1. What is Physiologic Dead Space?

Physiologic dead space refers to the portion of each breath that does not participate in gas exchange. It includes both anatomical dead space (airways) and alveolar dead space (non-perfused alveoli).

2. How Does the Calculator Work?

The calculator uses the Bohr equation for physiologic dead space:

Where:

• \( V_d \) — Physiologic dead space (ml)
• \( V_t \) — Tidal volume (ml)
• \( PaCO_2 \) — Arterial partial pressure of CO₂ (mmHg)
• \( PeCO_2 \) — Mixed expired partial pressure of CO₂ (mmHg)

Explanation: The equation calculates the fraction of each breath that doesn't participate in gas exchange by comparing arterial and expired CO₂ levels.

3. Importance of Dead Space Calculation

Details: Dead space measurement is crucial in mechanical ventilation, assessing pulmonary embolism, and evaluating lung function in critically ill patients.

4. Using the Calculator

Tips: Enter tidal volume in ml, arterial CO₂ in mmHg, and expired CO₂ in mmHg. All values must be valid (V_t > 0, PaCO₂ > 0, PeCO₂ ≥ 0).

5. Frequently Asked Questions (FAQ)

Q1: What is a normal dead space fraction?
A: Normally, V_d/V_t is about 0.2-0.35 at rest. Higher values indicate increased dead space ventilation.

Q2: When is dead space increased?
A: Dead space increases in pulmonary embolism, COPD, positive pressure ventilation, and low cardiac output states.

Q3: How is PeCO₂ measured?
A: PeCO₂ is measured by collecting expired gas in a Douglas bag or using capnography.

Q4: What's the difference between anatomical and physiologic dead space?
A: Anatomical dead space is just the conducting airways, while physiologic dead space includes both airways and non-perfused alveoli.

Q5: How does dead space affect ventilation?
A: Increased dead space requires higher minute ventilation to maintain normal CO₂ elimination.