What constitutes orthostasis?

What Constitutes Orthostasis

Orthostasis refers to the physiologic state of assuming an upright position, which triggers a series of cardiovascular adjustments including blood volume shifts, compensatory vasoconstriction, and heart rate changes to maintain adequate cerebral perfusion. 1

Normal Physiologic Response to Orthostasis

When moving from supine to standing position, the normal cardiovascular response includes:

• A slight reduction in blood pressure (approximately 4 mmHg systolic and 5 mmHg diastolic) 1
• An increase in heart rate to compensate for reduced venous return 1
• Activation of baroreceptor reflexes that transfer information from arterial baroreceptors in the carotid arteries and aortic arch to the vasomotor center in the medulla oblongata 1
• Increased peripheral vascular resistance through sympathetic activation to prevent blood pooling in lower extremities 1

Pathologic Orthostatic Responses

Classical Orthostatic Hypotension

Classical OH is defined as a sustained decrease in systolic BP ≥20 mmHg, diastolic BP ≥10 mmHg, or a sustained decrease in systolic BP to an absolute value <90 mmHg within 3 minutes of standing or head-up tilt. 1, 2

• In patients with supine hypertension, a systolic BP drop ≥30 mmHg should be considered diagnostic 1, 2
• The heart rate response is typically blunted (<10 bpm increase) in neurogenic OH due to impaired autonomic control 1, 2
• In contrast, heart rate increase is preserved or enhanced in OH due to hypovolemia 1

Initial Orthostatic Hypotension

Initial OH is characterized by a BP decrease on standing of >40 mmHg systolic and/or >20 mmHg diastolic within 15 seconds of standing. 1, 2

• Blood pressure spontaneously and rapidly returns to normal, so the period of hypotension is brief (<40 seconds) 1
• This represents a transient mismatch between cardiac output and total peripheral resistance 1
• Syncope is rare with initial OH, though lightheadedness and visual disturbances commonly occur 1

Delayed Orthostatic Hypotension

Delayed OH is defined as OH occurring beyond 3 minutes of head-up tilt or active standing. 1, 2

• It is characterized by a slow progressive decrease in BP over time 1
• The absence of bradycardia helps differentiate delayed OH from reflex syncope 1
• This pattern suggests progressive fall in venous return and low cardiac output 1

Clinical Context and Testing

Proper Measurement Technique

When assessing for orthostatic changes:

• Wait at least 2 minutes before measuring supine vital signs 1
• Wait at least 1 minute before measuring upright vital signs to allow hemodynamic changes to stabilize 1
• Keep the blood pressure cuff at heart level and use properly sized cuff 1
• Blood pressure cannot be measured reliably in atrial fibrillation using standard instruments 1

Diagnostic Testing

• Active standing test (lying to standing) is the primary diagnostic method for classical and initial OH 1
• Head-up tilt-table testing can aid in confirming diagnosis when standard orthostatic vital signs are nondiagnostic 1, 3
• Beat-to-beat BP monitoring is particularly useful for detecting initial OH 1

Key Clinical Pitfalls

A critical pitfall is that patients with heart failure and elevated filling pressures may paradoxically show an increase in systolic blood pressure with orthostatic changes due to improved hemodynamics from reduced preload. 1 When these patients achieve lower filling pressures, the loss of this paradoxical increase may indicate a euvolemic state. 1

Another common error is measuring orthostatic vitals too quickly, before cardiovascular adjustments have occurred, leading to false negative results. 1