Haematology
Interpretation of blood gases
The acidity of the blood is measured by pH. Ideally blood gases should be
measured on an arterial sample, but in babies capillary samples are sometimes
used, which makes the PO2 unreliable. A high pH refers to an alkalosis and a
low pH to an acidosis. The pH is a logarithmic scale, so a small change in pH
can represent a large change in hydrogen ion concentration.
Once the blood
becomes profoundly acidotic (pH < 7.0), normal cellular function becomes
impossible. There are metabolic and respiratory causes of both acidosis and
alkalosis (see below). The pattern pH and PCO2 can be used to determine the
type of abnormality.
