Question 1:

PH = 7.30, that is mild acidaemia.

HCO3 = 18, that is low (less than 24), So we have metabolic acidosis.

Once we have metabolic acidosis, next step is to calculate compensation and anion gap.

For compensation we use Winter’s formula {expected PCO2 = 1.5 x HCO3 + 8 (+/- 2)}. accordingly, expected PCO2 should be 35 (33 – 37). Since PCO2 is 28 so we have additional respiratory alkalosis.

Anion gap = Na – (Cl + HCO3) = 16 ( more than 12) so we have high anion gap metabolic acidosis. “HAGMA”

Because we have HAGMA, we need to calculate the Delta ratio. that is (AG – 12/ 24 – HCO3) = 0.66 ” between 0.4 – 0.8″ so we have pure HAGMA.

MetHb is very high, usual level is around 1%, so we have severe Methhaemoglobinaemia.

Lactate level is 3.2 that is elevated, so we have lactataemia.

The final conclusion is Mild high anion gap metabolic acidosis with associated respiratory alkalosis. The metabolic acidosis is most probably a lactic acidosis secondary to methaemoglobinaemia.

Question 2: 

Prilocaine, the local anaesthetic that used in Bier’s block is known to cause Methaemoglobinaemia as one of it’s side effects. (The involved enzyme is still unknown).

Other substances that known to cause Methaemoglobinaemia:

• Antibiotics (trimethoprim, sulfonamides).
• Dapsone.
• Local anesthetics (especially benzocaine, prilocaine, and lignocaine).
• Metoclopramide.
• Ingestion of compounds containing nitrates.

MetHb is formed by the deoxygenation of Hb or due to reactive oxygen species such as superoxide and/or peroxide. The iron in the haem group of the Hb is oxidized from ferrous state (Fe 2+) to ferric state (Fe 3+). This oxidized haem group is unable to bind oxygen molecule, that reduces the amount of O2 in the blood and this results in cyanosis. RBCs are unable to transport sufficient amount of oxygen to the body tissue, this results in tissue hypoxia and shift of the oxygen dissociation curve to the left.

Treatment of Methaemoglobinaemia is with supplemental oxygen and methylene blue,  1% solution (10 mg/ml) 1 to 2 mg/kg administered intravenously slowly over five minutes. Although the response is usually rapid, the dose may be repeated if the level of methaemoglobin is still more than 25 one hour after the initial infusion.

Question 3:

The Bier block is a very safe procedure but we should be aware that there are few potential complications that can occur with this procedure.

These complications can be divided into two groups:

• 1- Tourniquet related complications: The most common adverse event encountered during the procedure is tourniquet pain. This pain can be minimized by using a double tourniquet, with inflation of the distal tourniquet followed by deflation of the proximal cuff. This will generally provide increased patient comfort. If this is ineffective, additional sedative medications or alternate methods of providing analgesia may be required.
• Other complications include nerve damage, compartment syndrome, skin discoloration or petechiae, and thrombophlebitis.

2- Local anaesthetic related These include:

• Seizures, due to sodium channels blocking effect.
• Arrhythmias, due to decrease conduction times leading to prolonged PR intervals and widened QRS complexes, and even sinus brady/arrest. ( Ventricular arrhythmias, including fibrillation, are more likely to occur with bupivacaine than prilocaine or lignocaine).
• Methaemoglobinaemia.