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EBMG

Blood Gas Analysis and Acid-Base Balance

Essentials

  • Blood gas analysis is used to detect disturbances in acid-base balance and tissue oxygenation and to monitor their treatment.
  • Acid-base balance can be disturbed because of a respiratory or metabolic reason.
    • Respiratory acid-base imbalance is caused by a change in carbon dioxide concentration; acidosis by respiratory depression and alkalosis by hyperventilation.
    • In a metabolic disorder, the acid amounts produced by the metabolism or otherwise entering the body are excessive in acidosis and too scarce in alkalosis.

Blood specimen

  • Arterial blood is used primarily.
  • The blood specimen should be examined as soon as possible after sampling, preferably within less than 15 minutes. A frozen sample can be examined within 15 to 30 minutes after sampling.
  • Possible air bubbles in the specimen must be expelled, and the specimen must be handled with care (risk of haemolysis).
  • pH can be reliably determined also from venous blood.
  • Capillary blood is mainly used in children.

Measurements

  • Partial pressure of oxygen (pO2) (not from capillary blood)
  • pH
    • Logaritmic scale (pH 5 is 10-fold compared to pH 4)
  • Partial pressure of carbon dioxide (pCO2)
  • Base balance
    • Base excess, BE (metabolic alkalosis)
    • Base deficit, BD (metabolic acidosis)
  • BE is used to describe both conditions; the + and the - sign indicate whether the condition is alkalosis (+) or acidosis (-).
  • Instead of BE, a standard bicarbonate (SBC) value can also be used.

Reference values

  • Reference values should be checked with the laboratory used.
  • Arterial blood
    • pO2 average > 11 kPa (with lower values in advancing age)
    • pH 7.35-7.45
    • pCO2 4.5-6.0 kPa
    • BE 0 ± 2.5 mmol/l
    • Standard bicarbonate (SBC) 22-26 mmol/l
  • Capillary blood
    • pO2 varies and has no clinical significance
    • pH 7.35-7.45
    • pCO2 4.5-6.0 kPa
    • BE 0 ± 2.5 mmol/l
    • SBC 22-26 mmol/l

Severity of hypoxaemia and hypercapnia

  • See table T1.
  • aB-pO2< 7.3 (-7.9) kPa in a patient with chronic obstructive pulmonary disease is an indication for continuous oxygen therapy when additional criteria are met.
  • aB-pO2< 8 kPa, pCO2> 6.7 kPa indicates acute respiratory insufficiency.
  • When pCO2 rises acutely to > 10-12 kPa carbon dioxide narcosis results.
  • In a hypothermic patient, pO2 and pCO2 values are increased (an artefact).

Severity of hypoxaemia and hypercapnia

SeverityHypoxaemia aB-pO2 (kPa)Hypercapnia aB-pCO2 (kPa)
Mild8-116.1-6.6
Moderate6-7.96.7-8
Severe<6>8

Disturbances of the acid-base balance

  • See table T2 and calculator http://www.dynamed.com/calculators/#ArterialBloodGas (in Dynamed, requires subscription).
  • Causes of metabolic acidosis (BE < -2.5)
    • Ketoacidosis (diabetic, alcohol-induced)
    • Renal failure or tubular pathology (renal tubular acidosis)
    • Shock, insufficient oxygen supply to the tissues
    • Lactic acidosis
    • Severe diarrhoea
    • Intoxication (ammonium chloride, methanol, salicylates, ethylene glycol)
  • Causes of metabolic alkalosis (BE > +2.5)
    • Vomiting
    • Overdose of bicarbonate
    • Insidious hypovolaemia
    • Thiazide diuretics/furosemide
  • Causes of respiratory acidosis
  • Causes of respiratory alkalosis
    • See Hyperventilation Hyperventilation.
    • Insensitivity of the respiratory centre to changes in pCO2 as a result of a trauma or a disease process.
    • Psychogenic causes (panic disorder)
    • Hypoxaemia

Disturbances of the cid-base balance

DisturbanceBlood pHBlood pCO2Blood BEUrine pH
Metabolic acidosisUncompensateddecreasesnormaldecreasesdecreases
Fully compensatednormaldecreasesdecreases
Metabolic alkalosisUncompensatedincreasesnormalincreases
Fully compensatednormalincreasesincreasesincreases
Respiratory acidosisUncompensateddecreasesincreasesnormaldecreases
Fully compensatednormalincreasesincreases
Respiratory alkalosisUncompensatedincreasesdecreasesnormal
Fully compensatednormaldecreasesdecreasesdecreases

References

  • Berend K, de Vries AP, Gans RO. Physiological approach to assessment of acid-base disturbances. N Engl J Med 2015;372(2):195. [PubMed]
  • Spital A. Physiological approach to assessment of acid-base disturbances. N Engl J Med 2015;372(2):193. [PubMed]
  • Narins RG, Emmett M. Simple and mixed acid-base disorders: a practical approach. Medicine (Baltimore) 1980;59(3):161-87. [PubMed]