ARTERIAL GASES: PROCEDURE, INTERPRETATION AND VALUES - MEDICINE - 2024

The blood gases, or gases Stethoscopes- correspond to a technique used to quantify the concentration of oxygen, carbon dioxide and bicarbonate present in the blood. The reference sample is arterial blood, since it is the one with the highest oxygen concentration. The pH of the blood is also obtained with this test.

Acid-base balance means the equal concentration of acidic and alkaline substances in the body. The human body has a pH close to neutrality, with a slight predominance of alkaline substances. Its normal value ranges between 7.35 and 7.45, a value at which vital functions develop normally.

In humans, the organs responsible for regulating the acid-base balance and pH are the lungs and kidneys. The respiratory system regulates the concentration of gases, while the renal system controls the concentration of bicarbonate. The alteration of the acid-base balance will be the consequence of respiratory or renal dysfunction.

The best tool to demonstrate the alteration of the pH is the determination of arterial gases. The test allows the determination of parameters such as the partial pressures of oxygen, carbon dioxide and bicarbonate. Its use is intended for critically ill patients with acidosis or alkalosis.

Process

Taking the arterial blood sample requires skill and precision. Responsible personnel must have training to achieve the efficiency and effectiveness of this technique.

Doctors, nurses with experience in intensive care, and some bioanalysts can perform arterial blood collection without difficulty.

There are a number of steps that are required to successfully draw arterial blood. The preparation of the patient is essential, in addition to having the necessary implements.

Preparation of the patient

- The procedure should be performed in a quiet and clean area.

- The patient will be relaxed and calm. Because of how uncomfortable or painful the test can be, the test will be explained to the patient in detail. Collaboration is essential.

- When the patient receives oxygen, it must be removed previously. Breathing ambient air will ensure the reliability of the test. Oxygen will be omitted 10 to 20 minutes before the procedure.

Necessary equipment

- Small caliber syringes are the most widely used, such as those used to administer insulin.

- Needle No. 25 X 1 ″ or 26 X 1 ″, to cause the least possible damage to the arterial wall.

- Anticoagulant must be placed in the syringe.

- Have a container with ice to transport the sample.

- In some cases, local anesthesia will be used.

- Cotton or gauze for tamponade.

Sampling

- Locate the place where the sample will be taken. The most commonly used artery is the radial, but the brachial, femoral, or pediatric arteries are also useful.

- Dorsiflexion of the wrist -extension- exposes the radial artery and facilitates palpation of the pulse wave. A pad or roll of bandage under the wrist allows positioning and rest of the limb.

- Palpate the arterial pulse and hold it lightly as a guide or reference.

- Insert the needle at an angle of 45 ° in the direction of the pulse wave. The pressure of the arterial blood flow will make the blood flow rapidly after the puncture. A 0.5 to 1 cc sample of arterial blood is sufficient.

- Check that the blood is really arterial. The appearance is clear, bright, or deep red.

- Put the syringe in the container with ice.

Once the sample is obtained, it is transported to the laboratory or place where the measurement equipment is located.

Normal values

Normal values ​​or reference values ​​are the values ​​at which the body's functioning is optimal. They correspond to the concentrations of oxygen (O ₂), carbon dioxide (CO ₂) and bicarbonate (HCO ₃ ^-), or the pH value that is frequently measured in arterial blood.

PaO

It corresponds to the partial pressure of arterial oxygen. Its reference value is 75 to 100 mmHg.

PaCO

Partial pressure of carbon dioxide, whose normal value ranges between 35 and 45 mmHg.

HCO

The ionic bicarbonate measurement is expressed in milliequivalents per liter (mEq / L) and its value is in the range of 22 to 26 mEq / L.

EB

The excess of base is an indicator of the existence of metabolic alterations of the acid-base balance. It corresponds to the metabolic (non-respiratory) component of metabolic acidosis or alkalosis. It is expressed in millimoles per liter (mmol / L) and its value is +/- 2 mmol / L.

pH

The pH is the indicator of the existence of acidity or alkalinity in the body. Normal pH values ​​are between 7.35 and 7.45.

H

The concentration of hydrogen ions (H ⁺) is inversely proportional to the pH value. When the pH decreases H ⁺ increases and vice versa. It also indicates the acidity or alkalinity of the body. The value of this indicator is expressed in nanomoles per liter and its range is 35 to 45 nmol / L.

Interpretation

The result of arterial gases is useful to indicate alterations of the acid-base balance in the body. There are diseases that can produce a predominance of a state of acidity or alkalinity. The importance is that the organism works in a state close to neutrality and its alteration implies serious consequences.

The systematic analysis of the values ​​obtained by arterial blood gas will indicate the state of imbalance and the affected system. Taking into account the reference values, the results could indicate an increase or decrease of a parameter.

Changes in pH and hydrogen ion concentration

The variation of the pH values ​​outside the normal range depends directly on the alteration of the other parameters. Changes in the partial pressures of oxygen and carbon dioxide, as well as the concentration of bicarbonate, influence the observed changes.

The alteration of the pH can be of two types:

- A value lower than 7.35 indicates a tendency towards acidity, or acidosis. The hydrogen ions or H + will present values ​​greater than 45 nmol / L.

- When the pH value exceeds 7.45 it is called alkalosis. In this case, H + will be less than 35 nmol / L.

Changes in partial pressure of gases

- The partial pressure of the gases oxygen and carbon dioxide is inversely proportional. The elevation of one causes the decrease of the other.

- The partial pressure of oxygen (PaO ₂) is considered decreased when its value is less than 75 mmHg in arterial blood. When this occurs it is called hypoxemia. PaCO ₂ increases (value greater than 45 mmHg) and the consequence is respiratory acidosis, in the absence of other alterations.

- An increase in PaO ₂ above 100 mmHg is considered hyperoxemia, and is the determinant of respiratory alkalosis. This is due to the significant decrease in PaCO ₂, below 35 mmHg.

Baking soda and excess base

- Ionic bicarbonate or HCO ₃ ^- is a determinant of the acid-base balance due to its alkaline behavior. The regulation of the amount of ionic bicarbonate depends on the kidney, which is in charge of its synthesis and reabsorption. Any alteration of this organ can cause metabolic imbalance.

- The increase in bicarbonate in the blood often implies a compensatory response to the existence of acidosis in the body.

- Bicarbonate with a value greater than 26 mEq / L supposes the presence of a metabolic alkalosis. The excess of base will be greater than 2 mmol / L. A compensatory response is related to an increase in PaCO ₂, in the search for equilibrium.

- The decrease in HCO ₃ ^- to a value below 22 MEq / L is related to metabolic acidosis. The excess of base will be less than -2 mmol / L. PaCO ₂ may decrease.

Arterial gases are not only a diagnostic tool for acid-base balance disorders in the body. This technique allows physicians to control the response to the indicated treatments, the purpose of which is the timely correction of the alterations found.

References

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4. Danckers, M. (2016). Arterial blood gas sampling. Recovered from emedicine.medscape.com
5. Kaufman, D. (nd). Interpretation of Arterial blood gases. Recovered from thoracic.org