Easy rules for assessing compensation

Learn some super simple rules that will help you decide whether compensation is adequate or not. You'll learn exactly how the lungs compensate a metabolic problem and how the kidneys compensate a respiratory problem.

Learn how to resolve acid-base problems with a simple, four-step approach in our Acid-Base Essentials course. Dr. Franz Wiesbauer and Dr Rainer Oberbauer join forces to cover important topics like anion gap, mixed acid-base problems, and compensation.

[00:00:00] Welcome back. So, in some of the previous videos, we've learned how to tell if multiple metabolic problems were present at the same time. Remember, that was what the delta anion gap and the corrected bicarb were all about. In this lesson, you will learn how to tell if an additional respiratory problem is present on top of metabolic problems or if an additional metabolic problem is present on top of a respiratory problem. In this video, we're going to learn about

[00:00:30] compensation. Compensation refers to the fact that the lungs will try to compensate for a metabolic problem by hyper and hypoventilating and the kidneys will try to compensate for a respiratory problem by excreting or retaining bicarb. So, this is the kidney representing the metabolic system, and these are the lungs representing the respiratory system. When there's metabolic acidosis, the lungs will hyperventilate and when there's metabolic alkalosis,

[00:01:00] the lungs will hypoventilate. And when there's respiratory acidosis, the kidneys will cause bicarb retention. And when there's respiratory alkalosis, the kidneys will cause bicarb excretion. That's what the whole concept of compensation is all about. So, let's have a look at what the kidneys do in the case of a respiratory acidosis first. So, we said that in respiratory acidosis, the pCO2 goes up,

[00:01:30] which causes the pH to go down in response to that. And the kidneys will try to compensate by increasing bicarb as well. So, when there's no additional metabolic problem then the kidneys will increase bicarb by a predicted amount. Since renal compensation takes hours to days, bicarb will increase just a little bit in acute respiratory acidosis and more in chronic respiratory acidosis because the kidneys have more time. Let's see what happens

[00:02:00] in the acute phase. So, in this setting, for each 10-unit increase in pCO2, the bicarb will increase by 1 unit. What about the chronic phase? There, we said that the kidneys have more time to adapt, so here, for every 10-unit increase in the pCO2, bicarb will increase by 3 units. And what about respiratory alkalosis? Here, the pCO2 goes down in response to hyperventilation. As a consequence,

[00:02:30] the pH goes up and since the kidneys try to compensate for that, they will cause bicarbonate excretion so the bicarb will go down. What happens in acute respiratory alkalosis? Here, for every 10-unit decrease in pCO2, bicarb will decrease by 2 units. And what happens in the chronic setting? Here, for every 10-unit decrease in pCO2, the bicarb will decrease by 4 units. What

[00:03:00] if the bicarb doesn't change by these amounts? Well, that probably means that there's an additional metabolic problem. If the bicarb is lower than predicted by these rules, there's also metabolic acidosis, and if it's higher than predicted, then there's probably also metabolic alkalosis. And what about metabolic acidosis? We know that here, as the bicarb goes down, the pH goes down and as a compensatory mechanism, the pCO2

[00:03:30] also goes down, due to hyperventilation. Now, respiratory compensation takes place immediately. So, here, we don't discriminate between acute and chronic settings. So, let's see by how much the lungs adapt in metabolic acidosis. For each unit decrease in bicarb, the pCO2 goes down by 1 unit. And what about metabolic alkalosis? Here, we also don't discriminate between acute and chronic problems since the lungs adapt

[00:04:00] to this immediately as well. So, in metabolic alkalosis, the bicarb goes up and the pH will go up too. As a consequence, the patient will start to hypoventilate, in order to correct the pH. As you can imagine, there's a certain limit as to how much the patient can hypoventilate. So, this type of compensation is somewhat limited. Let's see by how much the pCO2 changes in this setting. So, for every 2-unit increase in bicarb. The pCO2

[00:04:30] will increase by 1 unit. What's the problem if the pCO2 doesn't change by the predicted amount? What if the pCO2 is lower than predicted? Well, then that means that respiratory alkalosis is also present and if it's higher than predicted, then respiratory acidosis is also present. So, here's a little memory aid that should help you to remember what we've just learned. So, on the left side, we write down acidosis. On the right side, we write down

[00:05:00] alkalosis. Then we have acute respiratory problems, chronic respiratory problems, and metabolic problems. And here are the compensatory changes that take place in each setting. We said in acute respiratory acidosis, for every 10-unit increase in pCO2, bicarb goes up by 1 unit. In acute respiratory alkalosis, we said for every 10-unit decrease in pCO2, bicarb goes down by 2 units.

[00:05:30] In chronic respiratory acidosis, we said for every 10-units increase in pCO2, bicarb goes up by 3 units. And in chronic respiratory alkalosis, we said for every 10- unit decrease in pCO2, bicarb goes down by 4 units. What about metabolic acidosis? Here, for every 1-unit decrease in bicarb, pCO2 goes also down by 1 unit. And what about metabolic alkalosis? Here, for every 2-unit increase

[00:06:00] in bicarb, pCO2 will also go up by 1 unit. So, do you recognize the pattern here? Well, to tell you the truth, you really only have to remember the numbers in the left column and add 1 to the numerator in order to arrive at the numbers of the right column. So, it's 1 / 10 for acute respiratory acidosis and 2 / 10 for acute respiratory alkalosis. It's 3 / 10 for chronic respiratory acidosis and

[00:06:30] 4 / 10 for chronic respiratory alkalosis. It's 1 / 1 for metabolic acidosis and 2 / 1 for metabolic alkalosis. We can now complete our algorithm for diagnosing any acid-based problem. Let's write down the updated algorithm. So, we said, number one, we calculate the anion gap. Number two, if there is an elevated anion gap, we calculate the delta anion gap and the corrected bicarb. Number three,

[00:07:00] we check the pH and the pCO2 in order to identify the primary problem—is it metabolic or respiratory? And now, there's an add-on. Number four, check if compensation is adequate, and if it's not, an additional acid-based problem is present. You now have all the tools it takes in order to assess over 90% of acid-based problems. Now, go through the practice cases and make sure that what you've learned sticks.