AcidBase Disorders

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Acid-Base Disorders

• Charles J. Foulks, MD, FACP
• Professor of Medicine
• Scott White

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H Metabolism

• Proximal tubule reabsorbs HCO3-
• Distal tubule secretes H
• Lung disease and disorders of mechanics of
  ventilation cause retention of CO2
• Vomiting causes loss of H

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Summary statement

• Acid-base physiology describes (1) how the lungs
  regulate arterial pCO2 in the face of variable
  CO2 production and (2) how the kidneys regulate
  arterial HCO3 by reabsorbing filtered bicarbonate
  and replacing bicarbonate lost in buffering acids
• The goal is to maintain a pCO2 of 40 mm Hg and a
  bicarbonate of 24 mEq/L (mMol/L). This results in
  a pH of 7.4.

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Endogenous acids

• No one glows in the dark, TXU doesnt consider
  you to be a resource.
• All equations must be charge neutral
• Sources of non-volatile acids
• Organic cation to neutral NH4 urea H
• S-containing amino acids to sulfuric acid
• S SO4-2 2H
• Neutral compound to organic acid
• pyruvate lactate H

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Endogenous acids

• Organic anion to neutral product
• Lactate H pyruvate
• Diet
• CHO, fats neutral (CO2 H2O), small production
  of ketone bodies/lactate
• Proteins major source of H

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Language of acid-base disorders

• Acidemia, alkalemia
• Acidosis, alkalosis
• Respiratory (CO2)
• Metabolic (HCO3-)
• Normal pH 7.40, normal pCO2 40, and normal
  HCO3- 24. When you try to understand a disorder
  you must start from here.

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Acid-base rules

• The pH is governed by the ratio of pCO2/HCO3-
• When working an acid-base problem, ALWAYS put the
  ratio pCO2/HCO3- down on paper first
• H 24 (pCO2/HCO3) Henderson formula
• The partner should travel in the same direction
  as the primary disturbance e.g., If HC03-
  falls, then the pCO2 should fall also.

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Language of acid-base disorders

• Compensation
• If there is a primary disturbance in one of the
  pairs of the bicarbonate/carbonic acid buffer
  system, the other will attempt restoration of the
  pH by moving in the same direction.
• You cannot compensate to normal therefore
• YOU CANNOT OVER-COMPENSATE
• The kidneys compensate for changes in HCO3-
• The lungs compensate for changes in CO2

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Acid-base rules

• Always look at the pH first
• If it is lt 7.40 then you have an acidemia
  therefore there is an acidosis.
• Only 2 ways to cause an acidosis
• pCO2 or HCO3-
• If the pCO2gt40, then respiratory acidosis
• Compensation would be HCO3gt24
• If the HCO3- lt 24, metabolic acidosis
• Compensation would be pCO2lt40
• In a simple disorder, the compensating partner
  must move in the same direction as the culprit.

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Acid-base rules

• If pH gt 7.40, then you have alkalemia, therefore
  there is an alkalosis.
• Only 2 ways to get one
• pCO2 or HCO3
• If pCO2lt40, then respiratory alkalosis
• Compensation would be HCO3lt24
• IF HCO3- gt24, then metabolic alkalosis
• Compensation would be pCO2gt40

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Acid-base rules

• Primary Compensation
• pCO2
  HCO3-
• pCO2
  HCO3-
• HCO3-
  pCO2
• HCO3-
  pCO2

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Acid-base rules

• The compensating partner has five choices
• Change in the proper direction, but too far.
• Change in the proper direction, just right.
• Change in the proper direction but not far
  enough.
• Not change at all.
• Change in the .
• Only one of this is a simple disorder with
  appropriate compensation 2.
• All of the others have a second primary disorder

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Acid-base rules

• Change too far a second primary disorder.
• Change just right, compensation.
• Change too little, a second primary disorder.
• Not change at all, a second primary disorder.
• Change in the wrong direction, a second primary
  disorder.

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Metabolic acidosis

• Primary fall in HCO3- because of addition of
  H-R- or HCl
• If H-R- added, then it leaves a tracks in the
  anion gap (the R-).
• AG Na (Cl- HCO3-) normal is 10-15
• The AG is made up of mostly anionic charges on
  albumin. What happens to the AG if the albumin
  concentration is low?
• If H-R- was added, the HCO3- falls by the
  amount of R- added. Some like to call this
  BASE DEFICIT, DELTA BICARBONATE, etc. THERE IS
  NO REASON TO MAKE THIS DIFFICULT. REASON IT OUT.

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Metabolic acidosis

• Types of H-R-
• lactic
  paraldehyde
• oxalic acid (ethylene glycol) salicylates
  
• ketones (diabetic, starvation, alcoholic)
• formic acid (methanol) uremic

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Metabolic acidosis

• Non-anion gap (hyperchloremic)
• Renal in origin Renal Tubular Acidosis
• Proximal defect in reclamation of HCO3-, normal
  acidification. Urine pH lt 5.5
• Distal defect in acidification, normal HCO3-
  reclamation. Urine pH gt 6.1
• Both associated with hypokalemia
• Type IV defect in acidification and in secretion
  of potassium hyperkalemic
• Elderly, diabetic, hospitalized are the
  populations
• in which Type IV is usually seen.

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Metabolic acidosis

• If there is a metabolic acidosis (pHlt7.4 and
  HCO3- lt 24), what should the pCO2 be?
• Winters formula ONLY IN METABOLIC ACIDOSIS
• Predicts the compensation of pCO2
• pCO2exp HCO3- x 1.5 8 2
• NONE OF THE OTHER RULES WORK.

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Metabolic Acidosis

• Example if the HCO3- is 12
• pCO2exp 12 x 1.5 8 2 26 2
• Using this example, it is a simple disorder if
  the pCO2 is 24-28
• If the pCO2 lt 24, then it is too far therefore,
  a primary respiratory alkalosis
• If the pCO2 gt 28, not far enough, therefore, a
  primary respiratory acidosis

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Metabolic acidosis problems

• 40 yo man admitted with RR of 30, Na 142,
• K 3.6, Cl- 100, HCO3- 12, pH 7.28, pCO2 26
• Step 1. pH is acid, bicarbonate is low metabolic
  acidosis
• AG is 30, short differential
• Step 2. Is there compensation?
• pCO2exp 12 x 1.5 8 2 26 2
• Yes, this is a simple disorder.
• Step 3. Causes?

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Metabolic acidosis problems

• If the pH lt 7.4, the HCO3- is 12, and the
• pCO2 34, what is the disturbance?
• Step 1 is the same. Acidemia, metabolic acidosis.
• Step 2 reveals that the pCO2 should be 24-28. It
  is not there is now a primary resp acidosis
• Step 3. Why is there now resp failure?
• Step 4. What is the pH?
• H (24)(34/12)68 nanoEq/L

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• Conversion of pH to H etc.
• H
  pH
• 24 x 0.8 19 7.6
• 32 x 0.8 32 7.5
• 40 x 0.8 40 7.4
• 40 x 1.25 50 7.3
• 50 x 1.25 63 7.2
  
• etc. 78 7.1
• Very inaccurate below 7.1 or above 7.6

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Metabolic acidosis problems

• What if the HCO3- is 12 and the pCO2 is 18?
• Step 1 is the same.
• pCO2exp 12x1.582262 (24-28)
• Step 2 reveals that the pCO2 is too low,
  therefore there is a primary resp alkalosis
• Step 3. Causes of met acid the same, why is there
  a primary resp alkalosis?

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What is the new pH?

• H 24 x 18/12 36
• This is lt 40 (neutral pH), therefore
• 40 x 0.8 32 pH of 7.3
• 36 is halfway between 32 and 40, therefore the pH
  is halfway between 7.3 and 7.4
• pH 7.35

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Metabolic alkalosis

• Bicarbonate is filtered, reabsorbed proximally,
  and is generated distally.
• Met alkalosis requires 2 events
• Generation of bicarbonate
• Maintenance of hyperbicarbonatemia

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Metabolic alkalosis

• Generation events
• Loss of HCl in vomiting
• Cholera-like diarrhea
• Volume depletion
• Excess of aldosterone
• Post-hypercapnic

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Metabolic alkalosis

• Maintenance events Something must raise the
  proximal tubules ability to increase
  reabsorption of increased filtered bicarbonate.
  Think of a dam.
• Maintenance events hypokalemia, excess
  aldosterone, volume depletion, hypercapnia,
• No good rule for estimating the pCO2exp.
• In general, the pCO2 will not be above 50-55 torr
  unless oxygen is given.

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Metabolic alkalosis

• Treatment Must identify the generation event and
  the maintenance event.
• Must treat both of them. Remember that metabolic
  alkalosis is usually associated with hypokalemia
  and hypochloremia.
• In all metabolic alkaloses that are not
  hypertensive or congenital, the volume status is
  low. The urine Cl- is helpful.

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Metabolic alkalosis problems

• Na 140, K 3.0, Cl 86, HCO3 40, pH 7.52, pCO2 51.
• pH gt 7.40 alkalosis bicarbonate gt24 metabolic
  alkalosis
• 2. Did the pCO2 change in the same direction?
  51lt55 therefore probably compensation.
• AG 14, no metabolic acidosis.
• 3. ALWAYS CALCULATE THE AG
• 4. Causes of generation and maintenance?

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Respiratory disorders

• Acute disorders cause a larger change in the pH
  than chronic disorders. Renal compensation may
  take 3 days to complete.
• Acute for every pCO2 change of 10 the pH will
  change 0.08 pH units. As an example,
• if pCO2 falls, the pH will rise.
• Chronic for a pCO2 change of 10 the pH will
  change 0.04 pH units.

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Respiratory disorders

• Assume the patient has a pCO2 of 50, what should
  the pH be?
• If Acute pCO2 change is 50-4010. pH should be
  7.32 (7.40-0.08 pH units).
• Chronic pCO2 change is 50-4010. pH should be
  7.36 (7.40-0.04 pH units)
• Therefore, if the pH is between 7.32 and 7.36,
  you have a simple respiratory acidosis.

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Respiratory disorders

• Assume the patient has a pCO2 of 30, what should
  the pH be?
• Acute pCO2 change is 40-3010. pH should be 7.48
  (7.400.08 pH units)
• Chronic pCO2 change is 40-3010. pH should be
  7.44 (7.400.04 pH units)
• Therefore, if the pH is between 7.48 and 7.44,
  you have a simple respiratory alkalosis.

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Summary

• Always write down pCO2/HCO3-
• Remember the partner should change in the same
  direction
• Winters formula for metabolic acidosis only.
• pH change as function of pCO2 change. This lets
  you bracket what compensation should be if it
  is a simple disorder. If the pH is outside this
  range, you have another primary disorder.
• Conversion of pH to H
• ALWAYS, ALWAYS, CALCULATE THE ANION GAP