Causes of Hypotension with Bradycardia

1. Toxicologic/Pharmacologic Causes

Beta-Blocker Toxicity

• Competitive blockade of β₁ receptors → reduced HR, AV conduction, and contractility → bradycardia + hypotension
• May be indistinguishable clinically from calcium channel blocker (CCB) toxicity
• Clues: normal/elevated glucose, normal-to-high K⁺, cool/clammy skin
• Treatment: fluids, glucagon (first-line — bypasses β receptor via adenylate cyclase), high-dose insulin (HDI), vasopressors
• Tintinalli's Emergency Medicine, p. 1314

Calcium Channel Blocker Toxicity

• Non-dihydropyridines (verapamil, diltiazem): block L-type Ca²⁺ channels in SA/AV node and myocardium → bradycardia, AV block, myocardial depression
• Dihydropyridines (amlodipine, nifedipine): primarily vasodilation → hypotension; reflex bradycardia may occur
• Clues: elevated lactate, hyperglycemia (insulin secretion is calcium-dependent), warm/flushed skin with amlodipine
• Verapamil is the most lethal in overdose
• Treatment: IV calcium, HDI, norepinephrine, possible ECMO; glucagon is not recommended (unlike β-blocker OD)
• Rosen's Emergency Medicine, p. 2941; Tintinalli's, p. 1318

Cardiac Glycoside Toxicity (Digoxin, Oleander, Foxglove, Bufo Toad)

• Enhanced vagal tone + direct AV node suppression → bradycardia, heart block, and hypotension
• Acute OD: associated with hyperkalemia (Na/K-ATPase blockade)
• May cross-react with digoxin immunoassay (plant/animal glycosides)
• Ventricular ectopy is characteristic
• Tintinalli's Emergency Medicine, p. 1314

Clonidine / Central α₂ Agonists (also Tetrahydrozoline)

• Stimulates presynaptic α₂ receptors in the rostral ventrolateral medulla → decreased sympathetic outflow → bradycardia and hypotension
• Clues: "opioid-like" triad — coma, miosis, bradypnea (may respond partially to naloxone)
• Rosen's Emergency Medicine; Tintinalli's Emergency Medicine, p. 1318

Class IA & IC Antiarrhythmics (e.g., Propafenone, Flecainide, Quinidine)

• Sodium channel blockade → wide-complex bradycardia, conduction slowing, and negative inotropy → hypotension
• Clue: wide-QRS bradycardia
• Tintinalli's Emergency Medicine, p. 1318

Organophosphates / Cholinergic Toxidrome

• Acetylcholinesterase inhibition → excess ACh at muscarinic receptors → bradycardia, hypotension, bronchospasm
• Full SLUDGE/DUMBELS picture: Salivation, Lacrimation, Urination, Defecation, GI distress, Emesis; Miosis, Bradycardia
• Tintinalli's Emergency Medicine, p. 1314

Cyanide Poisoning

• Profound metabolic (lactic) acidosis from mitochondrial ETC blockade → cardiovascular collapse with bradycardia in late/severe toxicity
• Clue: cherry-red skin, fire victim, highly elevated lactate
• Rosen's Emergency Medicine

Large Overdoses of Sedative-Hypnotics and Muscle Relaxants

• CNS/autonomic depression → bradycardia and hypotension
• Tintinalli's Emergency Medicine, p. 1318

2. Cardiovascular/Cardiac Causes

Acute Coronary Syndrome (especially Inferior STEMI)

• RCA occlusion → ischemia of SA/AV node (RCA supplies the SA node in ~60%, AV node in ~80%) → sinus bradycardia or heart blocks (I°, II° Mobitz I/Wenckebach)
• Hypotension from cardiogenic shock, RV infarction, or Bezold-Jarisch reflex
• Right ventricular MI: classic triad of hypotension + elevated JVP + clear lung fields, with bradycardia
• Tintinalli's Emergency Medicine, p. 1318

High-Grade AV Block / Sick Sinus Syndrome

• Intrinsic conduction disease (ischemia, infiltration, fibrosis) causing escape rhythms → rate too slow to maintain adequate cardiac output

3. Neurologic/Reflex-Mediated Causes

Neurogenic Shock (Spinal Cord Injury ≥ T4–T6)

• Loss of sympathetic outflow below the lesion → peripheral vasodilation + venous pooling + unopposed vagal tone
• Classic triad: bradycardia + hypotension + peripheral vasodilation (warm, pink skin — distinguishes it from other shock)
• Bradycardia worsens with cervical/upper thoracic injuries (loss of cardiac sympathetics at T1–T4)
• Occurs in 7–20% of spinal cord injuries
• Tintinalli's Emergency Medicine; Goldman-Cecil Medicine; Schwartz's Principles of Surgery

Vasovagal (Neurocardiogenic) Syncope / Reflex Syncope

• Bezold-Jarisch reflex: vigorous ventricular contraction in a near-empty ventricle (after prolonged standing/blood pooling) → paradoxical vagal activation → bradycardia + vasodilation → hypotension
• Cardioinhibitory type: bradycardia/asystole predominates
• Vasodepressor type: peripheral vasodilation predominates
• Mixed type: both
• Triggers: pain, venipuncture, micturition, defecation, carotid sinus hypersensitivity, postural stress
• Braunwald's Heart Disease; Adams & Victor's Neurology; Ganong's Medical Physiology

Raised Intracranial Pressure (Cushing Reflex)

• Severe ICP → brainstem compression → classic Cushing's triad: hypertension + bradycardia + irregular respirations
• Note: in end-stage herniation, hypotension replaces hypertension as cardiovascular collapse occurs

High Spinal / Total Spinal Anesthesia

• Spinal anesthetic ascending to cervical levels → blockade of cardiac accelerator fibers (T1–T4) and peripheral sympathetics → severe bradycardia + hypotension + respiratory insufficiency
• Morgan & Mikhail's Clinical Anesthesiology

4. Metabolic/Endocrine Causes

Myxedema Coma (Severe Hypothyroidism)

• Profound thyroid hormone deficiency → decreased HR, contractility, and peripheral resistance
• Features: bradycardia (sinus), hypotension refractory to vasopressors until thyroid hormone replaced, hypothermia, hyponatremia, altered mentation
• Precipitated by infection, cold, surgery, or sedatives
• Rosen's Emergency Medicine, p. ~501; Goldman-Cecil Medicine; Miller's Anesthesia

Hypothermia

• Core temperature < 32°C → progressive bradycardia (J/Osborn waves on ECG), decreased cardiac output, hypotension
• Risk of VF below 30°C
• Tintinalli's Emergency Medicine, p. 1318

Hyperkalemia

• K⁺ > ~6.5 mEq/L → depolarization of conduction system → conduction slowing, bradyarrhythmias (sine wave pattern), and cardiovascular collapse
• Tintinalli's Emergency Medicine, p. 1318

Summary Table

Clinical Approach

1. ECG immediately — identify AV blocks, ST changes, hyperkalemia pattern, QRS width
2. Toxicology screen — if any suspicion of overdose; check glucose, K⁺, lactate
3. Glucose — elevated in CCB toxicity (diagnostic clue)
4. History — medications (β-blockers, digoxin, CCBs, antiarrhythmics, clonidine), thyroid history, spinal injury
5. Skin exam — warm/vasodilated (neurogenic shock, DHP-CCB) vs. cold/clammy (cardiogenic, β-blocker) vs. flushed (vasovagal pre-syncope)

• Rosen's Emergency Medicine, Concepts and Clinical Practice
• Tintinalli's Emergency Medicine: A Comprehensive Study Guide
• Goldman-Cecil Medicine
• Braunwald's Heart Disease