Bicarbonate (HCO₃⁻) Level

A bicarbonate level measures amount of carbon dioxide in the blood and assists in identification and treatment of acid–base imbalance in respiratory and metabolic systems. Bicarbonate helps to maintain a balance in the pH levels of the body. The lungs and kidneys work to adjust bicarbonate, carbon dioxide, and carbonic acid levels in the case of respiratory alkalosis or metabolic acidosis.

Primary hypobicarbonatemia (low HCO₃⁻ level) is brought about by any combination of three mechanisms: (1) overproduction of acids, which causes replacement of NaHCO₃ by the Na⁺ salt of the offending acid (e.g., lactate replaces HCO₃⁻ in lactic acidosis); (2) loss of NaHCO₃ through diarrhea along with kidney retention of dietary NaCl, which causes hyperchloremic metabolic acidosis; and (3) generalized kidney disease or specific forms of renal tubular acidosis, which cause retention of acids that are normally produced by intermediary metabolism or by urinary excretion of alkali (Table 14.6).

Hyperbicarbonatemia (high HCO₃⁻ level) with sustained increases in HCO₃⁻ levels is brought about by a source of new alkali or by the presence of factors that stimulate kidney retention of excess HCO₃⁻. These mechanisms include excessive GI loss of acid exogenous alkali in persons whose kidneys avidly retain NaHCO₃ and renal synthesis of HCO₃⁻ in excess of daily consumption. Other pathophysiologic factors that affect kidney reabsorption of >25 mEq/L (>25 mmol/L) of HCO₃⁻ and contribute to sustained hyperbicarbonatemia include extracellular fluid volume contraction, hypercapnia, hypokalemia, hyperaldosteronemia, and hypoparathyroidism.

1. Obtain an arterial or venous blood sample (see Chapter 2 for venous and arterial blood collection).

2. Label the sample with patient’s name, identification number, date, time, mode of O₂ therapy, and flow rate.

3. Place the arterial sample on ice and transfer it to the blood gas laboratory.

4. Observe standard precautions.

1. Metabolic acidosis is associated with the following conditions:

   1. Uncontrolled diabetes

   2. Diarrhea and vomiting

   3. Renal tubular acidosis

   4. Lactic acidosis

   5. Poisoning

2. Metabolic alkalosis is associated with the following conditions:

   1. Dehydration or hypovolemia

   2. Excess of corticosteroids (hyperaldosteronism and Cushing syndrome)

   3. Antacid and laxative abuse or overuse

   4. Diuretic therapy

Pretest Patient Care

1. Explain test purpose and procedure.

2. Follow guidelines in Chapter 1 for safe, effective, informed pretest care.

1. Drugs and supplements may affect bicarbonate levels.

2. Eating fruits high in acid may affect bicarbonate levels.

Normal

• Adults: 22–32 mmol/L

• Children (2–10 years): 17–30 mmol/L)