Acidosis - Metabolic Acidosis

Metabolic Acidosis

Metabolic acidosis may result from increased production of metabolic acids or disturbances in the ability to excrete acid via the kidneys. Renal acidosis is associated with an accumulation of urea and creatinine as well as metabolic acid residues of protein catabolism.

An increase in the production of other acids may also produce metabolic acidosis. For example, lactic acidosis may occur from:

1. severe (PaO₂ <36mm Hg) hypoxemia causing a fall in the rate of oxygen diffusion from arterial blood to tissues
2. hypoperfusion (e.g., hypovolemic shock) causing an inadequate blood delivery of oxygen to tissues.

A rise in lactate out of proportion to the level of pyruvate, e.g., in mixed venous blood, is termed "excess lactate", and may also be an indicator of fermentation due to anaerobic metabolism occurring in muscle cells, as seen during strenuous exercise. Once oxygenation is restored, the acidosis clears quickly. Another example of increased production of acids occurs in starvation and diabetic acidosis. It is due to the accumulation of ketoacids (ketosis) and reflects a severe shift from glycolysis to lipolysis for energy needs.

Acid consumption from poisoning such as hypercapnia, elevated levels of iron in the blood, and chronically decreased production of bicarbonate may also produce metabolic acidosis.

Metabolic acidosis is compensated for in the lungs, as increased exhalation of carbon dioxide promptly shifts the buffering equation to reduce metabolic acid. This is a result of stimulation to chemoreceptors, which increases alveolar ventilation, leading to respiratory compensation, otherwise known as Kussmaul breathing (a specific type of hyperventilation). Should this situation persist, the patient is at risk for exhaustion leading to respiratory failure.

Mutations to the V-ATPase 'a4' or 'B1' isoforms result in distal renal tubular acidosis, a condition that leads to metabolic acidosis, in some cases with sensorineural deafness.

Arterial blood gases will indicate low pH, low blood HCO₃, and normal or low PaCO₂. In addition to arterial blood gas, an anion gap can also differentiate between possible causes.

The Henderson-Hasselbalch equation is useful for calculating blood pH, because blood is a buffer solution. The amount of metabolic acid accumulating can also be quantitated by using buffer base deviation, a derivative estimate of the metabolic as opposed to the respiratory component. In hypovolemic shock for example, approximately 50% of the metabolic acid accumulation is lactic acid, which disappears as blood flow and oxygen debt are corrected.

Treatment of uncompensated metabolic acidosis is focused upon correcting the underlying problem. When metabolic acidosis is severe and can no longer be compensated for adequately by the lungs, neutralizing the acidosis with infusions of bicarbonate may be required.