Blood gas analysis, pt 3: interpreting pH

After taking note of the direction of the pH shift – acidaemia or alkalaemia – it is important to determine the primary and secondary causes.

If an acidaemia is present (pH less than 7.35), an underlying respiratory or metabolic acidosis, or both, must exist. Similarly, if an alkalaemia is present (pH more than 7.45), an underlying respiratory or metabolic alkalosis, or both, must be present.

This is usually very simple, with the exception of cases presenting with a normal pH (between 7.35 and 7.45, slightly higher for cats).

Cases with normal pH

For cases with a normal pH, we need to determine which category it falls into:

1. No acid-base disturbance
   • Both respiratory and metabolic components are within the normal reference range.
2. Complete compensation for the acid-base disturbance
   • This requires specific calculations that will be discussed in a later blog.
   • This cannot be determined by glancing at the figures alone.
3. Two opposing acid-base disturbances (a mixed disorder), which are cancelling the effect of each other out in terms of pH.
   • Both the respiratory and metabolic components will be outside of their reference range, going in the opposite direction to each other.

Determining primary disorder

Since these animals are within the normal pH range – particularly those with complete compensation – how can you tell which is the primary disease process?

A golden rule of thumb is: even with maximal compensation, the pH will still usually move in the same direction as the primary problem.

Therefore, if the pH lies towards the acidaemic side of the mid-point of the pH range (less than 7.4), the primary disease process is an acidosis. By the same token, if the pH lies towards the alkalaemic side of the midpoint of the pH range (more than 7.4), it will have a primary alkalosis disorder.

The reason behind this is the body does not usually overcompensate for an acid-base disturbance.

Secondary disorder

Once the primary disorder has been identified, we need to look at whether a secondary disorder is also present and, if so, whether this is the result of compensation or a true mixed process.

To determine whether compensation occurred, you need to understand the timeline for when compensation usually occurs.

With respiratory compensation, this typically starts immediately, but may take up to 8 to 12 hours to occur. This is because adjusting levels of CO₂ is relatively easy, with the change of respiratory rate and patterns.

On the other hand, metabolic compensations take approximately one to three days to occur, since renal excretion of hydrogen ions or retention of bicarbonate takes longer.

Variation magnitude

If the magnitude of the observed variation is compatible with compensation alone (this requires calculation), a compensatory mechanism is likely. A mixed process (mixed acid-base disorder) is present if the magnitude:

• does not correspond to the clinical status of the patient
• falls outside of the compensation time frame
• is outside of the expected magnitude of compensation

All other causes for why the acid base is moving in the opposite direction must be ruled out before determining a secondary process is present.

Once the primary and, if present, secondary disorder are determined, the next step is to determine the cause of the respiratory and metabolic acidosis and alkalosis.