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Acid-Base Disorders

Charles J. Foulks, MD, FACP

Professor of Medicine

School of Community Medicine

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[H+] Metabolism

• Origin of H+: fats, CHO

– 15-20 mol of CO2 per day produced

• Volatile acid– Protein: S, N, P containing amino acids

• Fixed acid

• Must buffer and then excrete H+

• Fixed acid buffered by HCO3, proteins, bone

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[H+] Metabolism

• CO2 buffered by RBCs:

– C02 enters RBCs the with CA

• C02+H2O H++HCO3

-

• HCO3- actively transported into plasma

• H+ retained in RBCs

• Lungs: HCO3- reenters RBCs, equation

reversed and CO2 excreted

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[H+] Metabolism

• Meat diet: 1-2 mEq H+ per gram protein intake– Sulfuric acid: cysteine, methionine– HCl: lysine, arginine, histidine

– HCO3- : aspartate, glutamate, and citrate

from vegetable sources• Actually H+ consumed in metabolism and

HCO3- left behind

– Net effect: 50-100 mMol net H+ added

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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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Language of Acid-Base Disorders

• Acidemia, alkalemia

• Acidosis, alkalosis

• Respiratory (CO2)

• Metabolic (HCO3-)

• Normal pH= 7.40, nl pCO2 = 40, nl HCO3 = 24

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Acid-Base Rules

• The pH is governed by the ratio of pCO2/HCO3

- (IGNORE THE UNITS)• When working an acid-base problem,

ALWAYS put the ratio pCO2/HCO3-

down on paper first• [H+] = 24 (pCO2/HCO-

3) (24 is a fudge factor for the units)

• pH is expressed as [H+] nanoEq/mL (nEq/mL)

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Acid-Base Rules

• The body always wishes to maintain a pH of 7.40.

• Therefore, any primary perturbation in this ratio means that the body will act to restore that ratio.

• It’s like dancing: if your partner goes one way, you should too.

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Acid-Base Rules

Primary Compensation

pCO2 HCO3-

pCO2 HCO3-

HCO3- pCO2

HCO3- pCO2

[H+] = 24 (pCO2/HCO3-)

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Acid-Base Rules

• The compensating partner has five choices:1. Change in the proper direction, but too far.

2. Change in the proper direction, but just right.

3. Change in the proper direction but not far enough.

4. Not change at all.

5. Change in the wrong direction.

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Acid-Base Rules

• Always look at the pH first– If it is < 7.40 then you have an acidotic

process• Only 2 ways:

– pCO2 or HCO3- (Could have both)

• If the pCO2 > 40, then respiratory acidosis

– Compensation would be HCO3 > 24

– Special case: if pCO2 > than it should be but is < 40

• If the HCO3 < 24, metabolic acidosis

– Compensation would be pCO2 < 40

• If both: two primary disorders

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Acid-Base Rules

• If pH > 7.40, then you have an alkalotic process– Only 2 ways:

– pCO2 or HCO3 (Could have both)

– If pCO2 < 40, then respiratory alkalosis

• Compensation would be HCO3 < 24

– If HCO3 > 24, then metabolic alkalosis

• Compensation would be pCO2 > 40

– If both, then two primary disorders

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Acid-Base RulesCompensation

1. Change too far: a second primary disorder.

2. Change just right, compensation.

3. Change too little, a second primary disorder.

4. Not change at all, a second primary disorder.

5. Change in the wrong direction, a second primary disorder.

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Metabolic Acidosis

1. Primary fall in HCO3- because of addition of

H+-R- or HClIf H+-R- added, then leaves tracks in the anion gap.

AG= [Na+] –([Cl- + HCO3-]); normal is 10-15

(14+2)

2. If H+-R- was added, the HCO3- falls by the

amount of R- added.3. The R- group is the unmeasured anion.

What used to be Na-HCO3 is now Na-R and HCO3

-.

4. Compensation means pCO2 will fall.

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Metabolic Acidosis

• Types of H+-R-

– lactic acid– paraldehyde– oxalic acid – ethylene glycol – salicylates– ketones (diabetic, starvation, alcoholic)– formic acid (methanol) – uremic

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Metabolic Acidosis

• Non-anion gap (hyperchloremic) (Addition of HCl) NaHCO3 + HCl HCO3

- + NaCl• Renal in origin: Renal Tubular Acidosis• Proximal: defect in reclamation of HCO3

-, normal acidification

• Distal: defect in acidification, normal HCO3-

reclamation• Both associated with hypokalemia• Type IV: defect in acidification, hyperkalemic

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Metabolic Acidosis

• Winter’s formula ONLY IN METABOLIC ACIDOSIS

• Predicts the compensation of pCO2

pCO2exp = [ [HCO3- ] x 1.5] + 8 2

Example: if the HCO3- is 12

pCO2exp = [12 x 1.5] + 8 2 = 26 2

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Metabolic Acidosis

• Using this example, if the HCO3- is 12, it

is a simple disorder if the pCO2 is 24-28• If the pCO2 < 24, then it is too far:

therefore, a primary respiratory alkalosis

• If the pCO2 > 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. What is the AG?

AG is 30, short differential

Step 2. Is there compensation?

pCO2exp = [12 x 1.5] + 8 2 = 26 2Yes, this is a simple disorder.

Step 3. Causes?

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Metabolic Acidosis:Problems

Using the case in slide 20, change the pCO2 to 18. HCO3

- = 12.Step 1 is the same. Metabolic acidosis

Step 2 reveals that the pCO2 is too low (Should be 26 2), therefore there is a primary respiratory alkalosis.

Step 3. Causes of met acid the same, why is there a primary respiratory alkalosis?

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Metabolic Alkalosis

• Bicarbonate is filtered and 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– Volume depletion– Excess of aldosterone (mineralocorticoid)– Post-hypercapnic– Diuretics

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Metabolic Alkalosis

• Maintenance events: Something must raise the proximal tubule’s ability to increase reabsorption of the increased filtered bicarbonate load. Think of a dam. Normal top of dam is 24 mEq/L

• Maintenance events:– hypokalemia– excess aldosterone – volume depletion (hypoperfusion)– hypercapnia

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Compensation forMetabolic Alkalosis

• No good rule for estimating the pCO2exp.

• In general, the pCO2 will not be above 50-55 torr unless oxygen is given.

• If supplemental oxygen is given then the pCO2 may be > 60. Highest I’ve seen is 98 torr.

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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– hypochloremia

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Metabolic Alkalosis:Problems

40 yo man admitted with Na 140, K 3.0, Cl 86, bicarbonate 40, pH 7.52, pCO2 51.

Step 1. pH > 7.40, therefore an alkalosis present; bicarbonate > 24 therefore, metabolic alkalosis.

Step 2. Did the pCO2 rise? Yes, under 55 and therefore probably compensation. What is the AG?

AG = 14, no metabolic acidosis.

Step 3. 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.

• 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?

• Acute: pCO2 change is 50-40 = 10. pH should be 7.32. (7.40-0.08).

• Chronic: pCO2 change is 50-40 = 10. pH should be 7.36 (7.40-0.04).

• Therefore, if the pH is between 7.32 and 7.36, you have a simple respiratory acidosis.

• What if pH < 7.32? pCO2 = 50– Metabolic acidosis as well.

• What if > 7.36? pCO2 = 50– Metabolic alkalosis as well.

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Respiratory Disorders

• Assume the patient has a pCO2 of 30, what should the pH be?

• Acute: pCO2 change is 40-30 = 10. pH should be 7.48.

• Chronic: pCO2 change is 40-30 = 10. pH should be 7.44.

• Therefore, if the pH is between 7.48 and 7.44, you have a simple respiratory alkalosis.

• If pH > 7.48, then a metabolic alkalosis also.• If pH < 7.44, then a metabolic acidosis also.

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Respiratory Alkalosis

• Voluntary hyperventilation • Hypoxemia (includes altitude) • Liver failure • Anxiety hyperventilation syndrome • Any acute pulmonary problem, e.g., acute

pulmonary embolism, pneumonia, mild asthma attack, mild pulmonary edema, any infectious or catabolic illness.

• ASA

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Respiratory Acidosis

• Central nervous system depression, e.g., drug overdose, anesthesia

• Chest bellows weakness or dysfunction, e.g., myasthenia gravis, polio, massive obesity, diaphragm paralysis, flail chest, paralyzing agents, low energy (glucose, phosphate), low potassium

• Disease of lungs and/or upper airway, e.g., severe asthma attack, chronic obstructive pulmonary disease, severe pneumonia, severe pulmonary edema, upper airway obstruction

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ABG Interpretation

• Strategy for Interpretation

• Write this ratio down: pCO2/HCO3

– Look at pH first, determine if it is acidotic or alkalotic

– Look at the ratio and see which condition for the disorder was met.

– Next, did the partner go in the right direction?• If not then a second primary disorder is

present– If so then: too far, not far enough or just right?

• If just right, then compensation; if not then another primary disorder

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 Case 1

• An asymptomatic patient is found to have the following laboratory values:

Na+ 138 mEq/LK+ 3.9 mEq/LCl- 112 mEq/LHCO3- 14 mEq/L

• Most likely culprit?• RTA

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Case 2

• A 64-year-old disoriented woman is brought to the emergency room. No history is obtainable. She is tachypneic (respiratory rate is 35/min) and confused, but physical examination is otherwise not remarkable.

• Laboratory studies include:Na+ 144 mEq/L Arterial blood pH 7.24

K+ 4.4 mEq/L HCO3- 9

Cl- 107 mEq/L pCO2

22 mm Hg

1. What is her acid-base abnormality?

2. What are the possible causes?

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Case 2

• She has normal serum creatinine concentration and plasma ketone level with is undetectable.

• Possible diagnoses?• Salicylate overdose

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• Na+ up by 3%; Cl- up by 7% • Na+ and Cl- should always follow each

other by the same percentage– Effect of water distribution

• If they go in different directions or don’t move the same percentage then there is another acid-base disorder.

– Much easier than the delta/delta

• Chronic respiratory alkalosis before the intoxication.

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Case 4

• A 50-year-old man is more successful as a lawyer than as a husband. His recent divorce and the financial settlement have pushed his cigarette and Cutty Sark consumption to unreasonable levels, and flared his long-standing peptic ulcer disease. “Doc, I’ve about had it. I’ve been vomiting for four days.” You admit him to the hospital. His BP is 110/70, falling to 70/60 when he sits up. Resting pulse rate is 98/min. Skin turgor is poor, and he has obviously lost weight.

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Case 4

• His Hct is 46%, BUN 28 mg/dL, serum creatinine 1.4 mg/dL. Urinalysis shows S.G. 1.028, no protein, and an normal sediment.

• Other laboratory studies include:Na+ 135 mEq/L Arterial pH 7.58

K+ 2.6 mEq/L pCO2 42 torr

Cl- 83 mEq/L

HCO3- 42 mEq/L

•What is the acid-base disorder?–Primary metabolic alkalosis

–Primary respiratory alkalosis

•Use common sense

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Case 5

• Following a debridement for necrotizing fasciitis, a 58-year-old woman develops severe diarrhea caused by pseudomembranous colitis. The volume of diarrheal fluid is approximately 1/2 liter/hour. Except for a history of moderately severe chronic bronchitis and exertional breathlessness, her past history is unremarkable. Results of the initial laboratory tests are:

Na+ 138 mEq/L Arterial pH 6.97

K+ 2.8 mEq/L [H+] 107 nEq/L

Cl- 115 mEq/L pCO2 40 mm Hg

HCO3- 9 mEq/L

What is the acid-base disorder?

Primary hyperchloremiac metabolic acidosis AND

Primary respiratory acidosis

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Case 6

• A 44 year old moderately dehydrated man was admitted with a two day history of acute severe diarrhea. Electrolyte results: Na+ 134, K+ 2.9, Cl- 108, HCO3- 16, BUN 31, Cr 1.5.ABG: pH 7.31     pCO2 33 mmHg          HCO3 16   pO2   93 mmHg

• Diagnosis?– Metabolic acidosis, non-AG

• Note the divergence of the Na and Cl

– Compensation?• Yes

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Case 7

• A 22 year old female with type I DM, presents to the emergency department with a 1 day history of nausea, vomiting, polyuria, polydypsia and vague abdominal pain. P.E. noted for deep sighing breathing, orthostatic hypotension, and dry mucous membranes.

• Labs: Na 132 , K 6.0, Cl 93, HCO3- 11 glucose 720, BUN 38, Cr 2.6. UA: pH 5, SG 1.010, ketones negative, glucose positive . Plasma ketones trace. ABG: pH 7.27 HCO3- 10  PCO2 23

• What is the acid base disorder?– Primary AG metabolic acidosis, compensated. Where are the ketones?

• BOHB

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Case 8

• A previously well 55 year old woman is admitted with a complaint of severe vomiting for 5 days. Physical examination reveals postural hypotension, tachycardia, and diminished skin turgor. The laboratory finding include the following:

• Electroyes: Na 140 , K 3.4, Cl 77 HCO3- 9, Cr 2.1ABG: pH 7.23 , PCO2 22mmHg

• Disorder?

– Primary AG metabolic acidosis with compensationNa+ unchanged but Cl- down 23%

• Metabolic alkalosis present as well.

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Case 9

• A 70 year old man with history of CHF presents with increased shortness of  breath and leg swelling.ABG: pH 7.24, PCO2 60 mmHg, PO2  52 HCO3- 27

• What is the acid base disorder?– Primary respiratory acidosis

– Acute or chronic?• pH range: 0.04x2=0.08 chronic: pH 7.32

• pH range: 0.08x2=0.16 acute: pH 7.24

• Acute

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Pearls

• Always write down the ratio

• Partners should move in the same direction.

• Go too far, just right, not enough, not change, wrong way

• Na+ and Cl- should move the same direction by the same percentage. If they don’t another disorder present.

• Winter’s formula for met acidosis only

• pH changes based on pCO2 changes

• No good rules for metabolic alkalosis

• Always calculate the AG

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Pearls

• Maximum pCO2 for compensation without oxygen is 50-55 torr.

• Triple disorders are possible:

– Met. Acidosis, met. Alkalosis, and a respiratory disorder. Must look at AG and do Na and Cl move the same.