Dead space and alveolar ventilation
Also known as: anatomical dead space, physiological dead space, alveolar dead space, Bohr equation, alveolar ventilation
The part of each breath that does not take part in gas exchange: anatomical dead space (≈150 mL of conducting airways) plus alveolar dead space (ventilated but unperfused alveoli).
Overview
Anatomical dead space is the volume of the conducting airways from the nose and mouth to the terminal bronchioles — ≈150 mL in an adult (≈2 mL/kg, roughly 1 mL per pound of body weight). It is measured by Fowler's method (single-breath nitrogen washout). The last ≈150 mL of each inspiration stays in the airways and is exhaled unchanged, while the first ≈150 mL of each expiration is dead-space gas.
Alveolar dead space consists of alveoli that are ventilated but not perfused (V/Q = ∞); it is negligible in healthy upright lungs but increases in pulmonary embolism, emphysema, low cardiac output and positive-pressure ventilation. Physiological dead space = anatomical + alveolar, calculated with the Bohr equation: VD/VT = (PaCO2 − PĒCO2)/PaCO2, normally ≈0.2–0.35.
Alveolar ventilation V̇A = (VT − VD) × f — ≈(500 − 150) × 12 ≈ 4.2 L/min at rest. Because VD is fixed per breath, rapid shallow breathing wastes a larger fraction of each breath: 250 mL × 24/min gives the same minute ventilation (6 L/min) but only 2.4 L/min of alveolar ventilation. PaCO2 is inversely proportional to alveolar ventilation (PaCO2 ∝ V̇CO2/V̇A).
- Location
Conducting airways (anatomical); unperfused alveoli (alveolar).
Function
- Anatomical dead space warms, humidifies and filters air before it reaches the alveoli.
Clinical significance
Pulmonary embolism
Embolised lung units are ventilated but not perfused, increasing alveolar dead space; patients hyperventilate to maintain V̇A and are usually hypocapnic.
Apparatus dead space
Snorkels, face masks and ventilator tubing add dead space; in mechanically ventilated patients small tidal volumes with large apparatus dead space cause hypercapnia.
Test yourself
Tidal volume is 500 mL, anatomical dead space 150 mL and respiratory rate 12/min. What is the alveolar ventilation?
- 6.0 L/min
- 1.8 L/min
- 7.8 L/min
- 4.2 L/min
Answer: 4.2 L/min
Alveolar ventilation = (VT − VD) × f = (500 − 150) × 12 = 4200 mL/min. Minute ventilation is 6.0 L/min. Rapid shallow breathing wastes a larger fraction of each breath on dead space.
Compiled from standard anatomy and physiology references — see the Respiratory System sources. For education only; not medical advice.