Arterial and Venous Blood Gas Analysis

by Kemal Gunaydin


Measurements of PaO2, PaCO2, SaO2, pH, and bicarbonate values are made with arterial blood gas (ABG) analysis in order to determine the acid-base balance and respiratory regulation. Arterial blood gas (ABG) analysis is an important laboratory method that provides reliable information about the patient’s metabolic status and respiratory physiology.

Indications for arterial blood gas (ABG) analysis are

  • Diagnosis and follow-up of metabolic and respiratory acidosis and alkalosis
  • Determination of the type of respiratory failure
  • Determination of the need for mechanical ventilation
  • Evaluation of the indication for admission to intensive care
  • Determination of the effectiveness of the given treatment
  • Indication and follow-up of oxygen treatment
  • Evaluation of the reason for sudden and unexplained dyspnea

Generally, the radial, brachial and the femoral arteries are used for this purpose. The choice of the artery is associated with many factors. It mainly depends on the physician’s experience and the patient’s clinical condition. Primarily, the radial artery is preferred. The Allen test should be performed prior to the procedure to evaluate the adequacy of the collateral circulation in hand. The obtained blood gas sample should be delivered to the laboratory as soon as possible.

The normal values of the arterial blood gases (Please refer to the agreed norms from your lab);

  • pH    7.35 – 7.45
  • PaCO2     35 – 45 mmHg
  • PaO2     80 – 100 mmHg
  • SaO2     %95 – 97
  • Standard HCO3     22 – 26 mEq/L
  • Actual HCO3     22 – 26 mEq/L
  • BE (Base excess)     ±3 mmol/L
[H+]: Hydrogen ion concentration.
pH: The negative logarithm of the hydrogen ion concentration.
PaO2: Oxygenation
PaCO2: Alveolar ventilation
PaO2 and PCO2: Gas exchange
Ph, PCO2, and HCO3: These are used to evaluate the acid-base status.


In ABG, pH shows a status of acidosis or alkalosis. However, it is not possible to understand its type with pH. pH is also the only parameter showing compensation. Its normal values are between 7.35-7.45. It is decompensated acidosis if pH<7.35, and decompensated alkalosis if pH>7.45.

Arterial partial pressure of oxygen (PaO2)

This is the partial pressure of oxygen in the arterial blood. It is used in the evaluation of oxygenation.

  • PaO2: Between 60-79 mm Hg, “ mild hypoxemia.”
  • PaO2: Between 40-59 mm Hg, “moderate hypoxemia.”
  • PaO2: Below 40 mmHg, “severe hypoxemia.”

Arterial partial pressure of carbon dioxide (PaCO2)

This is the partial pressure of carbon dioxide in the arterial blood. It is the indicator of alveolar ventilation. Its normal value is 40 mmHg at sea level, while it is 46.5 mmHg in venous blood. Increased values show respiratory acidosis, while decreased values demonstrate respiratory alkalosis.

Alveolar-arterial oxygen gradient – p(A-a) O2

This is the difference between the alveolar and arterial partial pressures of oxygen, providing general information about the function of gas exchange in the lungs. Its normal value is 5 mmHg, which increases with age. A 4 mmHg increase is observed for every 10 years after 20 years of age.

  • p(A-a) O2: [150-(1.25x PaCO2)]-PaO2
  • Expected p(A-a) O2 value for age: 2.5+ [0.25xage(years)]

Bicarbonate (HCO3-)

This is the serum concentration of the bicarbonate ion. It is an important buffer in the blood, and it is used to evaluate the metabolic component of the acid-base balance. Standard bicarbonate is the bicarbonate value that should be present in the blood under standard conditions (37°C temperature and 40 mmHg PCO2). Its normal value is 22-26 mEq/L. Actual bicarbonate is the real bicarbonate value in the blood. Its normal value is 22-26 mEq/L. Increased values indicate metabolic alkalosis, while decreased values show metabolic acidosis.

Base excess (BE)

Metabolic acidosis or alkalosis may be determined by looking at the base excess. BE is the amount of required acid or base to bring the pH of the totally oxygenated blood to 7.40 at 37°C and 40 mmHg PCO2; it is the indicator of the metabolic status. If BE is <-2.5, it is metabolic acidosis, if BE >+2.5, it is metabolic alkalosis.

Anion Gap (AG)

The anion gap represents the difference between the serum cations and the anions. In daily practice, the measured cation is sodium, and the anions are chloride and the bicarbonate. The normal AG is 12±4 mEq/L. Albumin constitutes the majority of the immeasurable anions. In patients with low levels of albumin, AG should be considered according to the level of albumin. It shows whether the metabolic acidosis develops due to the accumulation of non-volatile acids (lactic acid, ketoacids, etc.) (increased AG metabolic acidosis), or due to loss of bicarbonate (normal AG or hyperchloremic metabolic acidosis).

  • AG = Na+ – (HCO3- + Cl-)
  • Expected AG = Calculated AG+2.5X [4.5- albumin level]

Delta-Delta Gap (ΔAG/ΔHCO3-)

In the presence of high AG metabolic acidosis, the “delta-delta gap” is calculated to determine a second metabolic acid-base balance imbalance. In this case, the increase in AG is compared with the decrease in HCO3.
AG/ΔHCO3- = (Calculated AG-12) / (24-measured HCO3-)

  • In the presence of high AG metabolic acidosis, ΔAG/ΔHCO3- = 1.
  • If there is also hyperchloremic acidosis, ΔAG/ΔHCO3- <1.
  • If there is also metabolic alkalosis, ΔAG/ΔHCO3- >1.


Lactate is a surrogate anaerobic indicator of metabolism, which is increased under stress and hypoperfusion. It is also used as an indicator of the resuscitative efforts in patients with shock and an indicator of survival in patients with septic shock. Levels above 4 mmol/L are associated with a mortality rate of 28%.


1th STEP

  • If pH or PaCO2 are out of normal range, acid-base balance imbalance is present

2th STEP

  • If pH is abnormal, and pH and PaCO2 move in opposite directions, the
    primary disorder is RESPIRATORY
  • If pH is abnormal, and pH and PaCO2 move in the same direction, the
    primary disorder is METABOLIC

3th STEP

  • If one of pH or PaCO2 is normal, a mixed acid-base disorder is present.
  • If pH is normal, the change in the direction of PaCO2 defines a respiratory disorder.
    • If PCO2 ↑ = Respiratory acidosis-metabolic alkalosis
    • If PCO2 ↓ = Respiratory alkalosis-metabolic acidosis
  • If PaCO2 is normal, the change in the direction of pH defines a metabolic disorder.
    • If pH ↑ = Metabolic alkalosis-respiratory acidosis
    • If pH ↓ = Metabolic acidosis-respiratory alkalosis

4th STEP

  • If primary metabolic acidosis or alkalosis is detected, the expected PaCO2 is calculated.
    • For metabolic acidosis;
      • Expected PaCO2= (1.5 x HCO3) + 8 ± 2
    • For metabolic alkalosis;
      • Expected PaCO2= 40 + (0.6 x ▲HCO3)
  • If PaCO2 is within the expected range, full compensation is present.
  • If it is more than the expected value, concomitant respiratory acidosis is present.
  • If it is less than the expected value, concomitant respiratory alkalosis is present.

5th STEP

  • If there is respiratory acidosis or alkalosis present, the expected pH is calculated.
  • In acute respiratory acidosis;
    • Expected pH= 7.4 – [ 0.008 x (PaCO2-40)]
  • Chronic respiratory acidosis;
    • Expected pH = 7.4 – [ 0.003 x (PaCO2-40)]
  • If pH is below the expected value in acute respiratory acidosis, there is concomitant metabolic acidosis present.
  • If it is above the expected value in chronic respiratory acidosis, there is concomitant metabolic alkalosis present.
  • In acute respiratory alkalosis;
    • Expected pH =7.4 + [ 0.008 x (40 – PaCO2)]
  • In chronic respiratory alkalosis;
    • Expected pH =7.4 + [ 0.003 x (40 – PaCO2)]
  • If pH is above the expected value in acute respiratory alkalosis, there is concomitant metabolic alkalosis present.
  • If it is below the expected value in chronic respiratory alkalosis, there is concomitant metabolic acidosis present.


  • The evaluation of the anion gap in metabolic acidosis

Acid-base disorders


  • Causes of Acidosis with Increased Anion Gap
    • Lactic Acidosis
    • Ketoacidosis
    • Uremic Acidosis
    • Methanol poisoning
    • Ethanol poisoning
    • Ethylene Glycol poisoning
    • Propyl Alcohol Poisoning
    • Salicylate Poisoning
    • Iron Poisoning
  • Causes of Acidosis with Normal Anion Gap
    • Diarrhea
    • Isotonic Saline Infusion
    • Renal failure
    • Renal tubular acidosis
    • Acetazolamide
    • Ureteroenterostomy
  • Causes of Alkalosis with Decreased Amount of Fluid
    • Gastric acid loss
      • Vomiting
      • Gastric aspiration
    • Renal Cl loss
      • Use of diuretics
      • Urine Cl < 20mmol/L
  • Causes of Alkalosis with Normal Amount of Fluid
  • Mineralocorticoid excess
    • Hyperaldosteronism
    • Bartter syndrome
    • Cushing syndrome
  • K loss
    • Urine Cl > 20 mmol/L



  • Causes of Acidosis
    • Hypoventilation
      • CNS diseases
      • Muscle diseases
    • Severe V/Q mismatch
      • Chronic lung diseases
  • Causes of Alkalosis
    • Brain lesion or diseases
    • Centrally acting drugs or chemicals
      • Salicylate,
      • Endotoxin,
      • Progesterone
    • Hypoxemia compensation
    • Pneumonia, pulmonary edema, PTE
    • Lack of fluid volume
    • Liver failure
    • Sepsis (+ met. acidosis)
    • Psychiatric diseases

Venous Blood Gas

Arterial blood gas sampling is an uncomfortable, painful, difficult and an invasive procedure for the patient. Furthermore, the success rate of the procedure may decrease due to movement of the patient or low arterial blood pressure. Therefore, the question “can venous blood gas be used instead of arterial blood gas?” has been raised, and many studies have been performed on this subject. Since venous blood gas is easy to sample from the peripheral veins or the central veins in patients with central venous catheters, it is a more comfortable and an easy procedure for some patients and the physicians.

In many studies, a very good correlation has been shown between venous blood gas and the arterial blood gas. To evaluate the acid-base disorders and ventilation, comments can be made easily by checking the PvCO2, pH and HCO3 levels. In addition, SvO2 levels in patients with central venous catheters are very important indicators in evaluating patients in shock. However, it is not a useful method to evaluate oxygenation. To overcome this problem, it would be a sensible approach to measure the saturation with pulse oximetry simultaneously.

Arterial blood gas is a more reliable and accurate method for assessing the oxygenation. Arterial and venous blood gases provide similar and very close measurements in terms of PC02, HCO3, and pH levels.

The comparison of arterial, peripheral vein and central blood gases

 Peripheral Venous Blood GasCentral Venous Blood Gas
PCO23 to 8 mmHg higher than the arterial pH4 to 5 mmHg higher than the arterial pH
pH0.02 to 0.04 pH units lower than the arterial pH0.03 to 0.05 pH units lower than the arterial pH
HCO31 to 2 mEq/L higher than the arterial pHlittle or no increase in HCO3


References and Further Reading

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