Respiratory Physiology

Lung Volumes & Capacities - Lung Locker Lowdown

⭐ FRC is crucial in anesthesia as it's the primary oxygen store during apnea (e.g., induction). It's significantly ↓ by supine position, obesity, pregnancy, and general anesthesia, predisposing to rapid desaturation.

Ventilation, Perfusion & V/Q - Air-Blood Duet

• Ventilation: Key to gas exchange.
  • Minute Ventilation ($V_E$): $TV \times RR$. Total air moved/min.
  • Alveolar Ventilation ($V_A$): $(TV - V_D) \times RR$. Air for gas exchange.
• Dead Space ($V_D$): Air not in gas exchange.
  • Anatomical ($V_{D,anat}$): Conducting airways.
  • Physiological ($V_{D,phys}$): $V_{D,anat}$ + non-perfused alveoli.
  • Bohr Equation: $V_D/V_T = (PaCO_2 - PECO_2)/PaCO_2$. Normal $V_D/V_T$: 0.2-0.35.
• West's Zones of Lung: (📌 Mnemonic: 'Downhill blood flow')

  Zone	Pressures	Blood Flow	V/Q
  1	$P_A > P_a > P_v$	Lowest	High
  2	$P_a > P_A > P_v$	Medium	Match
  3	$P_a > P_v > P_A$	Highest	Low
  

• V/Q Ratio: Ventilation to Perfusion. Normal ~0.8.
• V/Q Mismatch: Imbalance between ventilation and perfusion.

  Type	V/Q	Key Feature	O2 Correction
  Shunt	0	No ventilation, perfusion OK	Poor
  Dead Space Ventilation	∞	No perfusion, ventilation OK	Good

⭐ General anesthesia often increases physiological dead space and promotes atelectasis, leading to increased V/Q mismatch, primarily through an increase in shunt.


Gas Exchange & Transport - Gas Swap Gala

• Diffusion: Fick's Law. DLCO: diffusion capacity.
• Oxygen Cascade: $PIO_2 \rightarrow PAO_2 \rightarrow PaO_2 \rightarrow$ tissue $PO_2$.
• $O_2$ Content & Delivery:
  • $CaO_2 = (Hb \times 1.34 \times SaO_2) + (PaO_2 \times 0.003)$.
  • $DO_2 = CO \times CaO_2 \times 10$ (Normal ~1000 mL/min).
  • $VO_2$ (Normal ~250 mL/min).
• Oxyhemoglobin Dissociation Curve (ODC):
  • P50: ~26.6 mmHg. Steep part: $PaO_2$ <60 mmHg. | Right Shift ($\downarrow O_2$ Affinity) | Left Shift ($\uparrow O_2$ Affinity) | | :----------------------------------------- | :-------------------------------------------- | | $\uparrow PCO_2$, $\downarrow pH$ (Acid), $\uparrow$Temp, $\uparrow$2,3-DPG | $\downarrow PCO_2$, $\uparrow pH$, $\downarrow$Temp, $\downarrow$2,3-DPG | | Exercise | HbF, COHb, MetHb |
  • 📌 CADET, face Right! (Right shift: $\uparrow CO_2$, Acid, 2,3-DPG, Exercise, Temp).
• Gas Transport Effects:
  • Bohr Effect: $\downarrow pH / \uparrow PCO_2 \rightarrow$ ODC right shift ($O_2$ release).
  • Haldane Effect: Deoxy-Hb binds more $CO_2$.

  ⭐ The Haldane effect is quantitatively more important for $CO_2$ transport than the Bohr effect is for $O_2$ transport. It facilitates $CO_2$ uptake in tissues and release in lungs.

• $CO_2$ Transport Forms: Bicarbonate (~70%), CarbaminoHb (~23%), Dissolved $CO_2$ (~7%).

Respiratory Mechanics & Control - Breathing's Boss & Bellows

• Mechanics:
  • Compliance: Lung inflation ease. Static $C_{stat} = \Delta V / \Delta P_{plat}$; Dynamic $C_{dyn} = \Delta V / \Delta P_{peak}$.
  • Elastance: Recoil ($1/Compliance$).
  • Airway Resistance ($R_{aw}$): $(P_{peak} - P_{plat}) / Flow$. Poiseuille's Law: $R \propto 1/r^4$.
  • Work of Breathing: Overcomes elastic, resistive forces.
  • P-V Loops: Show compliance, work.
• Control:
  • Neural: Medulla (DRG: inspiration; VRG: insp/exp); Pons (Apneustic: $\uparrow$insp; Pneumotaxic: $\downarrow$insp).
  • Chemical:
    • Central Chemoreceptors: CSF $[H^+]$ (from $CO_2$).
    • Peripheral (Carotid/Aortic bodies): Respond to $PaO_2 < \textbf{60 mmHg}$, $\uparrow PaCO_2, \downarrow pH_a$.
  • Hering-Breuer Reflex: Inflation inhibits inspiration.

⭐ Most volatile anesthetics cause dose-dependent depression of the respiratory response to both hypercapnia and hypoxia, primarily by acting on central and peripheral chemoreceptors and respiratory centers.

High‑Yield Points - ⚡ Biggest Takeaways

• FRC: Key O2 reservoir; ↓ in obesity, supine, GA, risking hypoxia.
• Closing Capacity (CC): ↑ with age. CC > FRC causes atelectasis, V/Q mismatch.
• Dead Space (VD/VT): Normal 0.2-0.35. ↑ in PE, COPD, ↓cardiac output.
• HPV: Optimizes V/Q matching. Inhibited by high-dose volatiles, sepsis.
• O2-Hb Curve Right Shift: ↓O2 affinity (↑P50) via ↑H+, ↑CO2, ↑Temp, ↑2,3-DPG.
• Compliance: ↓Lung in ARDS, fibrosis. ↓Chest wall in obesity, ascites.