Gas exchange terminology

There are a number of measurements that are made when taking a blood gas sample (capillary or arterial). Check out the following definitions of the some of the most important measurements, which we will explore in more detail later.

pH

The pH tells you if a patient is acidotic or alkalotic. The relationship with Hydrogen ions (H+) is what determines a molecule’s acidic or basic (alkalotic) profile. A molecule which freely gives away H+ ions is called an acid (for example, Carbonic acid). A solution with lots of acids contains a lot of free Hydrogen ions. In contrast, a molecule which freely receives H+ ions is called an alkali, or base. A solution with lots of alkali contains very few Hydrogen ions. Confusingly, pH is an inverse of hydrogen ion concentration; the more hydrogen ions present, the lower the pH.

PaO2

The amount of oxygen dissolved in the blood and reflects how well the ventilation of the lungs matches the perfusion of the lungs. If ventilation matches perfusion, the PaO2 will be almost exactly the same as the Oxygen partial pressure of the alveoli.

PaCO2

The partial pressure of carbon dioxide in the blood.

Base excess

Represents an increase or decrease in the amount of base compared with the amount of acids present

HCO3

Concentration of bicarbonate is present in blood. This is used to determine the source of acid base imbalance, alongside pH and CO2.

SaO2

Percentage of Haemoglobin saturated with oxygen HbO2, compared to the percentage of unbound haemoglobin (Hb). This measurement is taken directly from a blood sample, unlike the measurement taken indirectly from a pulse oximetry probe. In practice, the SpO2 and SaO2 should be the same, or at least within 2%.

SpO2

Percentage of Haemoglobin saturated with oxygen HbO2, compared to the percentage of unbound haemoglobin (Hb). This is an indirect measurement taken using a pulse oximeter probe, which we will explore in more detail later.

In addition to these measurements, taken directly and indirectly from the patient, there is one vital piece of information that holds the key to interpretation of the results. The fractional inspired oxygen concentration (FiO2) is the percentage of oxygen in the patient’s inspired air. If they are breathing room air, this is simply 21% (or 0.21). If the patient is on supplementary oxygen, this percentage increases. Without this observation, the results from a blood gas measurement are meaningless.

Mark as Understood

© Institute of Clinical Science and Technology (ICST) 2020

Support: admin@clinicalscience.org.uk