Acidosis and Alkalosis

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Testing is used to identify whether a patient has an acid-base disorder, to determine how severe the disorder is, and to help diagnose underlying diseases such as diabetic ketoacidosis or poisoning. Testing is also done to monitor critically ill patients, as well as patients with long term diseases of acid-base balance, such as chronic lung disease and kidney disease. The primary tests used to identify, evaluate, and monitor acid-base disorders are blood gases and electrolytes.

Blood gases are a group of tests performed together on an arterial blood sample (blood obtained from an artery instead of a vein). They are a snapshot of the blood’s pH, the amount of oxygen dissolved in the blood (PO2), and the amount of carbon dioxide dissolved in the blood (PCO2). Using these results the amount of bicarbonate in the blood (HCO3-) can be calculated. The sample must be arterial to get an accurate measurement of oxygen and carbon dioxide as the concentration of these gases change as the blood moves through the body tissues. This means the sample has to be taken from an artery, usually at the wrist, which can be slightly more painful than normal venous blood sampling (venepuncture) as arteries have nerves and muscles in the vessel walls. Arteries can also spasm when irritated making the procedure slightly more likely to fail and strong pressure for a couple of minutes must be exerted afterwards to prevent bruising, this being much more important than in normal venepuncture.

Results seen:

Acid-Base DisorderpHHCO3-PCO2Body Compensation
Metabolic acidosis Less than 7.35 Low Low Increased breathing rate (hyperventilation) to increase CO2 elimination
Metabolic alkalosis Greater than 7.45 High High Slowed breathing (hypoventilation) to decrease CO2 elimination
Respiratory acidosis Less than 7.35 High High Kidney increases production of HCO3- and excretion of acid
Respiratory alkalosis Greater than 7.45 Low Low Kidney decreases production of HCO3- and excretion of acid

Electrolytes refers to a group of four tests: Na+ (sodium), K+ (potassium), Cl- (chloride) and bicarbonate (total carbon dioxide content).

An anion gap can be calculated from the electrolytes and provides a clue to the cause of the acid/base imbalance.

Depending on the suspected cause, a number of other tests may be requested: Renal profile, glucose, ketones in blood and ketones in urine, lactate, aspirin (salicylate), ethylene glycol, and methanol, to name a few.

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