Quantifying discrepancy between indicated and actual oxygen flow rates delivered by Comweld Ezi-flow low and standard...

During clinical use of extracorporeal membrane oxygenation (ECMO) in hyperbaric conditions at our centre, upward titration of indicated sweep gas flow rates is required to maintain adequate CO₂ clearance. This project measured the impact of hyperbaric pressure on oxygen flow rates delivered by the Comweld Ezi-Flow flowmeters used in our centre.

Oxygen flow rates through Comweld Ezi-flow standard and low oxygen gas flowmeters were set at 101.3 kPa (1 atmosphere absolute [atm abs]) and then measured at intervals up to 284 kPa (2.8 atm abs) using a calibrated gas flow analyser, with cross verification against a small Douglas bag test type apparatus. During testing, the chamber was compressed and decompressed at a rate of 10 kPa·min⁻¹. Flow rates during chamber compression and decompression were compared.

The indicated rate of oxygen gas flow through the unadjusted flowmeters changed minimally - typically rising by a maximum of half of the diameter of the indicator ball. The actual (volumetric) flow, tested across indicated flow rates from 3 to 12 L·min⁻¹, was consistently reduced by approximately 50% as the chamber pressure increased from 101.3 to 284 kPa (1 to 2.8 atm abs). A slightly smaller reduction was observed when assessing the low flowmeter across the same pressure range; reductions of 40.0 and 43.3% were demonstrated at 0.3 to 0.6 L·min⁻¹ respectively. Differences in flow rates between compression and decompression were minor except at the very lowest flows.

At 284 kPa (2.8 atm abs), actual volumetric flow of oxygen through Comweld Ezi-Flow flowmeters is dramatically reduced and this needs appropriate compensation to ensure therapeutic aims are achieved.