How to Calculate Physiological Dead Space
Physiological dead space is calculated using the Bohr equation: VD/VT = (PaCO2 - PECO2)/PaCO2, where PaCO2 is arterial CO2 and PECO2 is mixed expired CO2. 1, 2
Required Measurements
To calculate dead space, you need two essential values:
- Arterial PCO2 (PaCO2): Obtained from arterial blood gas analysis 2
- Mixed expired PCO2 (PECO2): Measured by collecting expired gas in a large gas-impermeable bag over time and analyzing the PCO2 of the collected gas, or by sampling from a mixing chamber in modern exercise systems 1, 2
For breath-by-breath systems, PECO2 can be calculated from the VCO2/VE ratio, where both gas volumes are expressed in liters (STPD) 1, 2
The Bohr Equation
The standard Bohr equation is:
VD/VT = (PaCO2 - PECO2) / PaCO2 1, 2
This yields a dimensionless fraction representing the proportion of each tidal volume that does not participate in gas exchange 1
Modified Equation for Mechanical Ventilation
When patients are connected to ventilator equipment with additional dead space from tubing and valves, use the modified equation:
VD/VT = [(PaCO2 - PECO2) / PaCO2] + (VDvalve/VT) 1, 2
Where VDvalve represents the dead space of the mouthpiece, valve, and ventilator circuit 1, 2
Important Technical Considerations
Do Not Use End-Tidal CO2 as a Substitute
End-tidal PCO2 (PETCO2) should NOT be used as an index of PaCO2, especially in patients with lung disease. 1 This is misleading because PETCO2 can actually exceed PaCO2 during exercise in normal individuals, and there are no valid procedures to adequately estimate PaCO2 noninvasively in patients with lung disease 1
Equipment Dead Space Limits
Total apparatus dead space should not exceed 2.0 mL/kg body weight to ensure accurate measurements 2
Gas Volume Standardization
Gas volumes must be expressed in STPD (standard temperature, pressure, dry) conditions for VCO2/VE calculations 2
Normal Values and Clinical Interpretation
- Normal VD/VT at rest: Approximately 0.34 (or less than 0.35) 2
- Normal VD/VT during exercise: Decreases to ≤0.10 in healthy individuals 2
- Abnormal values: In mechanically ventilated patients with ARDS, VD/VT >0.6 is clearly abnormal and associated with significantly higher mortality 2
Physiological Meaning
The VD/VT ratio represents the fraction of each breath that is "wasted" on ventilating both the anatomic dead space (conducting airways) and alveolar dead space (ventilated but unperfused or underperfused alveoli) 1
An increase in VD/VT reflects increased inefficiency of ventilation due to ventilation-perfusion mismatching or right-to-left shunt, requiring increased minute ventilation to maintain PaCO2 1
Common Pitfalls
Breathing pattern significantly affects VD/VT measurements: Rapid shallow breathing increases VD/VT even without true ventilation-perfusion abnormalities 1
Correction for mechanical dead space: When alveolar dead space is present, less than the full volume of attached mechanical dead space appears in the measured dead space, and traditional subtraction of mechanical dead space leads to underestimation of physiological dead space 3