A Brief Summary
A step by step guide to putting it all together
Step 1: Check the PaO2
Is the patient hypoxaemic? Remember hypoxaemia will kill faster than anything else so this will need managed first.
Be sure to make note of the FiO2 - you'll need to know if the patient is receiving supplemental oxygen to determine if the PaO2 is abnormal.
A low PaO2 indicates respiratory failure, which we can categorise further by checking the PaCO2.
Step 2: Check the PaCO2
Type 1 respiratory failure is a problem with gas exchange across the alveolar membrane, so PaO2 will be low and PaCO2 will be normal.
Type 2 respiratory failure is a problem with the pump that moves air in and out of the alveoli, so PaO2 will be low and PaCO2 will be high.
CO2 retention can produce an acidosis, so let's move on to look at the acid-base balance.
Note: Remember the PaO2 and PaCO2 are only useful in an arterial sample, so if they're abnormal make sure it's not a venous sample.
Step 3: Check the pH
The pH of the blood is tightly regulated and should be between 7.35 and 7.45.
If the pH is low (< 7.35) the patient is acidaemic.
If the pH is high (> 7.45) the patient is alkalaemic.
A deranged pH means the patient has an acid-base disorder. Let's narrow that down a bit to determine if it's respiratory or metabolic in origin.
Step 4: Respiratory vs Metabolic
We can distinguish respiratory from metabolic acid-base disorders by looking at the PaCO2 and HCO3- values.
The tables below summarise the findings we would expect for each disorder, along with the changes we would see if there is compensation.
| Acidosis | |||
|---|---|---|---|
| pH | PaCO2 | HCO3- | |
| Respiratory acidosis | |||
| Respiratory acidosis with metabolic compensation | |||
| Metabolic acidosis | |||
| Metabolic acidosis with respiratory compensation | |||
| Alkalosis | |||
|---|---|---|---|
| pH | PaCO2 | HCO3- | |
| Respiratory alkalosis | |||
| Respiratory alkalosis with metabolic compensation | |||
| Metabolic alkalosis | |||
| Metabolic alkalosis with respiratory compensation | |||
If what you see on the blood gas doesn't make sense in light of the above values, consider a mixed picture in which two acid-base disorders are occurring simultaneously.
Step 5: Calculate the anion gap
If there is a metabolic acidosis, calculating the anion gap can help narrow the list of differential diagnoses.
It tells us how many unmeasured anions are in the plasma, differentiating between pathologies that add anions and those that cause a drop in bicarbonate.
To calculate the anion gap we use the formula
Na+ - ( Cl- + HCO3- )
The normal range is 4-12 mmol/L.
A high anion gap metabolic acidosis means something is adding anions, for example lactic acidosis or diabetic ketoacidosis.
A normal anion gap metabolic acidosis means bicarbonate is being lost, for example due to diarrhoea or renal disease.
Step 6: Check the other values
At this stage we have established whether the patient is hypoxic, if they are in respiratory failure, if they have an acid-base disorder, whether the source is respiratory or metabolic, and if there is any compensation.
Looking at the other values can give us more information about what the primary pathology may be, or how the body is responding to an acid-base disorder.
On the next page we will briefly summarise some of the other values you might find on a blood gas readout.