Objective The ability to ventilate babies with tidal volumes (V_Ts) below dead space has been demonstrated both in vivo and in vitro, though it appears to violate classical respiratory physiology. We hypothesised that this phenomenon is made possible by rapid flow of gas that penetrates the dead space allowing fresh gas to reach the lungs and that the magnitude of this phenomenon is affected by flow rate or how rapidly air flows through the endotracheal tube.

Methods We conducted two bench experiments. First, we measured the time needed for complete CO₂ washout from a test lung to assess how fixed V_T but different inflation flow rates affect ventilation. For the second experiment, we infused carbon dioxide at a low rate into the test lung, varied the inflation flow rate and adjusted the V_T to maintain stable end tidal carbon dioxide (ETCO₂).

Results At all tested V_Ts, lower flow rate increased the time it took for CO₂ to washout from the test lung. The effect was most pronounced for V_Ts below dead space. The CO₂ steady-state experiment showed that ETCO₂ increased when the flow rate decreased. Ventilating with a slower flow rate required a nearly 20% increase in V_T for the same effective alveolar ventilation.

Conclusions Inflation flow rate affects the efficiency of CO₂ removal with low V_T. Our results are relevant for providers using volume-controlled ventilation or other modes that use low inflation flow rates because the V_T required for normocapnia will be higher than published values that were generated using pressure-limited ventilation modes with high inflation flows.