Blood gas analysis

General

For a blood gas analysis (in short: BGA) are among others measured the concentrations of certain gases in the blood. The gases, including oxygen (O2) and carbon dioxide (CO2), have a certain partial pressure (pO2 and pCO2) in the blood, which should normally be stable and in this way maintain the vitality of the organism. In addition, further parameters are determined, e.g. the current oxygen saturation in the blood, the acid-base balance based on bicarbonate (current or standard bicarbonate (aHCO3 or SBC or StHCO3)) and base deviation (BE = base excess) and the pH of the blood. The bicarbonate value and the base deviation are not measured directly, but calculated and always refer to standardized values ​​in the blood (temperature: 37 °, pCO2: 40mmHg, fully saturated blood). Furthermore, you can determine the hemoglobin value, lactate value or blood sugar values ​​during a blood gas analysis. In special cases, e.g. If smoke gas poisoning or similar is suspected, the BGA can also be used to determine the concentrations of carbon monoxide or nitrogen.

General information can be found at: Blood test

The blood gas analysis enumerates Intensive care units for standard clinical diagnostics and is carried out daily (or several times a day). Especially with severe ones Respiratory diseases it can quickly provide information about an increasing deterioration and the necessary measures can be initiated quickly. Blood gas analysis is also carried out regularly when monitoring anesthesia.

Physiological background

In the blood there should always be a constant concentration Hydrogen ions be available and therefore always a stable one PH value from 7,36 - 7,44. The body has several for this Buffer systems, via which superfluous hydrogen ions can be eliminated or, if there is a deficiency, hydrogen ions can also be retained. The main buffer system is the Bicarbonate household, which on the one hand take up hydrogen ions and then on carbonic acid water and carbon dioxide (is exhaled) can disintegrate. In the event of a lack of hydrogen ions, the carbon dioxide, which is constantly produced in the body during cell respiration, can also be coupled to water with the help of enzymes or spontaneously and then via the Reverse reaction to react to bicarbonate and a hydrogen ion.

Other important buffer systems are the Hemoglobin buffer, of the Phosphate buffer and the Protein buffer. In regulating the pH value in the blood, both the buffer systems themselves, but also the exhalation of carbon dioxide through the lungs and the excretion of hydrogen ions through the kidneys are involved. In this regulatory circle there are consequently many starting points that can shake the system if it loses its normal function. There are e.g. metabolic (metabolic) Malfunctions caused by a malfunction in the buffer systems Imbalance of hydrogen ions. On the other hand, there is respiratory-related (respiratory) Disorders in which there is an increased or decreased exhalation of carbon dioxide. Of course there can also be an error in both systems; this is referred to as a mixed malfunction.

Normal values ​​blood gas analysis

• Oxygen: The partial pressure of oxygen in the blood can fluctuate slightly depending on age. It should always be between 80 mmHg and 100 mmHg. In patients older than 75 years, it can also be below 80 mmHg. Deviations below the lower reference value are also possible in the case of severe, chronic diseases of the lungs or heart. In most cases, however, the patients feel good subjectively and are usually sufficiently oxygenated.
• Carbon dioxide: The normal carbon dioxide partial pressure should be between 35 - 45 mmHg, regardless of age. If the exhalation of carbon dioxide is reduced as a result of lung diseases, the value can shift upwards. If chronic lung diseases are present, the increased carbon dioxide levels are usually well tolerated and the patients can be clinically normal. However, if the partial pressure rises rapidly, this can be an expression of exhaustion of breathing, which is an absolute emergency.
• pH value: The pH value is a measure that indicates the strength of an acidic (acidic) or basic (alkaline) solution in relation to its hydrogen ion content.If the pH is low, the examined medium contains a high number of hydrogen ions, which can lead to acidosis, an acidic metabolic state, in the body. At a high pH there are few hydrogen ions; if this condition is found in the body, it is known as alkalosis. The normal pH level 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 indicates a high concentration, a high pH indicates a low concentration. The deviations from the normal pH value have already been briefly mentioned above and can have various causes. For example, reduced exhalation of carbon dioxide can lead to an acidic metabolic situation in the organism; one speaks of respiration-related (respiratory) Root cause. Can e.g. the kidneys no longer adequately excrete hydrogen ions, one speaks of a metabolic (metabolic) Root cause.

• Bicarbonate (HCO3): This value shows the concentration of bicarbonate in the blood. He should normally be between 22-26 mmol / l move. It is not influenced by breathing, so it is a purely metabolic parameter that can also be used to diagnose such a disorder. In patients with chronic diseases of the lungs, such as chronic obstructive pulmonary disease (COPD) the value can also elevated be. This is considered to be a sign of compensation for the respiratory disorder present.
• Base deviation (base excess = BE): The base deviation or base excess, indicates the value of acids or bases that would be necessary to restore a normal pH value. Some standard conditions are defined here. So the base excess gives that Number of bases / acids to the normalization of pH value to a value of 7,4 at a carbon dioxide partial pressure of 40mmHg and a blood temperature of 37 ° C. It is usually between -5 and +5. If the excess base value is negative, then there are to few Bases in the blood, so a low pH value (acidic) can be assumed. In the opposite case, i.e. with one to high BE value, there are too many bases, the blood adopts an alkaline pH value. The BE value is not influenced by the carbon dioxide partial pressure and can therefore also be used to diagnose metabolic Interferences are used. Here, too, it can be caused by chronic obstructive pulmonary disease metabolic compensation the respiratory disorder lead to an increase in the BE value.
• Oxygen saturation (SO2): The oxygen saturation gives the proportion of of oxygen present in the blood to the maximum possible recording capacity of oxygen in the blood and is always given as a percentage. They should be in healthy adults over 96% lie.

Performing a blood gas analysis

There are several ways to do the blood gas analysis. Usually arterial blood is required for the implementation. On the one hand, this can be taken from the earlobe. For this purpose, the earlobe is rubbed with an ointment that promotes blood circulation. The ointment causes the ear to be supplied with blood much better than usual and thus the blood is arterialized. Please note that you must not massage the earlobe or press it firmly, as otherwise tissue water or plasma can falsify the values ​​of the blood gas analysis. 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 collection. It should be noted that the capillary must be heparinized, that is, it must be coated with an agent that prevents the blood obtained from clotting, otherwise the evaluation is no longer possible. The capillary should therefore be swirled carefully so that the blood also mixes with the anticoagulant. The blood is now placed in a special analyzer which, after a while, displays the values. Alternatively, the capillary removal can also take place on the fingertips using the same procedure.

The values ​​can be analyzed most precisely in pure arterial blood. To do this, however, you have to puncture an artery, which is not done in the routine control, since the possible complications from e.g. Bleeding etc. is too high. In intensive care units, during operations or in very urgent cases, however, the arterial puncture is still performed as standard, because the patient is under medical supervision anyway and a permanent arterial access is usually provided. To do this, either the artery near the spoke or the elbow on the wrist or the leg artery in the groin region is chosen. Another possibility is to carry out the blood gas analysis from a central venous catheter lying on a patient. So-called mixed venous blood is taken here, which is helpful in diagnosing the patient's metabolic and respiratory status. Purely venous blood can also be used for a blood gas analysis, but is not recommended as a standard, as the oxygen content can vary greatly depending on the location of the extraction site.

Pulmonary embolism

In the case of an acute pulmonary embolism, a pulmonary vessel is blocked by a dragged one Blood clots. You can get one here with the patient Lack of oxygen found in the blood. Because the patient no longer has enough oxygen, he breathes more often. Through this Hyperventilation but it usually comes to one waste the concentration of carbon dioxide exhaled with each breath. In around 20% of the patients, however, these constellations do not occur; normal oxygen partial pressures are found in the blood. Especially when you only have a small one Cardiac output (Volume of blood transported by the heart per minute), it can remain with a normal distribution in the arterial blood gas analysis. If the oxygen partial pressure increases in the arterial blood gas analysis, this can be an indication of a deteriorating blood flow situation in the patient. In individual cases, falling values ​​can also indicate an improvement in the blood flow to the lungs. If the patient is already familiar with cardiovascular diseases, the diagnosis of pulmonary embolism can be made significantly more difficult. Overall, a blood gas analysis alone is not sufficient to diagnose a pulmonary embolism; the clinical symptoms such as shortness of breath, palpitations, chest pain etc. and other diagnostic means (EKG, chest X-ray, D-dimer test, CT or MR image) are still required the vessels) must be observed.