Information
Editors
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
Severity | Hypoxaemia aB-pO2 (kPa) | Hypercapnia aB-pCO2 (kPa) |
---|
Mild | 8-11 | 6.1-6.6 |
Moderate | 6-7.9 | 6.7-8 |
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
Disturbance | | Blood pH | Blood pCO2 | Blood BE | Urine pH |
---|
Metabolic acidosis | Uncompensated | decreases | normal | decreases | decreases |
Fully compensated | normal | decreases | decreases | |
Metabolic alkalosis | Uncompensated | increases | normal | increases | |
Fully compensated | normal | increases | increases | increases |
Respiratory acidosis | Uncompensated | decreases | increases | normal | decreases |
Fully compensated | normal | increases | increases | |
Respiratory alkalosis | Uncompensated | increases | decreases | normal | |
Fully compensated | normal | decreases | decreases | decreases |
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]