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GENERAL ARTICLES

Estimating Alveolar Dead Space from the Arterial to End-Tidal CO₂ Gradient: A Modeling Analysis

From the University Department of Anaesthesia, University Hospital, Nottingham, NG7 2UH, UK

Address correspondence and reprint requests to Jonathan G. Hardman, Clinical Senior Lecturer, University Department of Anesthesia, University Hospital, Nottingham, NG7 2UH, UK. Address email to j.hardman{at}nottingham.ac.uk

Using an original, validated, high-fidelity model of pulmonary physiology, we compared the arterial to end-tidal CO₂ gradient divided by the arterial CO₂ tension (Pa-E'CO₂/PaCO₂) with alveolar dead space expressed as a fraction of alveolar tidal volume, calculated in the conventional manner using Fowler’s technique and the Bohr equation: (VDalv/VTalv)_Bohr-Fowler. We examined the variability of Pa-E'CO₂/PaCO₂ and of (VDalv/VTalv)_Bohr-Fowler in the presence of three ventilation-perfusion defects while varying CO₂ production (CO₂), venous admixture, and anatomical dead space fraction (VDanat). Pa-E'CO₂/PaCO₂ was approximately 59.5% of (VDalv/VTalv)_Bohr-Fowler. During constant alveolar configuration, the factors examined (CO₂, pulmonary shunt fraction, and VDanat) each caused variation in (VDalv/VTalv)_Bohr-Fowler and in Pa-E'CO₂/PaCO₂. Induced variation was slightly larger for Pa-E'CO₂/PaCO₂ during changes in VDanat, but was similar during variation of venous admixture and CO₂. Pa-E'CO₂/PaCO₂ may be a useful serial measurement in the critically ill patient because all the necessary data are easily obtained and calculation is significantly simpler than for (VDalv/VTalv)_Bohr-Fowler.

IMPLICATIONS: Using an original, validated, high-fidelity model of pulmonary physiology, we have demonstrated that the arterial to end-tidal carbon dioxide pressure gradient may be used to robustly and accurately quantify alveolar dead space. After clinical validation, its use could replace that of conventionally calculated alveolar dead space fraction, particularly in the critically ill.

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