The oxygen consuming speed is 305 mL/min.
This is an example of a disguised Boyle's Law problem involving partial pressures.
The "trick" is to calculate the partial pressure of the dry oxygen. Both water vapour and the oxygen itself contribute to the pressure of the "wet" gas.
I assume that the atmospheric pressure is 1 atm = 760 Torr.
According to Dalton's Law of Partial Pressures,
At 20 °C, the vapour pressure of water is 17.5 torr.
The patient is consuming 1.83 L of oxygen in 6 min.
Rate of consumption is
For comparison, a "normal" adult breathes about 6 L of air or 1 L of O₂/min.