The expiratory capnogram: a measure of ventilation-perfusion inequalities.

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Abstract
A quantitative test of ventilation perfusion imbalance based on the single breath expired carbon dioxide curve (expiratory capnogram) is described. The rise in PCO2 CO2, pressure in 500 ml. of alveolar explrate (capnogram slope) is related, to a value of 6 mm. Hg, to dead-space ventilation. Values of capnogram slope greater than this are attributed to alveoli with low ventilation-perfusion ratios, which contribute a physiological shunt. In patients with chronic bronchitis the capnogram slope is closely related to mixed venous PCO2. The significance of this finding is discussed. The expiratory capnogram can provide a simple, sensitive, reproducible, and clinically significant test of ventilation-perfusion imbalance, which warrants further investigation.