Loading Calculator...
Please wait a moment
Please wait a moment
Calculate physiologic dead space and VD/VT ratio using Bohr equation
Arterial CO₂ from ABG
Mixed expired CO₂ (requires collection bag)
For calculating dead space volume
The original Bohr equation (1891) calculates physiologic dead space:
VD/VT = (PaCO₂ - PĒco₂) / PaCO₂
The Enghoff modification uses end-tidal CO₂ instead of mixed expired CO₂, which is easier to measure but overestimates dead space when there is V/Q mismatch or shunt.
| VD/VT Ratio | Interpretation | Clinical Context |
|---|---|---|
| 0.20-0.35 | Normal | Healthy individuals at rest |
| 0.35-0.50 | Mildly elevated | Mild V/Q mismatch, early lung disease |
| 0.50-0.60 | Moderately elevated | Significant V/Q mismatch (COPD, ARDS) |
| 0.60-0.70 | Severely elevated | Severe ARDS, massive PE |
| >0.70 | Critically elevated | Associated with high mortality |
Anatomic dead space is approximately 150 mL in adults (2 mL/kg). The VD/VT ratio is normally 0.20-0.35, meaning 20-35% of each breath does not participate in gas exchange. This increases with age and disease.
The original Bohr equation uses ideal alveolar CO₂ (PACO₂), which is difficult to measure directly. Enghoff substituted PaCO₂, which is easily obtained from ABG. In healthy lungs, PaCO₂ ≈ PACO₂. However, with V/Q mismatch or shunt, PaCO₂ > PACO₂, causing the Enghoff method to overestimate dead space.
PEEP can both increase and decrease dead space. Optimal PEEP recruits collapsed alveoli and improves V/Q matching (decreasing alveolar dead space). Excessive PEEP compresses pulmonary capillaries, creating zone 1 conditions and increasing dead space.
The difference between arterial and end-tidal CO₂ reflects dead space. Normal gradient is 2-5 mmHg. Increased gradient (>5 mmHg) suggests increased dead space ventilation. This is a simpler surrogate for the full Bohr calculation.
Dead space fraction is an independent predictor of mortality in ARDS. VD/VT >0.57 is associated with 3x higher mortality. High dead space reflects the heterogeneous lung involvement in ARDS and helps guide interventions like prone positioning.
Yes, strategies include: (1) optimizing PEEP to improve recruitment, (2) prone positioning to improve V/Q matching, (3) increasing cardiac output/perfusion, (4) reducing apparatus dead space (shorter circuits, smaller HME), and (5) treating underlying causes (thrombolytics for PE).
Calculate total and alveolar ventilation
Ideal tidal volume based on IBW
Assess oxygenation efficiency
ARDS severity classification
Age-specific normal ranges
GOLD staging and symptom assessment
This calculator provides estimates for educational purposes. Accurate dead space measurement requires arterial blood gas analysis and either mixed expired CO₂ collection (Bohr) or capnography (Enghoff). The Enghoff modification overestimates dead space in the presence of shunt or V/Q mismatch. Clinical decisions should be made in conjunction with other clinical parameters and specialist consultation.