Effect of vagotomy on respiratory mechanics and gas exchange in the neonatal calf.

The effect of vagotomy on the mechanical and gas exchange properties of the respiratory system was determined in 7 healthy, anesthetized neonatal Holstein calves. A pressure-compensated volume displacement plethysmograph measured the quasi-static pressure-volume (PV) characteristics of the lungs and thoracic cage. Functional residual capacity (FRC) was determined by He equilibration. Lung and chest wall compliances per kilogram of body weight were 2.25 +/- 0.19 ml/cm of H2O/kg and 8.85 +/- 2.44 ml/cm of H2O/kg, respectively. Total lung capacity (TLC) averaged 78.3 +/- 13 ml/kg and FRC was 51 +/- 5% of TLC. The lung PV curve deviated from single exponential behavior below 75% of TLC, indicating airway closure at large lung volumes. Residual volume, which averaged 28.1 +/- 6.0% of TLC, appeared to be determined by airway closure because the thoracic cage was compliant between FRC and residual volume. Vagotomy had no effect on the lung and thoracic cage PV curves, lung volumes, or the alveolar-arterial oxygen difference, but it decreased pulmonary resistance and increased respiratory system conductance and the dead space/tidal volume ratio, indicating dilation of large conducting airways. The calf appears similar to other neonates, in that the thoracic cage is compliant and airway closure occurs at large lung volumes. Both of these factors may predispose the calf to atelectasis.