DEAD SPACE AND SLOPE INDEXES FROM THE EXPIRATORY CARBON-DIOXIDE TENSION-VOLUME CURVE

The slope of phase 3 and three noninvasively determined dead space est imates derived from the expiratory carbon dioxide tension (PCO2) versu s volume curve, including the Bohr dead space (VD,Bohr), the Fowler de ad space (VD,Fowler) and pre-interface expirate (PIE), were investigat ed 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 bre athing frequency, and fixed frequencies of 10, 15 and 20 breaths.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 obstructio n, Discrimination between no-emphysema (asthma and controls) and emphy sema patients was possible on the basis of a plot of intercept and slo pe of the relationship between VD,Bohr and VT, A combination of both t he slope of phase 3 and VD,Bohr of a breath of 1 L was equally discrim inating, The influence of fixed frequencies in the controls did not ch ange the results, The conclusion is that Bohr dead space in relation t o tidal volume seems to have diagnostic properties separating patients with asthma from patients with emphysema with the same degree of airw ays obstruction. Equally discriminating was a combination of both phas e 3 and Bohr dead space of a breath of 1 L, The different pathophysiol ogical mechanisms in asthma and emphysema leading to airways obstructi on are probably responsible for these results.