Standard values blood gas analysis

Oxygen: The partial pressure of oxygen in the blood can vary slightly depending on age. It should always be between 80 mmHg and 100 mmHg. In patients over 75 years of age, it can also be below 80 mmHg.

Deviations below the lower reference value are also possible in the case of serious, chronic diseases of the lungs or heart. In most cases, however, patients subjectively feel well and are usually sufficiently oxygenated. – carbon dioxide: The normal carbon dioxide partial pressure should be between 35 – 45 mmHg regardless of age.

In case of reduced exhalation of carbon dioxide due to lung diseases, the value may shift upwards. If chronic lung diseases are present, the increased carbon dioxide levels are usually well tolerated and patients may be clinically unremarkable. However, if the partial pressure rises rapidly, this may be an expression of respiratory exhaustion, which is an absolute emergency.

PH-value: The pH-value is a measure that indicates the strength of an acidic (azide) or basic (alkaline) solution in relation to its content of hydrogen ions. If the pH is low, there is a high number of hydrogen ions in the medium under investigation, which can lead to acidosis, an acidic metabolic state, in the body. If the pH is high, there are few hydrogen ions, and if this state is found in the body, it is called alkalosis.

The normal pH value in the blood should be between 7.36 and 7.44. The pH value is a measure of the hydrogen ion concentration. A low pH value indicates a high concentration, a high pH value indicates a low concentration.

The deviations from the normal pH value have already been briefly mentioned above and can have various causes. For example, a reduced exhalation of carbon dioxide can lead to an acidic metabolic situation of the organism, this is called a respiratory cause. If, for example, the kidneys can no longer excrete hydrogen ions sufficiently, this is referred to as a metabolic cause.

Bicarbonate (HCO3): This value indicates the concentration of bicarbonate in the blood. It should normally be between 22 – 26 mmol/l. It is not influenced by respiration and is therefore a purely metabolic parameter that can also be used to diagnose such a disorder.

In patients with chronic lung diseases, such as chronic obstructive pulmonary disease (COPD), the value may also be increased. This is considered a compensatory sign of the respiratory disorder in question. – Base excess (BE): The base excess is the amount of acids or bases that would be required to restore normal pH.

Several standard conditions are defined. The base excess thus indicates the number of bases/acids that would be needed to normalize the pH to a value of 7.4 at a carbon dioxide partial pressure of 40mmHg and a blood temperature of 37°C. It is normally between -5 and +5.

If the excess base value is negative, there are too few bases in the blood, so a low pH value (acidic) can be assumed. In the opposite case, i.e. if the BE value is too high, there are too many bases in the blood, the blood assumes an alkaline pH value. The BE value is not influenced by the carbon dioxide partial pressure and can therefore also be used for the diagnosis of metabolic disorders.

In the case of chronic obstructive pulmonary disease, the metabolic compensation of the respiratory disorder can also lead to an increase in the BE value. – Oxygen saturation (SO2): The oxygen saturation indicates the proportion of oxygen present in the blood to the maximum possible absorption capacity of oxygen in the blood and is always given as a percentage. It should be above 96% in healthy adults.

Blood gas analysis can be carried out in different ways. Usually arterial blood is required for the analysis. This can be taken from the earlobe.

For this purpose, the earlobe is rubbed with a blood circulation-promoting ointment. The ointment has the effect that the blood supply to the ear is much better than usual and thus the blood is arterialised. It is important to note that the earlobe must not be massaged or pressed firmly, as tissue water or plasma can otherwise falsify the blood gas analysis values.

Then the earlobe is punctured with a lancet, a small pointed instrument, and the blood is collected in a capillary. This is why this procedure is also called capillary blood sampling. It is important to note that the capillary must be heparinized, i.e. it must be coated with an agent that prevents the coagulation of the collected blood, otherwise the evaluation is no longer possible.

One should therefore carefully swivel the capillary so that the blood also mixes with the anticoagulant. The blood is now put into a special analyser, which displays the values after some time. Alternatively, the capillary collection can also take place at the fingertips using the same procedure.

The most accurate way to analyse the values is in pure arterial blood. For this purpose, however, an artery has to be punctured, which is not done in the routine control, as the possible complications due to e.g. post-bleeding etc. are too high.

In intensive care units, during operations or in very urgent cases, arterial puncture is nevertheless performed as standard because the patient is under medical supervision anyway and usually a permanent arterial access is also made. For this purpose, one chooses either the artery near the radius or the wrist or the leg artery in the groin region. Another possibility is to perform blood gas analysis from a central venous catheter located in the patient’s body. Here, so-called mixed venous blood is taken, which is helpful in diagnosing the patient’s metabolic and respiratory status. Purely venous blood can also be used for blood gas analysis, but is not recommended as standard because the oxygen content can vary greatly depending on the location of the blood collection point.