Bicarbonate Level (HCO3-), Acid-Base Balance

Sample

1. It is done on serum or plasma of the patient (plasma arterial or venous can be used).
   • The best anticoagulant is heparin.
2. The test should be done as soon as possible and minimize the time interval between a sample taken and analyzed.
3. Collect samples anaerobically and heparin is the preferred anticoagulant.

Indication

1. It assists in evaluating the pH of the patient.
2. It also assists to evaluate electrolyte balance.
3. The bicarbonate ion is the measure of a metabolic (renal) component of the acid-base equilibrium.

Pathophysiology

1. Bicarbonate is the most important buffer system in the blood which maintains the pH (acid-base balance).
   1. H⁺ + HCO3 ^–↔ H2O + CO2
      1. Buffer pair = HCO3^– / H₂CO3
      2. The ratio = HCO3– / H2CO3  = 20:1
         1. HCO3 = 24 meq/L
         2. Carbonic acid = 1.2 meq/L
         3. Normal pH = 7.4
      3. Correction occurs when the values for both components of the buffer pair (HCO₃ / H₂CO₃) return to normal.
   2. Bicarbonate is the measure of a metabolic (Kidney) component of acid-base balance.
   3. Bicarbonate is easily regulated by the kidney which excretes it in excess and retains when needed.
   4. This buffer pair (HCO3– / H2CO3) operates both in the kidneys and the lungs, and major extracellular buffer.
2. In the body most of the CO2 is in the form of HCO3, So the CO2 level in blood is the measure of HCO3.
   1. The CO2 contents measure H2CO3, dissolved CO2, and the bicarbonate ions HCO3 that is present in the blood.
   2. CO2 is carried in the blood as:
      1. Dissolved in the plasma (pCO2).
      2. As bicarbonate (HCO3).
      3. Carbamino compound.

3. Bicarbonate level is affected by a variety of respiratory and metabolic disturbances which affect acid-base balance.
   1. HCO3 ion is a measure of the metabolic kidney part of the acid-base balance.
4. HCO3 is exchanged for other ions like Chloride and Phosphate to maintain electroneutrality.

5. When the HCO3 level increases, the pH also increases.
6. Kidneys play an important role in the balance of the acid-base system (c0mpensation).
   1. Kidneys compensate by producing more acidic or more alkaline urine.
   2. In respiratory acidosis, the kidney compensates by increased reabsorption of HCO3^–.
   3. In respiratory alkalosis, the kidney compensates by increased excretion of HCO3^–.

7. Lungs compensate by increased or decreased blow off of CO2.

NORMAL

• Arterial blood = 21 to 28 meq/L
• Venous blood = 22 to 29 meq/L
  • Peritoneal fluid = 24 to 29 meq/L
  • Duodenal fluid = 4 to 21 meq/L
  • Pancreatic fluid = 66 to 127 meq/L
• For SI unit multiplication factor is 1, that will be in mmol/L

lower-than-normal levels may be seen in:

1. Addison disease
2. Diarrhea
3. Ethylene glycol poisoning
4. Ketoacidosis
5. Kidney disease
6. Lactic acidosis
7. Metabolic acidosis
8. Starvation.
9. diabetic ketoacidosis.
10. Methanol poisoning
11. Salicylate toxicity (such as aspirin overdose)
12. Liver disease

Higher-than-normal levels may be seen in:

1. Breathing disorders (compensated respiratory acidosis)
2. Cushing syndrome
3. Excessive vomiting
4. Hyperaldosteronism
5. Ingestion of excessive amount of antacid, diuretics, and steroids
6. Severe vomiting.

The following conditions may also alter bicarbonate levels:

1. Alkalosis
2. Delirium
3. Dementia
4. Renal tubular acidosis, distal.
5. Renal tubular acidosis, proximal.

Acid-base balance

• Acidemia means arterial blood pH <7.4.
  • Acidosis means a systemic increase in H+ ions.
• Alkalemia means arterial blood pH >7.4.
  • Alkalosis means a systemic decrease in H+ ions.

1. Respiratory acidosis: There is an absolute CO2 excess that results in decreased pH and increased pCO2 and a base deficit.

1. Respiratory alkalosis: There is an absolute CO2 deficit that results in increased pH and decreased pCO2 and base excess.

1. Metabolic acidosis: There is an absolute HCO3 deficit resulting in decreased pH and HCO3.

1. Metabolic alkalosis: There is absolute HCO3 excess, resulting in increased pH and HCO3 levels.

	HCO3	pCO2	pH	Etiology
Normal values	22 to 26 meq/L	35 to 45 mm Hg	7.35 to 7.45
Metabolic alkalosis	Increased	Normal	increased	Prolonged vomiting, nasogastric drainage or NaHCO3 overdose
Metabolic acidosis	Decreased	Normal	Decreased	Diabetes, intestinal fistula, and kidney disease
Respiratory alkalosis	Decreased	Decreased	Increased	Hyperventilation
Respiratory acidosis	Increased	Increased	Decreased	Drugs causing respiratory depression

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