Dead space and slope indices from the expiratory carbon dioxide tension-volume curve.

The slope of phase 3 and three noninvasively determined dead space estimates derived from the expiratory carbon dioxide tension (PCO2) versus volume curve, including the Bohr dead space (VD,Bohr), the Fowler dead space (VD,Fowler) and pre-interface expirate (PIE), were investigated in 28 healthy control subjects, 12 asthma and 29 emphysema patients (20 severely obstructed and nine moderately obstructed) with the aim to establish diagnostic value. Because breath volume and frequency are closely related to CO2 elimination, the recording procedures included varying breath volumes in all subjects during self-chosen/natural breathing frequency, and fixed frequencies of 10, 15 and 20 breaths x min(-1) with varying breath volumes only in the healthy controls. From the relationships of the variables with tidal volume (VT), the values at 1 L were estimated to compare the groups. The slopes of phase 3 and VD,Bohr at 1 L VT showed the most significant difference between controls and patients with asthma or emphysema, compared to the other two dead space estimates, and were related to the degree of airways obstruction. Discrimination between no-emphysema (asthma and controls) and emphysema patients was possible on the basis of a plot of intercept and slope of the relationship between VD,Bohr and VT. A combination of both the slope of phase 3 and VD,Bohr of a breath of 1 L was equally discriminating. The influence of fixed frequencies in the controls did not change the results. The conclusion is that Bohr dead space in relation to tidal volume seems to have diagnostic properties separating patients with asthma from patients with emphysema with the same degree of airways obstruction. Equally discriminating was a combination of both phase 3 and Bohr dead space of a breath of 1 L. The different pathophysiological mechanisms in asthma and emphysema leading to airways obstruction are probably responsible for these results.

Does phase 2 of the expiratory PCO2 versus volume curve have diagnostic value in emphysema patients?

It has been postulated that serial inhomogeneity of ventilation in the peripheral airways in emphysema is represented by the shape of expiratory carbon dioxide tension versus volume curve. We examined the diagnostic value of this test in patients with various degrees of emphysema. The volumes between 25-50% (V25-50) and 25-75% (V25-75) of the expiratory carbon dioxide tension versus volume curve were determined in 29 emphysematous patients (20 severely obstructed and 9 moderately obstructed), 12 asthma patients in exacerbation of symptoms, and 28 healthy controls. Discriminant analysis was used to examine whether these diagnostic groups could be separated. With regard to phase 2 of the expiratory CO2 versus volume curve (mixture of anatomic deadspace and alveolar air), a plot of intercept versus slope of the relationships of (V25-50) and (V25-75) versus inspiratory volume (VI) from functional residual capacity (FRC), obtained during natural breathing frequency, proved to be most discriminating in the separation between healthy controls and severely obstructed emphysema patients. Separating healthy controls and severely obstructed emphysema patients on the basis of the discriminant line for V25-50, 9 of the 12 asthma patients in exacerbation were classified as normal, and only 5 of the 9 moderately obstructed emphysema patients as emphysematous. For V25-75 involvement of phase 3 of the curve (alveolar plateau) in asthma patients in exacerbation caused a marked overlap with the severely obstructed emphysema patients. In the healthy controls, a fixed breathing frequency of 20 breaths.min-1 led to an increase of both volumes.(ABSTRACT TRUNCATED AT 250 WORDS)