PH. 7.2 PO2 mm Hg 50 mm Hg, HCO3– 13, the values goes in favour of -
In this patient
PH →Decreased (N → 7.4)
PCO2 → Increased (N-40 mm Hg)
HCO3- →Decreased (N-24 mmol/L)
First step

Determine whether alkalosis or acidosis is present
Decreased pH indicates acidosis

Second step

Inspect the HCO3- / CO2 to determine the cause of acidosis
Here both the abnormalities are in sync with a decrease in pH

Increase in CO2 → Produces acidosis (respiratory)
The decrease in HCO3- → Produces acidosis (metabolic)

There is no compensatory response
It can be easily concluded, that it is a case of mixed respiratory acidosis together with metabolic acidosis
The other very simple way to detect these mixed disorders is to observe the HCO3-/CO2 ratio.
HCO3-/CO2 ratio moves in the opposite direction
There is no need to proceed to the third step.
Respiratory acidosis
Metabolic alkalosis
Respiratory alkalosis
Both respiratory and metabolic acidosis