Hypoxia and Oxygen Transport

Hypoxia: Classification & Causes - Breathless States

• Hypoxia: Reduced O₂ availability at tissue level.
• Types & Causes:
  • Hypoxemic Hypoxia: ↓$P_aO_2$.
    • ↓Inspired $F_iO_2$ (high altitude)
    • Hypoventilation (COPD, opioids)
    • V/Q mismatch (PE, pneumonia)
    • Shunt (cardiac defects)
    • Diffusion impairment (ILD)
  • Anemic Hypoxia: ↓O₂ carrying capacity of blood.
    • Anemia (↓Hb)
    • CO poisoning
    • Methemoglobinemia
  • Stagnant (Ischemic) Hypoxia: ↓Blood flow to tissues.
    • Shock, heart failure
    • Local obstruction (e.g., thrombus)
  • Histotoxic Hypoxia: Cells unable to utilize O₂ effectively.
    • Cyanide (cytochrome oxidase inhibitor)
    • Alcohol, narcotics
• Dyspnea: Subjective sensation of difficult breathing.

⭐ Cyanide poisoning (histotoxic hypoxia) causes increased venous oxygen tension as tissues fail to extract oxygen.

Oxygen Transport & OHDC - Hemoglobin's Haul

• O₂ transport: Dissolved (2%), Hb-bound (98%).
• Hb: 4 O₂ sites; cooperative binding.
• 1g Hb carries 1.34 mL O₂.
• Arterial O₂ content ($CaO_2$): $(Hb \times 1.34 \times SaO_2) + (PaO_2 \times 0.003)$.
• OHDC:
  • Sigmoid shape (cooperative binding).
  • P50: $PO_2$ at 50% Hb saturation (Normal: 26.6 mmHg).
  • Right Shift (↓ Affinity, ↑O₂ release): ↑$PCO_2$, ↑H⁺(↓pH), ↑Temp, ↑2,3-DPG. 📌 CADET, face Right!
  • Left Shift (↑ Affinity, ↓O₂ release): ↓$PCO_2$, ↓H⁺(↑pH), ↓Temp, ↓2,3-DPG, HbF, COHb, MetHb.
• Bohr Effect: ↑$CO_2$/H⁺ → right shift (O₂ unloading).
• Haldane Effect: DeoxyHb binds more $CO_2$ (facilitates $CO_2$ transport).

⭐ Fetal hemoglobin (HbF) has higher O₂ affinity (left-shifted curve, P50 ~19-20 mmHg) than adult HbA, aiding maternal O₂ uptake.

Systemic Effects of Hypoxia - Body Under Siege

• CNS: Highly sensitive. Confusion, impaired judgment, ataxia. Severe (PaO₂ < 30 mmHg): Loss of Consciousness (LOC).
• Cardiovascular System (CVS):
  • Acute: ↑Heart Rate (HR), ↑Cardiac Output (CO) due to sympathetic drive. Severe/prolonged: Bradycardia, ↓Blood Pressure (BP), arrhythmias.
  • Chronic: Pulmonary hypertension, Right Ventricular Hypertrophy (RVH).
• Respiratory System:
  • Acute: Hyperventilation (Hypoxic Ventilatory Response - HVR) → Respiratory alkalosis.
  • Chronic: Blunted HVR, especially in high-altitude natives.
• Hematologic: Chronic: ↑Erythropoietin (EPO) from kidneys → Polycythemia (Hematocrit > 55%).
• Cellular: Shift to anaerobic glycolysis → ↑Lactic acid production.

⭐ Cyanosis (bluish discoloration of skin & mucous membranes) typically becomes apparent when the mean capillary concentration of deoxygenated hemoglobin exceeds 5 g/dL.

an

Acclimatization to Altitude - Summit Strategy

• Goal: Maximize $O_2$ delivery & utilization for extreme altitudes over weeks/months.
• Hematological:
  • ↑ EPO → ↑ RBC mass, Hb (Polycythemia).
  • ↑ 2,3-DPG → Right shift ODC (aids $O_2$ unloading).
• Ventilatory:
  • Sustained ↑ alveolar ventilation (HVR); peripheral chemoreceptors reset.
  • Renal $HCO_3^-$ excretion compensates respiratory alkalosis.
• Cardiovascular & Tissue:
  • Heart rate normalizes (vs. acute).
  • ↑ Tissue capillarity, myoglobin, mitochondrial density, oxidative enzymes.
• Summit Strategy:
  • "Climb high, sleep low"; gradual ascent.
  • Staging: 2-3 weeks > 3000m before higher ascent.
  • Supplemental $O_2$: Common > 7000m.
  • Pharmacological aids:
    • Acetazolamide: Aids acclimatization, prevents AMS.
    • Dexamethasone: HACE/HAPE treatment.
• Sleep: Periodic breathing (Cheyne-Stokes) common.

⭐ The most important long-term acclimatization mechanism is increased hemoglobin concentration due to EPO stimulation.

High‑Yield Points - ⚡ Biggest Takeaways

• Hypoxic hypoxia (e.g., high altitude): ↓PaO2, ↓SaO2. Anemic hypoxia: ↓Hb, normal PaO2.
• OHDC Right Shift (↑P50, ↑O2 release): due to ↑H+, ↑CO2, ↑Temp, ↑2,3-DPG.
• Acclimatization: Hyperventilation, polycythemia, ↑2,3-DPG, ↑capillaries.
• CO Poisoning: Cherry-red skin, normal PaO2, ↓SaO2, strong left shift of OHDC.
• P50: PaO2 at 50% Hb saturation (normal 26.6 mmHg); reflects Hb-O2 affinity.
• Oxygen content (CaO2) primarily depends on Hb concentration and SaO2.