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

Abstract

The slope of phase 3 and three noninvasively determined dead space estimates derived from the expiratory carbon dioxide tension (PCO 2 ) 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 CO 2 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·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.

Capnographic Parameters in Ventilated Patients: Correspondence with Airway and Lung Tissue Mechanics

“…5A). This finding is in accordance with a wider phase II observed previously in emphysematous patients (31,32), and increases in SII after compliance elevation through recruitment maneuvers (30). Since PF is determined more by the lung tissue stiffness (r=0.34; p<0.0001 for H) than by the airway caliber (r=0.09; p=0.34 for Raw), the significant correlation of PF with the phase-II capnographic indices may also be attributed to the influence of the lung elastance during early expiration.…”

Volumetric capnography as a tool to detect early peripheric lung obstruction in cystic fibrosis patients

“…A heterogeneidade da ventilação também é avaliada pela CapV e sua gravidade pode ser utilizada para reunir informações sobre a anatomia e a função dos espaços aéreos distais. Em casos de asma e enfisema, o slope da fase III da CapV aumenta, assim demonstrando a presença de alterações funcionais e estruturais que levaram à heterogeneidade da distribuição da ventilação 14 .…”

“…A heterogeneidade da ventilação também pode ser identificada pelo aumento no slope do platô alveolar da curva de exalação de gases, como o dióxido de carbono (CO 2 ) medido durante a respiração de volume corrente por meio da capnografia volumétrica (CapV) 14 .…”

“…O padrão de eliminação de CO 2 na CapV, assim como o padrão de eliminação dos gases utilizados na técnica de MBW, fornece informações sobre a distribuição de ventilação, especialmente sobre a ventilação nos espaços aéreos distais [14][15][16][17][18][19] .…”

Relationships between capnogram parameters and respiratory mechanics in ventilated patients

“…Since velocity of the expiratory flow decreases during expiration, phases I and II are generally relatively longer, and phase III is relative shorter in vCap compared to those of tCap curve. Consequently, the SIII,V of vCap is somewhat steeper (18)(19)(20)(21)(22)24,27,31,(34)(35)(36)(37)(38) than SIII,T.…”