Metabolic acidosis

In medicine, metabolic acidosis is a state in which the blood pHis low (under 7.35) due to increased production of H⁺by the body or the inability of the body to form bicarbonate(HCO₃^-) in the kidney. Its causes are diverse, and its consequences can be serious, including comaand death. Together with respiratory acidosis, it is one of the two general types of acidosis.

Signs and symptoms

Symptoms are aspecific, and diagnosis can be difficult unless the patient presents with clear indications for arterial blood gassampling. Symptoms may include chest pain, palpitations, headache, altered mental status, decreased visual acuity, nausea, vomiting, abdominal pain, altered appetite (either anorexia or excessive eating) and weight loss(longer term), muscle weakness and bone pains. A slightly specific finding is when the patient reports rapid breathing, not due to shortness of breath but an unmotivated drive to hyperventilate. Kussmaul respirationis rare, but may indicate ketoacidosis.

Exteme acidosis leads to neurological and cardiac complications:

• Neurological: lethargy, stupor, coma, seizures.
• Cardiac: arrhythmias(ventricular tachycardia), decreased response to epinephrine; both lead to hypotension(low blood pressure).

Physical examinationoccasionally reveals signs of disease, but is otherwise normal. Cranial nerveabnormalitites are reported in ethylene glycolpoisoning, and retinaloedema can be a sign of methanol(methyl alcohol) intoxication. Longstanding chronic metabolic acidosis leads to osteoporosisand can cause fractures.

Diagnosis

Arterial blood gassampling is essential for the diagnosis. The pH is low (under 7.35) and the bicarbonate levels are decreased (<12 mmol/l). In respiratory acidosis(low blood pH due to decreased clearance of carbon dioxideby the lungs), the bicarbonate is elevated, due to increased conversion from H₂CO₃. An ECGcan be useful to anticipate cardiac complications.

Other tests that are relevant in this context are electrolytes(including chloride), glucose, renal functionand a full blood count. Urinalysis can reveal acidity (salicylatepoisoning) or alkalinity (renal tubular acidosis type I). In addition, it can show ketones in ketoacidosis.

To distinguish between the main types of metabolic acidosis, a clinical tool called the anion gap is considered very useful. It is calculated by subtracting the chloride and bicarbonate levels from the sodium plus potassium levels.

Anion gap = ( [Na⁺]+[K⁺] ) - ( [Cl^-]+[HCO₃^-] )

As sodium and potassium are the main extracellular cations, and chloride and bicarbonate are the main anions, the result should reflect the remaining anions. Normally, this concentration is about 8-16 mmol/l. An elevated anion gap (i.e. > 16 mmol/l) can indicate particular types of metabolic acidosis, particularly certain poisons, lactate acidosis and ketoacidosis.

As the differential diagnosisis narrowed down, certain other tests may be necessary, including toxicological screening and imaging of the kidneys.

Causes

The causes are best grouped by their influence on the anion gap:

Increased anion gap

• lactic acidosis
• ketoacidosis
• chronic renal failure(accumulation of sulfates, phosphates, uric acid)
• intoxication:
  • organic acids(salicylates, ethanol, methanol, formaldehyde, ethylene glycol, paraldehyde, INH, toluene)
  • sulfates, metformin(Glucophage®)
• massive rhabdomyolysis

Normal anion gap

• longstanding diarrhea(bicarbonate loss)
• pancreatic fistula
• uretero-sigmoidostomy
• RTA
• intoxication:
  • ammonium chloride
  • acetazolamide(Diamox®)
  • bile acid sequestrants
• renal failure(occasionally)

It bears noting that the anion gap can be spuriously normal in sampling errors of the sodium level, e.g. in extreme hypertriglyceridemia. The anion gap can be increased due to relatively low levels of cations other than sodium and potassium (e.g. calcium or magnesium).

Pathophysiology

Compensatory mechanisms

Metabolic acidosis is either due to increased generation of acid or an inability to generate sufficient bicarbonate. The body regulates the acidity of the blood by four buffering mechanisms.

• Blood buffering with bicarbonate. The enzyme carbonic anhydrasemaintains the equilibrium between bicarbonate and H₂CO₃. This is, in turn, converted into carbon dioxide and water.
• Intracellularbuffering by absorption of hydrogen atoms by various molecules, including proteins, phosphates and carbonate in bone.
• Respiratory compensation: chemoreceptors sense a deranged acid-base system, and there is increased breathing.
• Renal compensation: finally the kidney produces and excretes ammonium(NH₄⁺) and monophosphate, generating bicarbonate in the process while clearing acid.

Buffer

The elevated bicarbonate that distinguishes metabolic acidosis is therefore due to two separate processes: the buffer (from water and carbon dioxide) and additional renal generation. The buffer reactions are:

H⁺ + HCO₃^- = H₂CO₃ = CO₂ + H₂O

The Henderson-Hasselbalch equationmathematically describes the relationship between blood pH and the components of the bicarbonate buffering system:

pH = 6.1 + log (HCO₃^-/H₂CO₃)

In clinical practice, H₂CO₃ can be calculated by using H₂CO₃ = pCO₂ x 0.03.

Treatment

A pH under 7.1 is an emergency, due to the risk of cardiac arrhythmias, and may warrant treatment with intravenous bicarbonate. Bicarbonate is given at 50-100 mmol at a time under scrupulous monitoring of the arterial blood gas readings. This intervention however, is not effective in case of lactic acidosis.

If the acidosis is particularly severe and/or there may be intoxication, consultation with the nephrologyteam is considered useful, as dialysismay clear both the intoxication and the acidosis.

References

• Clinical Physiology of Acid-Base and Electrolyte Disorders by Rose, Post
• Intensive Care Medicine by Irwin and Rippe
• The ICU Book by Marino

This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Metabolic+acidosis Wikipedia article Metabolic acidosis.