Reading an ABG

What does an ABG look like?

The image above shows an example of a printout from a blood gas analyser. As you can see, the results are split into categories with the relevant parameters listed under each.

Some analysers will include markers to indicate results outside of the normal range. In this case we can see arrows telling us if the result is high or low.

At first glance it can seem a bit daunting, but we can make it easier to interpret by asking a series of simple questions. To start with we will only focus on the first two sections - Blood Gas Values and Acid Base Status.

Is there a respiratory problem?

Respiratory issues will present as derangements in pO₂ or pCO₂.

Hypoxia will kill the patient faster than anything else, so check the pO₂ first to make sure it's not low. Remember to make a note of the FiO₂ and keep this in mind when evaluating pO₂.

Next check the pCO₂. If it is high, the patient is retaining carbon dioxide. If it is low, they are blowing off more CO₂ than normal (hyperventilation). Both of these are abnormal and we need to figure out why.

A low pO₂ with a normal or high pCO₂ indicates respiratory failure, which we will discuss in more detail on the next page.

We know that CO₂ is involved in the plasma buffering system, so if the patient has an acid-base disorder and the pCO₂ is abnormal this could point to a respiratory cause.

Is there an acid-base disorder?

Now let's take a look at the pH – is it normal or abnormal?

If it’s reduced (< 7.35) then the patient has an acidosis – something is causing the blood to become more acidic than normal.

If it’s raised (> 7.45) the patient has an alkalosis – something is causing the blood to become more basic than normal.

If the pH is abnormal, the patient has an acid-base disorder. Let's narrow that down further.

Is the cause respiratory or metabolic?

Acid-base disorders can be caused by respiratory problems or metabolic problems. Take a look at the plasma buffer equation again:

H⁺ + HCO₃^- ⇋ H₂CO₃ ⇋ H₂O + CO₂

On one side we have bicarbonate (HCO₃^-) and on the other we have carbon dioxide (CO₂). If either of these are sufficiently deranged the buffer can become overwhelmed and cause an increase or decrease in H⁺ ions.

So if you have an acid-base disorder an abnormal pCO₂ could point to a respiratory cause, and an abnormal HCO₃^- could point to a metabolic cause.

We can further define these into 4 main acid-base disorders:

• Respiratory acidosis
• Respiratory alkalosis
• Metabolic acidosis
• Metabolic alkalosis

We will look at each of these in more detail later.

Compensation

Something to briefly touch on here is the concept of compensation. For simplicity we tend to discuss respiratory and metabolic acid-base disorders as separate entities, but in reality they are closely linked.

The acid-base balance of the blood can be controlled by both respiratory and metabolic mechanisms, and the body will use both to try to correct derangements.

If, for example, a patient has an acid-base disorder caused by a respiratory problem, the body may use it’s metabolic mechanisms to try to correct (or compensate) for this imbalance. Equally if there is a metabolic acid-base disorder, respiratory mechanisms may attempt to compensate for this.

Compensation can make interpreting ABGs a bit trickier, but we will break it down over the next few pages to help you understand how to identify it.

Summary

In just a few simple steps we now know if the patient is hypoxic, if they have an acid-base disorder, and we have some idea about what could be causing it.

Let's move on to discuss respiratory failure and acid-base disorders in more detail.