Acid Base Disorders

The accurate interpretation of laboratory tests in patients with acid-base disorders is critical for understanding path physiology, making a diagnosis, planning effective treatment and monitoring progress. This is an important topic particularly for junior medical staff who may encounter acidbase problems outside normal working hours when patients become acutely unwell. These clinical situations may be a source of confusion particularly because of the variety of terms used to describe and classify acid-base disorders. In this article, we aim to provide the reader with an overview of the key concepts necessary for developing a good working understanding of acid-base disorders that commonly present in clinical medicine.

The lungs help maintain acid-base balance in the body by maintaining external respiration (gas exchange in the lungs) and internal respiration (gas exchange in the tissues). Oxygen collected in the lungs is transported to the tissues by the circulatory system, which exchanges it for the carbon dioxide produced by cellular metabolism. Because carbon dioxide is 20 times more soluble than oxygen, it dissolves in the blood, where most of it forms bicarbonate (base) and smaller amounts form carbonic acid (acid).

We start with some acid-base disorder definitions and then provide the explanation.

Acidaemia:   An arterial pH below the normal range (pH<7.35)

Alkalaemia:  An arterial pH above the normal range (pH>7.45).

Acidosis:  A process lowering pH. This may be caused by a fall in serum bicarbonate and/or a rise in the partial pressure of carbon dioxide (PaCO2 .

Alkalosis:  A process raising pH. This may be caused by a rise in serum bicarbonate and/or a fall in PaCO2.

Acid Base Disorders and ABG findings:

Common Acid Base Disorders Includes, Respiratory acidosis, Respiratory alkalosis, Metabolic acidosis, Metabolic alkalosis.

Respiratory acidosis:

• (excess carbon dioxide retention)
• pH <7.35
• HCO3–> 26 mEq/L (if compensating)
• PaCO2 > 45 mm Hg

Causes:

• Central nervous system depression due to drugs, injury, or disease

• Asphyxia

• Hypoventilation due to pulmonary, cardiac, musculoskeletal, or neuromuscular disease

Respiratory alkalosis

• (excess carbon dioxide excretion)
• pH > 7.45
• HCO3 – <22 mEq/L (if compensating)
• PaCO2 < 35 mm Hg

Causes:

• Hyperventilation due to anxiety, pain, or improper ventilator settings

• Respiratory stimulation due to drugs, disease, hypoxia, fever, or high room temperature

• Gram-negative bacteremia

Metabolic acidosis

• (HCO3– loss, acid retention)
• pH < 7.35
• HCO3 – <22 mEq/L
• PaCO2 < 35 mm Hg (if compensating)

Causes:

HCO3– depletion due to diarrhea

• Excessive production of organic acids due to hepatic disease, endocrine disorders, shock, or drug intoxication

• Inadequate excretion of acids due to renal disease

Metabolic alkalosis

• (HCO3- retention, acid loss)
• pH  >7.45
• HCO3- > 26 mEq/L
• PaCO2> 45 mm Hg (if compensating)

Causes:

• Loss of hydrochloric acid due to prolonged vomiting or gastric suctioning

• Loss of potassium due to increased renal excretion (as in diuretic therapy) or steroids

• Excessive alkali ingestion