1. What is Dead Space Fraction?

The dead space fraction (V_d/V_t) represents the proportion of each tidal volume that does not participate in gas exchange. It's an important measure in respiratory physiology and critical care.

2. How Does the Calculator Work?

The calculator uses the Bohr equation:

Where:

• \( V_d/V_t \) — Dead space fraction (unitless ratio)
• \( PaCO_2 \) — Arterial partial pressure of CO₂ (mmHg)
• \( PeCO_2 \) — Mixed expired partial pressure of CO₂ (mmHg)

Explanation: The equation compares the difference between arterial and expired CO₂ to estimate the portion of ventilation that's wasted.

3. Clinical Importance

Details: Dead space fraction is crucial for assessing ventilator efficiency, diagnosing pulmonary embolism, and managing ARDS patients. Normal values are typically 0.2-0.4, increasing in various lung pathologies.

4. Using the Calculator

Tips: Enter PaCO₂ from arterial blood gas analysis and PeCO₂ from mixed expired gas analysis. Both values must be in mmHg and PaCO₂ must be greater than zero.

5. Frequently Asked Questions (FAQ)

Q1: What's a normal dead space fraction?
A: Normally 0.2-0.4 (20-40%). Values >0.6 indicate significant dead space ventilation.

Q2: When does dead space increase?
A: Increases occur in pulmonary embolism, COPD, ARDS, pulmonary hypertension, and low cardiac output states.

Q3: How is PeCO₂ measured?
A: PeCO₂ requires collection of mixed expired gas, typically via a Douglas bag or directly from ventilator expiratory port.

Q4: Can end-tidal CO₂ be used instead?
A: End-tidal CO₂ (PetCO₂) can estimate PeCO₂ but may underestimate true dead space in heterogeneous lung disease.

Q5: What's the physiological dead space?
A: Physiological dead space includes both anatomical (conducting airways) and alveolar (non-perfused alveoli) components.