Week 2: Arterial Blood Gases (ABGs)

What are ABGs

• Definition: They are diagnostic tests that measure:

  • Acid–base status

  • Oxygenation levels in the blood

  • Typically drawn from the radial artery

Indications for ABGs

• Respiratory distress or failure: Assessing the severity of respiratory issues.

  • Most common.

  • Done bedside.

  • Typically, when calling a rapid response.

• Sepsis and shock: Determining the metabolic state and oxygen delivery.

• Diabetic Ketoacidosis (DKA) and metabolic disorders: Evaluating acid-base disturbances.

• Altered mental status: Identifying potential metabolic or respiratory causes.

  • A common chronic problem that can cause an altered mental status is COPD due to the retained CO2. Resulting in needing an ABG.

  • Obstructive sleep apnea is another reason for this because many refuse to wear a CPap when sleeping.

• Ventilator management: Guiding adjustments in mechanical ventilation settings.

• Urgent decision making: Helping clinicians make rapid assessments in critical situations.

ABG Normal Values

• pH: 7.35–7.45

• PaCO₂: 35–45 mmHg

• HCO₃⁻: 22–26 mEq/L

• PaO₂: 80–100 mmHg

• SaO₂: 95–100%
  NOTE: These values are crucial for diagnosis and must be MEMORIZED by healthcare practitioners.

Psyiological Regulation of Acid-Base Balance

• Lungs: Regulate CO₂ levels, which affect pH; faster breathing decreases CO₂ (increases pH), slower breathing retains CO₂ (decreases pH).

• Kidneys: Regulate HCO₃⁻ levels; they can excrete or retain hydrogen ions.

• ROME: respiratory opposite, metabolic equal.

Role of the Lungs

• Control of PaCO₂: The lungs have a rapid response in regulating CO₂ levels, impacting pH directly:

  • Faster breathing → decrease in CO₂ → increase in pH (alkalosis)

    • Respiratory distress, anxiety.

  • Slower breathing → increase in CO₂ → decrease in pH (acidosis)

    • COPD, respiratory depression.

  • Key Point: Respiratory conditions are primarily reflected by changes in CO₂ levels.

Role of the Kidneys

• Control of HCO₃⁻: The kidneys respond more slowly but are critical in long-term pH balance by affecting bicarbonate concentrations.

• Response Mechanisms: They can excrete or retain hydrogen ions to regulate acid-base balance.

  • Low pH, below the waist.

  • BMP: The CO2 lab value helps with determining the kidney’s function, which is the opposite of the CO2 in an ABG.

Respiratory Acidosis

• Criteria: pH ↓, PaCO₂ ↑

  • Severe Cases: If pH < 7.25 and PaCO₂ > 55, mechanical ventilation may be required!!!!

    • Either intubated at bedside or placed on a BiPap until transfer.

  • Most commonly seen.

• Common Causes: Hypoventilation is seen in conditions such as:

  • Chronic Obstructive Pulmonary Disease (COPD)

  • Respiratory failure

  • Overdose on respiratory depressants (e.g., opioids)

    • Can give Narcan to wake them up and breathe better.

• Nursing Focus: Ensure proper airway management and ventilation support.

Respiratory Alkalosis

• Criteria: pH ↑, PaCO₂ ↓

• Common Causes: Hyperventilation resulting from conditions such as:

  • Anxiety

  • Pain

  • Early sepsis

• Blow into a paper bag; the bag will hold on to the CO2 and deliver it back into the lungs.

• Nursing Focus: Identify and address the underlying cause of hyperventilation.

Metabolic Acidosis

• Criteria: pH ↓, HCO₃⁻ ↓

• Common Conditions: Examples include:

  • DKA (Diabetic Ketoacidosis)

  • Lactic acidosis

    • Can be seen in patients with sepsis or in DKA.

  • Renal failure

  • Sepsis and shock

• Nursing Focus: Administer necessary fluids (to “wash them out”), insulin as needed.

Metabolic Alkalosis

• Criteria: pH ↑, HCO₃⁻ ↑

• Common Causes: Conditions such as:

  • Vomiting

  • NG suctioning

  • Diuretic use (abuse)

• Nursing Focus: Manage electrolyte imbalances and ensure proper fluid volumes.

  • Hypocalcemia and hypokalemia will be seen in these clients.

Compensated States

• Normal pH: Indicates full compensation.

• Uncompensated: Either CO₂ or HCO₃⁻ will be normal (not both).

• Partially Compensated: Neither CO₂ nor HCO₃⁻ is normal, indicating ongoing attempts at regulation.

  • If unsure which is the primary problem, look at the client’s clinical picture/diagnosis to help determine this.

Oxygenation Assessment

• PaO₂ < 80 mmHg: Indicates hypoxemia, a crucial condition affecting patient care.

• SaO₂: Reflects hemoglobin saturation and assists in guiding oxygen therapy and mechanical ventilation requirements.

• Intubation is done to help with CO2 but oxygen is delivered during this process to help compensate.

Nursing Responsibilites for ABG Imbalances

• Pre-Procedure: Perform an Allen test before arterial blood collection to assess collateral circulation.

• Post-Procedure: Monitor for bleeding or hematoma at the puncture site after ABG sampling.

  • Some clients may have an A-line, which is an IV inserted at the artery to create more ease in drawing labs rather than continuously poking them.

• Critical Value Recognition: Understand and identify critical ABG values for prompt intervention.

• Patient Correlation: Integrate ABG findings with the patient's clinical picture for comprehensive care.