This is a subject which is almost guaranteed to come up in one or more aspects of your exams. It a skill that, although not hard, needs to be practised because mistakes are easily made. Blood gases are obtained from an arterial blood sample usually taken from the radial artery. The normal values are given below:
The essential question you want to answer are: is there acidosis or alkalosis? Is this caused by a change in the PaCO2 or the HCO3? Is there compensation?
The routine that you should go through to answer these questions is straightforward:
This assumes that the pH is abnormal. If it is normal, check whether this the PaCO2 and HCO3 are within the normal ranges or whether one is fully compensating the other. There is never over-compensation: the pH will never go from acidosis to alkalosis or vice versa as a result of compensation.
The pH in this example is less than 7.35 and therefore acidotic. Next look to see whether this can be explained by the PaCO2. The PaCO2 is higher than the normal range. Think of CO2 as an acid and this explains the blood acidity. Finally is there any compensation? the base excess is within the normal range so there is non.
Common causes of blood gas results should be memorised:
|Gas Deviation||Characteristics||Common Causes|
|Respiratory Acidosis||Acidosis with increased PaCO2||Type II respiratory failure: Asthma/COPD, muscle weakness (e.g. Guillain Barre syndrome), respiratory depression (e.g. sedatives)|
|Respiratory Alkalosis||Alkalosis with low PaCO2||Commonly hyperventilation|
|Metabolic Acidosis||Acidosis with low HCO3||
|Metabolic Alkalosis||Alkalosis with high HCO3||Potassium depletion, vomiting|