Acid Base Interpretation

How to interpret acid base abnormalities in blood gases, by Dr Jess McNiff.

Using an arterial blood gas or ABG is one of the common way’s of determining acid base status in a patient. It is a skill that many individuals find difficult. This page hopes to help you by explaining a simple way of looking through an ABG result and determining the acid/base abnormality.

In renal disease acid/base profiles are important mainly due to the management of decreased bicarbonate production by the kidney or intoxication of the agent leading to renal impairment.

For the purpose of this teaching we will refer mainly to the terms acidaemic and alkaleamic (acidaemia/alkalaemia) describing the balance of ions in the blood.

The text below is covered in the video above but acts as a supplementary reminder.

Hydrogen Ions/pH

Most ABG strips will have normal values on them so do not worry about memorising them.

We have determined whether there is an acidaemia or an alkalaemia, next we determine the cause.

Is there a respiratory cause?

So Carbon Dioxide is the marker of respiratory causes of acid base disturbance, however there is another cause.

Is there a metabolic component?

So we have discussed about primary causes of acid/base disturbance and had a quick introduction to compensation but here is a bit of a further explanation.

Compensation and Anion Gap

The example in the video is acidaemia however it is the same for alkalaemia where CO2 can be increased to try to compensate for a metabolic alkalaemia. If only one of either carbon dioxide or bicarbonate is changed there is likely to be a primary abnormality but if both have changed there is likely to be compensation.

The anion gap, quoted normal values for AG vary; some say <16 meq/l (<11 or 12 if calculated without potassium. Anion gap can be useful, but it isn’t very reliable for spotting unmeasured anions when it is only moderately elevated, and is sometimes not as high as you’d expect, for example in some patients with lactic acidosis.  Renal failure increases it.

Common causes of acid base disturbance

Respiratory Acidosis
– Sleep apnoea
– Opiates
– Asthma with T2RF
– Neuromuscular diseases
– Some pneumonia

Metabolic Acidosis
-Renal tubular Acidosis eg. Fanconi’s syndrome
– Uraemia
– Diabetic ketoacidosis
– Lactic acidosis
– Toxins – ethylene glycol
– Methanol
– Salicylates aspirin etc
Respiratory Alkalalosis
– Hypoxaemia
– Any cause of hyperventilation – Pain, anxiety, panic etc

Metabolic alkalosis
– Vomiting and diarrhoea
– Diuretics – especially loop diuretics
– Continued antacid overuse

Now test yourself on the following cases