There was no significant difference of the mean values of the respiratory dead space between these two groups (223 +/- 43 ml in healthy subjects vs. As expected, there was also a correlation between the airway resistance and respiratory dead space in patients with lung emphysema (fig. This indicates, that the use of C18O2 is a valid method to measure the functional dead space. There was a good correlation between respiratory dead space, measured by the use of Ar-gas and C18O2-gas in healthy subjects (fig. 21 healthy nonsmoking subjects and 29 patients with clinical signs of lung emphysema participated in this study. Hence, C18O2 allows exact measurement of the respiratory dead space in patients with lung emphysema. Therefore C18O2 is only expired from the dead space of the lung, but not from the alveolar region. CO2 labeled with the stable oxygen isotope 18O (C18O2) is completely taken up in the gas exchanging region of the lung, but not from the conducting airways. The use of C18O2 as tracer gas allows to separate phase II and phase III even if the patients have severe emphysema (fig. 1 expirogram 3), the separation of the three phases in patients with lung emphysema is not possible due to gradual transition of phase II into phase III in these patients (fig. While this distinction is easy in healthy subjects (fig. Until now, it was not possible to measure the respiratory dead space in patients with lung emphysema using the method of Fowler, because in this method distinction of the three phases of an inert gas expirogram is essential. The change in airway resistance is caused by obstruction of central airways, which is supposed to reduce the respiratory dead space. In patients with lung emphysema, changes in lung volumes as well as changes in airway resistance are well known.
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