What does the above image in Fig. 74.1 depict?

What is P₅₀?

How do you determine the oxygen content?

At what point can cyanosis be detected?

What variables shift the curve to the left and to the right and how does that affect oxygen transport?

What is Fick equation?

What is Bohr effect?

What is Haldane effect?

This is the oxyhemoglobin dissociation curve which demonstrates the measured relationship between the partial pressure of oxygen (PO₂) on the x-axis and the oxygen saturation (SaO₂) on the y-axis. The graph is sigmoid or S shaped. Initially, in the steep portion of the curve, the hemoglobin’s affinity for oxygen increases with maximum O₂ loading, and then the graph levels off around PO₂ of 60 mmHg with little change even when the PO₂ is increased significantly.

P₅₀ is the oxygen tension at which hemoglobin is 50% saturated which is typically around 26 mm Hg and is a measure of hemoglobin’s affinity for oxygen.

The blood oxygen content (CaO₂) is calculated as the sum of the oxygen bound by hemoglobin (Hb) and the oxygen dissolved in the plasma.

O₂ content = oxygen bound to hemoglobin + oxygen dissolved in blood.

CaO₂ = (1.39 × Hb × SaO₂/100) + (PaO₂ × 0.003).

For example, if Hb is 15 g/dL, SaO₂ is 100%, and PaO₂ is 100 mm Hg, then

CaO₂ = (15 × 1.39 × 1) + (100 × 0.003)

   = 20.85 + 0.3

   = 21.15 mL/dL.

Cyanosis can be detected at an SaO₂ of approximately 80%. Clear cyanosis can appear at an SaO₂ of approximately 67%. The appearance is also affected by skin perfusion, skin pigmentation, and hemoglobin concentration.