Orthostatic changes in hemodynamics

Physiology - The Body's Rebound

• Trigger: Transitioning to an upright posture (e.g., standing).
• Initial Effect: Gravitational pooling of ~500-700 mL of blood in lower extremity veins.
• Cascade: This ↓ venous return (preload), leading to ↓ cardiac output and a transient ↓ in mean arterial pressure ($MAP$).

• Response (Baroreflex): Aortic and carotid baroreceptors detect the ↓ $MAP$, triggering a rapid autonomic response:
  • ↑ Sympathetic outflow & ↓ parasympathetic outflow.
  • Leads to ↑ heart rate, ↑ contractility, and systemic vasoconstriction (↑ TPR).
  • This compensates for the initial drop, restoring blood pressure and cerebral perfusion.

⭐ Orthostatic Hypotension: Defined as a sustained reduction in systolic blood pressure of at least 20 mmHg or a reduction in diastolic blood pressure of 10 mmHg within 3 minutes of standing.

Pathophysiology - When Gravity Wins

• Upon standing, gravity pulls ~500-1000 mL of blood into the compliant lower-body venous system.
• This ↓ venous return to the heart, leading to a transient drop in cardiac output (CO) and mean arterial pressure (MAP).
• The primary compensatory response is the baroreceptor reflex.

⭐ Orthostatic hypotension occurs when this reflex fails, defined by a sustained reduction in systolic blood pressure of at least 20 mmHg or a reduction in diastolic blood pressure of 10 mmHg within three minutes of standing.

Diagnosis - Spotting the Drop

• Core Criteria: Diagnosis confirmed with a bedside orthostatic vitals test.
  • Measure blood pressure and heart rate in the supine position after 5 minutes of rest.
  • Have the patient stand up.
  • Repeat measurements at 1 and 3 minutes.
• Positive Test: Within 3 minutes of standing:
  • Systolic BP drop ≥ 20 mmHg
  • Diastolic BP drop ≥ 10 mmHg
  • Symptoms of cerebral hypoperfusion (e.g., lightheadedness, dizziness).

⭐ A key diagnostic clue is the heart rate response. An insufficient heart rate increase (<15 bpm) despite hypotension suggests a neurogenic cause (autonomic failure).

Management - Staying Upright

• Non-Pharmacologic (First-line):

  • Physical counter-maneuvers (e.g., leg crossing, squatting, muscle tensing).
  • Increase salt and water intake.
  • Wear compression stockings or abdominal binders.
  • Elevate head of bed.

• Pharmacologic Options:

  • Fludrocortisone (expands plasma volume).
  • Midodrine or Droxidopa (vasoconstrictors).

⭐ Midodrine, an α1-agonist, carries a risk of supine hypertension. Counsel patients to take it while upright and avoid doses near bedtime.

High‑Yield Points - ⚡ Biggest Takeaways

• On standing, gravity pools blood in the lower body, causing a transient drop in venous return.
• This leads to decreased cardiac output and a fall in blood pressure.
• The baroreceptor reflex is the immediate compensatory response.
• It triggers increased sympathetic outflow, causing vasoconstriction and increased heart rate.
• Orthostatic hypotension results from a failure of this reflex.
• It's defined as a SBP drop of >20 mmHg or DBP drop of >10 mmHg within 3 minutes of standing.