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Acid Base and Blood Gas Analysis 

pH is the negative log of the hydrogen ion concentration. More hydrogen ion, more acid. Less acid, more alkaline. As the pH goes down you are more acidotic and as the pH goes up you are more alkalotic. So it is actually going inversely, negative log, but the Europeans have already switched over. I donít think we are going to switch over, I wish we would, and theyíve gone to hydrogen ion concentrations so it is directly - it actually adds and subtracts with all the other ions and frankly I wish we would change over, but acid base, blood gas analysis, acidosis, alkalosis

Now partial pressure is another concept that is sometimes difficult to deal with and that is, it isnít a real measurement of anything. Itís just a relationship to the total. Here the barometric pressure is about 730 mmHg or so. So thatís a partial pressure of 100%, whatever that is. And then partial pressures then are the part that a gas plays in that. So if a gas, an inert gas, is 80% then the partial pressure is 80% of whatever that is. If itís 10% or 20%, itís whatever that is. So if its partial pressure is 730 mmHg and somebody is 10%, itís 73. If itís 20% itís 146.

Now bicarbonate, on the other hand, is something. It is an ion. The problem is, we canít measure it. There has been no accurate way ever devised to measure it. So all of the bicarbonates that you get will be calculated. The good news is that the calculations are such that it is

CO2 content, however, I measured and thatís the CO2 as it comes. It doesnít matter what. Now the vast majority of CO2 is present in bicarbonate, like 95%. So most people use those terms interchangeably. CO2, CO2 content, bicarb. For practical purposes thatís okay. Unless somebody is trying to find out it you have deep scientific knowledge, it is okay to use them interchangeably because clinically we do

The third thing is blood pressure, pulse and cardiac output. Again, most all of those things can be clinically estimated, but you can have all the hemoglobin you want, you can have all the oxygen you want, and if the pump isnít working, you arenít getting it where you want. Itís amazing when people sometimes look at the numbers that we provide, they sometimes leave the patient mentally and go back and look at the

Now the last thing that I think you need to deal with is that specimens differ in where they came from. We can do all these things on arterial, venous, capillary or alveolar gas. I guess rather than remembering this, you always have to remember to look on the report and ask yourself, "Where did this come from?" where does it come from, because the normal values will be different. Again, if the computer is doing itís job

One last concept that we will talk a little bit about is the concept of base excess. Now, bicarbonate is called a base and you can talk to your chemist as to why itís called a base. Anyway itís called a base. Itís not a pejorative name. Itís just base. So base excess or base deficit really means that you have too much or too little bicarbonate and you can say, "Okay, but we are going to measure serum bicarbonate, why isnít serum bicarbonate alone? Why isnít that okay?" and the answer is, mostly it is okay. But since the body is multiple compartments and

Now these charts are all in your handout, but I have developed over the years a way of looking at what is going on that relates to the interface between the alveolus and the capillary. I think itís the best way to think about it because you only have to have one picture in your mind and itís almost impossible to forget how this picture looks. Because, I donít care how you draw the alveolus. You can draw it any way you want, and I don't care how you draw the capillary, you can draw it any way you want. But I think everybody knows that the air has got

Now, letís just reverse the charges. Youíve got a patient with a nasogastric tube in, or chronic vomiting, or you are taking some nonabsorbable antacids, or thereís too little hydrogen ion. Again, for our purposes it doesnít matter what the cause. It does for the patient because we are going to have to eventually figure that out. But for our purposes it doesnít matter. If our problem is loss of hydrogen ion, then everything stops. The PCO2 goes up, the carbonic acid goes up and bicarbonate goes up. Hydrogen ion is down because you just continue

Now weíll quickly go through the other two. Okay, now we have some trouble breathing. The patient has got a piece of steak in here or thereís pneumonia or thereís the hyaline membrane or thereís pulmonary edema, or the ventilator is going too slow. Again, it doesnít matter if there is some reduction in respiration. Now youíve got oxygen in play, because if youíve got reduction in respiration, youíve got the

Take the reverse now, last hyperventilation: teenager, anxiety, the ventilator is going to fast, brain tumor, stroke, doesnít matter. The person is hyperventilating. If you are hyperventilating into room air, thereís no problem that I know of on the oxygen side. So if you are hyperventilating you can forget about the oxygen. It will still stay mostly within limits. But then the hyperventilation, off goes the CO2 and

I put a few of these things on the charts here. I donít think we have any of them in there because I donít think they are at all necessary. People have invented little schemes to help you do your work. I believe that if you understand what you are doing, you can survive any one of these charts. The reason I donít like them is very simple, and that is; the lab is not that precise and if these little schemes are supposed to be different diagnostic categories - which they are - you see if your crosses are right there you get one answer, and if they are right there you

What I am going to do is I want you all to just take a quick - Iíve got some cases here - and we will take some quick goes through the cases and they are in order to kind of demonstrate how to do it. Now what I generally do, and I think most of you have your habits already, but

This patient is acidotic, without any question. So we have an acidotic patient. So itís either respiratory acidosis or metabolic acidosis. Letís just say for though purposes we are going to try to make this patient respiratory acidosis. If thatís true, the patient is hypoventilating, right? The patient is hypoventilating the PCO2 goes up. In this case, the PCO2 is down, not up. So even if you get taken down the wrong road, if