Acidosis

Acidosis

Description

Respiratory Acidosis

• Reduced pH owing to alveolar hypoventilation with elevated PaCO₂
• Defined as PaCO₂ >45 mm Hg or higher than expected for calculated respiratory compensation for metabolic acidosis
• Divided into three broad categories:
• Primary failure in central nervous system drive to ventilate:
• Sleep apnea
• Anesthesia
• Sedative overdose
• Primary failure in transport of CO₂ from alveolar space:
• Chronic obstructive pulmonary disease
• Myasthenic crisis
• Severe hypokalemia
• Guillain-Barre syndrome
• Primary failure in transport of CO₂ from tissue to alveoli:
• Severe heart failure

Metabolic Acidosis

• Process that reduces serum pH by decreasing plasma bicarbonate levels
• Primarily caused by:
• Gain of a strong acid through ingestion or metabolism
• Loss of bicarbonate from the body
• Metabolic acidosis is clinically evaluated by dividing into two main groups:
• Elevated anion gap metabolic acidosis:
• Bicarbonate reduced through buffering of added strong acid
• Anion gap is increased owing to retention of the unmeasured anion from the titrated strong acid.
• Normal anion gap metabolic acidosis owing to:
• Kidneys fail to reabsorb or regenerate bicarbonate.
• Losses of bicarbonate from gastrointestinal tract (diarrhea)
• Ingestion or infusion of substances that release hydrochloric acid
• No anion gap is observed owing to the absence of any unmeasured anion of a titrated acid and secondary chloride retention with HCO₃^- loss.

Etiology

Anion Gap Acidosis

To remember possible causes, use mnemonic A CAT MUD PILES:

• Alcohol ketoacidosis
• Carbon monoxide or cyanide
• Aspirin
• Toluene
• Methanol
• Uremia
• Diabetic ketoacidosis
• Paraldehyde
• Iron/isoniazid
• Lactic acidosis
• Ethylene glycol
• Starvation

Increased Osmolar Gap

To remember possible causes, use mnemonic ME DIE:

• Methanol
• Ethylene glycol
• Diuretics (mannitol; no acidosis)
• Isopropyl alcohol (no acidosis)
• Ethanol

Non-Anion Gap Metabolic Acidosis

• Gastrointestinal losses of bicarbonate:
• Diarrhea
• Villous adenoma
• Removal of small bowel, pancreatic or biliary secretions
• Tube drainage
• Small bowel/pancreatic fistula
• Anion exchange resins (i.e., cholestyramine):
• Ingestion of calcium chloride or magnesium chloride
• Renal loss of bicarbonate
• Renal tubular acidosis:
• Type I (distal): hypokalemic hyperchloremic metabolic acidosis:
• Decreased ability to secrete hydrogen
• Serum HCO₃ <15>
• Potassium low
• Renal stones common
• Type II (proximal): hypokalemic hyperchloremic metabolic acidosis:
• Decreased proximal reabsorption of HCO₃^-.
• Acidosis limited by reabsorptive capacity of proximal tubule for HCO₃^-
• Serum HCO₃ typically 14–18 mEq/L
• Low/normal potassium.
• Type IV (hypoaldosteronism): hyperkalemic hyperchloremic acidosis:
• Aldosterone deficiency or resistance causing decreased H⁺ secretion
• Serum bicarb >15 mEq/L
• Normal/elevated potassium
• Carbonic anhydrase inhibitors
• Tubulointerstitial renal disease
• Hypoaldosteronism
• Addition of hydrochloric acid such as:
• Ammonium chloride
• Arginine hydrogen chloride
• Lysine hydrogen chloride

Diagnosis

Signs and Symptoms

• Nonspecific findings
• Vital signs:
• Tachypnea or Kussmaul respirations with metabolic acidosis
• Hypoventilation with respiratory acidosis
• Tachycardia
• Somnolence
• Confusion
• CO₂ narcosis
• Myocardial conduction and contraction disturbances

Essential Workup

• Electrolytes, BUN, creatinine, glucose:
• Decreased bicarbonate with metabolic acidosis
• Hyperkalemia and hypercalcemia with severe metabolic acidosis
• Arterial blood gases:
• pH
• CO₂ retention in respiratory acidosis
• CO level
• Calculate anion gap: Na⁺ - (HCO₃^- + Cl^-):
• Correct anion gap for hypoalbuminemia:
• For every 1 g/dL decrease in albumin (from 4.0 g/dL), add 2.5 points to calculated anion gap.
• Normal range = 5-12 ± 3 mEq/L
• Anion gap >25 mEq/L is seen only with:
• Lactic acidosis
• Ketoacidosis
• Toxin-associated acidoses
• Calculate the degree of compensation:
• Expected PaCO₂ = 1.5[HCO₃^-] + 8
• If PaCO₂ inappropriately high, patient has a concomitant respiratory acidosis and inadequate compensation
• 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
• Evaluate the delta gap:
• For every one-point increase in anion gap, HCO₃^- should decrease by 1 mEq/L in simple acid/base disorder.
• Evaluate 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 non-anion gap acidosis.

Tests

Lab

• Urinalysis for glucose and ketones
• Measure serum osmolality:
• Calculated serum osmolality = 2 Na + glucose/18 + blood urea nitrogen/2.8
• Osmolal gap = difference between calculated and measured osmolality:
• Normal = <10
• 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

Alert

• Failure to appreciate acidosis in mixed acid/base disorders
• Failure to appreciate inadequate respiratory compensation for metabolic acidosis and need for ventilatory support

Differential Diagnosis

• Anion gap acidosis:
• To remember possible causes, use mnemonic A CAT MUD PILES:
• Alcohol ketoacidosis
• Carbon monoxide or cyanide
• Aspirin
• Toluene
• Methanol
• Uremia
• Diabetic ketoacidosis
• Paraldehyde
• Iron/isoniazid
• Lactic acidosis
• Ethylene glycol
• Starvation
• Increased osmolar gap:
• To remember possible causes, use mnemonic ME DIE:
• Methanol
• Ethylene glycol
• Diuretics (mannitol)
• Isopropyl alcohol
• Ethanol

Treatment

Initial Stabilization

Airway, breathing, 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

ED Treatment

• 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 osmolal 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.

Medication (Drugs)

• 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

Follow-Up

Disposition

Admission Criteria

• Worsening metabolic acidosis
• ICU admission if pH <7.1>
• Respiratory acidosis

Discharge Criteria

Resolving or resolved anion gap metabolic acidosis

References

1. Adrogue HJ, Madias NE. Management of life-threatening acid-base disorders. New Engl J Med. 1998;338:26.

2. Swenson ER. Metabolic acidosis. Respir Care. 2001;46:342.

3. Whittier WL, Rutecki GW. Primer on clinical acid-base problem solving. Dis Mon. 2004;50:122.

Codes

ICD9-CM

276.2 Acidosis

ICD10

E87.2