NCLEX Prep: Interpreting Blood Gases

Whether you are preparing for the NCLEX-RN exam or a practicing Registered Nurse, having a basic understanding of blood gases and how to interpret them will be beneficial to you throughout your career. Understanding gas levels can help determine whether a patient is experiencing a respiratory or metabolic disorder and if that disorder is acidic or alkaline in nature. This can help determine both the cause of the condition and the appropriate treatment for that condition and its symptoms.

Interpreting blood gas results requires a fundamental understanding of what the normal ranges are for each of the gases being measured. The normal ranges for arterial blood gases are as follows:

• pH: 7.35 to 7.45

• PaO2 (partial pressure of oxygen in arterial blood): 75 to 100 mmHg

• PaCO2 (partial pressure of carbon dioxide in arterial blood): 35 to 45 mmHg

• HCO3- (bicarbonate): 22 to 28 mEq/L

Abnormal levels of these gases can indicate various respiratory or metabolic disorders. Here are some possible scenarios:

1. Respiratory acidosis: This occurs when there is an increase in PaCO2 and a decrease in pH. The lungs aren’t removing enough carbon dioxide (CO2), which then accumulates in the blood and can lead to a decrease in the blood's pH level. This is often due to various reasons such as hypoventilation (reduced breathing rate) or impaired lung function, leading to inadequate elimination of CO2.

   1. In normal circumstances, the lungs help maintain the balance of CO2 and oxygen levels in the blood. However, in respiratory acidosis, the lungs are unable to remove enough CO2 from the body, causing an accumulation of CO2 in the blood.

   2. The excess CO2 reacts with water in the blood to form carbonic acid, which can lower the pH level of the blood, making it more acidic. Symptoms of respiratory acidosis can include confusion, drowsiness, headache, shortness of breath, and a rapid heart rate.

   3. Common conditions that can result in respiratory acidosis include chronic obstructive pulmonary disease (COPD) such as chronic bronchitis or emphysema, respiratory failure, airway obstruction, neuromuscular disorders such as muscular dystrophy or ALS, or medications that suppress breathing such as opioids or sedatives.

   4. Treatment for respiratory acidosis depends on the underlying cause, but may involve oxygen therapy, bronchodilators, and mechanical ventilation.

2. Respiratory alkalosis: This occurs when there is a decrease in PaCO2 and an increase in pH. The lungs are removing too much CO2 from the body, causing a decrease in the concentration of CO2 in the blood and can lead to an increase in the blood’s pH level. This is often due to various reasons such as hyperventilation (increased breathing rate) or excessive mechanical ventilation.

   1. In normal circumstances, the lungs help maintain the balance of CO2 and oxygen levels in the blood. However, in respiratory alkalosis, the lungs are removing too much CO2 from the body, causing a decrease in the concentration of CO2 in the blood.

   2. The decreased levels of CO2 lead to a decrease in the formation of carbonic acid, which is the primary acid in the blood. This, in turn, increases the pH level of the blood, making it more alkaline. Symptoms of respiratory alkalosis can include lightheadedness, numbness, tingling in the extremities, muscle spasms, and in severe cases, seizures.

   3. Common conditions that can result in respiratory alkalosis include: hyperventilation as a result of anxiety or a panic attack, high altitudes due to decreased oxygen and increased ventilation, a pulmonary embolism, liver disease, or certain medications.

   4. Treatment for respiratory alkalosis depends on the underlying cause, but in severe cases, oxygen therapy or mechanical ventilation may be required.

3. Metabolic acidosis: This occurs when there is a decrease in HCO3- and a decrease in pH. There is an excess of acid in the blood, leading to a decrease in the blood's pH level. This can occur due to various reasons such as an increase in acid production or a decrease in the elimination of acids from the body. This can be caused by conditions such as renal failure, diabetic ketoacidosis, or lactic acidosis.

   1. One of the primary mechanisms that maintain the acid-base balance in the body is the bicarbonate buffer system. This system involves the production and elimination of bicarbonate ions (HCO3-) from the body, which helps neutralize acid in the blood. In metabolic acidosis, there is either a decrease in bicarbonate levels or an increase in acid levels, leading to an imbalance in the buffer system.

   2. Symptoms of metabolic acidosis can include rapid breathing, confusion, headache, fatigue, and in severe cases, coma.

   3. Common conditions that can result in metabolic acidosis include: kidney disease or failure, diabetes, lactic acidosis, or ingestion of certain toxins or medications such as methanol or aspirin

   4. Treatment for metabolic acidosis depends on the underlying cause, but may involve administration of fluids, electrolytes or bicarbonate.

4. Metabolic alkalosis: This occurs when there is an increase in HCO3- and an increase in pH. There is an excess of bicarbonate (HCO3-) in the blood, leading to an increase in the blood's pH level. This can occur due to various reasons such as a loss of acid from the body or an increase in bicarbonate levels. This can be caused by conditions such as vomiting, prolonged use of diuretics, or excess ingestion of bicarbonate.

   1. One of the primary mechanisms that maintain the acid-base balance in the body is the bicarbonate buffer system. This system involves the production and elimination of bicarbonate ions (HCO3-) from the body, which helps neutralize acid in the blood. In metabolic alkalosis, there is either an increase in bicarbonate levels or a decrease in acid levels, leading to an imbalance in the buffer system.

   2. Symptoms of metabolic alkalosis can include muscle weakness, cramps, nausea, vomiting, and confusion.

   3. Common conditions that can result in metabolic alkalosis include: vomiting or diarrhea due (the loss of stomach acid can lead to an increase in bicarbonate levels), overuse of antacids which can increase bicarbonate levels, liver or respiratory disease, or the use of certain medications (e.g. diuretics, corticosteroids)

   4. Treatment for metabolic alkalosis depends on the underlying cause, but may involve administering fluids or electrolytes. It is important to seek medical attention promptly if you experience symptoms of metabolic alkalosis.