Disorders of Acid Base Balance

and Christopher Isles²

(1)

Institute of Cardiovascular and Medical Sciences, University of Glasgow, Glasgow, UK

(2)

Dumfries and Galloway Royal Infirmary, Dumfries, UK

Q1 How do you diagnose an acid-base disorder?

You will be able to diagnose acid-base disorders correctly by measuring hydrogen ion concentration (normal range 35–45 nmol/l) or pH (normal range 7.36–7.44), PaCO₂ (normal range 4.6–6.0 kPa) and serum bicarbonate (normal range 22–27 mmol/l) on an arterial blood gas sample, which will also give you PaO₂ and lactate. Venous gases can be used in an emergency if an arterial sample is not possible and will give reliable measurements of H⁺, PaCO₂ and bicarbonate provided you use a blood gas tube.

Q2 There are only four disorders of acid-base balance. What are they?

These are metabolic acidosis, metabolic alkalosis, respiratory acidosis and respiratory alkalosis. Patients are considered to be acidotic if their H⁺ concentration >45 kPa or pH <7.36 and alkalotic when H+ <35 kPa or pH >7.44. If the patient is acidotic and the PaCO₂ is increased this is a respiratory acidosis whereas if PaCO₂ is normal this is a metabolic acidosis. If the patient is alkalotic and the PaCO₂ is reduced this is a respiratory alkalosis whereas if PaCO₂ is normal this is a metabolic alkalosis. We summarise these disturbances and the compensatory changes that follow in Table 9.1. Compensatory changes are either pulmonary (affecting ventilation) or renal (affecting bicarbonate reabsorption). Their purpose is to return H+ concentration towards normal so far as is possible.

Table 9.1

The four disorders of acid-base balance

H+	Interpretation	CO₂	Acid-base disorder	Compensatory change
Increased	Acidotic	Normal or low	Metabolic acidosis	Respiratory: Fall in CO₂
Increased	Acidotic	High	Respiratory acidosis	Metabolic: Rise in bicarb
Decreased	Alkalotic	Normal or high	Metabolic alkalosis	Respiratory: Rise in CO₂
Decreased	Alkalotic	Low	Respiratory alkalosis	Metabolic: Fall in bicarb

Q3 How do you diagnose metabolic acidosis?

If the patient is acidotic and the PaCO₂ is normal this is a metabolic acidosis. Patients compensate for this by blowing off CO₂ which often leads to deep respirations e.g. the Kussmaul breathing of DKA. Further assessment requires that you calculate the anion gap which enables you to determine whether the patient has normal or high anion gap metabolic acidosis.