A novel technique to determine dead space ventilation in ARDS

Abstract

Introduction: Bohr's dead space is the portion of tidal volume (V_T) not participating in gas exchange with pulmonary blood flow. This parameter is important in patients with acute respiratory distress syndrome (ARDS) for treatment follow-up, ventilator settings optimization and prognostication. However, dead space calculation is cumbersome using current techniques. Recently, a novel technique based on volumetric capnography (VC) was validated. Our aim was to compare VC with current techniques in ARDS patients and controls.

Methods: In 15 ARDS patients and 15 controls (post-operative cardiac surgery) dead space (V_D) was calculated with VC using the Bohr equation: V_D/V_T = (PACO₂-PeCO₂)/PACO₂, where PACO₂ and PeCO₂ are alveolar and mixed expired CO₂ tension. Both variables were calculated from a mathematical fit of the volumetric capnogram. Dead space was also calculated using the Bohr-Enghoff modification (arterial CO₂ tension (PaCO₂) instead of PACO₂). PeCO₂ was measured in expired air using a Douglas bag (DB) and via indirect calorimetry with a metabolic monitor (MM).

Results: In ARDS patients, dead space determined with VC (53±8%) was lower compared to DB (68±8%, p<0.0001) and MM (72±8%, p<0.0001). PeCO₂ determined with MM (3.2±0.4 kPa) was higher compared to DB (2.5±0.3 kPa, p<0.05) and VC (2.5±0.3 kPa, p<0.05) in the controls. In ARDS patients, PeCO₂ was higher with DB (2.1±0.4 kPa) compared to VC (1.8±0.4 kPa, p<0.05). PaCO₂ was higher than PACO₂ in the controls (5.2±0.5 vs. 4.3±0.5 kPa, p<0.0001) and ARDS patients (6.9±1.7 vs. 3.9±0.8 kPa, p<0.0001).

Conclusion: Current techniques, using PaCO₂ instead of PACO_2, overestimate dead space through pulmonary shunting. Volumetric capnography measures true Bohr's dead space.