So let's go back to our Winston. If we remember him, he was case two. That might seem a long way away now. He had a history of acute collapse while playing at the park. And we think that he has an acute airway obstruction due to his brachycephalic nature. But we want to evaluate an arterial blood gas because we want to try to find out, does he have lung disease, too? We don't know. Right now, we just know that he's hypoxemic and we know that he does have obstruction of his airway, but maybe he's developed non cardiogenic pulmonary edema due to an airway obstruction and he now has lung disease, too. 

So let's go back to look at our arterial blood gas. So here is our arterial blood gas. We already know that our PACO2 is increased and our PAO2 is really low. It's at 66. So he's got significant severe hypoxemia. So let's do our AA gradient in his case. So we're going to take our 150 we're going to take our PACO2 and divide it by a respiratory quotient, which is point eight. And then we're going to minus 66, which is our arterial oxygen concentration. And the good news is that in Winston's case, his AA gradient is 14, and if you remember, less than 15 is considered normal. So we can say that even though he's severely hypoxemic, he has a normal pulmonary function. And if we do our 120 rule and we look at it that way, we'll find that his value is 122, which also indicates normal pulmonary function. And so our problem is hyperventilation and luckily not pulmonary dysfunction as well. So that's good news for Winston because that means that we can treat him in the way that we had planned. 

All right. Here is a overall summary of how we going to evaluate these blood gases. So if you get a blood gas on boards, this is what you're going to do. First question we're going to ask now that we've looked at venous and arterial, is is the sample venous or arterial? Second, is the pH normal? Third is the primary disturbance metabolic or respiratory? Fourth, is there compensation? And fifth, if you've got an arterial blood gas, is there hypoxemia? If the answer is yes, we're going to do our AA gradient, we'll use the 120 rule to figure out if there's evidence of pulmonary dysfunction. 

Here is a very brief summary table of our expected changes with each of the four different disorders. So we've gone through metabolic acidosis, respiratory acidosis, metabolic alkalosis and respiratory alkalosis.