Oxygen Concentration Curves • LITFL • BSCC Examination

aka BSCC Physiology 014

Basic Science in Clinical Context Examination: 2 minutes long in 2 parts.

Exam candidate answering a question (under exam conditions)
Professor providing a more detailed explanation (with transcript)

Please draw the curve demonstrating the relationship between oxygen concentration and the partial pressure of Oxygen.

Demonstrate how this changes in polycythaemia and anaemia
Explain what happens to a normal patient when 1/3 of the haemoglobin is bound to carbon monoxide?

Examinee response: Drawing and explanation in real-time video/audio

Examiner Explanation

Transcript

Oxygen Concentration Curves

Oxygen is carried in the blood (in two ways): bound to Hb or dissolved

0.3 ml of oxygen is dissolved in every 100ml of blood

When bound to Hb

1 gm of HB binds 1.39ml O2
A patient with a normal range of Hb e.g. 15 (15g/100ml blood), the Hb will carry 20.8 ml O2 /100ml blood. (15 X 1.39=20.8)

Plotted on the Hb-O2 curve you can see 20ml of oxygen. Notice that at a PaO2 of 90mmhg the range is >95%.

A patient who is polycythaemic might have a Hb of 20. This Hb will carry 27.8 ml of O2/100ml of blood.

Note: that the concentration of oxygen per 100ml of blood is greater in the polycythaemic patient, but the partial pressure of oxygen and the HB O2 saturation are identical to the patient with a normal range of Hb because the total Hb available to bind to oxygen as a PERCENTAGE will be 100%. You may have more Hb in the polycythaemic patient, but as percentage- all of it will bind to oxygen giving you a saturation of 100%

A patient who is anaemic might have a Hb of 10 (10g/100ml). This Hb will carry 13.9 ml of oxygen per 100ml of blood.

The concentration of oxygen per 100ml of blood is less than a patient with normal HB, but the partial pressure of oxygen and the HBO2 saturation are identical because the Hb available to bind to oxygen as a PERCENTAGE will be 100%. You may have less Hb, but as a percentage all the available Hb will bind to oxygen given you saturation of 100%

In Summary:

The Hb concentrations may differ in a polycythaemic, normal and anaemic patient, causing the oxygen concentration per 100ml of blood to differ in each patient but the PAO2 remains the same and the HbO2 saturations will still read 100% with every patient because the Hb available to bind oxygen as a percentage will be 100%

Can you please explain what happens to a normal patient when a third of their Hb is bound to carbon monoxide?

The curve will be compressed and shift a bit to the left if carbon monoxide is present, because CO has a MUCH GREATER affinity for Hb (up to 240 times), therefore a lot of HB will be tied up to small amount of CO.