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 |
↑ |
↓↓ |
↓ |
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 - 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 |
|