The alveolar dead space (VDalv) as a percentage of tidal volume was calculated in horses by substituting the values for partial pressure of carbon dioxide in end-expiratory gas (PE’CO₂) and for partial pressure of carbon dioxide in arterial blood (PaC0₂) in the equation:
% VDalv =( (PaC0₂ - PE'C0₂)/PaC0₂) X 100. The mean % VDalv of 12 chronic obstructive pulmonary disease (COPD) subjects was 3 times greater than that of 22 normal subjects. Since the larger % VDalv of COPD subjects was due to an elevated PaC0₂, it was considered that maldistribution of ventilation was the principal cause of their increased % VDalv compared with that of clinically normal subjects.
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