Abstract
Introduction: Bohr's dead space is the portion of tidal volume (VT) 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 (VD) was calculated with VC using the Bohr equation: VD/VT = (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 PACO2, overestimate dead space through pulmonary shunting. Volumetric capnography measures true Bohr's dead space.
- © 2014 ERS