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Various blood gas values, whether high or low, give a different diagnosis.

In contrast to traditional blood tests, arterial blood gases (ABGs) provide the doctor with values of six different components that must be interpreted as a single result.

1. pH
   • 7.5 to 7.45 (a high pH often points to metabolic or respiratory alkalosis).
   • pH value determines the presence of acidosis or alkalosis in the blood. If the pH is less than the normal range, it indicates acidosis, and a pH value above than normal range indicates alkalosis.
2. PaCO₂ (partial pressure of carbon dioxide)
   • 35 to 45 mmHg (a high PaCO₂ may indicate hypercapnia).
   • The PaCO₂ level represents the level of alveolar ventilation. A high PaCO₂ indicates alveolar hypoventilation, whereas a reduced PaCO₂ indicates alveolar hyperventilation.
   • Acute fluctuations in PaCO₂ will cause pH to shift. PaCO₂ usually has a delayed reaction, so an increase in PaCO₂ occurs slowly, and an extremely high PaCO₂ indicates a long-standing condition related to lungs or metabolism. As a result, even when treated, PaCO₂ levels may take a long time to recover.
3. PaO₂ (partial pressure of oxygen)
   • 75 to 100 mmHg (a high PaO₂ is often seen in intensive care units or ICU settings where external O₂ is supplied).
   • PaO₂ indicates gas exchange in the lungs, and it determines the status of arterial blood oxygenation. A low PaO₂ indicates hypoxemia (low oxygen in the blood), which may be caused by hypoventilation or a mismatch between ventilation and perfusion.
   • The PaO₂ generally falls with aging because the elderly have less elastic recoil in their lungs, resulting in a higher ventilation-perfusion mismatch. Therefore, a low PaO₂ is concerning in a young individual, but it may be normal in older people.
4. HCO₃ (bicarbonate)
   • 22 to 26 mEq/L (high bicarbonate is indicative of metabolic alkalosis).
   • HCO₃ (bicarbonate) acts as a buffer that regulates the pH balance of the blood and prevents the blood from becoming too acidic or alkaline. HCO₃ is exhaled out as CO₂, and even kidneys excrete or reabsorb HCO₃ to maintain the proper pH balance of the blood.
5. SpO₂
   • 94 to 100 percent.
   • SpO₂ indicates the amount of healthy hemoglobin that is carrying oxygen in the blood.
6. Base excess (BE)
   • −2 to +2 mEq/L (seen in metabolic disturbances).
   • Base excess ranges from being negative to positive where the lower negative number is the base deficit, and the higher positive number is the base excess. The amount of HCO₃ in the blood determines the BE. If the BE is excess indicating a positive number, then it indicates alkalosis, and if the number is low toward a negative side, it indicates acidosis.

An imbalance between oxygen and carbon dioxide in the blood results in various metabolic disorders. By knowing the values of these six components a doctor can diagnose respiratory diseases, functional disorders of major organs, such as kidneys and heart, drug overdose, and metabolic disorders.

Table. Imbalances and their causes Imbalances Metabolic acidosis (serious electrolyte disorder characterized by an imbalance in the body's acid-base balance) Metabolic alkalosis Respiratory acidosis (lungs cannot remove enough of the carbon dioxide produced by the body) Respiratory alkalosis

pH
Decreases
Increases
Decreases
Increases

PaCO₂
Decreases
Increases
Increases
Decreases

HCO₃
Decreases
Increases
Increases
Decreases

Base excess
Decreases
Increases
Normal or increases
Normal or increases

Causes

• Diabetic ketoacidosis,
• kidney diseases,
• drug overdose,
• lactic acidosis,
• severe dehydration

• Loss of stomach acids,
• diuretics,
• excess antacid consumption,
• hypokalemia (low potassium levels),
• a reduced blood volume in the arteries,
• genetic predisposition (such as Liddle’s syndrome,
• Bartter’s syndrome,
• Gitelman’s syndrome),
• Apparent mineralocorticoid excess (AME),
• glucocorticoid remediable aldosteronism

• Acute pulmonary edema,
• neuromuscular disorders (such as multiple sclerosis or muscular dystrophy), 
• severe obesity,
• scoliosis

• Anxiety or panic, 
• fever,
• pain,
• tumor,
• trauma,
• over-breathing (hyperventilation),
• severe anemia,
• liver disease,
• overdose of medications (such as
• salicylates or
• progesterone),
• seen normally during pregnancy

2 types of blood gas tests

Blood gases test can be done with either arterial blood, which is called arterial blood gas, or venous blood gas, which is called venous blood gas.

1. Venous blood gas (VBG): Done to study the gases, such as oxygen and carbon dioxide, that are dissolved in the blood.
2. Arterial blood gases (ABG): Measures how effectively the lungs move oxygen into the blood and eliminate carbon dioxide from the blood, as well as how much hemoglobin can transmit oxygen throughout the body.

ABG is often done to detect any abnormalities because VBG contains less partial pressure of oxygen (PO₂) than ABG.

Arteries carry oxygen-rich blood from the lungs to all the other organs and tissues in the body. Unlike conventional blood samples collected through veins, a blood sample is collected from arteries before the blood reaches the body tissues.

Such arteries include:

• Radial artery
• Brachial artery
• Femoral artery

An ABG is one of the most regularly used tests, which determines the amount of the buffer system's compensation, measures acid-base balance (pH), oxygen (O₂) levels, and carbon dioxide (CO₂) in arterial blood.

An ABG is a key indicator of a patient's clinical condition and proper interpretation of an ABG can lead to faster and more accurate adjustments in the plan of treatment.

A venous blood gas (VBG) can estimate pH and CO₂ properly but cannot offer good O₂ data. As a result, ABG has become the gold standard in patients who are ill and at a risk of rapid decompensation, have a respiratory component, or are admitted in hospital settings.