What is physiological dead space?

Physiological dead space is the portion of each breath that does not participate in gas exchange. It includes anatomical dead space (airways) plus alveolar dead space (ventilated but un-perfused alveoli). Normal Vd/Vt is roughly 0.20–0.35 at rest.

What is the Bohr equation?

The Bohr equation calculates the dead-space fraction as Vd/Vt = (PaCO2 − PeCO2) / PaCO2, where PaCO2 is arterial CO2 tension and PeCO2 is mean expired CO2 tension. A higher ratio means more wasted ventilation.

What PeCO2 value should I use?

Use the mixed expired CO2 from a Douglas bag collection or the ventilator's exhaled CO2 measurement — NOT end-tidal CO2 (EtCO2). EtCO2 approximates PaCO2 in healthy lungs but underestimates dead space in disease states.

What Vd/Vt ratio is clinically significant?

Vd/Vt >0.60 is associated with difficulty weaning from mechanical ventilation. Values >0.70 in ARDS correlate with increased mortality. A Vd/Vt rising over serial measurements suggests worsening pulmonary perfusion.

How is anatomical dead space estimated?

A rough estimate is 2.2 mL per kilogram of ideal body weight (or approximately 1 mL/lb). This represents the conducting airways from mouth/nose to terminal bronchioles and is not affected by disease state.

Does this replace formal dead space measurement?

This tool is for educational and rapid bedside estimation only. Formal dead space requires calibrated CO2 analyzers and careful sample collection. Always apply clinical judgment and consult your institution's protocols.

Respiratory Dead Space Calculator

Name: Respiratory Dead Space Calculator
Availability: InStock
Author: Nham Vu

Enter PaCO2, PeCO2, and tidal volume to calculate Vd/Vt ratio and absolute dead space volume using the Bohr equation.

Bohr Equation Inputs

PaCO₂ — Arterial CO₂ (mmHg)

From arterial blood gas. Normal: 35–45 mmHg.

PECO₂ — Mixed Expired CO₂ (mmHg)

Mean expired CO₂ — use ventilator measurement, not EtCO₂.

Tidal Volume (Vt) (mL)

Measured tidal volume. Typical ICU setting: 400–600 mL.

Optional — Anatomical Estimate

Body Weight (kg)

Estimates anatomical dead space at ~2.2 mL/kg.

Vd/Vt Ratio

—

Absolute Dead Space (Vd)

—

Alveolar Volume (Va)

—

Effective portion of tidal volume reaching alveoli.

Bohr Equation

Vd/Vt = (PaCO₂ − PēCO₂) / PaCO₂

Vd = Vd/Vt × Vt | Va = Vt − Vd

Summary