So yesterday I went home with a headache, photophobia (light sensitivity), and chills. No VapoRub or Windex, but I feel better today; however, my drive to blog has been suppressed. Since I have to present to first year medical students tomorrow, this post will cover the same topic: the use of the anion gap in metabolic acidosis.
The body tightly regulates acid-base balance with short and long-term adjustment processes. The bicarbonate buffering system, which consists of CO2, water, and H+, provides the most important source of regulation.
Bicarbonate is generated by the kidney, H+ and water are excreted by the kidney, and CO2 can be excreted by the lungs. This equilibrium can be thought of as the balance between renal (kidney) and pulmonary (lung) processes.
Normal metabolic processes generate approximately 1 mEq/kg body weight per day of H+ that must be eliminated from the body or buffered and neutralized. Ingestion or generation of excess H+ or abnormal losses of bicarbonate may result in acid buildup. This process is called a metabolic acidosis. It can be easily diagnosed on a blood gas. In a patient with no pulmonary history, it may be suspected when the total CO2 (a measure of bicarbonate) on the electrolyte panel is low.
Blood is an electroneutral fluid. As part of the extracellular space, the primary cation in the serum is sodium. It must be balance by anions including chloride, bicarbonate, and a number of unmeasured substances. In normal states, plasma proteins constitute the bulk of the unmeasured anions. The anion gap is a measure of these unmeasured anions and can be calculated by subtracting the measured anions from the measured cation [Na – (Cl + Bicarbonate)] = Anion Gap. The anion gap is normally 12±4.
It is useful to classify metabolic acidosis by the anion gap. If the gap is increased, there are “new” unmeasured anions in the blood, either from the ingestion or production of an acid that has dissociated into H+ and its anion. Because the acid is electroneutral, the chloride remains normal.
If the kidney is unable to excrete H+, then bicarbonate is consumed and levels drop. Similar biochemical events result if bicarbonate is wasted, either from the gastrointestinal tract or via the kidney. In either of these cases, there is no “new” anion in the blood to electrically balance Na+, so chloride increases. This keeps the anion gap in the normal range.
I have demonstrated these types of acidosis in the video below:
High gap acidosis occurs with ingestion of drugs or toxins, or dysregulation of metabolism such as diabetic ketoacidosis or organic acidemia. Normal gap acidosis occurs with diarrhea, other forms of intestinal bicarbonate losses, and renal tubular acidosis.