INTERN BASICS

Acid-Base

August 9, 2005

Jeremy Marcus MD

ACID BASE – Why it’s “hard”

- It’s Math

- Everyone does it differently

- Everyone thinks they know the “best” way to do it and/or teach it

- Emphasis on numbers instead of clinical correlation

A word about “internal consistency”

pH = 6.1 + log ([HCO3]/0.03 x pCO2)

[H+] = 24 x pCO2/[HCO3]

What is the equation getting at?

e.g. Pt with COPD, acute-on-chronic tachypnea & dyspnea; team got ABG:

7.48/87/56, arterial HCO3 = 63

Stepwise approach

1. Acidosis or alkalosis2. Primary disorder respiratory or metabolic3. If respiratory, is it acute or chronic? 4. Appropriate compensation?5. Calculate anion gap6. Assess for complex (triple) disorders

Acidosis or alkalosis

pH < 7.38 Acidosis

pH > 7.42 Alkalosis

Stepwise approach

1. Acidosis or alkalosis2. Primary disorder respiratory or metabolic3. If respiratory, is it acute or chronic? 4. Appropriate compensation?5. Calculate anion gap6. Assess for complex (triple) disorders

Primary disorder respiratory or metabolic?In respiratory acidosis, pCO2 and HCO3 both go up

In metabolic acidosis, pCO2 and HCO3 both go down

In respiratory alkalosis, pCO2 and HCO3 both go down

In metabolic alkalosis, pCO2 and HCO3 both go up

Examples:

7.32/28/83 HCO3 14

7.31/70/75 HCO3 34

7.10/50/55 HCO3 15

Stepwise approach

1. Acidosis or alkalosis2. Primary disorder respiratory or metabolic3. If respiratory, is it acute or chronic?4. Appropriate compensation?5. Calculate anion gap6. Assess for complex (triple) disorders

If respiratory, is it acute or chronic? Two numbers to remember: 0.08 and 0.03

(works for respiratory acidosis or alkalosis)

Acute resp acidosis: for every 10 incr pCO2, pH decr 0.08

Chronic resp acidosis: for every 10 incr pCO2, pH decr 0.03

Acute resp alkalosis: for every 10 decr pCO2, pH incr 0.08

Chronic resp alkalosis: for every 10 decr pCO2, pH incr 0.03

Example:

7.31/70/78 HCO3 34

Stepwise approach

1. Acidosis or alkalosis2. Primary disorder respiratory or metabolic3. If respiratory, is it acute or chronic? 4. Appropriate compensation?5. Calculate anion gap6. Assess for complex (triple) disorders

Appropriate compensation

Metabolic acidosis: Winter’s formula

pCO2 = 1.5 [HCO3] + 8 2Alternative: For every HCO3 decr 1, pCO2 decr 1

Respiratory acidosis/alkalosis: Acute resp acidosis: for every 10 incr pCO2, HCO3 incr 1Chronic resp acidosis: for every 10 decr pCO2, HCO3 incr 4Acute resp alkalosis: for every 10 incr pCO2, HCO3 decr 2Chronic resp alkalosis: for every 10 decr pCO2, HCO3 incr 5

Metabolic alkalosis:pCO2 should never be >55If pCO2 is elevated, pH should be alkalemicFor every HCO3 incr 10, pCO2 should incr 7

Stepwise approach

1. Acidosis or alkalosis2. Primary disorder respiratory or metabolic3. If respiratory, is it acute or chronic? 4. Appropriate compensation?5. Calculate anion gap6. Assess for complex (triple) disorders

Calculate the anion gap … every time!

Unmeasured Anions Unmeasured Cations

Proteins (albumin) 15 mEq/L   Calcium 5 mEq/L Organic acids 5 mEq/L Potassium 4.5 mEq/L Phosphates 2 mEq/L Magnesium 1.5 mEq/L Sulfates 1 mEq/L  

Totals: 23 mEq/L 11 mEq/L

Difference = 12 mEq/L = normal anion gap

- Correct for albumin (2.5 for every drop of 1 below 3.0)

- If anion gap > 20 with metabolic alkalosis, there’s an additional acidosis

Stepwise approach

1. Acidosis or alkalosis2. Primary disorder respiratory or metabolic3. If respiratory, is it acute or chronic? 4. Appropriate compensation?5. Calculate anion gap6. Assess for complex (triple) disorders

Assess for complex (triple) disorders

Corrected bicarbonate = (AG-12) + HCO3

If < 24, suggests concurrent acidosis

If > 24, suggests concurrent alkalosis

(Yes, this is the same as “delta delta.”)

Stepwise approach

1. Acidosis or alkalosis2. Primary disorder respiratory or metabolic3. If respiratory, is it acute or chronic? 4. Appropriate compensation?5. Calculate anion gap6. Assess for complex (triple) disorders7. CORRELATE CLINICALLY!

Differential diagnosis: metabolic acidosis

Anion gap acidosis Non-anion gap acidosis

K etoacidosis U reterosignoidostomyU remia S alineS alicylates E arly renal failureM ethanol D iarrheaE thanol, ethylene glycol C arbonic anhydrase inhibitorsL actate A mino acids

R enal tubular acidosisS upplements (TPN)

P ancreatic fistula

USUALLY saline or diarrhea

Differential diagnosis: metabolic alkalosis

• Volume contraction (vomiting, overdiuresis, ascites)

• Hypokalemia

• Alkali ingestion (bicarbonate)

• Excess gluco- or mineralocorticoids

• Bartter's syndrome

• USUALLY vomiting or overdiuresis

Differential diagnosis: respiratory acidosis

• Central Nervous System Depression (Sedatives, CNS disease, Obesity Hypoventilation syndrome)

• Pleural Disease (Pneumothorax)

• Lung Disease (COPD, pneumonia)

• Musculoskelatal disorders (Kyphoscoliosis, Guillain-Barre, Myasthenia Gravis, Polio)

• Practically, think about “tiring” (even a little respiratory acidosis in asthma is often a harbinger of badness)

Differential diagnosis: respiratory alkalosis

• Catastrophic CNS event (CNS hemorrhage)

• Drugs (salicylates, progesterone)

• Pregnancy (especially the 3rd trimester)

• Decreased lung compliance (interstitial lung disease)

• Liver cirrhosis

• Anxiety/Pain

Examples

21 yo woman presents with confusion, fever, flank pain, “breathing heavy”

7.32/28 140 104 14

Examples

21 yo woman presents with confusion, fever, flank pain, “breathing heavy”

7.32/28 140 104 14

Primary disorder = metabolic acidosisWinter’s formula: expected pCO2 = 29 (ok)AG = 22; expected HCO3 = 10 + 14 = 24 (ok)

Examples

21 yo woman presents with confusion, fever, flank pain, “breathing heavy”

7.32/28 140 104 14

Primary disorder = metabolic acidosisWinter’s formula: expected pCO2 = 29 (ok)AG = 22; expected HCO3 = 10 + 14 = 24 (ok)

Anion gap metabolic acidosisDKA with pyelonephritis

Examples

58 yo man presents with 4d cough, diarrhea. Chest x-ray shows LLL infiltrate. Pt’s breath smells of alcohol.

7.31/10 123 99 5

Examples

58 yo man presents with 4d cough, diarrhea. Chest x-ray shows LLL infiltrate. Pt’s breath smells of alcohol.

7.31/10 123 99 5

Primary disorder = metabolic acidosisWinter’s formula: expected pCO2 = 15, so concurrent respiratory alkalosisAG = 19; expected HCO3 = 7 + 5 = 12, so non-anion gap metabolic acidosis

Examples

58 yo man presents with 4d cough, diarrhea. Chest x-ray shows LLL infiltrate. Pt’s breath smells of alcohol.

7.31/10 123 99 5

Primary disorder = metabolic acidosisWinter’s formula: expected pCO2 = 15, so concurrent respiratory alkalosisAG = 19; expected HCO3 = 5 + 5 = 10, so non-anion gap metabolic acidosis

Anion gap metabolic acidosis, non-anion gap metabolic acidosis, respiratory alkalosis

Alcoholic ketoacidosis, diarrhea, pneumonia

Examples

56 yo man found vomiting on the street

7.40/40 145 100 24

Examples

56 yo man found vomiting on the street

7.40/40 145 100 24

Can’t tell primary disorder by pH… but AG = 21Expected HCO3 = 9 + 24 = 33 so concurrent metabolic alkalosis

Examples

56 yo man found vomiting on the street

7.40/40 145 100 192 3.6 24 9.1

Can’t tell primary disorder by pH… but AG = 21Expected HCO3 = 9 + 24 = 33 so concurrent metabolic alkalosis

Metabolic alkalosis and metabolic acidosisVomiting in the setting of worsening uremia due to CKD

Examples

58 yo man with 4d cough, vomiting, altered mental status

7.50/20 145 100 15

Examples

58 yo man with 4d cough, vomiting, altered mental status

7.50/20 145 100 15

Respiratory alkalosisAG = 30, so concurrent anion gap metabolic acidosisExpected HCO3 = 18 + 15 = 33 so concurrent metabolic alkalosis

Examples

58 yo man with 4d cough, vomiting, altered mental status

7.50/20 145 100 15

Respiratory alkalosisAG = 30, so concurrent anion gap metabolic acidosisExpected HCO3 = 18 + 15 = 33 so concurrent metabolic alkalosis

Respiratory alkalosis, anion gap metabolic acidosis, metabolic alkalosisPneumonia, alcoholic ketoacidosis, vomiting

Examples

35 yo woman presents obtunded

7.10/50 145 100 15

Examples

35 yo woman presents obtunded

7.10/50 145 100 15

Primary respiratory acidosisAG = 30 so concurrent primary metabolic acidosisExpected HCO3 = 18 + 15 = 33 so concurrent metabolic

alkalosis

Examples

35 yo woman presents obtunded

7.10/50 145 100 15

Primary respiratory acidosisAG = 30 so concurrent primary metabolic acidosisExpected HCO3 = 18 + 15 = 33 so concurrent metabolic

alkalosis

Same as last patient but obtunded so hypoventilating!Hypoventilation due to altered mental status, DKA, vomiting

Landing safely

Do the exercise on every ABG for practice

Landing safely

Do the exercise on every ABG for practice

Do the exercise on every ABG for practice

Landing safely

Do the exercise on every ABG for practice

Do the exercise on every ABG for practice

Ask questions