- Nonspecific findings
- Vital signs:
- Tachypnea or Kussmaul respirations with metabolic acidosis
- Hypoventilation with respiratory acidosis
- Altered mental status (CO2 narcosis)
- Myocardial conduction and contraction disturbances (dysrhythmias)
Check the degree of compensation by calculating the expected values and comparing them to the observed laboratory values as follows:
- Electrolytes, BUN, creatinine, and glucose:
- Decreased bicarbonate with metabolic acidosis
- Hyperkalemia and hypercalcemia with severe metabolic acidosis
- Arterial blood gases:
- CO2 retention in respiratory acidosis
- CO level
- Respiratory acidosis:
- Acute: Expected HCO3− increased by 1 mEq/L for every 10 mm Hg increase in PaCO2
- Chronic: Expected HCO3− increased by 4 mEq/L for every 10 mm Hg increase in PaCO2
- Calculate anion gap: Na+ – (HCO3− + Cl−):
- Correct anion gap for hypoalbuminemia:
- For every 1 g/dL decrease in albumin (from 4 g/dL), add 2.5 points to calculated anion gap.
- Do not correct sodium concentration when calculating the anion gap in the setting of marked hyperglycemia because hyperglycemia affects the concentration of chloride and bicarbonate, as well as sodium.
- Normal range = 5 – 12 ± 3 mEq/L
- Anion gap >25 mEq/L is seen only with:
- Lactic acidosis
- Toxin-associated acidosis
- Calculate the degree of compensation:
- Expected PaCO2 = 1.5[HCO3−] + 8
- If PaCO2 inappropriately high, patient has a concomitant respiratory acidosis and/or inadequate compensation.
- Evaluate the delta gap (ΔGap):
- For every 1-point increase in anion gap, HCO3− should decrease by ∼1 mEq/L in simple acid–base disorder.
- As the volumes of distribution of the unmeasured anions and serum HCO3− are not in unity, a ΔGap > 6 signifies a mixed acid–base disorder
- Evaluate ΔGap by comparing the change in the anion gap (ΔAG) with the change in the HCO3− (ΔHCO3−) from normal:
- If ΔAG > ΔHCO3−, then patient has a concomitant metabolic alkalosis.
- If ΔHCO3− > ΔAG, then patient has concomitant nonanion gap acidosis.
Diagnostic Tests and Interpretation
- ABG: See interpretation above.
- Obvious benefit is less patient discomfort and ease in acquiring sample
- pH varies by <0.04 units when compared to arterial sampling.
- Correlation between venous pCO2 lacking
- Limited role in screening for hypercapnia. pCO2 >45 mm Hg is sensitive (but not specific) for detection of arterial pCO2 > 50 mm Hg in hemodynamically stable patients
- Useful in simple acid–base disorders
- Urinalysis for glucose and ketones
- Measure serum osmolality:
- Calculated serum osmolality = 2 Na + glucose/18 + BUN/2.8
- Osmolar gap = difference between calculated and measured osmolality:
- Normal = <10
- Elevated osmolar gap may indicate toxic alcohol as etiology of acidosis.
- Absence of an osmolar gap should never be used to rule out toxic ingestions:
- Osmolar gap imprecisely defined
- Delayed presentations may have normal gap
- Large variance in gap among normal patients
- Toxicology screen:
- Methanol, ethylene glycol, ethanol, and isopropyl alcohol if increased osmolality gap
- Aspirin or iron levels for suspected ingestion
- Co-oximetry for CO exposure
- Serum ketones or β-hydroxybutyrate level
- Serum lactate
- May identify cardiomyopathy or CHF
- Underlying pneumonia
- May identify regional wall motion abnormalities or valvular dysfunction
- Evaluate for conduction disturbances
- Anion gap acidosis:
- Increased osmolar gap:
Airway, breathing, and circulation (ABCs):
- Early intubation for severe metabolic acidosis with progressive/potential weakening of respiratory compensation
- Naloxone, D50W (or Accu-Chek), and thiamine if mental status altered
- Respiratory acidosis:
- Treat underlying disorder
- Provide ventilatory support for worsening hypercapnia
- Identify and correct aggravating factors (pneumonia) in chronic hypercapnia.
- Metabolic acidosis:
- Identify if concurrent osmolar gap.
- Treat underlying disorder:
- Diabetic ketoacidosis
- Lactic acidosis
- Alcohol ketoacidosis
- Correct electrolyte abnormalities.
- IV fluids:
- Rehydrate with 0.9% normal saline if patient hypovolemic.
- Consider hemodialysis
- Dextrose: D50W 1 amp (50 mL or 25 g); (peds: D25W 4 mL/kg) IV
- Naloxone (Narcan): 2 mg (peds: 0.1 mg/kg) IV or IM initial dose
- Thiamine (vitamin B1): 100 mg (peds: 50 mg) IV or IM
Ongoing CareAdmission Criteria
Consider ICU admission if:
- pH <7.1
- Altered mental status
- Respiratory acidosis
- Hemodynamic instability
- Electrolyte abnormalities
Resolving or resolved anion gap metabolic acidosis
Pearls and Pitfalls
- Failure to appreciate acidosis in mixed acid–base disorders
- Failure to appreciate inadequate respiratory compensation for metabolic acidosis and need for ventilatory support
- Clues to the presence of a mixed acid–base disorder are normal pH with abnormal PCO2 or HCO3−, when the HCO3− and PCO2 move in opposite directions, or when the pH changes in the direction opposite that expected from a known primary disorder.
- Ayers C, Dixon P. Simple Acid-Base Tutorial. J Parenter Enteral Nutr. 2012;36(1):18–23.
- Kellum JA. Determinants of plasma acid-base balance. Crit Care Clin. 2005;21(2):329–346. [PMID:15781166]
- Robinson MT, Heffner AC. Acid base disorders. In: Adams J ed. Emergency Medicine. Philadelphia, PA: Elsevier; 2012.
- Whittier WL, Rutecki GW. Primer on clinical acid-base problem solving. Dis Mon. 2004;50:122. [PMID:15069420]
- 51387008 Acidosis (disorder)
- 12326000 Respiratory acidosis (disorder)
- 59455009 Metabolic acidosis (disorder)
Matthew T. Robinson
© Wolters Kluwer Health Lippincott Williams & Wilkins
is a sample topic from the 5-Minute Emergency Consult
To view other topics, please sign in or purchase a subscription.
Emergency Central is a collection of disease, drug, and test information including 5-Minute Emergency Medicine Consult, Davis’s Drug, McGraw-Hill Medical’s Diagnosaurus®, Pocket Guide to Diagnostic Tests, and MEDLINE Journals created for emergency medicine professionals. Complete Product Information.