Blood gas analysis

Simple method for measuring the gas distribution (partial pressure) of O2 (oxygen), CO2 (carbon dioxide) as well as the pH value and the acid-base balance in the blood.

During inhalation, oxygen (O2) enters the lungs; during exhalation, carbon dioxide (CO2) is released via the lungs. The acid-base balance of the body is also controlled by the CO2 partial pressure.

Both external respiration (e.g. bronchial asthma, chronic obstructive bronchitis) and the so-called "internal respiration" can be disturbed.

Oxygen partial pressure (pO2), carbon dioxide partial pressure (pCO2) and the pH value of the blood are determined. Lack of oxygen or an excess of carbon dioxide leads to a drop in the pH value (the blood becomes "acidic"). The body tries to compensate for this "acidic state" with "bicarbonate" (hydrogen bicarbonate, HCO3-).

If the pH of the blood drops (the blood becomes acidic) as a result of metabolic disorders such as hyperglycaemia in diabetes mellitus, the body will try to compensate for this state by hyperventilation.

The pH value of the arterial blood is normally between 7.37 and 7.45. The venous blood has a pH value of 7.26 to 7.46.

• The carbon dioxide partial pressure (pCO2) in arterial blood is 35 to 46 mmHg. A decreased pCO2 is called hypocapnia, an increased pCO2 is called hypercapnia.
• The oxygen partial pressure (pO2) in arterial blood is 75 to 105 mmHg.
• The bicarbonate concentration in the blood is 21 to 26 mmol/l. This value applies to both arterial and venous blood.
• The oxygen saturation (O2 saturation) of healthy adults is > 96 %.

Metabolic or "respiratory acidosis: pCO2 increased, pH decreased, bicarbonate normal. If an asthmatic exhales too little carbon dioxide, the partial pressure of carbon dioxide increases, the blood becomes "acidic" = decreasing pH.

Respiratory alkalosis: pCO2 decreases, pH increases, bicarbonate normal: in hyperventilation the pCO2 decreases, the blood becomes "alkaline" = increasing pH.

Metabolic acidosis: pCO2 decreases, pH decreases, bicarbonate decreases: In some cases (e.g. diabetes mellitus) metabolic acidity, a decrease in pH, pH value and pCO2 decrease, as does the bicarbonate.

After an athletic running load, the lactate concentration can increase due to a lack of oxygen, depending on the load. The lactic acid produced causes a pH drop which is buffered by bicarbonate ions. The concentration of bicarbonate decreases. In case of incomplete buffering: formation of a metabolic acidosis.

Metabolic alkalosis: pCO2 increased, pH increased, bicarbonate increased: In case of strong vomiting there is a loss of acidity. pH, pCO2 and bicarbonate values increase. The blood becomes "basic".

Note: Reference values and test results may vary from laboratory to laboratory. There are also diurnal and (seasonal) seasonal variations without disease value.

Standard values (arterial) in adults

• pH = 7.35-7.45 ; <7.35: acidosis; >7.45: alkalosis
• pO2 = 75-97 mmHg (depending on age)
• Oxygen saturation = 95-99
• pCO2 = 35-45 mm Hg
• HCO3(act) = 21-26 mmol/l (actual bicarbonate)
• HCO3(std) = 23-27 mmol/l (standard bicarbonate)
• BE (base deficit, base deviation or base excess) = 0 mval/l (-2 to +3 mmol/l)

Standard values (newborns / infants / children)

• pH = 7.20-7.41 (newborn 1st day)
• pH = 7.34-7.45 (newborn 10-90 days)
• pH = 7.38-7.45 (infant 4-12 months)
• pCO2 = 29-61 mmHg (newborn 1st day)
• pCO2 = 26-43 mmHg (newborn 10-90 days)
• pCO2 = 27-40 mmHg (3.6-5.3 kPa) (infant 4-12 months)
• pO2 = 16-35 mmHg (umbilical vein)
• pO2 = 70-85 mmHg (newborn 10-90 days)