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In medicine, metabolic acidosis is a process which if unchecked leads to acidemia, i.e. blood pH is low (less than 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 coma and death. Together with respiratory acidosis, it is one of the two general causes of acidemia.
Symptoms are aspecific, and diagnosis can be difficult unless the patient presents with clear indications for arterial blood gas sampling. Symptoms may include chest pain, palpitations, headache, altered mental status, decreased visual acuity, nausea, vomiting, abdominal pain, altered appetite (either loss of or increased) and weight loss (longer term), muscle weakness and bone pains. Those in metabolic acidosis may exhibit deep, rapid breathing called Kussmaul respirations which is classically associated with diabetic ketoacidosis. Rapid deep breaths increase the amount of carbon dioxide exhaled, thus lowering the serum carbon dioxide levels, resulting in some degree of compensation. Over compensation via respiratory alkalosis to form an alkalemia does not occur.
Extreme acidemia leads to neurological and cardiac complications:
Physical examination occasionally reveals signs of disease, but is otherwise normal. Cranial nerve abnormalities are reported in ethylene glycol poisoning, and retinal edema can be a sign of methanol (methyl alcohol) intoxication. Longstanding chronic metabolic acidosis leads to osteoporosis and can cause fractures.
Arterial blood gas sampling is essential for the diagnosis. The pH is low (under 7.35) and the bicarbonate levels are decreased (<24 mmol/l). Due to respiratory compensation (hyperventilation), carbon dioxide is decreased and conversely oxygen is increased. An ECG can be useful to anticipate cardiac complications. Other tests that are relevant in this context are electrolytes (including chloride), glucose, renal function and a full blood count. Urinalysis can reveal acidity (salicylate poisoning) 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. Anion gap = ( [Na+] ) – ( [Cl-]+[HCO3-] )
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