Overview
This lecture concludes the course on understanding arterial blood gases (ABGs) by integrating key concepts into a unified model of human pathophysiology, with a focus on oxygen content, delivery, and ABG interpretation for clinical diagnosis.
Oxygen Content and Delivery
- Arterial oxygen content (CaO2) equals oxygen bound to hemoglobin plus oxygen dissolved in blood.
- CaO2 ≈ 1.34 × hemoglobin concentration × O2 saturation; dissolved oxygen is negligible.
- Oxygen delivery (DO2) to tissues is cardiac output × CaO2.
- Cardiac output depends on heart rate, intravascular volume, contractility, and afterload.
- Regional DO2 = regional blood flow (Q) × CaO2.
Unified Model of Pathophysiology
- Primary aim: human physiology maintains aerobic respiration by O2 delivery and CO2 removal.
- Four core domains analyzable by ABG: acid-base balance (Henderson-Hasselbalch), ventilation (alveolar ventilation equation), oxygenation (alveolar gas equation, AA gradient), and O2 transport/delivery (O2 dissociation curve, CaO2, DO2).
- Intravascular volume and cardiac function are also significant, affecting stroke volume and perfusion.
ABG Analysis Approach
- Start with oxygenation: check AA gradient, compare to expected, check saturation gap (<5% normal).
- Assess acid-base status: check pH, PaCO2, then compensation, anion gap, and delta ratio.
- Develop differential diagnoses for findings, using history, labs (lactate, ketones), and imaging.
- Synthesize findings into a final diagnosis that ties abnormalities together.
Case Summaries and Interpretation
- Case 1: Elderly woman with fever and distress—elevated AA gradient hypoxemia, respiratory alkalosis, high anion gap metabolic acidosis; diagnosis: septic shock from pneumonia, with lactic acidosis.
- Case 2: Unconscious middle-aged man—normal AA gradient hypoxemia, respiratory acidosis, high anion gap metabolic acidosis, metabolic alkalosis; diagnosis: drug overdose (alcohol, opiates, benzodiazepines), alcoholic ketoacidosis, lactic acidosis, dehydration.
- Case 3: Comatose man with AIDS—normal AA gradient, high saturation gap, concurrent respiratory alkalosis and metabolic acidosis, high and normal anion gap acidosis; diagnosis: methemoglobinemia (Bactrim overdose), lactic acidosis (aspirin/methemoglobinemia), acute kidney injury, respiratory alkalosis (aspirin/methemoglobinemia).
Key Terms & Definitions
- CaO2 — Arterial oxygen content: total O2 per 100 ml of blood.
- DO2 — Oxygen delivery: amount of O2 delivered to tissues per minute.
- AA Gradient — Difference between alveolar and arterial O2, indicating oxygenation efficiency.
- Saturation Gap — Difference between pulse oximetry O2 sat and ABG-calculated sat; >5% suggests abnormal hemoglobin.
- Delta Ratio — Value to distinguish mixed acid-base disorders in elevated anion gap metabolic acidosis.
- Methemoglobinemia — Condition with oxidized hemoglobin that impairs O2 delivery.
Action Items / Next Steps
- Practice ABG interpretation using structured approach outlined.
- Review key equations: Henderson-Hasselbalch, alveolar gas equation, CaO2 formula.
- Complete assigned readings or sample case analysis as directed by instructor.