Microemboli reduce phase III slopes of CO2 and invert phase III slopes of infused SF6

We investigated the effect of increasing doses of intravenously infused glass microspheres (mean diameter 125 microns) on gas exchange in anesthetized, heparinized, mechanically ventilated goats (VT = 16-18 ml/kg). Breath-by-breath CO2 expirograms were collected using a computerized system (Study A) during the infusion of a total of 15 g of microspheres. We found a 50% decrease in extravascular lung water by indicator dilution with a corresponding doubling of alveolar dead space (VDalv). Airways deadspace (VDaw) decreased by 13 ml (10%) and mean normalized phase III slope for CO2 decreased from 0.23 to -0.08 L-1 becoming negative in 3 of 5 animals. In a second study (Study B), simultaneous breath-by-breath CO2 and infused SF6 expirograms were collected using an infrared CO2 analyzer and a mass spectrometer. Under baseline conditions VDaw for CO2 was smaller than for SF6 and the ratio of the phase III slope for SF6 to the phase III slope for CO2 was 1.39. Following embolization there were no differences in VDaw between the two gases, however, the phase III slope for CO2 became either slightly negative or extremely flat, while the phase III slope for SF6 became negative in 73% of the breaths (-0.17 L-1, P < 0.05). Negative phase III slopes have been predicted by a single path model when blood flow is confined to the most mouthward generations of the acinus (Schwardt et al., Ann. Biomed. Engin, 19: 679-697, 1991). The agreement between the numerical model and the experimental data is consistent with a serial distribution of blood flow within the acinus.