A prolonged postinspiratory pause enhances CO₂ elimination by reducing airway dead space

Abstract

SummaryBackground: CO 2 elimination per breath (V CO 2 , T ) depends primarily on tidal volume (V T ). The time course of flow during inspiration influences distribution and diffusive mixing of V T and is therefore a secondary factor determining gas exchange. To study the effect of a postinspiratory pause we defined Ômean distribution timeÕ (MDT) as the mean time given to inspired gas for distribution and diffusive mixing within the lungs. The objective was to quantify changes in airway dead space (V Daw ), slope of the alveolar plateau (SLOPE) and V CO 2 , T as a function of MDT in healthy pigs. Methods: Ten healthy pigs were mechanically ventilated. Airway pressure, flow and partial pressure of CO 2 were recorded during resetting of the postinspiratory pause from 10% (baseline) to, in random order, 0, 5, 20 and 30% of the respiratory cycle. The immediate changes in V Daw , SLOPE, V CO 2 , T , and MDT after resetting were analyzed. Results: V Daw in percent of V T decreased from 29 to 22%, SLOPE from 0AE35 to 0AE16 kPa per 100 ml as MDT increased from 0AE51 to 1AE39 s. Over the same MDT range, V CO 2 ,T increased by 10%. All these changes were statistically significant. Conclusion: MDT allows comparison of different patterns of inspiration on V Daw and gas exchange. Estimation of the effects of an altered ventilator setting on exchange of CO 2 can be done only after about 30 minutes, while the transient changes in V CO 2 ,T may give immediate information. MDT affects gas exchange to an important extent. Further studies in human subjects in health and in disease are needed.

Effects of inspiratory pause on CO2 elimination and arterial Pco2 in acute lung injury

“…Our results agree with previous findings that a postinspiratory pause enhances CO 2 elimination in healthy or surfactantdepleted animals (4,10,11,20) and with those obtained in surfactant-depleted pigs in which inspiration at pressure-controlled ventilation was prolonged (13). They also agree with results in patients with ALI/ARDS (2,8,12,14).…”

“…In humans, equilibration through collateral channels may play a role in this. However, Uttman and Jonson made similar observations in healthy pigs, which do not have collaterals (20).…”

“…In healthy pigs, a prolonged Tp increases CO 2 elimination per tidal breath (VtCO 2 ) by decreasing airway dead space (V Daw ) (20). It was suggested that a prolonged Tp increased the mean distribution time (MDT) of inspired gas, so as to allow more time for diffusion of CO 2 towards more central airways (7).…”

Pattern of inspiratory gas delivery affects CO2 elimination in health and after acute lung injury

“…In accordance with the hypothesis based upon previous studies, ΔV E CO 2 increased in relation to lnMDT [16,17]. A better fit of a logarithmic equation rather than a linear one agrees with concepts based upon physiology and morphology.…”

“…Uttman et al showed in healthy pigs that CO 2 elimination depends on time available for gas distribution and diffusion within the respiratory zone, mean distribution time (MDT) [16]. Variation of MDT was achieved by changing the duration of the postinspiratory pause (T P ).…”