Metabolic Acidosis

Metabolic Acidosis
A Review by
George B. Buczko MD FRCP(C)

Case Presentation 1

• 54 year old man with fever and abnormal liver function for liver biopsy
• Biopsy “well tolerated” until 3 hours afterwards when he developed abdominal distension , with systolic BP 40 and Hg 4.6

Case Presentation 2

• Vasopressin and bicarbonate infusions and blood transfusion restored BP to 85/40
• The patient was rushed to the OR for exploratory laparotomy

Case Presentation 3

Arterial blood analysis:
pH 6.95, p aO 2 337, p aCO 2 44, TCO 2 10 H +102nM
Na 142, K 6.3, Cl 106 anion gap 26
Albumin 1.2g/dl
Expected anion gap 6 because of low albumin
Anion gap 20 above expected
Lactate 18.3meq/l
Minute ventilation 6.4 liters

Case Presentation 4

• The problem: high H +
• Cerebral enzyme dysfunction
• Cardiac enzyme dysfunction
• Myocardial dysfunction in the face of hemorrhagic shock
• Downward spiral from more than just blood loss

Metabolic Acidosis

• Definition
• Acid-Base physiology
• Anion gap
• Differential diagnosis of metabolic acidosis with high anion gap
• Lactic acidosis
• Oxidative phosphorylation
• Types of Lactic acidosis
• Treatment of Lactic Acidosis

Metabolic Acidosis
(primary fall in serum bicarbonate)

• A condition that causes a primary fall in serum bicarbonate level
• H + + HCO 3 - D H 2CO 3 D H 2O + CO 2

Metabolic Acidosis
(primary fall in serum bicarbonate)

• H + + HCO 3 - D H 2CO 3 D H 2O + CO 2
• According to the above, a fall in HCO 3 - will result from:

1. Addition of H + (shift right: $ in HCO 3 - )
2. Loss of bicarbonate (shift left: Ÿ in H+)

• Increase in H + occurs in both situations
• Metabolic Acidosis
  (primary fall in serum bicarbonate)

Increase in H +:

• Enzyme dysfunction which leads to
• Organ dysfunction

Heart/Brain
Metabolic Acidosis
(primary fall in serum bicarbonate)
Increase in H +:

• H + is accompanied by an anion in order to maintain electrical neutrality
• The anion may be Cl - (HCl administration)
• The anion may be LACTATE, a KETONE, PHOSPHATE, SULPHATE, or an ingested anion

Metabolic Acidosis
(primary fall in serum bicarbonate)
The Anion Gap:

• In the body

cations = anions

• Not all of the anions are measured in routine laboratory analysis
• [Na +] – ([Cl -] + [HCO3 -]) = 12

Metabolic Acidosis
(primary fall in serum bicarbonate)
The Anion Gap:

• The usual unmeasured anions that account for the “gap” are:

Albumin
Phosphates
Sulphates

Metabolic Acidosis
(primary fall in serum bicarbonate)

• The Anion Gap:
• #anion gap in the presence of #[H +] is a marker for the presence of anions that accompany H +but are not routinely measured

Metabolic Acidosis
(primary fall in serum bicarbonate)
High Anion Gap Acidosis:
Type Anion:

• Lactic lactate
• Diabetic ketones
• Uremia sulphate/phosphate
• ASA salicylate
• Methanol formate
• E. Glycol oxalate

Lactic Acidosis
Why do we need oxygen?

• For oxidative phosphorylation

What is oxidative phosphorylation?

• ADP + P i = ATP (requires energy)
• The formation of ATP

What does the oxygen do?

Lactic Acidosis
Glycolysis:
Glucose "Pyruvate "Acetyl CoA
Kreb’s:
Acetyl CoA "NADH & FADH
Electron transport chain (ETC)
NADH & FADH "ATP

Lactic Acidosis

• The bulk of ATP is generated in the electron transport chain (ETC) in the mitochondrion
• The energy for creating the high-energy phosphate bond is generated at several points in the ETC. So are hydrogen ions

Metabolic Acidosis
(primary fall in serum bicarbonate)

Lactic Acidosis

• Type A: failure of oxidative phosphorylation (Pyruvate èLactate)
• Type B: lactate production overwhelms lactate metabolism

Lactic Acidosis  
Type A (more severe)
Failure of ETC:
Decreased Oxygen delivery
Shock of any type
Severe hypoxemia
Severe Anemia
Inhibitors (CO, CN)
 Back to original case
Arterial blood analysis:
pH 6.95, p aO 2 337, p aCO 2 44, TCO 2 10 H +102nM
Na 142, K 6.3, Cl 106 anion gap 26
Albumin 1.2g/dl
Expected anion gap 6 because of low albumin
Anion gap 20 above expected
Lactate 18.3meq/l
Minute ventilation 6.4 liters

Lactic Acidosis: Treatment

• Treat the underlying cause
• Lower the H + concentration

Lactic Acidosis: Treatment
Underlying cause in this case:
Profound rapid blood loss
Transfusion of blood and products
Circulatory support

Lactic Acidosis: Treatment
Lower the H + concentration
H + + HCO 3 - D H 2CO 3 D H 2O + CO 2
Lower the p aCO 2 by increasing minute ventilation

Lactic Acidosis: Treatment
Lower the p aCO 2 by
increasing
minute ventilation

Lactic Acidosis: Treatment
For every 1meq/l drop in HCO 3 - from 25, p aCO 2 should decrease by ~ 1 torr
“Normal” p aCO 2 in the face of HCO 3 - 10 is 25 (40 – 15) and not 40 torr

Lactic Acidosis: Treatment
Intravenous bicarbonate administration:
Pro: lowers H + concentration ( ›pH)
improves pressor response
improves myocardial function
Con: worsens intracellular acidosis
may worsen outcome
hypertonic

Lactic Acidosis: Treatment
Bottom line:
If there is adequate circulation
and if minute ventilation is appropriate,
some bicarbonate administration is
warranted.
Don’t aim for full correction, continue
arterial blood analysis

Metabolic Acidosis: Summary

• Definition
• Acid-Base physiology
• Anion gap
• Differential diagnosis of metabolic acidosis with high anion gap
• Lactic acidosis
• Oxidative phosphorylation
• Types of Lactic acidosis
• Treatment of Lactic Acidosis

Lactic Acidosis
take-home points
With hemodynamic instability:
Severe acute bleed
Sepsis
Trauma
Increase minute ventilation
Analyze arterial blood
Judicious intravenous NaHCO 3 -