nephrology core curriculum simple acid-base disorders

Nephrology Core CurriculumSimple Acid-Base Disorders

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


• 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


• In respiratory acidosis why does bicarbonate increase?

Acid-BaseBicarbonate/Carbon Dioxide Buffer System

Carbonic Acid (H2CO3) Buffer system

CO2 + H2O H2CO3H+ + HCO3-

1. 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

2. 6800 molecules of HCO3- per molecule of H2CO3

So once formed, H2CO3 immediately converts to bicarbonate and a hydrogen ion

2.1.


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

1. 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)?

2 1 3


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-



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



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


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


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)


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)


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


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 CHRONICALLY HCO3 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

Acid-BaseCompensation for Primary Acid-Base Disturbances

Disorder Primary Change CompensationMetabolic Acidosis HCO3

- Winters, 1.2mmHg decrease in pCO2 per 1meq/L fall in HCO3

-, or pCO2 = last 2 digits of pH

Metabolic Alkalosis HCO3- 70%, or 0.7mm Hg

increase in pCO2 per 1meq/L rise in HCO3

-


-Acute pCO2 1 meq/L per 10 mm Hg

-Chronic pCO2 3.5 meq/L per 10 mm Hg

Respiratory Alkalosis


-Chronic pCO2 5 meq/L per 10 mm Hg

Metabolic Acidosis- SimpleWinters Calc CO2

0

5

10

15

20

25

30

35

40

42.539.5

36.533.5

30.527.5

24.521.5

18.515.5

12.59.5pCO2

pH

Winters +2

Winters -2

Winters

Metabolic Acidosis- SimpleWinters Calc CO2 vs. 120% rule

0

5

10

15

20

25

30

35

40

42.539.5

36.533.5

30.527.5

24.521.5

18.515.5

12.59.5pCO2

pH

Winters +2

Winters -2

Winters

1.2x

PCO2 from 12-32, same as Winters

Metabolic Acidosis- SimpleWinters Calc CO2 vs. CO2=pH

0

5

10

15

20

25

30

35

40

42.539.5

36.533.5

30.527.5

24.521.5

18.515.5

12.59.5pCO2

pH

Winters +2

Winters -2

Winters

pCO2

PCO2 from 14-40, same as Winters

Acid-BaseCompensation for Primary Acid-Base Disturbances

Disorder Primary Change CompensationMetabolic Acidosis HCO3

- Winters, 1.2mmHg decrease in pCO2 per 1meq/L fall in HCO3

-, or pCO2 = last 2 digits of pH

Metabolic Alkalosis HCO3- 70%, or 0.7mm Hg

increase in pCO2 per 1meq/L rise in HCO3

-



-Chronic pCO2 3.5 meq/L per 10 mm Hg

Respiratory Alkalosis


-Chronic pCO2 5 meq/L per 10 mm Hg

Metabolic AlkalosisWorks even better

Calculated PCO2 vs. = last two of pH(serum bicarb 25-51)

40

42

44

46

48

50

52

54

56

58

60

pCO2

La

st

two

dig

its

of

pH

Calculated PCO2

PCO2= pH

pco2 + .2 = pH

pco2 -.2 =pH

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

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)

Acid-Base• Even a value that appears to be “ideal” compensation

can actually be 3 disorders. Must use anion gap, potential bicarbonate, and history

20

25

30

35

40

45

40 50 60 70 80 90

Acute

Chronic

1. COPD with chronic respiratory acidosis

20

25

30

35

40

45

40 50 60 70 80 90

Acute

Chronic

2. Develops vomiting (contraction alkalosis)

20

25

30

35

40

45

40 50 60 70 80 90

Acute

Chronic

3. Develops vomiting (contraction alkalosis)

nephrology core curriculum simple acid-base disorders

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