Acid-Base Disorders 4

Type II RTA is different than type I RTA. It’s not that you can’t excrete hydrogen ions. It’s that the kidneys leak bicarbonate. There is a low threshold in the blood for spilling bicarbonate in the urine. Normally you and I start to spill about 22 … or kids, actually, start to spill 22-24. In type II RTA they start to spill at 15-16, so they begin to lose their bicarbonate with relatively low serum bicarbonates. The clinical findings, again, hyperchloremia. I can’t stress enough, hyperchloremia – metabolic acidosis. With what in the potassium? Low potassiums, low or normal. That’s the way to recognize this. High chlorides, low or low normal potassium with an acidosis. The urine pH is greater than 5.5 when the bicarbonates, or the serum bicarbonates or the CO2 is normal. But it can decrease to less than 5.5 when you get very acidotic. Because then you get underneath the threshold and the body is able to reabsorb all the bicarbonate, and in this situation the distal tubule is working fine. It’s only the proximal tubule that is messed up and spilling all the bicarbonate. The K can be low or normal. Never high. This is seen usually with Fanconi’s syndrome, but it may be isolated.

So what is the Fanconi’s syndrome? The Fanconi’s syndrome is a disease of the proximal tubule, that has all of the listed criteria, or many of the listed criteria; proximal RTA, they have amino aciduria, glucosuria, phosphaturia and hypophosphatemia and hypokalemia. They have uricosuria. They spill uric acid and decreased plasma uric acid. They have rickets, growth retardation, polyuria, dehydration, and sometimes low molecular weight globulin proteinuria.

What are examples? This is the one they will ask you. Number one, cystinosis. Most frequent cause. Galactosemia, Wilson’s disease are others. Glycogen storage disease, Wilson’s disease, galactosemia, but cystinosis, nephropathic cystinosis, is the archetypal one. Here are some other selected causes. The full group is in your syllabus; heavy metal poisoning, cisplatin, biphosphamide, and then some other nephrologic ones, and glue-sniffing. Now if you have an isolate, those with Fanconi’s syndrome, I’ve listed some causes of isolated proximal RTA that are shown here. It is highly unlikely they will ask you any of these. And they are listed here: sporadic, genetic, carbonic anhydrase inhibition.

How do you evaluate type II RTA? Again, hyperchloremia, metabolic acidosis. But the anion gap is negative. Remember I told you that for distal RTA the anion gap was positive. This one is not a problem with making ammonia. This is a problem just with leaking lots of bicarb. So the anion gap is negative. The urine pH is greater than 5.5 when the plasma bicarb is what is normal to you and me, but when the plasma bicarb falls below the threshold, the urine pH is low.

I am going to … this is another way to evaluate it. Again, this is something that pediatric nephrologists do. It’s a fractional excretion but instead of a fractional excretion of sodium, it’s a fractional excretion of bicarbonate. And we can do this, and this allows us to do that. Again, they will not be asking you that.

A quick word about type IV RTA. There is no type III. There was a mistake. Somebody made a mistake and so they called… we called just type IV. Type IV is so-called low-renin hypo-renin hypoaldosteronism. This again is a non-anion gap acidosis but it has hyperkalemia. Urine pH can look like type II RTA with a pH less than 5.5. Highly unlikely that they will ask you about this. Very unusual. It’s caused by true aldosterone deficiency, conditions which decrease renin secretion and conditions where the kidney cannot respond to mineralocorticoids. This is a reiteration; the diagnostic workup of suspected RTA. We look first for he serum anion gap. We see whether it is elevated. The metabolic acidosis is elevated. If it is you work up for a gap acidosis. If there is a normal gap, you want to evaluate for RTA. Then I put in how you go with RTA’s, either type II or type I and type IV. I’m going to skip over this, and I’m going to skip over metabolic alkalosis except to remind you that the laboratory studies for metabolic alkalosis will show you that besides a blood pH that is high, there is a low chloride, a low potassium, increased bicarbonate or CO2 in the blood. The few physical signs; tetany. Underline tetany and convulsions. This is one of the things that can give you tetany and convulsions, metabolic alkalosis.

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