Welcome to ICU Primary PrepCast. Hi, I'm Maddy. Hi, I'm Swapnil.
And welcome to yet another primary snippet. So Maddy, this week we are going to discuss a slightly complicated topic because a lot of candidates are struggling to answer this question. is around mixed venous oxygen saturation.
So the question is, what are the factors that affect mixed venous oxygen saturation? I think firstly, it's important to know what mixed venous oxygen saturation is. So this is measured from blood within the pulmonary artery. So you're through a PA catheter and this contains blood mixed in the right ventricle, which is therefore thought to be representative of oxygen extraction for the whole body. So it looks at the saturation level of that blood.
And this therefore is used as a surrogate marker for the overall balance between oxygen delivery and oxygen consumption. So if we think about it like this, we can think about as being dependent on factors affecting oxygen delivery and factors affecting oxygen consumption. So in terms of, we'll start with factors affecting oxygen delivery. So any factor that decreases oxygen delivery will decrease the mixed venous oxygen saturation. And we can think about the oxygen delivery equation, so which is cut.
oxygen delivery is cardiac output, so the amount of blood going around the body, multiplied by the oxygen content within that blood. And the oxygen content is the maximal oxygen carrying capacity of blood, which they have different numbers quoted depending on the equation used, but it's variably either 1.34, 1.39, depending on what you look at, multiplied by the hemoglobin concentration, multiplied by the hemoglobin oxygen saturation. And this is added... to the solubility constant for oxygen at 37 degrees, which is 0.03 mils per litre per millimetre of mercury, multiplied by the PaO2, so the partial pressure of oxygen. So if we think about cardiac output, we've learned before about what the factors are that affect cardiac output.
So this is heart rate times stroke volume, and stroke volume is affected by preload, afterload, and contractility. So anything that affects these factors will affect your mixed venous oxygen saturation. Then thinking again about this oxygen delivery equation. So the next part is the hemoglobin concentration.
So for example, anemia, which decreases your hemoglobin concentration, can reduce your mixed venous oxygen saturation in severe anemia. And then we think about the oxygen saturation of hemoglobin. So the factors affecting this, they're all the factors we think about that shift the oxygen hemoglobin dissociation curve. So factors shifting the curve to the right will decrease hemoglobin's affinity for oxygen and therefore can...
decrease the mixed venous oxygen saturation. So these are things like acidosis, increased carbon dioxide levels, increased 2,3-DPG concentration and increased temperature. And factors shifting the curve to the left can increase hemoglobin's affinity for oxygen and therefore might increase the mixed venous oxygen saturation, which are the opposite factors, so alkalosis, decreased carbon dioxide levels, et cetera. And then finally, looking at the PaO2 or the partial pressure of oxygen, increased PaO2 increases mixed venous oxygen saturation.
So things like... I mean, this is only really relevant in conditions where it's pretty extreme. So hyperbaric therapy or hyperoxia and decreased PO2 will decrease the mixed venous oxygen saturation.
So this can be caused by anything that causes some hypoxia. So lung disease, hyperventilation, diminished inspired oxygen content or a right to left shunt. So that's thinking about the oxygen delivery factors. And then we come to the factors affecting oxygen consumption.
So oxygen consumption is a cardiac output times that. content of oxygen in arterial blood minus the content of oxygen in venous blood. So obviously, once again, factors affecting cardiac output can affect oxygen consumption.
Anything increasing oxygen consumption will decrease the mixed venous saturation, and anything that decreases oxygen consumption will increase the mixed venous oxygen saturation. So we can think about these in two main categories, so metabolic demand and oxygen utilization. In terms of metabolic demand, anything that...
obviously increases your metabolic demand. So think about when you're exercising, sepsis, hyperthyroidism, fever, shivering, pain, stress, that will increase oxygen consumption and therefore decrease your mixed venous oxygen saturation. And anything that conversely decreases your metabolic demand, such as paralysis, analgesia, hypodermia, will decrease this oxygen consumption. And then coming to the second part of factors affecting oxygen consumption, it's the actual utilization of the oxygen. So conditions such as histiotoxic hypoxia, the main thing would be cyanide poisoning, but other examples would be things like beriberi, where the tissues are unable to actually use the delivered oxygen.
Therefore, oxygen consumption is decreased and mixed venous saturation is increased, but there's still a hypoxia at the tissue level. So that's a bit paradoxical. And then conditions like microvascular shunting and sepsis will also increase your mixed venous saturation. So if there's any abnormal utilization of the oxygen, that can also affect your mixed venous oxygen saturation. Thanks, Maddy.
That's the end of our today's episode. We'll be back with another snippet in a week's time. Till then, goodbye and have a nice time. Thanks for listening. See you next time.