Rosen & Barkin's 5-Minute Emergency Medicine Consult (27 page)

DIAGNOSIS

• Nonspecific findings
  
• Vital signs:
  
  • Tachypnea or Kussmaul respirations with metabolic acidosis
    
  • Hypoventilation with respiratory acidosis
    
  • Tachycardia
    
• Somnolence
  
• Confusion
  
• Altered mental status (CO
  ₂
  narcosis)
  
• Myocardial conduction and contraction disturbances (dysrhythmias)
  

• Electrolytes, BUN, creatinine, and glucose:
  
  • Decreased bicarbonate with metabolic acidosis
    
  • Hyperkalemia and hypercalcemia with severe metabolic acidosis
    
• Arterial blood gases:
  
  • pH
    
  • CO
    ₂
    retention in respiratory acidosis
    
  • CO level
    

Check the degree of compensation by calculating the expected values and comparing them to the observed laboratory values as follows:

• Respiratory acidosis:
  
  • Acute: Expected HCO
    ₃
    ⁻
    increased by 1 mEq/L for every 10 mm Hg increase in PaCO
    ₂
    
  • Chronic: Expected HCO
    ₃
    ⁻
    increased by 4 mEq/L for every 10 mm Hg increase in PaCO
    ₂
    
• Calculate anion gap: Na
  ⁺
  – (HCO
  ₃
  ⁻
  + 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
      
    • Ketoacidosis
      
    • Toxin-associated acidosis
      
• Calculate the degree of compensation:
  
  • Expected PaCO
    ₂
    = 1.5[HCO
    ₃
    ⁻
    ] + 8
    
  • If PaCO
    ₂
    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, HCO
    ₃
    ⁻
    should decrease by ∼1 mEq/L in simple acid–base disorder.
    
  • As the volumes of distribution of the unmeasured anions and serum HCO
    ₃
    ⁻
    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 HCO
  ₃
  ⁻
  (ΔHCO
  ₃
  ⁻
  ) from normal:
  
  • If ΔAG > ΔHCO
    ₃
    ⁻
    , then patient has a concomitant metabolic alkalosis.
    
  • If ΔHCO
    ₃
    ⁻
    > ΔAG, then patient has concomitant nonanion gap acidosis.
    

• ABG: See interpretation above.
  
• VBG:
  
  • 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 pCO
    ₂
    lacking
    
  • Limited role in screening for hypercapnia. pCO
    ₂
    >45 mm Hg is sensitive (but not specific) for detection of arterial pCO
    ₂
    > 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
  

CXR:

• May identify cardiomyopathy or CHF
  
• Underlying pneumonia
  

ECG:

• May identify regional wall motion abnormalities or valvular dysfunction
  
• Evaluate for conduction disturbances
  

• Anion gap acidosis:
  
  • Mnemonic
    A CATPILES MUD
    
• Increased osmolar gap:
  
  • Mnemonic
    ME DIE
    

TREATMENT

Airway, breathing, and circulation (ABCs):

• Early intubation for severe metabolic acidosis with progressive/potential weakening of respiratory compensation
  
• Naloxone, D
  ₅₀
  W (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
      
    • Ingestion
      
  • Correct electrolyte abnormalities.
    
• IV fluids:
  
  • Rehydrate with 0.9% normal saline if patient hypovolemic.
    
  • Consider hemodialysis
    

• Dextrose: D
  ₅₀
  W 1 amp (50 mL or 25 g); (peds: D
  ₂₅
  W 4 mL/kg) IV
  
• Naloxone (Narcan): 2 mg (peds: 0.1 mg/kg) IV or IM initial dose
  
• Thiamine (vitamin B
  ₁
  ): 100 mg (peds: 50 mg) IV or IM
  

FOLLOW-UP

Consider ICU admission if:

• pH <7.1
  
• Altered mental status
  
• Respiratory acidosis
  
• Hemodynamic instability
  
• Dysrhythmias
  
• Electrolyte abnormalities
  

Resolving or resolved anion gap metabolic acidosis

• 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 PCO
  ₂
  or HCO
  ₃
  ⁻
  , when the HCO
  ₃
  ⁻
  and PCO
  ₂
  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.
  
• 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.
  

See Also (Topic, Algorithm, Electronic Media Element)

Alkalosis

CODES

276.2 Acidosis