Bicarbonate Level (HCO3^–)

Sample for Bicarbonate Level (HCO3-)

1. It is done on the patient’s serum or plasma (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.

Indications for Bicarbonate Level (HCO3^–)

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

Definition of the Bicarbonate Level (HCO3^–):

1. This is the second most plasma anion after chloride.
2. As an index bicarbonate ions concentration, this measures the total CO2 in the blood (serum).
3. >90% of blood CO2 exists in the ionized HCO3^– form, which is converted to CO2 by adding a standard amount of acid in the serum.
4. Arterial blood has less CO2 than venous blood.
5. For the result’s uniformity, total CO2 is done on venous blood serum where the normal range is 19 to 25 meq/L.
6. The CO2 contents measure H2CO3, dissolved CO2, and the HCO3^– anions.
   1. H2CO3 and dissolved CO2 contents in the blood are so small that CO2 contents are an indirect measure of HCO3^– anions.
7. HCO3^– Play a major role in the acid-base balance.

Pathophysiology of Bicarbonate Level (HCO3^–):

1. Bicarbonate is the most important buffer system in the blood, maintaining 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 (ECF)
            1. HCO3^– = 12  meq/L (ICF)
         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 measures a metabolic (Kidney) component of acid-base balance.
   3. The kidney easily regulates bicarbonate, which excretes excess and retains it when needed.
   4. This buffer pair (HCO3– / H2CO3) operates both in the kidneys and the lungs and this is the 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 (HCO3^–) ions 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 measures 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.

Kidney role in acid-base:

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

Lung’s role in acid-base balance:

1. Lungs compensate by increased or decreased blow-off of CO2 and this will change the pH.

NORMAL Bicarbonate Level (HCO3^–)

• 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, which 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 the following:

1. Breathing disorders (compensated respiratory acidosis)
2. Cushing syndrome
3. Excessive vomiting
4. Hyperaldosteronism
5. Ingestion of excessive amounts of antacids, 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.

Respiratory acidosis:

• There is an absolute CO2 excess that results in decreased pH, increased pCO2, and a base deficit.

Respiratory alkalosis:

• There is an absolute CO2 deficit that results in increased pH and decreased pCO2 and base excess.

Metabolic acidosis:

• There is an absolute HCO3 deficit resulting in decreased pH and HCO3.

Metabolic alkalosis:

• There is absolute HCO3 excess, resulting in increased pH and HCO3 levels.

Acid-base balance values in different conditions:

	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

Panic values:

Clinical parameter	Panic value
pH	<7.25   or  >7.55
pO2	<50 mm Hg
pCO2	>60 mm Hg

Summary of the parameters needed for the acid-base balance:

Lab test	Importance
pH	This will tell: Increased pH value indicates alkalosis Decreased value of pH indicates acidosis
pCO2	This is the partial pressure of CO2, and it will tell: The respiration modulates this pCO2 This is the index of ventilation
pO2	This is the partial pressure of the O2 in the arterial blood and tell: Low values indicate hypoxia pO2 is the indirect measure of O2 contents of arterial blood.

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