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

TLDR

The study compares the arterial to end‑tidal CO₂ gradient divided by PaCO₂ with conventional alveolar dead space fraction calculated by the Bohr–Fowler method, using a validated high‑fidelity pulmonary model. The authors modeled how the ratio and Bohr–Fowler dead space fraction change under three ventilation‑perfusion defects while varying CO₂ production, venous admixture, and anatomical dead space. The ratio was roughly 59.5 % of the Bohr–Fowler dead space fraction, varied similarly with the same factors, and may serve as a simpler serial measurement that could replace conventional calculation after clinical validation.

Abstract

In Brief 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 (V̇co2), 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 (V̇co2, 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 V̇co2. 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.