Effect of flow rate on blood gases during constant flow ventilation in dogs

We studied the effect of flow rate (V) on arterial blood gases during constant flow ventilation (CFV) in 9 anesthetized, paralyzed dogs weighing 9.5 to 26.5 kg. The constant flow was administered through catheters placed in each mainstem bronchus. Alveolar ventilation increased linearly with increasing V over the range of 0.18 to 1.0 L/s but was relatively constant at flows above 1.0 L/s. We found that CFV produced normocapnia at a mean V of 0.48 +/- 0.21 L/s (mean +/- SD). However, we did not find any significant relationship between body weight and the V required for normocapnia. At all flow rates we found a relatively large alveolar to arterial oxygen difference (48.9 +/- 8.8 mmHg, mean +/- SD), suggesting significant inhomogeneities in ventilation-perfusion. Our data are consistent with a 2-zone model of gas exchange where gas exchange is dominated by bidirectional convective streaming in the airways closest to the jets, cardiogenic induced flows in the most peripheral airways, and jet-induced turbulence in those airways between these 2 regions.