Exercise 1.
Calculators Page:
The purpose of this exercise is to demonstrate the interaction between cardiovascular, metabolic and respiratory factors in the determination of arterial PO2.

First, increase the FiO2 to 80% using the 'spin control' to the right of the FIO2 box. Leave the arterial PO2 (PaO2) set at 100 mm Hg, but note that this represents a considerable loss of oxygenating efficiency and that the calculated right to left shunt (QS/QT) has increased from about 1% to 22.9%. - A value that might be seen in a patient with 'mild' Adult Respiratory Distress Syndrome. Note also that oxygen utilisation (VO2) is 301 mls/min, the mixed venous PO2 (PVO2) is 38 mm Hg and the cardiac output is 5 l/min.

Suppose now that you are called to see the patient because the arterial PO2 has fallen from 100 mm Hg to 60 mm Hg. What might have happened? - Obviously, in real life, you would examine the patient, check the ventilation - all that sort of thing. But could the problem be related to cardiac output?

To demonstrate the effect of a changing output in the presence of pre-existing lung disease, make the following adjustments to the model.

Reduce the arterial PO2 to 60 mm Hg using the 'spin control' to the right of the PaO2 box.

Reduce the mixed venous PO2 to 23 mm Hg using the 'spin control' to the right of the PVO2 box. - This figure has been chosen so that the magnitude of the right to left shunt remains more or less constant at ~ 23%.

Now reduce the cardiac output to 2.8 l/min.- This figure has been chosen so that oxygen utilisation remains more or less constant at ~ 300 mls/min.

We have now confirmed that a reduction in cardiac output from 5.0 l/min to 2.8 l/min in the presence of a fixed right-to-left shunt and unchanged oxygen utilisation results in a fall in PaO2 from 100 mm Hg to 60 mm Hg. If oxygen transfer is perfect (QS/QT = 0%), this effect will not occur, because the arterial blood is never diluted with shunted venous blood.

The effects of anaemia and changing metabolic rate can be similarly explored using this part of the simulator.

Exercise 2.
Calculators Page:
The purpose of this exercise is to demonstrate the effect of blood gas measurement errors on metabolic calculations.

Because of the sigmoid shape of the haemoglobin dissociation curve, measurement errors (particularly in the mixed venous sample), can lead to major errors in the calculation of the derived measurements.

Suppose that you are trying to calculate oxygen utilisation in a patient on your intensive care unit, but you are unaware that the pH electrode of your blood gas analyser is unstable and is subject to an error of +/- 0.05 pH units for any measurement. Will this have much of an effect on the metabolic calculations?

Experiment with the effect of adjusting the pH up or down by 0.05 pH units. Note that calculated VO2 will vary from 273 - 330 ml/min when this adjustment is made to the venous sample, but that the variation is only 298 - 304 ml/min when the same adjustment is made to the arterial sample.

Thus the impact of small pH measurement errors on metabolic calculations is about ten times larger if they are made on the venous specimen. This difference is explainable by the different positions of the arterial and venous points on the haemoglobin dissociation curve.

Now repeat the exercise, but this time examine the effect of an error of 1-2 mm Hg in the measurement of mixed venous PO2. Note the marked effect of a small partial pressure measurement error on the content calculation. Remember that complete mixing of the venous blood does not occur until the pulmonary artery is reached and that a venous sample obtained before this point may represent 'streamed' superior or inferior caval blood ².

Exercise 3.
Haemoglobin page:
The purpose of this exercise is to demonstrate the impact of temperature changes in the arterial blood as it passes through the vascular tree. We will examine the effect of a 10 degrees C change in blood temperature during arterial transit. - The sort of temperature change which may occur during cardio-pulmonary bypass (or, for that matter, if you immerse your arm in a bucket of ice-cold water).

What will happen to the arterial PO2 if we now cool the blood under hermetic (sealed) conditions by 10 degrees C?

Change the blood temperature to 27 degrees C using the 'spin control' to the right of the temperature box. Note how the dissociation curve moves to the left. Note also how the oxygen content has increased to 209.5 mls/L. However, if conditions are hermetic, the oxygen content of the blood cannot alter, so adjust the content back to the original value of 205.6 mls/L using the arterial PO2 control. You will find that an arterial PO2 of 59 mm Hg gives you the closest content value. Thus we have demonstrated that cooling blood by 10 degrees C will reduce arterial PO2 from 97 mm Hg to 59 mm Hg without affecting arterial oxygen content. - As the blood is cooled, the left-shift of the dissociation curve causes haemoglobin to bind oxygen more avidly thus carrying the same amount at a lower partial pressure. At the same time, the solubility of oxygen in plasma has increased marginally which will also have the effect of lowering arterial PO2.

These partial pressure changes are important during bypass when patients are heated or cooled and are one of many reasons why gaseous microembolism may occur at different times during such procedures. ( - To understand this, think about what would happen if you equilibrated blood with 100% oxygen at 27 degrees C and then warmed it under hermetic conditions to 37 degrees C.)

Exercise 4.
Calculators Page:
The purpose of this exercise is to demonstrate interactions between cardiac output, oxygen utilisation and haemoglobin concentration in determining the mixed venous oxygen saturation.

First, note some of the 'default' values on the 'Calculators' page. In particular, oxygen utilisation (VO2) is 301 mls/min, cardiac output 5.0 L/min, haemoglobin 150 G/L and SvO2 69.5%.

Now reduce the cardiac output to 4.0 L/min, and return the VO2 to as close to 301 mls/min as you can get by reducing the mixed venous PO2 from 38 mm Hg to 34 mm Hg. Note that the SvO2 has now fallen to 62.7%.

Thus we can see that a reduction in cardiac output from 5 to 4 litres/min would cause SvO2 to fall from 69.5% to 62.7% under these circumstances.

Now experiment with the effect of changing cardiac output on the mixed venous saturation under conditions of anaemia or with a different metabolic rate (VO2). Note how anaemia magnifies the effect of a change in cardiac output on the SVO2, whereas a low metabolic rate diminishes this effect.

1. Hardman JG, Bedforth NM Estimating venous admixture using a physiological simulator. Br J Anaesth 1999 Mar;82(3):346-9

2. Edwards JD; Mayall RM Importance of the sampling site for measurement of mixed venous oxygen saturation in shock. Crit Care Med, 26:1356-60, 1998 Aug

Last edited on: 14/11/2000