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Noninvasive measurement of mean alveolar carbon dioxide tension and Bohr's dead space during tidal breathing

Dept of Respiratory Medicine, Respiratory Function Laboratory, University of Athens Medical School, "Sotiria" Hospital for Diseases of the Chest, Athens, Greece

CORRESPONDENCE: J. Jordanoglou, Dept of Respiratory Medicine, Respiratory Function Laboratory, University of Athens Medical School, "Sotiria" Hospital for Diseases of the Chest, 152, Mesogion Ave, Athens, GR-11527, Greece. Fax: 30 17770423

Keywords: alveolar carbon dioxide tension, arterial carbon dioxide tension, Bohr's dead space, chronic obstructive pulmonary disease, end-tidal carbon dioxide, physiological dead space

Received: July 13, 2000
Accepted January 30, 2001

The lack of methodology for measuring the alveolar carbon dioxide tension (P_A,CO2) has forced investigators to make several assumptions, such as that P_A,CO2 is equal to end-tidal (P_ET,CO2) and arterial CO₂ tension (P_a,CO2).

The present study measured the mean P_A,CO2 and Bohr's dead space ratio (Bohr's dead space/tidal volume (V_D,Bohr/V_T)) during tidal breathing. The method used is a new, simple and noninvasive technique, based on the analysis of the expired CO₂ volume per breath (V_CO2) versus the exhaled V_T. This curve was analysed in 21 normal, healthy subjects and 35 chronic obstructive pulmonary disease (COPD) patients breathing tidally through a mouthpiece apparatus in the sitting position.

It is shown that: 1) P_A,CO2 is similar to P_a,CO2 in normal subjects, whilst it is significantly lower than P_a,CO2 in COPD patients; 2) P_A,CO2 is significantly higher than P_ET,CO2 in all subjects, especially in COPD patients; 3) V_D,Bohr/V_T is increased in COPD patients as compared to normal subjects; and 4) V_D,Bohr/V_T is lower than the "physiological" dead space ratio (V_D,phys/V_T) in COPD patients.

It is concluded that the expired carbon dioxide versus tidal volume curve is a useful tool for research and clinical work, because it permits the noninvasive and accurate measurement of Bohr's dead space and mean alveolar carbon dioxide tension accurately during spontaneous breathing.

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