Objective The ability to ventilate babies with tidal volumes (VTs) 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 VT 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 VT to maintain stable end tidal carbon dioxide (ETCO2).
Results At all tested VTs, lower flow rate increased the time it took for CO2 to washout from the test lung. The effect was most pronounced for VTs 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 VT for the same effective alveolar ventilation.
Conclusions Inflation flow rate affects the efficiency of CO2 removal with low VT. Our results are relevant for providers using volume-controlled ventilation or other modes that use low inflation flow rates because the VT required for normocapnia will be higher than published values that were generated using pressure-limited ventilation modes with high inflation flows.
- Respiratory physiology
- Volume targeted ventilation
- Volume guarantee
- Tidal volume
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Contributors EHH and MK made substantial contributions to the design of the study and the acquisition, analysis and interpretation of data. EHH wrote the first draft but MK revised it critically for intellectual content. Both authors approved the final version before submission. Both also agree to be accountable for all aspects of the work in ensuring that questions related to the accuracy or integrity of any part of the work are appropriately investigated and resolved.
Competing interests MK has been a consultant to Draeger Medical and has received research grant funding from the company for an unrelated clinical study. No individual from the company had any role or input into the present study.
Provenance and peer review Not commissioned; externally peer reviewed.