severe hypoglycemia can cause focal neurological signs mimicking stroke and lead to brain injury 2
• Requires immediate finger-stick glucose measurement and rapid correction 2

Electrolyte Abnormalities

• Severe hyperkalemia from hypoaldosteronism causing cardiac conduction abnormalities 5
• Hyponatremia from adrenal insufficiency or SIADH 5

Key Diagnostic Pitfalls

The passive leg raise test is the most reliable bedside assessment for determining if hypotension is due to hypovolemia, with a positive likelihood ratio of 11 and 92% specificity 2, 1. Traditional signs like tachycardia, oliguria, and decreased skin turgor are not predictive of fluid responsiveness 2.

Preoperative blood pressure measurements are unreliable for determining individual hypotension thresholds, as >80% of patients have inadequately characterized baseline pressures 2. Ambulatory blood pressure monitoring reveals multiple hypertension endotypes that dramatically affect perioperative risk but are missed by single clinic measurements 2.

Hypotension may be a biomarker rather than direct cause of organ injury in many perioperative settings, as interventional trials targeting specific blood pressure thresholds have failed to improve outcomes 2. The underlying pathophysiology causing both hypotension and organ dysfunction may be more important than the blood pressure number itself 2.