The transport of oxygen through the blood is one of the cornerstone physiological processes, and is therefore expected to be thoroughly understood in the Primary FRCA. Oxygen content and oxygen delivery are key components of this broad topic.
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In this article, we will discuss the concept of oxygen content, and how that relates to oxygen delivery to the tissues.
Oxygen Content
Oxygen content describes the amount of oxygen that is present within a given volume of blood, and is often measured in ml/100ml.
Oxygen is present within the blood in two forms, and by considering both of these separately, the slightly intimidating equation for oxygen content (below) becomes a bit more accessible.
Equation 1: Oxygen content (ml/100ml) = (1.34 x Hb x SpO2) + (0.0225 x PaO2)
where Hb = haemoglobin (g/dL), SpO2 = oxygen saturations (%), PaO2 = arterial partial pressure of oxygen in kPa* (*if mmHg are used the equation changes to 0.003 x PaO2)
Essentially, the two components are bound oxygen and dissolved oxygen.
Bound Oxygen
Bound oxygen refers to oxygen bound to haemoglobin. The multiplier of 1.34 is known as Hüfner’s constant, which represents the 1.34 ml of oxygen that is carried by each gram of haemoglobin. This is then multiplied through by the oxygen saturations, which adjusts for the percentage of haemoglobin that is oxygenated.
Together, these three terms multiplied together give the amount of oxygen bound to haemoglobin, in ml/100ml.
Dissolved Oxygen
In addition to bound oxygen, we also have oxygen dissolved within the plasma. This is proportional to the partial pressure of oxygen, which is an application of Henry’s law.
The multiplier of 0.0225 is used to estimate the proportion of oxygen that becomes dissolved in 100ml of plasma, per kPa of partial pressure. As oxygen is poorly soluble in plasma, only a small fraction of the partial pressure is carried within the plasma.
Oxygen Delivery
Oxygen delivery quantifies the amount of oxygen (in ml/min) that is circulated to the tissues of the body.
In order for the oxygen stored within the blood to reach tissues, it makes sense that we need a cardiac output. Therefore, the equation for oxygen delivery is as follows:
Equation 2: Oxygen delivery (ml/min) = Oxygen content (ml/100ml) x cardiac output (L/min) x 10
The multiplier of 10 arises due to the use of L/min for cardiac output and ml/100ml for oxygen content.
Suggested Reading
Chapter 3. The Primary FRCA structured oral examination Study Guide 1. 2nd edition. Wijayasiri and McCombe. 2016.
Chapter 1.1.14. Graphic anaesthesia: Essential diagrams, equations and tables for anaesthesia. 2nd edition. Hooper, Nickells, Payne, Pearson and Walton. 2023.
Pages 223-225. Physics, Pharmacology and Physiology for Anaesthetists. Key Concepts for the FRCA. 2nd edition. Cross and Plunkett. 2014.
BJA article: https://www.bjaed.org/article/s2058-5349(17)30035-5/fulltext