Submitted on April 19, 2006
Accepted on March 19, 2007
Tidal volume dependency of gas exchange in bronchoconstricted pig lungs
Axel T Kleinsasser1, Ivan Mark Olfert2, Alex Loeckinger3, Gordon Kim Prisk2, Susan Roberta Hopkins2, and Peter D. Wagner2*
1 Anesthesiology & CCM, Innsbruck Medical School, Innsbruck, Austria
2 Department of Medicine, University of California, San Diego, La Jolla, California, United States
3 Anesthesiology & CCM, Innsbruck Medical School, Austria
* To whom correspondence should be addressed. E-mail: pdwagner{at}ucsd.edu.
Independent of airway pressure, pulmonary resistance is known to fall with increasing tidal volumes, traditionally thought to result from radial traction on the airways. Anafi and Wilson recently presented a model of a single terminal airway which explains the tidal volume-associated fall in resistance with an additional mechanism pertinent to narrow airways: A stable, nearly closed airway that is challenged with an increase in tidal volume "pops open" to become a stable, well opened airway, and thus resistance drops suddenly. To test this model in vivo, the effects of high (24 ml/kg) and low (9 ml/kg) tidal
volumes (VT) in bronchoconstricted lungs were assessed using, 1) the multiple inert gas elimination technique (MIGET) and 2) a 15-breath multiple breath inert gas washout (MBW) technique, in anesthetized pigs. With high VT, ventilation/perfusion (VA/Q) mismatch was reduced (LogSDQ from 1.30±0.11 to 1.09±0.12, p<0.05) and blood flow to lung units with VA/Q ratios <0.1 was significantly
reduced (37±4% of cardiac output to 7±4%, p<0.05). Dynamic compliance was twice as high during high VTventilation (p=0.002). MBW analysis revealed that, while heterogeneity of ventilation during
bronchoconstriction was not significantly different between either low or high VT (LogSDVMBW = 1.39 ± 0.09 and LogSDVMBW = 1.34 ± 0.02, respectively), pre-inspiratory lung volume (PILV) decreased by 42% with low VT ventilation (p<0.05), whereas it did not change with high VT ventilation. The higher PILV during high VTis also consistent with Anafi and Wilson's model. In summary, the outcomes from MIGET, and to some extent the MBW, in our anesthetized and mechanically ventilated pigs are
consistent with a bistable terminal airway model as proposed by Anafi and Wilson. However, our data do not allow exclusion of other mechanisms that may lead to improved ventilatory distribution when tidal volume is increased.
