Single breath CO2 measurements of deadspace in ducks

We measured deadspace (VD) in ducks using CO2 expirograms (plots of expired PCO2 vs expired volume) obtained during artificial ventilation at different tidal volumes (VT) and respiratory system volumes (VRS). Conventional analysis of the expirograms for Bohr and Fowler VD indicated both were larger than anatomic VD. Most expirograms at VT less than or equal to 100 ml had terminal slopes greater than predicted for lung gas and violated the usual assumptions of the Fowler calculation. Bohr VD was not affected by VRS but increased with VT. This can be explained by expired PCO2 not reaching lung values at low VT and an expiratory mesobronchial ventilatory shunt. We propose a measure of mesobronchial shunt corresponding to a volume of gas exhaled in one breath from caudal air sacs through the mesobronchus (VM). VM/VT changes with pump vs constant flow ventilation indicating sensitivity of VM to flow pattern. We estimate mesobronchial shunting is greatest at the beginning of expiration and approaches zero only near the end of a 200 ml expiration with constant flow ventilation.