Maintaining acid–base balance

Maintaining acid–base balance

The renal and respiratory systems operate jointly to maintain blood pH within normal limits. If one system is overwhelmed, leading to a change in blood pH, the other usually adjusts, automatically, to limit the disturbance (e.g. if kidneys fail to excrete metabolic acids, ventilation is increased to exhale more CO₂). This is known as compensation.

Importantly, compensatory changes in respiration happen over minutes to hours, whereas metabolic responses take days to develop.

Disturbances of acid–base balance

An acidosis is any process that acts to lower blood pH. If it is due to a rise in PaCO₂, it is called a respiratory acidosis; if it is due to any other cause, then HCO₃ is reduced and it is called a metabolic acidosis.

An alkalosis is any process that acts to increase blood pH. If it is due to a fall in PaCO₂, it is called a respiratory alkalosis; if it is due to any other cause, then HCO₃ is raised and it is called a metabolic alkalosis.

Acid–base disturbances can be considered as a set of scales.

Normal acid–base balance

When acid–base balance is entirely normal, with no alkalotic or acidotic pressures, it is like having a set of scales with no weights on it (Figure 11).

Figure 11 Normal acid-base balance.

Uncompensated acid–base disturbance

When an acidosis or alkalosis develops, the scales become unbalanced, leading to acidaemia or alkalaemia respectively.

In Figure 12 there is a primary respiratory acidosis with no opposing metabolic process.

Figure 12 Uncompensated respiratory acidosis.

Compensated acid–base disturbance

As described earlier, a respiratory or metabolic disturbance is often compensated for by adjustment of the other system to offset the primary disturbance.

Figures 13 and 14 represent two scenarios in which the lungs have responded to a primary metabolic acidosis by increasing alveolar ventilation to eliminate more CO₂ (compensatory respiratory alkalosis). In Figure 13 an acidaemia persists despite compensation (partial compensation); in Figure 14, blood pH has returned to the normal range (full compensation).

Figure 13 Partially compensated metabolic acidosis.

Figure 14 Fully compensated metabolic acidosis.

When faced with such an ABG, how can we tell which is the primary disturbance and which is the compensatory process?

The first rule to remember is that overcompensation does not occur. The midpoint of the acid–base scales lies at a pH of 7.4 (H⁺ 40). If the scales tip toward acidaemia (pH < 7.4), this suggests a primary acidotic process; if they tip toward alkalaemia (pH > 7.4), a primary alkalotic process is likely.

The second rule is that the patient is more important than the ABG. When considering an ABG, one must always take account of the clinical context. For example, if the patient in Figure 14 were diabetic, with high levels of ketones in the urine, it would be obvious that the metabolic acidosis was a primary process (diabetic ketoacidosis).

Mixed acid–base disturbance

When a primary respiratory disturbance and primary metabolic disturbance occur simultaneously there is said to be a mixed acid–base disturbance.

If these two processes oppose each other, the pattern will be similar to a compensated acid–base disturbance (Figure 14) and the resulting pH derangement will be minimised. A good example is salicylate poisoning, where primary hyperventilation (respiratory alkalosis) and metabolic acidosis (salicylate is acidic) occur independently.

On the other hand, if the two processes cause pH to move in the same direction (metabolic acidosis and respiratory acidosis or metabolic alkalosis and respiratory alkalosis), a profound acidaemia or alkalaemia may result (Figure 15).

Figure 15 Mixed respiratory and metabolic acidosis.

The nomogram

An alternative way to analyse ABGs is to use the acid–base nomogram (Figure 16). By plotting the PaCO₂ and H⁺/pH values on the ABG nomogram, most ABGs can be analysed. If the plotted point lies outside the designated areas, this implies a mixed disturbance.

Figure 16 The nomogram.