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EBMG

Blood Gas Analysis and Acid-Base Balance

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.

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.

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.

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).

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
- Acute or chronic ventilatory insufficiency Respiratory Failure
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

Disturbance		Blood pH	Blood pCO₂	Blood BE	Urine pH
Metabolic acidosis	Uncompensated	decreases	normal	decreases	decreases
Metabolic acidosis	Fully compensated	normal	decreases	decreases
Metabolic alkalosis	Uncompensated	increases	normal	increases
Metabolic alkalosis	Fully compensated	normal	increases	increases	increases
Respiratory acidosis	Uncompensated	decreases	increases	normal	decreases
Respiratory acidosis	Fully compensated	normal	increases	increases
Respiratory alkalosis	Uncompensated	increases	decreases	normal
Respiratory alkalosis	Fully compensated	normal	decreases	decreases	decreases

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]