Assessing Lung Function – Laboratory Diagnostics

00:02 Finally, we are dealing with the gases, this puts us into yet another formula and this is one in which you can’t, there is no way you can circumvent this topic.

00:15 A-a gradient is what we are looking at and it is the big 'A' that you are paying attention to and the little a. Literally. Have to. The big 'A' represents PAO2. In another words, this is the oxygen in your alveoli and then you subtract from this, the oxygen that is in your little 'a' and that then represents the artery. If you take a look at the first bullet point here, you have, PaO2 and from hence forth, you are not going to hear me say PaO2 anymore. I am just going to refer to as being PO2, but to make sure that we are clear and that our teaching points are coming across. The PaO2 is a partial pressure of oxygen in the artery and you will tell me that is approximately PO2, there you go, of 100. 95, depending on the little bit of shunt. Now, the PAO2 would be the alveoli and that is obtained from the alveoli, obviously, but more importantly, let’s just talk about what A-a gradient means to you.

01:19 What does that gradient even refer to, the gradient coming out of the alveoli or going into the alveoli? Put yourself in the alveoli right now. There you are. You are sitting in that sack. Are you there? Nice to see you. Okay, now, in that sack, you are trying to get that oxygen through the alveolar membrane, through the interstitium and into the pulmonary capillary. That is the gradient. Who are you? Oxygen, okay. So, the gradient there should normally be, well how much? What is your PO2 in the alveoli, do you remember from physio? 100. Good! And that PO2 that is being carried by my pulmonary artery and pulmonary capillary into the alveoli, the PO2 is how much? A measly 40, right? So, that will be the blood on the, or oxygen content on the venous side, isn’t it? Deoxygenated blood, pulmonary artery and company. Therefore, the difference between the two would then give you a gradient of 60. That is a pretty big gradient for the oxygen to hop out of the alveoli and diffuse through the membranes into the pulmonary capillary and then bind to whom? Good. Haemoglobin.

02:33 Speaking of which, the PO2, the partial pressure of oxygen, is this actually referring to the haemoglobin attached or the oxygen attached to the haemoglobin? Not at all. The oxygen bound to haemoglobin, give me a test that you are going to use for its percentage.

02:50 That is your saturation of oxygen, isn’t it? So, your saturation of oxygen approximately 97% being the optimal percentage of your oxygen bound to haemoglobin. Right? But, here, we are referring to PO2, that little bit of oxygen is dissolved in your blood. I hope that is clear. I hope that you understand the diffusion. I hope you understand the gradient. And what that means to you clinically? Ah, that is what we are getting at.