ABG Notes Compilation

Acid-Base Balance Basics

  • Acid-base balance is a challenging topic for various healthcare students (medical, nursing, paramedics, respiratory therapists).

  • The approach focuses on conceptual understanding.

  • The discussion covers acid-base values, interpretation of imbalances, and compensation.

Arterial Blood Gas (ABG) Sampling

  • An ABG requires arterial blood.

  • Before drawing arterial blood, perform the Allen test to ensure collateral circulation.

  • Allen test involves applying pressure to both radial and ulnar arteries.

  • Collateral circulation is crucial to prevent hand ischemia if one artery is damaged during the ABG draw. Potential damage can occur, and the Allen test is a safety measure.

ABG Lab Values

  • Three key values: CO2 (carbon dioxide), HCO3 (bicarbonate), and pH.

  • Normal values are essential to know.

  • Differences in normal values may exist across books and labs, but focus on understanding the concepts.

Normal Values:

  • CO2: 35-45

  • HCO3: 22-26

  • pH: 7.35-7.45

  • Association: 7. 35-7.45 (pH) is related to 35-45 (CO2) for easier memorization.

Understanding pH

  • pH scale: 1 to 14.

  • pH measures hydrogen ions concentration using a logarithmic calculation, resulting in numbers opposite of intuition.

  • Alkaline (basic) is a high number, while acidic is a low number.

  • Acidic is associated with lemons, indicating a low pH.

  • Alkaline is associated with being basic, relating to a high number.

Understanding Carbon Dioxide (CO2)

  • CO2 has acidic properties.

  • Managed by the respiratory system.

  • When CO2 combines with water in the body, it becomes carbonic acid.

  • High respirations blow off CO2.

  • "You mad bro? Go outside and blow off some CO2" to remember CO2 management by the respiratory system.

Understanding Bicarbonate (HCO3)

  • Bicarbonate is basic.

  • Managed by the renal system (kidneys).

  • Kidneys reabsorb bicarbonate and excrete acids.

  • High pH indicates a basic problem (alkalosis).

  • The renal system is slow to respond.

Kidney Function

  • Kidneys reabsorb bicarbonate to conserve it.

  • Kidneys excrete acids.

  • In renal failure, bicarbonate levels are low because the kidneys cannot reabsorb it from the urine.

Three-Step Process for Interpreting ABGs

  1. Assess pH: low pH indicates acidosis, high pH indicates alkalosis.

  2. Interpret CO2: determine if it's normal, high, or low and whether it's moving in the opposite direction of pH, indicating a respiratory problem.

  3. Interpret HCO3: determine if it's normal, high, or low to see if it indicates a metabolic problem.

  4. Determine if compensation is present.

Interpreting ABG Values: Examples

Example 1

  • pH: low

  • CO2: high

  • HCO3: normal

  • This indicates respiratory acidosis.

  • Patients in respiratory acidosis are not adequately oxygenating.

  • Slow respiration leads to CO2 build-up, as seen in drug overdose patients.

  • Mechanism: CO2 build-up triggers the need to breathe; COPD patients rely on low oxygen levels instead.

Example 2

  • pH: high

  • CO2: low

  • HCO3: normal

  • This indicates respiratory alkalosis.

  • Patients with respiratory alkalosis are hyperventilating.

  • Hyperventilation results in paresthesias and tetany (Trousseau's sign).

  • Management: re-breathe CO2 using cupped hands or a paper bag.

Example 3

  • pH: high

  • CO2: high

  • HCO3: high

  • This indicates metabolic alkalosis.

  • Kidney issue causes retention of too much bicarbonate.

Example 4

  • pH: low

  • CO2: low

  • HCO3: low

  • This indicates metabolic acidosis.

  • The renal failure results in the kidneys not reabsorbing biacrb.

Key Concepts

  • Respiratory problems (CO2) lead to quick changes.

  • Renal problems (HCO3) lead to slow changes.

  • Respiratory disorders cause opposite changes in pH and CO2.

  • Metabolic disorders cause changes in the same direction for pH and HCO3.

Guidelines for Assessing Compensation

  • Compensation: True compensation rarely puts pH back in normal range.

  • ROME: Respiratory Opposite, Metabolic Equal.

  • Full compensation: pH is back in the normal range.

  • All values up: metabolic alkalosis.

  • All values down: metabolic acidosis.

Ratio to Maintain Balance

  • CO2 to HCO3 Ratio: It takes 20 parts CO2 to one-part bicarbonate.

Compensation Example

  • pH: Low

  • CO2: High

  • HCO3: High

  • The HCO3 is out of normal range and increasing to compensate for the acidosis.

ABG Analysis Tips

  • pH: is the patient acidotic or alkalotic?

  • Are pH and CO2 in opposite directions?

  • Are all values moving in the same direction?

ABG interpretation: Case Study

  • 26-year-old male has motorcycle accident and ICU admission on mech vent.

  • ABG Values: pH 7.24, PaCO2 68, HCO3 25.

  • What do you derive from this information?

  • ABG Interpretation: primary acidosis, respiratory origin. Therefore, respiratory acidosis.

  • Action: ensure effective rate and depth of respiration.

  • ABCs: ensure airway and breathing.