Metabolic Acidosis

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Contents 1 More Specific Terms 2 Etiology 3 Pathology 4 Clinical-manifestations 5 Laboratory 6 Management 7 More General Terms 8 Additional Terms 9 References

More Specific Terms

• D-lactic acidosis
• diabetic ketoacidosis (DKA)
• ketoacidosis
• lactic acidosis
• pyroglutamic acidosis
• renal tubular acidosis (RTA)

Etiology

• increased anion gap

• osmolal gap > 15 mosm

• uremia
• ethylene glycol
• methanol

• osmolal gap < 15 mosm

• lactic acidosis, D-lactic acidosis
• ketoacidosis
• aspirin
• paraldehyde
• pyroglutamic acidosis
  

• M - methanol
• U - uremia
• D - dehydration
• P - paraldehyde ( pharmacologic agents*)
• I - isoniazid ( INH)
• L - lactic acidosis
• E - ethanol
• S - salicylates
  *Pharmacologic causes: acetazolamide, paraldehyde, salicylate, spironolactone, INH
  
• normal anion gap acidosis

• GI HCO3- loss

• diarrhea
• cholestyramine
• GI fistula
• ureterosigmoidostomy

• renal HCO3- loss

• proximal renal tubular acidosis ( RTA-2)
• carbonic anhydrase inhibitor
• Cl- infusion
• toluene poisoning
• diabetic ketoacidosis

• renal H+ transport defects

• distal renal tubular acidosis ( RTA-1, RTA-4)
• Fanconi's syndrome

• pCO2 higher than expected*

• respiratory & metabolic acidosis
• cardiac arrest
• post-ictal state

• pCO2 lower than expected*

• aspirin poisoning
• septic shock
  

• * Rules for predicting pCO2 (respiratory) compensation for pure metabolic acidosis ( PaCO2, arterial)

• pCO2 ( mm Hg) = digits to right of decimal point in pH
• pCO2 decreases by 1-1.3 mm Hg for each meq/L decrease in HCO3-
• pCO2 +/- 2 = 1.5 x HCO3- meq/L + 8
• pCO2 ( mm Hg) = HCO3- + 15 (chronic)
• compensation takes 12-24 hours to complete, thus no compensation may be present immediately after an acute drop in pH
• failure of the pCO2 to decrease by the expected value suggests complicating respiratory acidosis
• excessive decrease in the pCO2 suggests complicating respiratory alkalosis [4]

• low anion gap acidosis

• low serum albumin
• unmeasured cations ( Ig light chains)

• urine anion gap: urine Na+ + urine K+ - urine Cl-

• large, negative urine anion gap suggests extrarenal origin of metabolic acidosis
• positive urine anion gap suggests renal origin of metabolic acidosis

Pathology

• increased susceptibility to cardiac arrhythmias
• decrease in myocardial contractility
• decreased response to inotropic agents
• secondary decrease in intracerebral pH may cause a diminished level of consciousness
• chronic metabolic acidosis

• loss of Ca+2 from bone
• skeletal muscle breakdown

Clinical-manifestations

• difficult to separate from primary disease process, may involve respiratory system, cardiovascular system, nervous system & skeletal system systems
• nausea & abdominal pain may accompany intoxication
• respiratory compensation causes deep ( Kussmaul) respirations, may result in fatigue & respiratory failure
• when pH < 7.2, arteriolar dilatation & hypotension
• cardiac arrhythmias [6]
• insulin resistance

Laboratory

• arterial blood gas ( ABG)

• decreased pH
• diminished pCO2 secondary to compensatory respiratory alkalosis

• electrolytes

• diminished serum bicarbonate
• serum K+: hyperkalemia
• if increased anion gap is present, measure

• serum ketones
• serum lactate

• osmolality: if calculated osmolal gap is increased, measure

• ethanol ( blood alcohol level)
• methanol
• ethylene glycol
• isopropyl alcohol ( anion gap not increased)

• CBC: a leukemoid reaction may occur

Management

• correct underlying primary disease process
• NaHCO3

• consider if if pH < 7.2
• HCO3- deficit = 0.5 x LBM* (desired - measured) [[[Hco3- | HCO3-]]]
• excess HCO3- can cause seizures, tetany, cardiac arrhythmias
• HCO3- should be discontinued when pH > 7.20

• with lactic acidosis, correction of pH to > 7.2 can result in overshoot alkalosis since metabolism of lactate generates bicarbonate

• supplementary K+

• K+ falls during correction of metabolic acidosis

• monitor Ca+2

• increasing pH decreases ionized Ca+2

• dialysis, epecially for dialyzable etiology

• aspirin
• ethylene glycol
• ethanol

• * lean body mass in kg

More General Terms

• acidosis

Additional Terms

• acid-base balance
• anion gap
• Henderson-Hasselbalch equation

References

1. Harrison's Principles of Internal Medicine, 13th ed. Companion Handbook, Isselbacher et al (eds), McGraw-Hill Inc. NY, 1995, pg 831
2. Manual of Medical Therapeutics, 28th ed, Ewald & McKenzie (eds), Little, Brown & Co, Boston, 1995, pg 58-62
3. Mayo Internal Medicine Board Review, 1998-99, Prakash UBS (ed) Lippincott-Raven, Philadelphia, 1998, pg 602-603
4. Medical Knowledge Self Assessment Program (MKSAP) 11, 14, American College of Physicians, Philadelphia 1998, 2006
5. Harrison's Principles of Internal Medicine, 14th ed. Fauci et al (eds), McGraw-Hill Inc. NY, 1998, pg 280
6. Prescriber's Letter 11(3):14 2004
7. National Guideline Clearinghouse Metabolic acidosis and growth in children. Caring for Australians with Renal Impairment ngc-guideline: [1]

metabolic acidosis