The effect of an increase in Sweep Gas Rate on PaCO2.
The data shown above are from a simulated patient with severe respiratory failure and an oxygen consumption of ~200 ml/min on stable VV ECMO. The patient has been completely paralysed with pancuronium and is ventilated on 100% oxygen with a tidal volume of 200 mls at 5 breaths per minute. The VV ECMO system has been set at a constant blood flow rate of 5.0 lpm while the ‘Sweep Gas’ rate has been increased step-wise from 2.0 lpm to 10 lpm. The patient's native cardiac output is about 5 lpm.
Note how the PaCO2 falls steadily from about 60 mm Hg at a sweep gas rate of 2 lpm to about 27 mm Hg at a sweep gas rate of 10 lpm.
Thus we can appreciate one of the important concepts of VV ECMO – that, for any given blood flow, carbon dioxide clearance is largely determined by the rate of gas flow through the oxygenator.