Metabolic acidosis

Approach to Metabolic Acidosis – Ned Hardison

ABG (VBG) reference ranges:

  • pH = 7.36-7.44 (~7.32-7.40)
  • PCO2 = 36-44 mmHg
  • HCO3 = 22-26 mEq/L
  • VBG can be used to screen for hypercarbia, but not to assess degree of hypercarbia
  • pH between ABG and VBG correlates well with mean difference 0.035 pH units

Primary Disorder

pH

PaCO2

HCO3

Metabolic acidosis

↓↓

Metabolic alkalosis

↑↑

Respiratory acidosis

↑↑

Respiratory alkalosis

↓↓

 

Equations

 

Non-anion gap metabolic acidosis (NAGMA)

  • In practice, there are two sources of bicarb deficit – kidneys or gut
      • Urine anion-gap can differentiate between the two
      • It corresponds to unmeasured NH4+ which kidneys should excrete to remove acid excess if they are functioning appropriately
  • Urine anion gap = Unmeasured cations (NH4+) – unmeasured anions = U­Na + UK – UCl
      • Positive value, low NH4+, kidneys not working appropriately, = renal etiology
      • Ne-GUT-ive value, high NH4+, kidneys working appropriately = gut loss 
  • Caveat: Proximal RTA has a normal distal urine acidification and has a negative urine AG

 

Managing Metabolic Acidosis

  • Metabolic acidosis is evidence of an underlying metabolic derangement
  • Severe metabolic acidosis due to shock and hypoperfusion can lead to poor response to vasopressors, arrhythmias, and cerebral edema
      • 1 ampule NaHCO3 = 50 mEq NaHCO3, can give 1-3 amps
      • NaHCO3 solution alone is hypertonic; Administer in 1/2 NS or D5W to avoid hypernatremia but NS usually avoided b/c of risk of worsening acidosis
      • Or a bicarb gtt, order 3 amps of NaHCO3 in 1L of D5W
  • Evidence for use of bicarbonate is mixed: No proven benefit in lactic acidosis or DKA, but some experts suggest use in DKA when pH<7.1
      • In acute NAGMA, reasonable to give bicarbonate when bicarb <12 or pH <7.1-7.2

 

Anion Gap

Non-anion gap

Etiologies

Glycols (ethylene, diethylene, propylene)

Oxoproline (d/t APAP overdose)

L-lactate (std. measured lactate)

D-lactate (d/t short bowel syndrome)

Methanol (and other alcohols)

Aspirin

Renal failure (uremia, PO4, SO4)

Ketoacidosis (DM, EtOH, starvation)

Gut Losses: Diarrhea

 

Renal Losses:

- AKI, CKD

- Renal tubular acidosis

- Crystalloid infusion

- Carbonic anhydrase inhibitors

Further Workup

For all cases: BMP, consider ABG, UA, +/- EKG

- Elevated osmolar gap (>10) can suggest ingestion (G or M, above)

- Calculate ∆∆ , interpret as above

- Consider UA (ketonuria), LFTs, lactate, Utox, β-OH-butyrate, APAP/ASA/EtOH levels

- Evaluate medication list for causes(e.g. Metformin, Linezolid)

- Urine anion gap

- +/- further w/u of AKI/RTA