#Interpreting a blood gas

#Normal values

	Normal range
pH	7.35 to 7.45
pCO2	35 to 40 mmHg
HCO3-	22 to 26 mmol/L
Base excess	-2 to 2

#Compensation

Compensation in acid–base disturbance refers to the body’s attempt to restore arterial pH toward normal.

• e.g. increasing renal bicarbonate reabsorption in response to a respiratory acidosis
• e.g. increasing minute ventilation (and CO2 elimination) in response to a metabolic acidosis

The degree of compensation is predictable and can be estimated by certain rules. Compensation does not necessarily normalise the pH.

#Boston rules for primary respiratory disturbance

Describes the expected change in bicarbonate for a primary respiratory acidosis or alkalosis according to the rules below. If the observed bicarbonate level is different to the expected level, this may indicate a secondary process.

	Change in HCO3- (mmol/L)
	Acute	Chronic
Resp acidosis: for every ↑10mmHg in pCO2	↑ 1	↑ 4
Resp alkalosis: for every ↓10mmHg in pCO2	↓ 2	↓ 5

An observed bicarbonate level that is higher than expected may indicate a concurrent metabolic alkalosis.

• e.g. in an acute respiratory acidosis with a pCO2 of 70, the expected bicarbonate is $24 + 3 \times 1 = 27$
• if the observed bicarbonate is 32, this indicates a secondary metabolic alkalosis

An observed bicarbonate level that is lower than expected may indicate a concurrent metabolic acidosis.

• e.g. in an chronic respiratory alkalosis with a pCO2 of 20, the expected bicarbonate is $24 - 2 \times 1 = 22$
• if the observed bicarbonate is 15, this indicates a secondary metabolic acidosis

#Rules for primary metabolic disturbance

For a primary metabolic acidosis, the expected pCO2 can be estimated with Winter's formula:

$$\text{expected pCO}_2 = 1.5 \times [\text{HCO}_3^-] + 8$$

For a primary metabolic alkalosis, the expected PCO2 is estimated by the following formula:

$$\text{expected pCO}_2 = 0.7 \times [\text{HCO}_3^-] + 20$$

The same principles apply as with primary respiratory pathology:

• if the observed pCO2 is higher than expected, this indicates a secondary respiratory acidosis
• if the observed pCO2 is lower than expected, this indicates a secondary respiratory alkalosis

#Anion gap

The anion gap (AG) reflects the difference between measured cations and measured anions in plasma, calculated by the following equation:

$$\text{Anion Gap} = [\text{Na}^+] - \big([\text{Cl}^-] + [\text{HCO}_3^-]\big)$$

A normal anion gap (AG) is 8 to 12 mmol/L.

If the anion gap is high, this indicates a high quantity of unmeasured anions. Acids dissociate in solution to an anion and a hydrogen ion. Hence a high anion gap may indicate the presence of acids.

A metabolic acidosis with AG >12 indicates a high anion gap metabolic acidosis (HAGMA) caused by the accumulation of acids, for example:

• lactate
• ketones
• toxins (e.g. ethylene glycol)
• impaired excretion of acids (e.g. phosphate) in renal failure

A metabolic acidosis with AG ≤12 indicates a normal anion gap metabolic acidosis (NAGMA), which may be due to hyperchloraemia or increased bicarbonate losses.

#Correcting for hypoalbuminaemia

Albumin is a major unmeasured anion. If a patient is hypoalbuminaemic, the body maintains electroneutrality by increasing the chloride concentration. The calculated anion gap therefore decreases. If not accounted for, hypoalbuminaemia could mask a HAGMA.

The corrected anion gap is calculated by the following equation:

$$\text{AG}_{\text{corrected}} = \text{AG}_\text{calculated} + 0.25 \times (40 - \text{albumin in g/L})$$

In other words, for every 1g/L reduction in serum albumin, the corrected anion gap decreases by 0.25mmol/L.

#Delta ratio

The delta ratio relates the anion gap to the change in bicarbonate level. It is given by the equation:

$$\Delta \space \text{ratio} = \frac{\Delta \text{AG}}{\Delta [\text{HCO}_3^-]} = \frac{\text{AG}-12}{24 - [\text{HCO}_3^-]}$$

#Interpretation

Principles

• $\Delta \text{AG}$ represents the rise in the anion gap
• $\Delta [\text{HCO}_3^-]$ indicates the severity of acidosis
• the ratio of these quantities therefore indicates the degree to which the anion gap explains the acidosis

<0.4 suggests NAGMA

• a very low delta ratio indicates that the anion gap has risen less than expected for the severity of acidosis
• in other words, the degree of acidosis is not explained by the anion gap
• if the anion gap is barely elevated and the acidosis is severe, this suggests the primary pathology is a NAGMA rather than a HAGMA

0.4–0.8 may indicate concurrent NAGMA + HAGMA

• when the delta ratio is low-ish, the degree of acidosis is only partly explained by the anion gap
• a NAGMA may be diluting the effect of the HAGMA on the delta ratio

0.8–2 suggests HAGMA

• the anion gap is in proportion to the degree of acidosis, indicating HAGMA

>2 may indicate concurrent HAGMA + metabolic alkalosis

• when the delta ratio is very high, the anion gap is out of proportion to the acidosis
• this may be due to a concurrent metabolic alkalosis, which would cause the bicarbonate to be higher than with a HAGMA alone
• ↑$[\text{HCO}_3^-] \longrightarrow$ ↓$\Delta [\text{HCO}_3^-] \longrightarrow$ ↑$\Delta \space \text{ratio}$