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Nephrology Core Curriculum Simple Acid-Base Disorders

Nephrology Core Curriculum Simple Acid-Base Disorders. Acid-Base Introduction. H + concentration is maintained within very narrow limits Normal extracellular level is 40 nanomol/L 40 x 10 -6 Approximately one millionth the concentration of K + , Na + , Cl +

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Nephrology Core Curriculum Simple Acid-Base Disorders

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  1. Nephrology Core CurriculumSimple Acid-Base Disorders

  2. Acid-BaseIntroduction • H+ concentration is maintained within very narrow limits • Normal extracellular level is 40 nanomol/L • 40 x 10-6 • Approximately one millionth the concentration of K+, Na+, Cl+ • .0000016-.0000160 free H+ (16 to 160 nanomol or pH 6.8-7.8) is the only range compatible with life • Such low levels are necessary because, given its small size of the H+ ion, it is highly reactive

  3. Acid-BaseIntroduction • Acid- Substance that can donate H+ ions • Base- Substance that can accept H+ ions • Two primary types of acid • Carbonic- generated from the metabolism of carbohydrates and fats • Results in the generation of approximately 15,000 mmol of CO2 per day • Non-carbonic- generated by the metabolism of proteins • Results in the generation of approximately 50-100meq/day of acid on a normal diet

  4. Acid-BaseIntroduction • In respiratory acidosis why does bicarbonate increase?

  5. Acid-BaseBicarbonate/Carbon Dioxide Buffer System • Carbonic Acid (H2CO3) Buffer system • CO2 + H2O H2CO3 H+ + HCO3- • 340 molecules of CO2 per molecule of H2CO3 • Can see, equilibrium tends to keep CO2 as CO2, that’s why carbonic anhydrase present in RBCs and kidney • 6800 molecules of HCO3- per molecule of H2CO3 • So once formed, H2CO3 immediately converts to bicarbonate and a hydrogen ion 1. 2.

  6. Acid-BaseBicarbonate/Carbon Dioxide Buffer System • Carbonic Acid (H2CO3) Buffer system • Because of these equilibrium constants, the net effect is: • CO2 + H2O H+ + HCO3- • Works well when you can control the CO2 and shift the equation to the right. • H+ + HCO3- CO2 + H2O • Add an acid 2. Combines with serum bicarbonate 3. Exhale carbon dioxide, driving equation to the right and removing all the acid 4. Kidney must later regenerate the used bicarb • But what about respiratory acidosis (i.e. the inability to control the CO2)? 1 2 3

  7. Respiratory Acidosis CO2 CO2 + H2O H2CO3 (inc CO2 drives to right) H2CO3 H+ + HCO3- Bicarb can’t buffer H2CO3, because bicarb + H2CO3 equals bicarb + H2CO3 (H+ + HCO3-)+ HCO3- (H+ + HCO3-) + HCO3- What happens instead is: H2CO3 + Hemoblogin(Hb-)HHb + HCO3- Acid-BaseBicarbonate/Carbon Dioxide Buffer System

  8. Acid-BaseBicarbonate/Carbon Dioxide Buffer System Respiratory Acidosis CO2 What happens instead is: H2CO3 + Hemoblogin(Hb-)HHb + HCO3- Net effect is for every molecule of CO2 retained, after the resulting H+ is buffered by the plasma proteins, one molecule of HCO3 is left over This is why the bicarbonate (a base) actually increases during respiratory acidosis -It is effectively an “anion gap” for respiratory acidosis Note: no renal involvement whatsoever at this stage. Acute compensation in respiratory acidosis would occur even in an anephric patient

  9. Acid-BaseBicarbonate/Carbon Dioxide Buffer System Respiratory Acidosis CO2 As a rule of thumb, the bicarbonate should increase 1 meq/L for every 10mm Hg increase in CO2 above normal Note: not a one for one due to the differing units- meq/L vs. mmHg

  10. Acid-BaseBicarbonate/Carbon Dioxide Buffer System Respiratory Acidosis ( CO2), Acute Effect of Serum Buffering Ex. Increase PCO2 from 40 to 80 mmHg -Without buffering, H+ increases to 80 nanomol/L H+= 24 X PCO2/Bicarb = 24 X 80 / 24 = 80 Equals a pH of 7.10 -With buffering, 40 increase in CO2 causes a 4 increase in bicarbonate H+= 24 X PCO2/Bicarb = 24 X 80 /28 = 69 Equals a pH of 7.17 So it helps, but overall, not great

  11. Acid-BaseBicarbonate/Carbon Dioxide Buffer System Respiratory Acidosis ( CO2), Chronic At 4-5 days, the kidneys kick in and increase the bicarbonate 3.5meq/L per every 10 increase in PCO2 Ex. Increase PCO2 from 40 to 80 mmHg -With buffering, 40 increase in CO2 causes a 14 increase in bicarbonate H = 24 X PCO2/Bicarb = 24 X 80 /38 = 50 Equals a pH of 7. 3 (vs. 7.17 with acute serum buffering or 7.10 without buffering)

  12. Acid-BaseBicarbonate/Carbon Dioxide Buffer System Respiratory Acidosis ( CO2), Chronic Renal Measures– Why does the kidney increase bicarbonate which is an ineffective buffer? Why not just dump H+ -Kidney can’t excrete much free H+ -pH of 4.5 (minimal urine pH)= urine H+ concentration 1000x serum, still only represents a free H+ of 0.04meq/L -Given a daily acid production of 100meq, it would require: 100meq/0.04meq/L or 2500 liters of urine per day to excrete as free hydrogen -Net effect, kidney sees decreased pH, combines CO2 + H20 to make carbonic acid. It dumps the H+ into the urine (converting NH3+ to NH4+) and the bicarbonate is returned to the serum H+ is dumped, and bicarbonate increases (hence the 3.5meq increase per each 10mmHg increase in CO2)

  13. Acid-BaseBicarbonate/Carbon Dioxide Buffer System The reverse of these actions occur in the face of respiratory alkalosis -CO2 (acute) CO2 + H2O H2CO3 (dec CO2 drives to left) H2CO3 H+ + HCO3- (H2CO3 dec drives left) Bicarbonate drops because it is being used to generate H2CO3 (which is ultimately converted to try and raise CO2). The pH drops because H+ is also consumed. Serum buffers give up H+ to try and raise the pH. Net result is that HCO3 drops 2meq/L per every 10 decrease in pCO2

  14. Acid-BaseBicarbonate/Carbon Dioxide Buffer System The reverse of these actions occur in the face of respiratory alkalosis -CO2 (chronic) CO2 + H2O H2CO3 (dec CO2 drives to left) H2CO3 H+ + HCO3- (H2CO3 dec drives left) Kidney essentially “pisses” away bicarbonate (which is the equivalent of adding acid to the body). Thereby correcting the alkalosis Net result is that CHRONICALLYHCO3 drops 5meq/L per every 10 decrease in pCO2 -easy to remember, the change is bigger for alkalosis because it is easier to urinates the bicarb away rather than making new as in the case of acidosis induced changed

  15. Acid-BaseCompensation for Primary Acid-Base Disturbances

  16. PCO2 from 12-32, same as Winters

  17. PCO2 from 14-40, same as Winters

  18. Acid-BaseCompensation for Primary Acid-Base Disturbances

  19. Metabolic AlkalosisWorks even better

  20. Acid-Base • Cannot be performed in a vacuum, interpretation must take into account the history • Ex. pH 7.27, pCO2 70, Bicarb 31, PO2 35 • What does this represent? • Next slide

  21. Acid-Base • Cannot be performed in a vacuum, interpretation must take into account the history • Ex. pH 7.27, pCO2 70, Bicarb 31, PO2 35 • What does this represent? • pH decreased– Acidosis • PCO2 increased- Respiratory acidosis • If acute, bicarb should increase 1/10, or = 27 • If chronic, bicarb should increase 3.5/10, or = 35 • Intermediate value means this could be an acute respiratory acidosis transitioning to chronic, a chronic acidosis with superimposed metabolic acidosis, or a acute respiratory acidosis superimposed on a metabolic alkalosis • None of these can be distinguished without the respective histories (or this assistance of a anion gap/potential bicarb determination)

  22. Acid-Base • Even a value that appears to be “ideal” compensation can actually be 3 disorders. Must use anion gap, potential bicarbonate, and history 2. Develops vomiting (contraction alkalosis) 3. Develops vomiting (contraction alkalosis) 1. COPD with chronic respiratory acidosis

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