Physiologic Dead Space

Physiologic Dead Space Calculator

Bohr-Enghoff equation  ·  VD/VT = (PaCO2 − PetCO2) / PaCO2  ·  ATS/ERS 2022 aligned

Capnometry & ABG
Bohr-Enghoff: VD/VT = (PaCO2 − PetCO2) / PaCO2  ·  Normal: ≤ 0.30 (≤ 30%)
PaCO2 = arterial CO2 from ABG  ·  PetCO2 = end-tidal CO2 from capnography  ·  Gap reflects non-perfused alveoli
Clinical context: Elevated VD/VT in ARDS, PE, COPD, over-distension  ·  VD/VT > 0.60 in ARDS correlates with increased mortality
Clinical scenario presets
Enter measured values (ABG + capnography)
mmHg
From arterial blood gas (ABG) · Normal: 35–45 mmHg · Obtained via radial or femoral artery puncture
mmHg
From capnography (side-stream or mainstream) · Normal: 35–45 mmHg · Should approximate PaCO₂ in healthy lungs
VD/VT interpretation — clinical reference (Bohr-Enghoff)
VD/VTCategoryCommon causesPaCO2−PetCO2 gapKey action
≤ 30% Normal Healthy lungs, well-matched V/Q < 5 mmHg Reassess if clinical deterioration
31 – 45% Mildly elevated Early ARDS, mild COPD, mild PE, age-related increase 5 – 10 mmHg Optimise PEEP; treat underlying cause; reassess ventilation
46 – 60% Significantly elevated Moderate–severe ARDS, massive PE, severe COPD, over-distension 10 – 20 mmHg Lung recruitment; prone positioning; anticoagulation if PE; bronchodilators
> 60% Critical Severe ARDS, cardiac arrest, massive PE, end-stage lung disease > 20 mmHg VV-ECMO evaluation; prone positioning; urgent pulmonology/ID consultation

VD/VT = (PaCO2 − PetCO2) / PaCO2  ·  Bohr-Enghoff equation  ·  Requires simultaneous ABG + mainstream/side-stream capnography  ·  No patient data stored or transmitted

Clinical references
  • 1.Bohr C. Über die Lungenatmung (On pulmonary respiration). Skand Arch Physiol. 1891;2:236–268. (Original Bohr dead space equation derivation)
  • 2.Enghoff H. Volumen inefficax. Bemerkungen zur frage des schädlichen raumes. Uppsala Läkarefören Förhandl. 1938;44:191–218. (Enghoff modification: substituting PaCO2 for alveolar CO2 making the formula clinically applicable with capnography)
  • 3.Nuckton TJ, Alonso JA, Kallet RH, et al. Pulmonary dead-space fraction as a risk factor for death in the acute respiratory distress syndrome. N Engl J Med. 2002;346(17):1281–1286. doi:10.1056/NEJMoa012835 (VD/VT > 0.60 as independent mortality predictor in ARDS)
  • 4.Kallet RH, Zhuo H, Liu KD, et al. The association between physiologic dead-space fraction and mortality in patients with acute lung injury and ARDS. Respir Care. 2009;54(10):1360–1368. (Relationship between dead space and outcomes)
  • 5.Graham BL, Brusasco V, Burgos F, et al. 2017 ERS/ATS standards for single-breath carbon monoxide uptake in the lung. Eur Respir J. 2017;49(1):1600016. (ATS/ERS pulmonary function standards relevant to gas exchange)
  • 6.Beydon L, Uttman L, Rawal R, Jonson B. Effects of positive end-expiratory pressure on dead space and its partitions in acute lung injury. Intensive Care Med. 2002;28(9):1239–1245. doi:10.1007/s00134-002-1385-3 (PEEP effects on physiological dead space partitioning)
  • 7.Lumb A. Nunn's Applied Respiratory Physiology. 9th ed. Elsevier; 2020. (Comprehensive reference on dead space physiology, Bohr equation, and clinical applications)
  • 8.West JB, Luks AM. West's Pulmonary Pathophysiology: The Essentials. 9th ed. Wolters Kluwer; 2017. (Dead space, V/Q mismatch, and pathophysiology)
  • 9.Acute Respiratory Distress Syndrome Network. Ventilation with lower tidal volumes as compared with traditional tidal volumes for acute lung injury and ARDS. N Engl J Med. 2000;342(18):1301–1308. (ARDSNet — dead space implications of lung-protective ventilation)
Frequently asked questions
Medical disclaimer

For qualified clinical professionals only. This Physiologic Dead Space Calculator is an educational decision-support tool. It is not a substitute for clinical judgment, arterial blood gas interpretation, volumetric capnography analysis, or comprehensive respiratory assessment.


The Bohr-Enghoff equation as implemented here uses PetCO2 (end-tidal CO2 from mainstream or side-stream capnography) as a surrogate for mean alveolar PCO2. This is the Enghoff modification of the original Bohr equation (which required measuring mixed expired PCO2 with a Douglas bag). The result represents total physiologic dead space fraction (VD/VT), which includes both anatomical dead space (conducting airways, ~150 mL) and alveolar dead space (ventilated but non-perfused alveoli).

The PaCO2−PetCO2 gradient is influenced by multiple factors beyond dead space: cardiac output (low CO increases the gradient), distribution of tidal volume (maldistribution increases apparent dead space), ventilator settings (PEEP, inspiratory flow), and capnography technique (accuracy of end-tidal sampling, circuit leaks). Mainstream capnography is more accurate than side-stream in mechanically ventilated patients.

VD/VT values > 0.60 in ARDS patients were identified as an independent predictor of ICU mortality by Nuckton et al. (NEJM 2002). However, clinical management should never be based on VD/VT alone — integrate with ABG, oxygenation indices (PaO2/FiO2), chest imaging, and haemodynamic data.

This tool does not store, transmit, or process patient-identifiable information. All calculations run locally in the user's browser.

Last reviewed: January 2026  ·  Bohr 1891  ·  Enghoff 1938  ·  Nuckton NEJM 2002  ·  ARDSNet NEJM 2000  ·  Nunn's 9th ed.  ·  ATS/ERS 2022