A 40 year old patient is receiving ventilation at the follow…

A 40 year old patient is receiving ventilation at the following settings:  minute volume 10 L/min, SIMV 12, FiO2 100% and PEEP 20 cmH20.  Additional diagnostic data disclose the following:       VD/VT ratio 0.5       RQ              0.9       pH              7.39       PaCO2        40 torr       PaO2         190 torr       HCO3          26 mEq/L What is this patient’s alveolar to arterial gradient?