Core57: Drug-induced adverse reaction: orthostatic hypotension

🧍 High-yield / 要点：Orthostatic hypotension = BP drop after standing: SBP ↓ ≥20 mmHg or DBP ↓ ≥10 mmHg within 3 minutes. Many cardiovascular, sedative, and anticholinergic drugs can cause it.

Definition / 定義

• Orthostatic/postural hypotension = low blood pressure that occurs when standing after sitting or lying.
• Diagnostic criterion:
  • Systolic BP decreases by ≥20 mmHg, or
  • Diastolic BP decreases by ≥10 mmHg,
  • within 3 minutes of standing.

Symptoms / 症状

• Dizziness.
• Lightheadedness.
• Weakness.
• Syncope.

Pathomechanism / 機序

• On standing, lower body vessels should constrict to maintain blood pressure.
• If sympathetic regulation is delayed or absent:
  • Blood pools in leg vessels.
  • Venous return decreases.
  • Cardiac output decreases.
  • Blood pressure falls.

Drug causes / 原因薬

Drug class	Examples	Note
Selective α1 antagonists	Prazosin, terazosin, doxazosin, tamsulosin	Most pronounced after first dose
Nonselective α antagonists	Phenoxybenzamine, phentolamine	Vasodilation
Organic nitrates	Nitroglycerin	Venodilation and reduced preload
Antihypertensives	ACE inhibitors, ARBs, β-blockers, calcium channel blockers	BP-lowering effect
SGLT2 inhibitors	Canagliflozin, dapagliflozin, empagliflozin	Dehydration/volume loss
Antipsychotics	Clozapine, olanzapine, chlorpromazine	Anti-α1 effect
First-generation antihistamines	Diphenhydramine, promethazine, dimetindene	Sedating/anticholinergic effects
Diuretics	Furosemide, hydrochlorothiazide, spironolactone	Volume depletion
Antidepressants	TCAs, MAO inhibitors	Autonomic/vascular effects

Remember / 覚え方

• Stand up → BP drops → dizziness/syncope
• α1 blockers, nitrates, diuretics, antipsychotics are high-yield causes
• First-dose prazosin effect is classic