Background: CO2 elimination per breath (VCO2,T) depends primarily on tidal volume (VT). The time course of flow during inspiration influences distribution and diffusive mixing of VT and is therefore a secondary factor determining gas exchange. To study the effect of a postinspiratory pause we defined 'mean distribution time' (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 (VDaw), slope of the alveolar plateau (SLOPE) and VCO2,T as a function of MDT in healthy pigs.
Methods: Ten healthy pigs were mechanically ventilated. Airway pressure, flow and partial pressure of CO2 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 VDaw, SLOPE, VCO2,T, and MDT after resetting were analyzed.
Results: VDaw in percent of VT decreased from 29 to 22%, SLOPE from 0.35 to 0.16 kPa per 100 ml as MDT increased from 0.51 to 1.39 s. Over the same MDT range, VCO2,T increased by 10%. All these changes were statistically significant.
Conclusion: MDT allows comparison of different patterns of inspiration on VDaw and gas exchange. Estimation of the effects of an altered ventilator setting on exchange of CO2 can be done only after about 30 minutes, while the transient changes in VCO2,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.