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Acid-Base Balance:- Part 4 – Arterial Blood gases (Blood Gases)

June 2, 2023Chemical pathologyLab Tests2

Table of Contents

  • Arterial Blood Gases
      • Sample for arterial blood gases
      • Purpose of the test (Indications) for Arterial Blood gases
      • Precautions for the collection of blood for Arterial Blood gases:
      • Venous blood  gases (VBG)
      • Arterial vs. Venous blood
      • Arterial blood gases (ABG):
      • Arterial blood gases introduction:
  • Arterial blood gases
        • Definition of arterial blood gases:
    • Blood arterial gases parameters:
    • The pH of the blood
    • pCO2 (Partial pressure of the carbon dioxide CO2)
    • HCO3 or CO2 content
    • pO2
    • O2 saturation
    • O2 content
      • Normal electrolytes and blood gases
      • Advise the following Arterial blood gases:
      • Interpretations and role of arterial gases in the acid-base balance:
        • Normal picture =  pH normal,  PCO2 normal, HCO3 normal.
    • Respiratory acidosis
    • Metabolic acidosis
    • Respiratory alkalosis
    • Metabolic Alkalosis
    • Interpretation of the various parameters:
      • Urine pH:
    • Anion gap
      • Calculation of Anion gap = Na (140 ) + K (4) — Cl (110 ) + HCO3(24) = 10 meq/L
      • Critical values
      • Signs and symptoms summary of the acid-base system changes:
      • Questions and answers:

Arterial Blood Gases

Sample for arterial blood gases

  1. The better choice is the Radial artery.
    1. It can draw the blood from the femoral artery or brachial.
    2. Blood can be drawn from the indwelling arterial line.
  2. The tests are done immediately because oxygen and carbon dioxide are unstable.
    1. Place the sample on ice and immediately transfer it to the lab.
  3. Arterial blood is better than venous blood.
  4. For venous blood syringe or tube is filled, and apply a tourniquet for a few seconds.
  5. Arterial blood is risky, and a trained person should do it.
    1. Never apply a tourniquet.
    2. Don’t apply the pull to the plunger of the syringe.

Purpose of the test (Indications) for Arterial Blood gases

  1. This test is done on mostly hospitalized patients.
  2. Mostly the patients are on a ventilator or unconscious.
    1. It monitors critically ill nonventilator patients.
  3. For patients in pulmonary distress.
  4. To assess the respiratory (ventilation), metabolic (renal) acid/base, and electrolyte imbalance.
  5. Its primary use is to monitor arterial blood gases and the pH of the blood.
  6. Also used to monitor oxygenation.
  7. Used to qualify a patient for the use of oxygen at home.
  8. This is used as preoperative baseline parameters.

Precautions for the collection of blood for Arterial Blood gases:

  1. Avoid pain and anxiety in the patient, which will lead to hyperventilation.
    1. Hyperventilation due to any cause leads to decreased CO2 and increased pH.
  2. Keep blood cool during transit.
  3. Don’t clench your finger or fist. This will lead to lower CO2 and increased acid metabolites.
  4. pCO2 values are lower in the sitting or standing position than in the supine position.
  5. Don’t delay the performance of the test.
  6. Avoid air bubbles in the syringe.
  7. Excess of heparin decreases the pCO2 by maybe 40% less.
  8. Not properly mixing the blood before running the test may give a false result.
  9. A prolonged tourniquet or muscular activity decreases venous pO2 and pH.
  10. The best way to collect arterial or venous blood is anaerobic.
  11. Arterial blood precautions:
    1. Only syringe and needle, no tourniquet, no pull on the plunger.
  12. Venous blood precautions:
    1. The needle and syringe of the heparinized evacuated tube filled, drawn a few seconds after the
      tourniquet.
    2. Liquid heparin is the only suitable anticoagulant with the proper amount.
      1. Less amount will lead to clot formation.
      2. The increased amount will lead to increased CO2 and a decrease in pH.
      3. This will lead to a dilutional error.
    3. Glass collection devices are better than plastic.
  13. Avoid in patients with coagulopathy.
  14. Avoid in a patient with AV fistula.

Venous blood  gases (VBG)

  1. It gives information about the local area from where the blood sample is taken.
    1. Venous blood color is dark red.
    2. Metabolism of the extremity varies from area to area.
    3. In shock,  the extremities are cold, and less blood perfusion.
    4. During the local exercise of the extremities, such as opening and closing the fist with power.
    5. In case there is an infection of the sample area.
  2. A blood sample from the central venous catheter is not a good mix of blood from various body parts.  For well-mixed blood sample should be taken from the right ventricle or the pulmonary artery, which
    is not an easy procedure.
  3. A blood sample from the central venous catheter:
    1. Shows low O2 concentration, which means that:
      1. Either the lungs have not oxygenated the arterial blood well.
      2. Or the Heart is not circulating the blood effectively.

Arterial vs. Venous blood

Difference between arterial and venous blood:

Biochemical parameters Arterial blood Venous blood
  • Use (Purpose)
  • For blood gases
For all routine lab test
  • Color
  • Bright red
Dark red
  • pH
  • 7.35 to 7.45 (7.40)
  • 7.32 to 7.43  (7.37)
  • pCO2 mmHg
  • 35  to 45 (40)
  • 41  to  51 (45)
  • CO2 contents  meq/L
  • 22 to 28 (25)
  • 24 to 30 (27)
  • Bicarbonate (HCO3–)
  1. 22 to 28 mmol/L
  2. 20 to 28 meq/L
  1. 23 to 29 mmol/L
  2. 22 to 30 meq/L
  • pO2mmHg
  • 80 to 100
  • 37 to 43 (40)
  • O saturation
  • 95%
  • 70 to 75%

Arterial blood gases (ABG):

  1. It gives a good mixture of blood from various areas of the body.
  2. Arterial blood color is bright red.
  3. Arterial blood measurement gives a better status of lung oxygenation.
    1. If arterial O2 concentration is normal, it indicates lung function is normal.
    2. If mixed venous O2 concentration is low, indicating the heart and circulation are failing.
  4. Arterial blood gives information about the lung’s ability to regulate the acid-base balance through the
    retention or release of CO2.

    1. Can also be checked the effectiveness of the kidneys in maintaining the appropriate bicarbonate
      level.

Arterial blood gases introduction:

  1. Arterial blood gases are a common test that provides a useful and potentially life-saving treatment.
  2. The following parameters are measured, and the rest are calculated:
    1. pH will give us information about the acid-base balance.
    2. pO2 will tell us about oxygenation (ventilation).
    3. pCO2 will also tell us about the acid-base balance.
      1. pH = 7.35 to 7.45.
      2. pO2 = >80 mm Hg.
      3. pCO2 = 35 to 45 mm Hg.
  3. The above three parameters are needed to help the patient’s management.

Arterial blood gases

Definition of arterial blood gases:

  1. Blood arterial gases measure the balance between Oxygen (o2) and carbon dioxide (CO2), giving information about the function of the lungs.
  2. It also measures the acid-base balance in the body.
  3. Lunga and kidneys work together to keep the acid-base balance.
  4. Blood arterial gases measure various gases in the arterial blood. It will tell the metabolic and respiratory status.
  5. It includes:
    1. The partial pressure of oxygen (paO2).
    2. Oxygen saturation (
    3. The partial pressure of carbon dioxide (paCO2).
    4. Bicarbonate level (HCO3–).
    5. pH level.

Blood arterial gases parameters:

The pH of the blood

  1. The pH of a solution is the negative logarithm of the hydrogen ion activity.
        1. pH = -logaH+.
  2. The acid-base status of the body is assessed by:
        1. pH.
        2. pCO2.
  3. While blood passes through the lung, O2 moves to the blood, and CO2 enters the lung.
Role of the lungs in Acid-base balance

Role of the Lungs in Acid-base Balance

  1. As the blood hydrogen concentration increases, the pH decreases, and if hydrogen ions decrease, the pH increases.
    1. The decrease of one pH unit represents a 10 times increase in H+ activity.
  2. The average pH of the blood of 7.40 is equal to the H+ ions concentration of 40 nmol/L.
    1. The lungs and the kidneys regulate the pH of the plasma.
Acid-base metabolism control

Acid-base metabolism control

  1. The acids found in the blood are carbonic acid (H2CO3), dietary acids, keto acid, and lactic acid.
  2. pH indicates acidity and alkalinity.
  3. In respiratory or metabolic alkalosis, the pH will be high.
  4. Respiratory acidosis or metabolic acidosis will decrease the pH value.
    1. pH alkaline when it is >7.4.
    2. pH is acidic when it is <7.35.
    3. Acidemia = pH <7.35
    4. Alkalemia = pH >7.45

pCO2 (Partial pressure of the carbon dioxide CO2)

  1. pCO2 measures the partial pressure of CO2 gas in the blood (arterial blood, plasma, or serum) measured in mm Hg.
    1. This is proportional to the amount of dissolved CO2 or the concentration of the CO2.
    2. pCO2 is a measurement of ventilation capability.
    3. The units are the same as pO2.
  2. pCO2 in the blood is 10% in the plasma and 90% carried by the red blood cells.
  3. With respiration, CO2 is breathed out, and the pCO2 level drops will depend on the breathing rate.
  4. The faster and more deeply one breaths, the more CO2 is blown off, and the pCO2 level drop.
Acid-base balance: Role of breathing and pCO2

Acid-base balance: Role of breathing and pCO2

  1. pCO2 is referred to as the respiratory component in acid-base determination because the lungs control this value.
  2. As the CO2 level increases, the pH level will decrease.
  3. The  pCO2 level in blood and CSF is a major stimulant to the breathing center in the brain.
  4. As the pCO2 level rises, breathing is stimulated.
  5. When the brain can not cope with increased pCO2 and cannot blow off excess CO2, the brain is depressed, ventilation decreases, and the patient goes into a coma.
  6. In metabolic acidosis, the lungs try to compensate by more blowing of CO2 to raise pH.
  7. In metabolic alkalosis, the lungs try to compensate by retaining the CO2 to lower pH.

HCO3 or CO2 content

Method to measure CO2 contents:

  1. The total CO2 contents are determined from the heparinized arterial or venous blood drawn anaerobically.
    1. This can be done in a vacuum tube or a syringe which is quickly capped.
    2. The blood is centrifuged, and the plasma is separated.
    3. Next, plasma is analyzed for CO2 by converting HCO3– and H2CO3 to the gas form.
  2. HCO3– and CO2:
  3. Most of the CO2 contents are HCO3¯ in the blood because the dissolved amount of the CO2 and H2CO3 contents are very small.
    1. The HCO3– ions can be measured directly as HCO3– or indirectly by CO 2 contents.
  4. Total CO2 =  HCO3¯ + Dissolved CO2.
  5. The most important buffer system of the plasma is HCO3¯ / H2CO3.
  6. It is also present in the RBC but at a lower concentration.
  7. The ratio of base: acid = 20: 1 in plasma.
  8. The kidney regulates HCO3¯ ions, and it is the measure of the metabolic (Renal) component of the acid-base balance.
  9. CO2 contents should not be confused with pCO2.
  10. CO2 contents are indirectly measured by HCO3¯.
    1. HCO3¯  : Dissolved CO2 =  25 : 1
    2. Any change in the above equation leads to a change in the pH.
    3. As the HCO3¯ level increase and the pH also increases.
  11. The HCO3¯ level:
  12. In metabolic alkalosis, the HCO3– level is elevated.
    1. In metabolic acidosis, the HCO3– level is decreased.
  13. In respiratory acidosis, kidneys attempt to compensate for increased reabsorption of HCO3¯.
    1. In respiratory alkalosis, kidneys excrete more HCO3¯ to lower the pH.
 Clinical conditions pH Bicarbonate (HCO3) level
Metabolic acidosis decreased decreased
Respiratory acidosis decreased increased
Metabolic alkalosis increased increased
Respiratory alkalosis increased decreased

pO2

  1. Oxygen in the blood is carried in two forms:
    1. Dissolved in plasma = <2%.
    2. Combined with hemoglobin = 98%.
  2. This partial pressure of the oxygen gas determines the force it exerts in attempting to diffuse through the pulmonary membrane.
    1. The pO2 reflects the amount of oxygen passing from the pulmonary alveoli to the blood.
  3. pO2 is the measure of the pressure of O2 present in the plasma.
    1. pO2 is the indirect measure of O2 contents of arterial blood.
  4. The pO2 level is decreased in the following ways:
    1. Pneumonia.
    2. Shock lung.
    3. Congestive heart failure.
    4. Congenital heart diseases.
    5. Patient under-ventilated.

O2 saturation

  1. O2 saturation indicates % of hemoglobin saturated with oxygen. OR:
    1. This measurement is the ratio between the actual O2 content of the hemoglobin and the potential maximum carrying capacity of the hemoglobin.
    2. O2% saturation is the percentage indicating the relationship between O2 and hemoglobin.
    3. This is not the O2 content.
  2. The combined measurement of:
    1. O2 saturation.
    2. pO2.
    3. Hemoglobin.
    4. This indicates the amount of O2 available to the tissues for oxygenation.
  3. When hemoglobin 92 to 100% carries O2, then perfusion or oxygen supply to the tissue is normal.
  4. With the decrease of the pO2 level, the saturation of hemoglobin also decreases.
  5. When the O2 saturation is 70% or low, the tissues cannot get adequate oxygen.
  6. Precaution:
    1. Please avoid smoking or exposure to close second-hand smoke or CO (carbon monoxide). In such cases, the COHb level is increased.
    2. Avoid the use of paint or varnish.

O2 content

  1. The actual amount of O2  in the blood is termed the O2 content.
    1. Normally all O2 is bound to hemoglobin.
  2. About 98% of all O2 delivered to the tissue is transported in combination with the hemoglobin.
  3. The following formula calculates O2 contents:
      • O2 content = O2 saturation x Hb x 1.34 + pO2 × 0.003

Normal electrolytes and blood gases

Source 1

pH

  • Adult / child = 7.35 to 7.45
  • Newborn = 7.32 to 7.49
  • 2 months  to 2 years = 7.34 to 7.46
  • pH venous blood = 7.31 to 7.41
    Body fluids pH
    • Arterial blood
    • 7.38 to 7.42
    • Venous blood
    • 7.37
    • Cerebrospinal fluid
    • 7.32
    • Pancreatic fluid
    • 7.8 to 8.0
    • Gastric juice
    • 1.0 to 3.0
    • Urine
    • 5 .0 to 6.0

pCO2

  • Adult /child = 35 to 45 mm Hg
  • Child <2 years = 26 to 41 mm Hg
    • Venous blood = 40 to 50 mm Hg

HCO3

  • Adult / child = 21 to 28 mEq/L
  • Newborn / infants =16 to 24 mEq/L

pO2

  • Adult/child = 80 to 100 mm Hg
  • Newborn = 60 to 70 mm Hg
    • Venous blood = 40 to 50 mm Hg

O2 saturation

  • Adult/child = 95 to 100%
  • Old people = 95%
  • Newborn = 40 to 90%

O2 content

  • Arterial blood = 15 to 22 vol%
  • Venous blood = 11 to 16 vol%

Source 2

 Chemicals Arterial Venous
pH 7.35 to 7.45 7.31 to 7.41
pCO2 35 to 45 mm Hg 40 to 50 mm Hg
pO2 75 to 100 mm Hg 40 to 50 mm Hg
O content 15 to 22 % 11 to 16 %
HCO3 21 to 28 meq/L
Anion Gap 3 to 10

Source 3

Normal Values of Analytes 

Lab test Blood Venous Arterial
pH 7.32 to 7.43 7.35 to 7.45
Bicarbonate (HCO3) 22 to 26 mmol/L
Albumin 3.5 to 5.0 G/dL
pCO2 Male = 35 to 48 mm HgFemale = 32 to 45 mm Hg
Anion Gap 3 to 10
Oxygen saturation O2 94 to 98% (decrease with age)
O2 content 11 to 16% 15 to 22%
pO2 80 to 108 mm Hg (depends on altitude)

Advise the following Arterial blood gases:

Biochemical parameter Adult Pediatric group
pH 7.35 to 7.45 7.32 to 7.42
pCO2 35 to 45 mm Hg 30 to 40 mm Hg
pO2 >80  mm Hg 80 to 100 mm Hg
O2 saturation >94%
CO2 content 45 to 51 vol% (19.3 to 22.4 mmol/L)
O2 content 15 to 22 vol% (6.6 to 9.7 mmol/L)
Base Excess >2 meq/L (>2 mmol/L)
Base deficit < – 2 meq/L  (< – 2 mmol/L)
HCO3 22 to 26 meq/L (22 to 26 mmol/L)

Interpretations and role of arterial gases in the acid-base balance:

Normal picture =  pH normal,  PCO2 normal, HCO3 normal.

  • 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

  1. pH and HCO3– go in the opposite direction.
  2. pH lower, pCO2 high, HCO3– high.
  3. Seen in respiratory depression due to any cause.
    1. Hypoventilation.
    2. Excessive retention of CO2.
Acid-base balance: Respiratory acidosis and compensatory mechanism

Acid-base balance: Respiratory acidosis and compensatory mechanism

Metabolic acidosis

  1. pH and HCO3– go in the same direction.
  2. pH low, pCO2 low,  HCO3– low.
  3. Seen in diabetes, shock, renal failure, and an intestinal fistula.
Metabolic acidosis changes and findings

Arterial Blood gases: Metabolic acidosis changes and findings

Acid-base balance: Metabolic acidosis and compensatory mechanism

Acid-base balance: Metabolic acidosis and compensatory mechanism

Respiratory alkalosis

  1. pH and HCO3– go in the opposite direction.
  2. pH is high.
  3. pCO2 low.
  4. HCO3– is normal or slightly decreased.
    1. Seen in hyperventilation.
    2. Excessive loss of CO2.
Arterial Blood gases: Respiratory alkalosis and compensatory mechanism

Arterial Blood gases: Respiratory alkalosis and compensatory mechanism

Metabolic Alkalosis

  1. pH and HCO3– go in the same direction.
  2. HCO3 is >30 meq/L.
  3. pH high,  pCO2 high,  HCO3– high.
  4. Urine pH >7.0 (Unless there is severe hypokalemia).
  5. Serum K is usually low.
  6. Seen in sodium bicarbonate overdose, prolonged vomiting, and nasogastric drainage.
Acid-base balance: Metabolic alkalosis and compensatory mechanism

Acid-base balance: Metabolic alkalosis and compensatory mechanism

Interpretation of the various parameters:

Urine pH:

  • pH = < than 7.4 = acidosis.
  • pH = > than 7.4 = alkalosis.

pCO2

  1. pCO2 is high = It is respiratory acidosis.
  2. pCO2 is low = It is metabolic acidosis.
  3. pCO2 is low = It is respiratory alkalosis.
  4. pCO2 is high = It is metabolic alkalosis.

HCO3–

  1. HCO3– high = It is in respiratory acidosis.
  2. HCO3–  low = It is in metabolic acidosis.
  3. HCO3–  low = It is in respiratory alkalosis.
  4. HCO3–  high = It is in metabolic alkalosis.

Anion gap

Calculation of Anion gap = Na (140 ) + K (4) — Cl (110 ) + HCO3(24) = 10 meq/L

    • Normal anion gap =   10 to 12 meq/L   = < 12

Table showing the values of pH, HCO3, pCO2, and etiology:

Clinical condition pH HCO3 pCO2 Etiology
Metabolic acidosis <7.4 low low Diabetic ketoacidosis. lactic acidosis.
Metabolic alkalosis >7.4 high high Vomiting
Respiratory acidosis <7.4 high high COPD, weakness of respiratory muscles
Respiratory alkalosis >7.4 low low Anxiety and pain

Critical values

Biochemical parameter Less than More than
pH 7.25 7.55
pCO2 20 mm Hg 60 mm Hg
HCO3 15 meq/L 40 meq/L
pO2 40 mm Hg
O2 saturation 75% or lower
Base Excess ± 3meq/L

Signs and symptoms summary of the acid-base system changes:

  1. Acidosis leads to coma and death due to depression in the CNS.
  2. Alkalosis leads to irritability, tetany, and possible death due to the stimulation of the CNS.
  3. The acidosis state is more threatening than alkalosis.

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

Lab test Importance
  • pH
This will tell:

  1. Increased pH value indicates alkalosis
  2. Decreased value of pH indicates acidosis
  • pCO2
This is the partial pressure of CO2, and it will tell:

  1. The respiration modulates this pCO2
  2. This is the index of ventilation
  • pO2
This is the partial pressure of the O2 in the arterial blood and tells:

  1. Low values indicate hypoxia
  2. pO2 is the indirect measure of O2 contents of arterial blood.

 

Questions and answers:

Question 1: What is the value of pH?
Show answer
Increased pH indicates alkalosis and decreased pH indicates acidosis.
Question 2: What is the critical value of pH?
Show answer
Critical value of pH 7.25 and 7.55 is dangerous for life.

Possible References Used
Go Back to Chemical pathology

Comments

Angelique Breske Reply
September 29, 2020

I agree with you

Charlsie Burruel Reply
October 12, 2020

I agree with you

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