Overview
This lecture covers acid-base imbalances, focusing on ABG (arterial blood gas) interpretation, key memory tricks for respiratory and metabolic disorders, and distinguishing compensation states.
Acid-Base Balance Basics
- Acid-base balance is the body's regulation of acid and base to maintain normal blood pH (7.35–7.45).
- A pH below 7.35 is acidosis; a pH above 7.45 is alkalosis.
- Main regulators: lungs (control CO₂, an acid) and kidneys (control HCO₃⁻, a base, and hydrogen ions, an acid).
Key Players and Memory Tricks
- Bicarbonate (HCO₃⁻) is a base; high levels → alkalosis.
- CO₂ is an acid; high levels → acidosis. Memory: "carbon dia-acid."
- Hydrogen ions are acids; high levels → acidosis.
Organ Involvement
- Lungs remove CO₂; low respiratory rate retains CO₂ (acidosis), high rate removes CO₂ (alkalosis).
- Kidneys excrete acid (H⁺) and retain base (HCO₃⁻) to balance pH.
Types and Causes of Acid-Base Disorders
- Metabolic Alkalosis: Vomiting or NG suction (loss of stomach acid), leads to base excess.
- Metabolic Acidosis: Diarrhea (loss of base), renal failure (acid retention), DKA (diabetic ketoacidosis).
- Respiratory Acidosis: Slow breathing (sleep apnea, head trauma, opioid/alcohol overdose, COPD, asthma, pneumonia).
- Respiratory Alkalosis: Fast breathing (panic attack, hyperventilation).
Compensation Concepts
- Compensation is the body's attempt to restore pH to normal (lungs and kidneys offset each other's imbalances).
- Full compensation: pH is normal; Partial: pH abnormal, but compensation is evident; Uncompensated: pH abnormal, no compensation.
ABG Interpretation Strategy ("Marching Band Suit" Method)
- pH: 7.35–7.45 (primary indicator; <7.35 acid, >7.45 base).
- CO₂: 35–45 mmHg (respiratory; <35 base, >45 acid).
- HCO₃⁻: 22–26 mEq/L (metabolic; <22 acid, >26 base).
- Match pH with CO₂ or HCO₃⁻ to determine respiratory or metabolic disorder.
- Determine compensation by whether pH is normal (full) or abnormal (partial/uncompensated).
Steps for ABG Problem Solving
- Step 1: Check pH (acid/base/normal).
- Step 2: Match with CO₂ (respiratory) or HCO₃⁻ (metabolic).
- Step 3: Assess if compensation is full (pH normal), partial (pH abnormal, compensation evident), or none (pH abnormal, no compensation).
ABG Procedure and Post-Care
- ABGs drawn from the radial artery; Allen’s test checks ulnar artery patency.
- Hold firm pressure after ABG draw to prevent bleeding due to arterial puncture.
Key Terms & Definitions
- Acidosis — Blood pH <7.35, excess acid.
- Alkalosis — Blood pH >7.45, excess base.
- Compensation — Body's response to restore normal pH by adjusting CO₂ and HCO₃⁻.
- Uncompensated — pH abnormal, no compensation.
- Partially Compensated — pH abnormal, evidence of compensation.
- Fully Compensated — pH normal, both systems involved.
- Metabolic — Related to kidney and HCO₃⁻.
- Respiratory — Related to lung and CO₂.
Action Items / Next Steps
- Practice ABG interpretation using the "marching band suit" method at least 5–10 times daily before exams.
- Memorize normal pH, CO₂, and HCO₃⁻ values.
- Review memory tricks for common causes of acid-base imbalances.
- Hold firm pressure after any arterial blood draw.