What is ABG (Arterial blood gas) Analysis?

 An analysis of the blood's levels of carbon dioxide and oxygen, as well as other factors that reflect the body's acid-base balance, is known as an arterial blood gas (ABG) analysis.

A small amount of arterial blood is typically drawn with a syringe from an artery in the wrist or groin to conduct the test. There is a small chance of bleeding or infection, and the procedure may be uncomfortable or painful. Following blood collection, the blood is then immediately examined for pH, bicarbonate (HCO3-), partial pressures of oxygen and carbon dioxide, as well as other electrolytes like sodium (Na+) and potassium (K+).

These variables offer crucial details about the patient's respiratory and metabolic health and can aid in the diagnosis of several conditions, including respiratory distress, metabolic acidosis or alkalosis, and hypoxemia.

Understanding normal values and how they relate to one another, as well as the pathophysiology and underlying physiology of acid-base balance, is necessary for the interpretation of ABG results. In a hospital or clinical setting, trained healthcare professionals such as respiratory therapists or doctors typically perform ABG analysis.

What do the ABG parameters mean?

Arterial pH: Blood hydrogen ions (H+) is measured by pH. Normal pH ranges between 7.35 to 7.45. The pH of the blood reflects the nature of blood whether it's acidic or alkaline. A pH below 7.35 is called acidic, while a pH above 7.45 is called alkaline or basic blood.

PaCO2: PaCO2 measures the blood's partial pressure of carbon dioxide. The typical PaCO2 ranges from 35 to 45 mmHg. Respiratory acidosis, which can be brought on by lung disease or respiratory failure, is indicated by a high PaCO2. Respiratory alkalosis is characterised by a low PaCO2, which can be brought on by hyperventilation or other conditions.

HCO3-: The HCO3- reflects the blood's bicarbonate concentration. The range of normal HCO3- is 22 to 26 mEq/L. A high HCO3- is a sign of metabolic alkalosis, which can be brought on by overeating, taking diuretics, or other conditions. Low HCO3- is a sign of metabolic acidosis, which can be brought on by various conditions like diabetic ketoacidosis, kidney disease, or other conditions.

BE: The BE measures the amount of bicarbonate in the blood that is in excess or deficient compared to the normal range. The range of normal BE is -2 to +2 mEq/L. While a positive BE denotes metabolic alkalosis, a negative BE denotes metabolic acidosis.

Values of blood gases:

The pH indicates acid/base.

Carbon dioxide partial pressure reflects alveolar ventilation.

The partial pressure of oxygen is used to calculate arterial oxygen levels.

 Base excess/ base deficit:

The blood can typically act as a buffer for acid metabolites. Base Hco3- concentrations in blood are typically between 22 and 26 millimoles per litre. When either acidemia or alkalemia is present, this buffering capacity decreases.

An abnormal acid/base balance known as acidosis occurs when the acids predominate. An abnormal acid/base balance known as alkalemia occurs when the bases predominate. When the Hco3- decreases, it results in a negative base excess and is known as a base deficit. 

On the blood gas report, it is typically displayed as a negative number, for example, -3. Identifying the patient's condition's acuteness or chronicity is useful. Examining whether the situation has been fully, partially, or uncompensated compensated is another strategy. The key to making this determination is the pH. The patient is acute if the pH is not in the normal range of 7.35 to 7.45. It might be partially compensated as it returns to normal. When a normal pH is present, it is either chronic or compensated. Recall the characteristics of gases to better comprehend blood gases. Gas molecules with mass that make up the earth's surface are drawn to the planet's centre of gravity. This atmospheric weight exerts a pressure that can support a mercury column 760 millimetres high at the surface. Dalton's law states that the total pressure of a mixture of gases is equal to the total of the partial pressures of the individual gases.

The atmosphere contains 20.9% oxygen. O2 partial pressure (760 times 20.9%) 159. The atmosphere is 79% nitrogen. N2 partial pressure (760 times 79%) 600 while 0.1% of the atmosphere is made up of other gases.

Gases diffuse across semipermeable membranes, displaying gradients of higher and lower concentrations. Oxygen and CO2 exchange across the alveolar-capillary membrane during respiration. Capillary units deliver oxygenated blood to the tissues while alveolar units ventilate the body returning carbon dioxide to the alveoli for removal via the lungs.


Extremes of age from newborns to geriatrics;

 A continuous change in fetal circulation affects the reports. In elderly patients aged above 60 years, PaO2 declines by 1 mmHg per year.

Any increase in activity or exercise; 

patients suffering from cardiopulmonary disorders, even with the same load, and oxygen consumption continues to increase which is not the same case in a healthy individual. 

Febrile state; 

that the increase in metabolism leads to a significant increase in oxygen consumption.

During pregnancy; 

especially the third trimester; when most females feel breathless


many patients suffering from COPD or spinal cord injury have hypoxemia during sleep

High altitude;

where barometric pressure is lower and oxygen saturation is low

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