Alterations in Arterial Blood Gases

Alterations in Arterial Blood Gases (ABGs)

Terminology Review

1. Arterial Blood Gases (ABGs): The assessment measures the partial pressures of oxygen (O2), carbon dioxide (CO2), pH (acid/base balance), and bicarbonate (HCO3) in the blood.

2. Acidosis: A condition characterized by an increase in hydrogen ion concentration, leading to a pH of less than 7.35.

3. Acids: Substances that donate hydrogen ions (H+) to a solution.a. Example: CO2 is a by-product of metabolism that can combine with water to form carbonic acid (H2CO3).

4. Alkalosis: Results from a decrease in hydrogen ions, signifying a pH greater than 7.45.

5. Anion Gap: The difference in concentration between cations and anions in the blood, useful in diagnosing metabolic acidosis.

6. Base Excess: Indicates the level of excess or insufficient bicarbonate in the bloodstream.

7. Bases: Substances that donate hydroxide ions (OH-) or accept hydrogen ions; a primary example is bicarbonate (HCO3).

8. Buffer: Chemical substances that help maintain the pH balance by either releasing or binding H+ ions.

9. Compensatory Mechanisms: The body's methods of adjusting pH to maintain homeostasis.

10. Partial Pressure: The pressure exerted by a particular gas in a mixture, typically observed as pO2 and pCO2 on ABG results.

11. pH: The measurement of hydrogen ions in the bloodstream; a higher concentration of H+ results in a lower pH (acidosis), while lower concentrations yield a higher pH (alkalosis).

Regulation of pH

• Goal: Homeostasis in acid-base balance is critical for physiological function.

  1. Buffers:

     • Bind or release H+ ions, operating continuously.

     • Examples: Proteins, carbonic acid-bicarbonate system, phosphate.

  2. Lungs:

     • Excrete CO2 during exhalation, regulating acid-base balance.

     • Alter respiratory rate based on pH levels. Responds to pH changes within 1-3 minutes, with effects lasting 12-24 hours.

     • Cannot remove H+ ions directly.

  3. Kidneys:

     • Produce, excrete, and retain HCO3 and excrete H+.

     • These mechanisms take approximately 24 hours to initiate, providing longer-lasting regulation.

Normal Ranges for ABG Results

• pH: 7.35 – 7.45

• pCO2: 35 – 45 mmHg

• HCO3: 22 – 26 mEq/L

• SaO2: 95 – 100%

• pO2: 80 – 100 mmHg

• Base Excess: -2 to +2 mEq/L

ABGs Analysis Process

• Step 1: Check the pH

  • Normal?

  • Low? (Acidosis)

  • High? (Alkalosis)

  • Conclusion: Gives the 'last name' of the ABG.

• Step 2: Check the pCO2

  • Normal?

  • Low?

  • High?

• Step 3: Check the HCO3

  • Normal?

  • Low?

  • High?

• Step 4: Determine the type of problem

  • pH - pCO2 - HCO3 Components:

    • Respiratory Acidosis: Low pH, High pCO2, Normal HCO3

    • Respiratory Alkalosis: High pH, Low pCO2, Normal HCO3

    • Metabolic Acidosis: Low pH, Normal pCO2, Low HCO3

    • Metabolic Alkalosis: High pH, Normal pCO2, High HCO3.

  • Conclusion: Gives the 'middle name' of the ABG.

Example ABG Result

• pH: 7.25

• pCO2: 56

• HCO3: 25

• SaO2: 90%

• pO2: 75

• Summary: R-O-M-E (Respiratory Opposite Metabolic Equal)

Assessment for Compensation

• Step 5: Compensation Assessment

  • Uncompensated: pH and one other value (pCO2 or HCO3) out of range.

  • Partially Compensated: All three values out of range (pH, pCO2, and HCO3).

  • Fully Compensated: Normal pH, but pCO2 and HCO3 are abnormal. Determine if the pH leans more towards acidosis or alkalosis (pH < 7.40 = acidosis, pH > 7.40 = alkalosis).

  • This step concludes by giving the 'first name' of the ABG.

Analyze pO2 and SaO2

• Assess whether these values are normal, low (hypoxemia), or high.

Respiratory Acidosis

• Pathophysiology: Arises when the lungs fail to expel enough CO2, resulting in an increase of carbonic acid in the blood.

• Causes: Conditions leading to inadequate ventilation including CNS depression, narcotics use, trauma, and respiratory diseases (e.g., COPD).

• Manifestations: Symptoms often include headache, confusion, weakness, tremors, paralysis, stupor, coma, and warm/flushed skin.

• Compensation: Kidneys respond by producing more bicarbonate to counterbalance the acidosis.

Respiratory Alkalosis

• Pathophysiology: A condition typically characterized by excessive CO2 exhalation.

• Causes: Driven by hyperventilation due to factors such as pain, anxiety, fever, sepsis, and CNS lesions.

• Manifestations: Constriction of cerebral blood vessels, dizziness, light-headedness, tetany, and numbness/tingling of extremities are common, along with potential seizures and cardiac dysrhythmias.

• Compensation: The kidneys counteract the alkalosis by reducing HCO3 production.

Metabolic Acidosis

• Common Causes: Decreased bicarbonate production, increased acid production, renal failure, or excessive loss of bicarbonate. Common scenarios include diabetic ketoacidosis, lactic acidosis, and gastrointestinal losses (e.g., diarrhea).

• Manifestations: Symptoms may present as hyperkalemia, abdominal pain, confusion, weakness, and warm/flushed skin. There's a heightened risk for rhythm disturbances of the heart.

• Compensation: The respiratory compensation involves increased breathing rate and depth (Kussmaul’s respirations) to remove CO2.

Metabolic Alkalosis

• Causes: Can occur from processes that increase bicarbonate levels, excessive intake of alkaline substances, or loss of acid (e.g., vomiting).

• Manifestations: Symptoms include hypokalemia, hyperactive reflexes, confusion, and seizures. Additionally, hypotension and cardiac dysrhythmias may arise.

• Compensation: The respiratory response involves a decrease in breathing rate to retain CO2.

Key Patterns in pH and ABGs

• pH & CO2: Inverse relationship observed in respiratory conditions—One goes up, the other must come down

• pH & HCO3: Direct relationship in metabolic conditions—Both increase or decrease together.