Arterial Blood Gas in MRCP (2)

Arterial Blood Gas in MRCP (2)


In my last post, I talked about Metabolic Acidosis and respiratory acidosis. Today I will cover metabolic alkalosis and respiratory alkalosis.

Take it easy because these two conditions are rare in your MRCP examination.

1) Metabolic alkalosis

The only one time you will be see metabolic alkalosis in your clinical practice is after patient has severe prolonged vomiting (especially in pyloric stenosis). This is because our gastric juice is acidic, where there is excessive loss of acid from our stomach, we turn alkalosis. You notice PH↓ and HCO3↑. You body may compensate by keeping more CO2 therefore there is a possibility that in your ABG, PaCO2↑



2) Respiratory Alkalosis

This is commonly seen in your clinical practice especially in A+E department but it is not an emergency. You usually see young ladies come to A+E complaining of shortness of breath but it is actually due to hyperventilation ( may be due to anxiety or not). However, never always assume that young ladies have hyperventilation when they complain to you that they are breathless. My old professor always told me that because some young ladies may also have pulmonary embolism ( due to risk factor of taking oral contraceptive pill or underlying autoimmune disease) when they are breathless. Therefore, always do an ABG if you are in doubt.

MRCP candidates always worried because in MRCP Part 1 and 2 examinations, they may show combination of abnormalities. There is always one rule to remember, if you can’t explain the abnormalities, always suspect this possibility.

Let me illustrate to you a case, let say a diabetic patient is admitted to you’re A+E due to cough and fever for 1 week and his CXR shows pneumonia. His ABG result is as below,

PH=7.2
HCO3=10
Random blood sugar = high
PaCO2=7 kpa
PaO2= 8kpa

Ok, from these first three results, we notice that this patient has metabolic acidosis ( PH↓, HCO3↓) and it is most probably due to diabetic ketoacidosis because the sugar is high as well. However, you will anticipate the PaCo2 to be low ( due to air hunger) but in this case the PaCO2 is high as well, you can’t explain that ( this is not a normal physiological respond) but from logical thinking, you know that this is a combination of metabolic and respiratory acidosis! ( Patient’s lung is also failing due to severe pneumonia and it is unable to compensate for the metabolic acidosis!)