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Metabolic acidosis has three main root causes: increased acid production, loss of bicarbonate, and a reduced ability of the kidneys to excrete excess acids. [5] Metabolic acidosis can lead to acidemia, which is defined as arterial blood pH that is lower than 7.35. [6]
The magnitude of this difference (i.e., "gap") in the serum is calculated to identify metabolic acidosis. If the gap is greater than normal, then high anion gap metabolic acidosis is diagnosed. The term "anion gap" usually implies "serum anion gap", but the urine anion gap is also a clinically useful measure. [4] [5] [6] [7]
When this happens the numerator is large, the denominator is small, and the result is a delta ratio which is high (>2). This means a combined high anion gap metabolic acidosis and a pre-existing either respiratory acidosis or metabolic alkalosis (causing the high bicarbonate) – i.e. a mixed acid–base metabolic acidosis. [citation needed]
The most common causes of high anion gap metabolic acidosis are: ketoacidosis, lactic acidosis, kidney failure, and toxic ingestions. [3]Ketoacidosis can occur as a complication of diabetes mellitus (diabetic ketoacidosis), but can occur due to other disorders, such as chronic alcoholism and malnutrition.
Urine NH 4 + is difficult to measure directly, but its excretion is usually accompanied by the anion chloride. A negative urine anion gap can be used as evidence of increased NH 4 + excretion. In a metabolic acidosis without a serum anion gap: A positive urine anion gap suggests a low urinary NH 4 + (e.g. renal tubular acidosis).
Hyperparathyroidism – can cause hyperchloremia and increase renal bicarbonate loss, which may result in a normal anion gap metabolic acidosis. Patients with hyperparathyroidism may have a lower than normal pH, slightly decreased PaCO2 due to respiratory compensation, a decreased bicarbonate level, and a normal anion gap. [3]
An elevated anion gap metabolic acidosis and ketosis is the classic present. [3] However, a mixed acid-base disorder may be present especially if vomiting is contributing to a hypochloremic alkalosis. [2] The ketone which is present is mostly beta-hydroxybutyrate rather than acetoacetate resulting in only a weakly positive nitroprusside test. [2]
In general, the cause of a hyperchloremic metabolic acidosis is a loss of base, either a gastrointestinal loss or a renal loss [citation needed]. Gastrointestinal loss of bicarbonate (HCO − 3) [citation needed] Severe diarrhea (vomiting will tend to cause hypochloraemic alkalosis) Pancreatic fistula with loss of bicarbonate rich pancreatic fluid