intern basics acid-base august 9, 2005 jeremy marcus md
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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” - PowerPoint PPT PresentationTRANSCRIPT
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