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  1. The better choice is the Radial artery.
    1. The sample may be taken 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 to the lab.
  3. Arterial blood is better than venous blood.
  4. For venous blood syringe or tubes are completely filled and apply a tourniquet for a few seconds.
  5. Arterial blood is risky and it should be done by the trained person.
    1. Never apply a tourniquet.
    2. Don't apply the pull to the plunger of the syringe.

Arterial VS Venous blood

  1. Arterial blood (ABG)  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 the lung oxygenation.
    1. If arterial O2 concentration is normal, indicate lung function is normal.
    2. If mixed venous O2 concentration is low, indicating heart and circulation are failing.
  4. Arterial blood gives information about the ability of the lung to regulate the acid-base balance through retention or release of CO2.
    1. The effectiveness of the kidneys in maintaining the appropriate bicarbonate level can also be checked.
  5. Venous blood  (VBG) 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, as opening and closing the fist with power.
    5. In case if there is an infection of the sample area.
  6. A blood sample from the central venous catheter is not a good mix of the blood from various parts of the body.  For well-mixed blood sample should be taken from the right ventricle or the pulmonary artery which is not an easy procedure.
  7. A blood sample from the central venous catheter:
    1. Shows low O2 concentration, it means that:
      1. Either the lungs have not oxygenated the arterial blood well.
      2. Or Heart is not circulating the blood effectively.

Precautions for the collection of blood

  1. Avoid pain and anxiety to 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 finger or fist. This will leads to lower CO2 and increased acid metabolites.
  4. pCO2 values are lower in the sitting or standing position in comparison with 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  maybe 40% less.
  8. Not proper mixing of the blood before running the test may give a false result.
  9. A prolonged tourniquet or muscular activity decreases venous pO2 and pH.
  10. Best way to collect arterial or venous blood anaerobically.
  11. Arterial blood precautions:
    1. Only syringe and needle, no tourniquet, no pull on the plunger.
  12. Venous blood precautions:
    1. Needle and syringe of the heparinized evacuated tube completely 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 an increase in CO2 and decrease in pH.
      3. This will leads to dilutional error.
    3. Glass collection devices are better than plastic.

Purpose of the test (Indications)

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


  1. Avoid in patients with coagulopathy.
  2. Avoid in a patient with AV fistula.


  1. The process of respiration supplies oxygen to the tissues and remove the carbon dioxide produced by the cellar metabolic activity.
  2. External respiration:
    1. Where oxygen in the air is exchanged at the alveolar level with carbon dioxide in the blood.
  3. Internal respiration:
    1. Take place at the tissue level, where oxygen in the blood is delivered to the cells and carbon dioxide is transferred from the cells to the blood for the disposal.

  1. The medullary center,  the brain stem controls the respiration by the increased CO2 and decreased O2.

  1. Henderson- Hasselbalch equation give the idea about the blood gas measurement.
    1. Henderson-Hasselbalch equation = pH = pKa + log (base/acid) 

      1. Where pH = 7.4
      2. pKa = 6.1
      3. Base (bicarbonate) = 24 
      4. Acid = disoolved PaCO2 = 0.03 x PaCO2 = 0.03 x 40 = 1.2
        1. pH = 7.4 = 6.1 + log(24/1.2) = 6.1 + 1.3 = 7.4
      5. Body makes every efforts to maintain the validity of the above equation.
      6. Our body will maintain pH 7.4 and pK is constant, so it can alter the bicarbonate and PaCO2.
        1. The body normally maintains the arterial pH between 7.35 to 7.45. This takes place through the buffer system of bicarbonate.
  1. CO2 and water react to form carbonic acid which then dissociates into hydrogen ions and HCO-3.
    1. The pH is dependent upon the total concentration of:
      1. CO2.
      2. HCO-3.
      3. Dissolved CO2.
      4. H+ ions.
        1. All these are inter-related.
      5. Bicarbonate / carbonic acid (HCO3 / H2CO3) is the most important buffering system.
        1. Acids or chemical substances can donate H+ ions.
        2. Bases or substances that can accept H+ ions.
        3. Strong acids readily give up H+, whereas strong base readily accepts H+.
      6. Respiratory and metabolic disorder depends on the correct measurement of:
        1. O2
        2. CO2
      7. Acid/base assessed by:
        1. Total CO2
        2. Plasma pH
        3. pCO2

  1. Common acid/base disorder the example is:
    1. Lactic acidosis and diabetic ketoacidosis.
      1. Intermediate organic acids are lactic acid and β-hydroxybutyric acid.
      2. These above acids metabolized to CO2 and water.
      3. These may accumulate and cause acidemia.


  1. The pH of a solution is the negative logarithm of the hydrogen ion activity.
    1. pH = -logaH+.
  2. Acid-base status of the body is assessed by:
    1. pH.
    2. pCO2.
  3. While blood passing through the lung, O2 moves to blood and CO2 goes into the lung.


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

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


  1. pCO2 is the measure of the partial pressure of CO2 in the blood.
    1. pCO2 is a measurement of ventilation capability.
  2. pCO2 in the blood is 10% in the plasma and 90% carried by the red blood cells.
  3. With the respiration, CO2 is breathed out and pCO2 level drops will depend upon the breathing rate.
    1. The faster and more deeply one breaths, the more CO2 is blown off and pCO2 level drop.
    2. pCO2 is referred to as the respiratory component in acid-base determination because this value is controlled by the lungs.
  4. As the CO2 level increases, the pH level will decrease.
    1. The  pCO2 level in blood and CSF is a major stimulant to the breathing center in the brain.
    2. As the pCO2 level rises, breathing is stimulated.
  5. When the brain can not cope with increased pCO2 and cannot blow off excess CO2 then the brain is depressed, ventilation decreases, and the patient goes into a coma.
    2. In metabolic acidosis, lungs try to compensate by more blowing of CO2 to raise pH.
    3. In metabolic alkalosis, lungs try to compensate by retaining the CO2 to lower pH.

HCO3 or CO2 content

  1. Most of the CO2 contents are as HCO3¯ in the blood.
    1. The HCO3 ions can be measured directly as HCO3 or indirectly by CO 2 contents.
  2. Total CO2 =  HCO3¯ + Dissolved CO2.
    1. The most important buffer system of the plasma is HCO3¯ / H2CO3.
    2. It is also present in the RBC but at a lower concentration.
    3. The ratio of base: acid = 20: 1 in plasma.
  3. HCO3¯ ions are regulated by the kidney and it is the measure of metabolic (Renal) component of the acid-base balance.
  4. CO2 contents should not be confused with pCO2.
  5. CO2 contents are indirectly measured by HCO3¯.
    1. HCO3¯  : Dissolved CO2 =  25 : 1 
      1. Any change in the above equation leads to change in the pH.
      2. As the HCO3¯ level increase and the pH also increases.
  6. The HCO3¯ level:
    1. Metabolic alkalosis, the HCO3 level is elevated.
      1. Metabolic acidosis, the HCO3 level is decreased. 
    2. Respiratory acidosis, kidneys attempt to compensate for increased reabsorption of HCO3¯.
      1. Respiratory alkalosis, kidneys excrete increased amount of HCO3¯ to lower the pH.


 Clinical conditions pH Bicarbonate (HCO3) level
Metabolic acidosis decreased decreased 
Metabolic alkalosis increased increased
Respiratory alkalosis increased decreased
Respiratory acidosis decreased increased


  1. Oxygen in the blood carried in two forms:
    1. Dissolved in plasma = <2%.
    2. Combined with hemoglobin = 98%.
    3. This partial pressure of the oxygen gas determines the force it exerts in attempting to diffuse through the pulmonary membrane.
    4. The pO2 reflects the amount of oxygen passing from the pulmonary alveoli to the blood.
  2. 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.
  3. The pO2 level is decreased in:
    • Pneumonia.
    • Shock lung.
    • Congestive heart failure.
    • Congenital heart diseases.
    • 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 Oand hemoglobin.
    3. This is not the O2 content.
  2. The combined measurement of:
    1. O2 saturation.
    2. pO2.
    3. Hemoglobin.
      1. This indicates the amount of O2 available to the tissues for oxygenation.
  3. When hemoglobin 92 to 100% carries O2, then perfusion or supply of oxygen to tissue is normal.
  4. With the decrease of the pO2 level, a saturation of hemoglobin also decreases.
  5. When the O2 saturation is at 70% or low, then the tissues are unable to get an adequate amount of oxygen.
  6. Precaution:
    1. Please avoid smoking or exposure to close second-hand smoke or to 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 Oin 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.
  2. This is calculated by the following formula:
    • O2 content = O2 saturation x Hb x 1.34 + pO2 × 0.003


Source 1





O2 saturation

O2 content

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 

  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    

Male = 35 to 48 mm Hg

Female = 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)    
Source 4

Arterial blood gases ordered in routine:

  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)  


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


Respiratory acidosis

Metabolic acidosis

Respiratory alkalosis


Metabolic Alkalosis

Calculation of Anion gap = (Na) -- [(Cl) + (HCO3)]

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

  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  


Possible References Used

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