17-year-old boy with wrist fracture. This patient became blue during manipulation of the fracture under bier’s block anaesthetic effect.
Arterial blood gas was performed and it showed the following:

PH = 7.30
PCO2 = 28 mmHg

PaO2 = 83 mmHg
HCO3 = 18 mmol/L
COHb = 1.2 %
MetHb = 47 %
Na = 136 mmol/L
K = 4.1 mmol/L
Cl = 102 mmol/L
Lactate = 3.2 mmol/L
Cr = 52 nmol/L
BSL = 5.6 mmol/L

Questions: 

1.  Interpret these blood gases
2. Why did this patient turn blue and what is the mechanism behind that?
3. What other potential complications you need to consider (Prepare for) when you perform Bier’s block?