Let us go to case number four. Here is Ms. Ivy. She is a six year old female spayed Labrador retriever. She is up to date on her vaccines and she has a two day history of vomiting after getting into leftover barbecue. And now she has a 12 hour history of coughing, lethargy and exercise intolerance. Okay. 

Here is her physical exam findings and her you'll just of note she is hypothermic. She has an increased respiratory rate and on exam she has an increased respiratory rate and effort and diffuse crackles in our ventral lung fields bilaterally. 

All right. So we're going to start with a venous blood gas here is our venous blood gas. So, let's get to our interpretation. First question. Is our pH normal? The answer is no. It is alkalotic. So we can say that there is an alkalemia. So we're losing hydrogen. Is the primary disturbance metabolic or respiratory? So we would expect if it was metabolic that our bicarb would be increasing and our base excess would be increasing. And if it was respiratory, our PCO2 would be decreasing. So the answer is that our PCO2 is less than 38, and so this is a respiratory alkalosis. Is there compensation? So we would expect that our bicarb and our base excess would also decrease to try to offset this increase in pH and so we would expect this bicarb to be lower than normal. And our bicarb is still normal and our basic excess is also still normal. So we just talked about what we would expect. So we would expect a decrease in our bicarb or a decrease in our base excess. But remember, like we already talked about, this is an important thing to remember, compensation takes 4 to 5 days. So we can say that this is an acute problem. So we would say there's no metabolic compensation. So we've got a respiratory alkalosis without metabolic compensation. 

All right. So respiratory alkalosis means that we have a decrease in our PCO2. So if we have an increase in our ventilation triggered by hypoxia, then we're going to increase our carbon dioxide excretion. So, again, I know this is like a hard thing to get your head around. I understand it because your your gut reaction is that your CO2 is going to be high, but actually because your CO2 is so diffusable, it is going to diffuse much more rapidly. So most commonly, a primary lung disease causes hypocapnia, therefore, a respiratory alkalosis. And most commonly aspiration pneumonia is very common. This dog has a history of vomiting, and so aspiration pneumonia is something we see a lot. Congestive heart failure would be another differential. And we can say it's acute because there's no evidence of metabolic compensation. 

So let's get on to oxygenation. And so this is another part of assessing our blood gas. And now we're going to switch to arterial blood gas rather than our venous blood gas. So we're on to part three, our arterial blood gas. Okay? Because remember that the PO2 on the venous blood gas is not very useful. The arterial blood gas, on the other hand, is the gold standard to evaluate our pulmonary function. But arterial blood gases can be difficult to obtain. You have to get blood directly from an artery. And so most commonly, usually in practice, I get it from my dorsal metatarsal artery. In smaller animals, you can sometimes get it from the femoral artery, but it's definitely pretty difficult in smaller patients. And I would say, generally speaking, I don't perform these in cats because if you have a cat that struggling to breathe in order to restrain them adequately to get an arterial blood gas there's a high probability that you're going to unfortunately cause that cat to arrest or die. So I would say that very rarely I perform them in cats or even in small dogs. But for example, Ivy is a big dog. She's a lab. And so we stand a better chance of being able to obtain an arterial blood gas in her case. What's really important to remember about both arterial and venous blood gases is you have to collect them anaerobically. Because if you expose them to air, the O2 and the CO2 are going to change in the blood and that's going to make our results invalid. So that's not going to be very helpful. So just remember that don't put them in a tube and then invert the tube. You want to keep them in the syringe. Sometimes I put the syringe into a stopper and then you run them straight away. 

All right. So on an arterial blood gas and this is an arterial blood gas now, on the right, our PAO2 provides information on the oxygen concentration dissolved in our arterial blood. Okay? Normal oxygen in our arterial blood is going to be 90 to 100 millimeters of mercury. And so we define hypoxemia as a PAO2 of less than 80 millimeters of mercury. But, you know, we've already said that you could be hypoxemic for two reasons. You could be not ventilating properly. So like Winston, or you could have primary lung disease, what we call venous admixture. So we've got areas of the blood that are not being adequately oxygenated by the lung. And so that's also going to cause a decrease in our PAO2. So the question is, how can we figure that out? How can we decide if a patient has a hypoventilation problem or a venous admixture problem? And the good news is that we can figure that out. We can do a calculation called an alveolar arterial oxygen gradient, or the shorthand for that is an AA gradient. That can help us figure out these two things. 

Okay. So the important thing to remember when you're trying to calculate an AA gradient is that we can only use it at sea level breathing room air. There are ways to change the formula if you're breathing supplemental oxygen. But it gets really complicated. And so don't run an AA gradient on a patient breathing supplemental oxygen. So here is the formula. So we have our alveolar, which is the capital A. Alveolar oxygen should equal 150 minus our alveol, sorry, our alveolar, excuse me, our arterial carbon dioxide divided by 0.8. Where 0.8 is a respiratory quotient. So estimating the amount of oxygen in our alveoli by this formula. So 150 minus the partial pressure of carbon dioxide in our artery divided by 0.8 and then a gradient is this p a. So to that we just calculated minus the P small A so our arterial O two that we've measured on our arterial blood gas. So in Ivy's case, if we do that calculation, we get 115 and we know that her O2 is 84. And so that gives us an eight a gradient of 31. Normal is less than 50. And so when our alveolar arterial gradient is greater than 15, that indicates that a lung isn't working properly. And so we know that the problem is a venous admixture, meaning that some of our venous blood is not getting oxygenated because we have a primary pulmonary dysfunction. Now there is another easier way to figure this out that's a little bit quicker. And it's called the 120 rule. Okay. And so this is like a really quick way to look at this. And what we would do again. And we have to be at sea level breathing room, air. But the p a CO2 so arterial carbon dioxide plus our arterial oxygen should be around about 120. Okay. If it's less than 120, that indicates pulmonary dysfunction. And using that calculation in Ivy's case, we get a result of 112 millimeters of mercury. And so that, again, indicates pulmonary dysfunction. So what does venous admixture mean? So as I just mentioned, venous admixture means the movement of venous blood from the right to the left heart without adequate oxygenation. So that occurs when blood is moving through areas of blood. So the blood supply to the lung is normal. But the ventilation of the lung is not so. We have things like bronchospasm that can cause a small airway narrowing. So that would be a cat with asthma. And then we also have a collapse of small airway as well, alveolar or alveoli, I'm sorry, due to fluid accumulation in those areas. So a severe aspiration pneumonia or a congestive heart failure. So in Ivy's case, we have a respiratory alkalosis without metabolic compensation, which suggests that this is acute. We have a decrease in up a02. So we're going to say she's got borderline hypoxemia and she has an increase in her age gradient at 31, which indicates that she has pulmonary dysfunction. Her thoracic radiographs are consistent with aspiration pneumonia. So this is a dog who has aspiration pneumonia causing hypoxemia. So she's going to benefit maybe from some oxygen supplementation and also from some antibiotics and appropriate therapy.