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15.e2 Acidosis

Acidosis
VetBooks.ir Etiology and Pathophysiology

BASIC INFORMATION
• Metabolic acidosis is caused by a primary to be absent only on the basis of normal
blood pH (absence of acidemia); PCO 2 and
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Definition decrease in plasma [HCO 3 ] or an increase [HCO 3 ] also must be normal.
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Acidosis is a net increase in the concentration in plasma [H ], resulting in a decreased pH. • Each primary abnormality may be accom-
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of hydrogen ions ([H ]) in the body that results It may be accompanied by a secondary or panied by a compensatory response.
in a lower blood pH. The causes of acidosis adaptive (compensatory) decrease in PCO 2 . ○ The exception is with cats; this species
may be metabolic (reflected in decreased • Metabolic acidosis may be caused by less commonly shows respiratory compen-
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[HCO 3 ] due to the accumulation of a non- ○ Loss of HCO 3 -rich fluid from the sation for metabolic acidosis.
volatile fixed acid) or respiratory (reflected in body (e.g., gastrointestinal [GI] tract or • Respiratory compensation for metabolic
increased PCO 2 levels). kidney) acidosis results in a 0.7-mm Hg drop in
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○ Failure of excretion of fixed acid PCO 2 for every 1-mEq/L drop in [HCO 3 ].
Epidemiology (e.g., caused by renal impairment or • Metabolic compensation for respiratory
SPECIES, AGE, SEX hypoperfusion) acidosis results in an increase in [HCO 3 ]
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All animals susceptible to acidosis ○ Excessive production of fixed acids by by 0.15 (acute) to 0.35 mEq/L (chronic)
the body (e.g., lactate, ketones) or addi- for every 1-mm Hg increase in arterial PCO 2 .
RISK FACTORS tion of fixed acids (e.g., ethylene glycol,
• Serious illnesses usually cause some form of salicylates)
acid-base disturbance. • Metabolic acidosis may be characterized by DIAGNOSIS
• Common causes of increased anion gap an increase in anion gap (suggesting accu-
metabolic acidosis mulation of organic acids such as lactate; Diagnostic Overview
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○ Ethylene glycol toxicity the corrected serum [Cl ] is normal) or a An initial database including CBC, serum
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○ Diabetic ketoacidosis normal anion gap (suggesting HCO 3 loss; biochemistry profile, and urinalysis is vital in
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○ Uremic acidosis (caused by the retention the corrected serum [Cl ] is elevated). identifying the cause of acid-base disturbances
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of uremic acids such as phosphate and ○ Corrected Cl is defined as patient [Cl ] because acidosis is never a primary disorder. It
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sulfate) × (normal [Na ]/patient [Na ]). is always the consequence of another disease;
○ Lactic acidosis (e.g., due to cardiovascular ○ Anion gap is calculated using the formula each disease that causes acidosis will have its
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collapse during shock) ([Na ] + [K ]) − ([Cl ] + [HCO 3 ]); own differential diagnosis list.
• Common causes of normal–anion gap acidosis normal values for dogs are between 12
○ Diarrhea (bicarbonate loss) and 24 mEq/L. Differential Diagnosis
○ Renal tubular acidosis (bicarbonate loss) ○ The anion gap represents additional ions • More than one primary acid-base imbalance
○ Dilutional acidosis (e.g., caused by rapid in a blood sample that are not accounted may occur concurrently, and these combina-
0.9% saline administration) for in the anion gap formula. tions are called mixed acid-base disorders. For
○ Hyperchloremia ○ Increased anion gap acidosis is normally example, metabolic and respiratory acidoses
○ Hypoadrenocorticism associated with an accumulation of organic may occur concurrently.
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acids such as lactate or ketones, and [Cl ]
ASSOCIATED DISORDERS remains unchanged. Initial Database
• Metabolic acidosis is commonly associated ○ The only important cause of a low anion • Blood gas analysis, serum electrolyte
with hypovolemia or hypoperfusion, which gap is hypoalbuminemia, which is concentrations
results in excessive lactic acid generation and common in critical illness. Hypoalbumin- ○ A venous blood sample is usually adequate
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reduced H loss or increased H retention. emia often confounds identification of for assessment of pH, metabolic distur-
• Respiratory acidosis is caused by decreased high–anion gap acidosis. bances, ventilation abnormalities (PCO 2 ,
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alveolar ventilation. ○ Normal anion gap metabolic acidosis is HCO 3 , base excess [BE]), and electrolytes,
associated with an increased corrected but an arterial blood sample is preferable
Clinical Presentation plasma [Cl ]. for concurrent assessment of respiratory
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DISEASE FORMS/SUBTYPES • Respiratory acidosis is associated with disease because PaO 2 (oxygenation) may
Metabolic acidosis may be associated with an reduced alveolar ventilation (respiratory then be assessed.
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increased anion gap (normochloremic metabolic minute volume), usually from an abnormally ○ Bicarbonate (HCO 3 ): low, unless mixed
acidosis) or normal anion gap (hyperchloremic low respiratory rate or decreased tidal volume acid-base disorder (opposing/offsetting
metabolic acidosis). Respiratory acidosis has (e.g., caused by large-volume pleural effusion, disorders)
no subclassifications. altered lung compliance, neuromuscular ○ Electrolyte abnormalities: see Etiology and
junction disruption as seen with neurotoxins Pathophysiology above
History, Chief Complaint and some peripheral neuropathies). The result • CBC, serum biochemistry panel, urinalysis
Acidosis may contribute to owner-observed is an increase in PCO 2 , leading to a reduced ○ Total protein, albumin: hypoalbuminemia
signs of systemic illness, but the clinical signs pH that may be accompanied by a compensa- explains a low anion gap; a normal–anion
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of acidosis are nonspecific. tory increase in [HCO 3 ] or reduction in gap metabolic acidosis with concurrent
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blood [Cl ] with chronicity. hypoalbuminemia should raise the suspi-
Physical Exam Findings • Simple respiratory acidosis will not alter cion that a high–anion gap metabolic
• Severe acidosis causes reduced cardiac output anion gap, but the metabolic compensatory acidosis exists but is concealed by the effect
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and arterial vasodilation (resulting in lethargy, response can be expected to drop [Cl ]. of hypoalbuminemia.
weakness, poor pulse quality) and central • The presence of severe acidosis may be
nervous system depression (mental dullness). masked by an equally severe alkalosis from Advanced or Confirmatory Testing
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• Respiratory compensation may be associated retention of HCO 3 or low PCO 2 (i.e., mixed • Special diagnostic evaluations should be
with deep, rhythmic breathing or tachypnea. acid-base disorder). Acidosis cannot be said tailored to the specific disease processes
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