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

Abstract

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 satur-ation 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.

Figure 1

ODCs in a theoretical healthy subject with a normal blood haemoglobin (Hb) concentration of 15 g ⋅ dL⁻¹. The y-axis can be plotted as either % saturation or oxygen content (concentration); with the latter showing the very small amount of oxygen dissolved in solution.

Figure 2

ODCs in a theoretical subject with anaemia and haemoglobin (Hb) concentration of 7.5 g ⋅ dL⁻¹ compared to normal haemoglobin concentration of 15 g ⋅ dL⁻¹. When oxygen content is plotted against P_O2 the curve in anaemia is scaled down by 50%, reflecting the halving of oxygen carrying capacity (dissolved oxygen is ignored); when Sa_O2 is plotted the anaemic and normal curves are superimposed.

Figure 3

Comparison of average oxygen dissociation curves based on measured arterial blood saturation and arterial PO₂ in patients with normal pH (7.35–7.45) with values calculated using the simplified Severinghaus equation.

Figure 4

Mean relationships between arterial PO₂ and measured oxygen saturation in blood specimens with different pH ranges.