FreeMedsite

USMLE Step 1 Review

Physiologic Dead Space (V_D)

1

What is Physiologic Dead Space?

Definition:

The portion of each breath that does not participate in gas exchange.

Physiologic dead space = Anatomic dead space + Alveolar dead space

1. Anatomic Dead Space

Air in the conducting airways (nose, trachea, bronchi) that never reaches alveoli.

Normal: ~150 mL

2. Alveolar Dead Space

Air in alveoli that are ventilated but not perfused (no blood flow), so gas exchange cannot occur.

Normal: ~0 mL

Key Concepts for Step 1:

• A. Normal Physiology: Physiologic dead space is approximately equal to anatomic dead space because alveolar dead space is negligible.
• B. Disease States: Alveolar dead space increases in pathologies like Pulmonary Embolism (ventilation continues, but perfusion is blocked).
• C. Gravity: In a standing person, the apex of the lung has more alveolar dead space than the base due to lower perfusion at the top.
• D. Tachypnea: Rapid shallow breathing increases the percentage of total ventilation that is wasted in dead space.

2

Visualization Diagram

Total Physiologic Dead Space = Anatomic (Trachea/Bronchi) + Alveolar (Non-perfused Alveoli)

3

The Bohr Equation

VD = VT × [(PaCO₂ - PeCO₂) / PaCO₂]

FreeMedsite

USMLE Step 1 Review | Respiratory Physiology