Diseases information. Disorders. Treatment.

Renal tubular acidosis. How do we recognize renal tubular acidosis? Number one, what I said before; first have a blood pH. You’ve got to have acidosis. I can’t tell you how often I have been consulted by some of you out there who will remain anonymous, in the audience, who say, “Low CO2, low bicarbonate, we obviously have an acidosis. Please evaluate the acidosis for us.” And it turns out, of course, that it’s a compensation because the blood pH, when we got it, was 7.6. So number one, be sure you know the acid-base status. Start stepwise. Start with your first step which is; know the primary disorder. Then these patients are hyperchloremic. They have a normal anion gap. Not a high anion gap. They have a relatively low serum K for the degree of acidosis, and this is true of type I and type II RTA’s. And they have an inappropriately high urinary pH. If you have all of these, you more than likely have an RTA. So these are the characteristics. This right here is something to look at. If you don’t look at anything else in RTA, look at this. Because this is the link, this is the association that will allow you to get it through the test.

Now there are different types. We nephrologists have an incredible amount of imagination and wit, so we of course call them type I and type II, with our great creativity. Type I is called distal RTA. It is caused by a problem in the distal tubule with the inability to decrease the urine pH to 5.5 during acidosis. Because the kidney can’t push out hydrogen ions out of the body. There is one of a number of molecular defects that cause the kidney to be unable to kick out hydrogen ions. You eat 3 milliequivalents per kilo of hydrogen ion per day in your phosphates and your sulfates and the things that you eat. You can’t get rid of them in RTA. Thus, you become a big walking hydrogen ion. The associated problems with type I - this is an important association - nephrocalcinosis. Another; we’ve said before, hypokalemia and low urinary citrate. Remember the two types of acidosis that give you low potassium, DKA - because you are peeing out all that K - and RTA. RTA, relative hypokalemia. It turns out that you get a secondary hyperaldosteronism and that’s why you get hypokalemia. What’s important to remember is low acidosis, low K; low or normal K - even a normal-ish K - you’ve got to really think about it because the K should be high when you are very acidotic. Think DKA or RTA.

How do you evaluate these patients? They have a hyperchloremic metabolic acidosis and they have a positive urinary anion gap. That’s because these patients don’t make very much ammonia. The urine - a lot of the positives that are over here to balance out the chlorides, etc. - is ammonia and usually the sodium plus potassium minus chloride normally is a negative number. But in distal RTA the sodium plus the potassium minus the chloride is a positive number in distal RTA. We’ll get to proximal RTA in a minute. And in normals, it’s a negative number. The urine pH, for the sake of this discussion here, is always greater than 5.5. There are situations where it may be below that but not in a million years will they ask you. So for the situation that we are talking about here, with regard to 98% of RTA’s, urine pH always greater than 5.5. You can bicarbonate load these patients and do a study called a urine PCO2 minus blood PCO2 and the values are given here. This is a question for pediatric nephrology Boards. Not for general peds Boards. You won’t get asked it.

The types; it is associated with many many things, and I’ve listed a number of them here. Genetically transmitted disease, autoimmune diseases and disorders associated with nephrocalcinosis. If you hear nephrocalcinosis and acidosis, think type I RTA, and there are a couple of situations here. Drugs that have tubular toxicity; we talked about hypokalemia with cisplatin and amphotericin. Amphotericin will also give you a situation of RTA. Amphotericin also gives you renal failure in very sick patients, but in patients that are not so sick and we are using lower doses of amphotericin, there is a tubular toxicity, and that’s amphotericin-B I’m talking about. Others include pyelonephritis obstruction and kidney transplant.

Metabolic alkalosis; you are alkalotic, you have a high CO2 with retained bicarbonate. The compensation is low ventilation. You decrease your ventilation and you drive up your PCO2. The respiratory alkalosis, the PCO2 is low as you are blowing PCO2 off. The pH is high, the bicarbonate excretion is your compensation, so your CO2 will be low. When you are studying, make sure you have a clear view of this before you go to this. So make sure you understand clearly what the primary problem is, and then look at the compensation and that will help you with your study. Do it in a stepwise fashion. Don’t even bother with the compensations until you are clear what the primary problem is. See the primary problem then look to the compensation.

Okay, now, the next thing you do if you have an acidosis, and in particular is to determine the anion gap. The anion gap is the sodium minus the bicarbonate and chloride. Normally it’s 9-12. An increase in the anion gap over normal represents unmeasured anions and those are usually things like lactate, beta-hydroxybutyrate, that’s the so-called gap acidosis. Where you are dumping acids into the blood. A gap acidosis, those include lactic acidosis, uremia, diabetic ketoacidosis or organic acidemias. Other exogenous acids like salicylate or ethanol. Non-gap acidoses; most are either loss of diarrhea or other base from the body, usually from the GI tract but it can be from other places, or renal tubular acidosis. So gap acidoses are when you are dumping acids into the blood, a pathologic process is dumping acids into the blood. A non-gap acidosis is when you are losing something from somewhere that is usually basic. And that something from somewhere is usually diarrhea and bicarbonate, bicarbonate in the diarrhea.

Acidosis; first we evaluate the primary disorder, and the definitions of the primary disorder. If you think about acid-base balance in a logical stepwise fashion, you may be able to remember it and figure it out. If you don’t do it in a logical stepwise fashion you are almost for sure not going to be able to do it, and even if you do think about it in a logical stepwise fashion, a lot of people have problems with it - like me. So it’s got to be logical and stepwise. So number one, we think of the primary disorder. What is the primary disorder? If there is metabolic acidosis, the pH is low compared to 7.4. If the primary cause is loss of bicarbonate, the lab finding; a low CO2. I make a big differential here between CO2 and of course PCO2. Because PCO2 is the respiratory component, the carbonic acid. If there is a respiratory acidosis - you have again, a low pH - you have increased carbonic acid and that’s an increased PCO2. So contrast these two. If it’s alkalosis the pH is high and metabolic alkalosis is caused by increased bicarbonate or low chlorides, and an increased CO2 is found in your electrolytes. Respiratory, again an increased pH, a low bicarbonate, a low PCO2. This is important to review. This seems simple. You guys have gone over it again and again, but this is important to review and be sure you’ve got cold.

The compensations are likewise important. Remember, you can never overcompensate so you determine the primary changes first. Metabolic acidosis, low CO2. What is the compensation? The compensation is in the other system; increased ventilation, and the result is decreased PCO2. But the primary is an acid pH in the blood and a low CO2. Respiratory acidosis, the primary change, high PCO2. Retained PCO2. The compensation is renal bicarbonate absorption and a high CO2. If you are acidotic and you see these two things, you are going to know that first, this is what would give you the high … the respiratory acidosis. Below the 7.2 type of blood pH. Remember please, that you cannot make a distinction of whether a patient is acidotic or alkalotic unless you have either a venous blood gas or an arterial blood gas that shows a pH. These numbers in and of themselves cannot show you whether the patient is acidotic or alkalotic. Very important to remember…