Relating oxygen partial pressure, saturation and content: the haemoglobin–oxygen dissociation curve

Key Points

• In clinical practice, the level of arterial oxygenation can be measured either directly by blood gas sampling to measure partial pressure (P_aO2) and percentage saturation (S_aO2) or indirectly by pulse oximetry (S_pO2).

• This review addresses the strengths and weaknesses of each of these tests and gives advice on their clinical use.

• The haemoglobin–oxygen dissociation curve describing the relationship between oxygen partial pressure and saturation can be modelled mathematically and routinely obtained clinical data support the accuracy of a historical equation used to describe this relationship.

Educational Aims To understand how oxygen is delivered to the tissues.

To understand the relationships between oxygen saturation, partial pressure, content and tissue delivery.

The clinical relevance of the haemoglobin–oxygen dissociation curve will be reviewed and we will show how a mathematical model of the curve, derived in the 1960s from limited laboratory data, accurately describes the relationship between oxygen saturation and partial pressure in a large number of routinely obtained clinical samples.

To understand the role of pulse oximetry in clinical practice.

To understand the differences between arterial, capillary and venous blood gas samples and the role of their measurement in clinical practice.

The delivery of oxygen by arterial blood to the tissues of the body has a number of critical determinants including blood oxygen concentration (content), saturation (S_O2) and partial pressure, haemoglobin concentration and cardiac output, including its distribution. The haemoglobin–oxygen dissociation curve, a graphical representation of the relationship between oxygen satur­ation and oxygen partial pressure helps us to understand some of the principles underpinning this process. Historically this curve was derived from very limited data based on blood samples from small numbers of healthy subjects which were manipulated in vitro and ultimately determined by equations such as those described by Severinghaus in 1979. In a study of 3524 clinical specimens, we found that this equation estimated the S_O2 in blood from patients with normal pH and S_O2 >70% with remarkable accuracy and, to our knowledge, this is the first large-scale validation of this equation using clinical samples. Oxygen saturation by pulse oximetry (S_pO2) is nowadays the standard clinical method for assessing arterial oxygen saturation, providing a convenient, pain-free means of continuously assessing oxygenation, provided the interpreting clinician is aware of important limitations. The use of pulse oximetry reduces the need for arterial blood gas analysis (S_aO2) as many patients who are not at risk of hypercapnic respiratory failure or metabolic acidosis and have acceptable S_pO2 do not necessarily require blood gas analysis. While arterial sampling remains the gold-standard method of assessing ventilation and oxygenation, in those patients in whom blood gas analysis is indicated, arterialised capillary samples also have a valuable role in patient care. The clinical role of venous blood gases however remains less well defined.