Studies on the Expired Alveolar Carbon Dioxide Tension in Chronic Pulmonary Emphysema with Special Reference to Pulmonary Arterial Hypertension

Abstract

Continuous analysis of expired alveolar CO2 tension, using a rapid infrared meter, was made in 10 normal subjects, and 20 patients with chronic pulmonary diseases. An analytical method similar to that previously described by Sivertson and Fowler was used. Arterial blood was sampled and arterial CO2 tension was measured in 17 patients simultaneously with expired alveolar CO2. Right heart catheterization was carried out in 38 patients with pulmonary emphysema.(1) In many patients, the resting end-tidal CO2 tension was frequently less than normal. In both normal subjects and patients, the upward slope of alveolar CO2 tension decreased toward the end of forced expiration. However, in patients with emphysema a larger rate of increase in expired alveolar CO2 tension, smaller expired volumes and the prolonged duration of forced expiration were observed. In patients, the apparent values of the difference between mixed venous and alveolar CO2 tension were large and the percentage rates of decrease in the difference were normal. This probably indicates that perfusion is not reduced in the overventilated alveoli of the emphysematous lung. These results well coincided with Fowler's observations.(2) In cases of pulmonary emphysema, the mean rates of increase in expired alveolar CO2 tension observed during a forced expiration after hyperventilation, remained similar to those observed during a forced expiration after normal breathing. An expiratory level was elevated in patients with emphysema during hyperventilation. Within the succeeding forced expiration, airway resistance and pulmonary pressure must have been increased, consequently the ratio of pulmonary blood flow per alveolar lung volume should be reduced.(3) In many patients, the endtidal CO2 tension was lower than the arterial CO2 tension, but the maximal CO2 tension of the forced expiration following normal breathing equaled or slightly exceeded the arterial CO2 tension. On the other hand, the measured arterial CO2 tension never exceeded the estimated mixed venous CO2 tension significantly.(4) In pulmonary emphysema, the elevated values of mixed venous CO2 tension well correlated with the degree of pulmonary hypertension. This correlation is proposed for the use in estimating the pulmonary hypertension from the value of expired alveolar CO2 tension in patients with a mixed venous CO2 tension of more than 55mm. Hg.