Dead Space Calculator

Dead space is the portion of each breath that does not participate in gas exchange. You breathe in 500 mL, but not all of it reaches functioning alveoli. The fraction that goes nowhere useful is dead space.

The Bohr equation calculates physiological dead space: Vd/VT = (PaCO2 - PECO2) / PaCO2. Here, PaCO2 is the arterial CO2 (from an arterial blood gas), PECO2 is the mixed expired CO2 (average across all exhaled gas), and VT is the tidal volume.

The logic is conservation of CO2. The CO2 exhaled per breath equals the CO2 from alveoli minus what was washed out of the dead space (which had no CO2 to give). Rearranging gives the dead space fraction.

Normal values: healthy adults have a dead space fraction (Vd/VT) of about 0.20 to 0.35. This includes anatomical dead space (airways that never exchange gas: nose, trachea, bronchi, about 150 mL) plus alveolar dead space (ventilated alveoli with poor blood flow, normally very small). Together these make up physiological dead space.

Elevated dead space (Vd/VT > 0.35) is clinically significant:

• Pulmonary embolism: blood flow to ventilated alveoli is blocked, creating large alveolar dead space. ARDS patients often have Vd/VT > 0.6.
• COPD: airspace destruction and vascular remodeling increase dead space.
• Mechanical ventilation: equipment tubing adds anatomical dead space.

A high dead space fraction means the lungs must work harder to maintain CO2 clearance. If minute ventilation cannot compensate, CO2 accumulates and PaCO2 rises (hypercapnia).

Minute ventilation = VT * respiratory rate. Alveolar ventilation = (VT - Vd) * respiratory rate. These two differ by exactly the dead space ventilation rate: Vd * RR mL/min.