Applied Respiratory Physiology

Lung Volumes & Mechanics - Air Chambers & Pipes

• Lung Volumes (approx. values for 70kg adult):
  • Tidal Volume (TV): 500 mL (normal breath)
  • Inspiratory Reserve Volume (IRV): 3000 mL
  • Expiratory Reserve Volume (ERV): 1100 mL
  • Residual Volume (RV): 1200 mL (cannot be measured by spirometry)
• Lung Capacities (sums of ≥2 volumes):
  • Inspiratory Capacity (IC) = TV + IRV
  • Functional Residual Capacity (FRC) = ERV + RV ≈ 2300 mL
  • Vital Capacity (VC) = IRV + TV + ERV ≈ 4600 mL
  • Total Lung Capacity (TLC) = VC + RV ≈ 5800 mL
• Mechanics:
  • Compliance ($C = \Delta V / \Delta P$): Lung distensibility. ↓ in Fibrosis, ARDS, Pulm Edema; ↑ in Emphysema, Ageing.
  • Resistance ($R = \Delta P / \text{Flow}$): Airway opposition, mainly in large airways. ↑ in Asthma, COPD, Bronchitis.
  • Anatomical Dead Space: Conducting airways. ~150 mL or 2 mL/kg. 📌 Mnemonic: "Anatomic Two mL/kg".

⭐ FRC (Functional Residual Capacity) is the volume of air in lungs after normal passive exhalation; it's the equilibrium volume where inward lung elastic recoil balances outward chest wall recoil. Measured by helium dilution or body plethysmography, not spirometry alone.

Gas Exchange & Transport - Oxygen's Great Exchange

• Diffusion: Governed by Fick's Law. Depends on pressure gradient, surface area, membrane thickness, gas solubility.
• Oxygen Transport:
  • Primarily bound to Hemoglobin (Hb). Small amount dissolved.
  • Oxyhemoglobin Dissociation Curve (ODC): Sigmoidal. $P_{50}$ ">

⭐ Cyanide poisoning causes histotoxic hypoxia by inhibiting cytochrome c oxidase, leading to increased mixed venous oxygen saturation as tissues cannot utilize delivered oxygen.

Ventilation/Perfusion - Lung's Perfect Pair

• V/Q Ratio: Balance between alveolar ventilation ($V_A$ ~4 L/min) & pulmonary blood flow ($Q$ ~5 L/min). Ideal $V_A/Q$ ≈ 0.8.
• Optimal Gas Exchange: Requires well-matched V and Q.
• V/Q Mismatch Types:
  • Dead Space (V/Q > 0.8 to ∞): Ventilated, not perfused.
    • Physiological = Anatomic + Alveolar.
    • Causes: PE, ↓CO, emphysema. Leads to ↑$PaCO_2$.
  • Shunt (V/Q < 0.8 to 0): Perfused, not ventilated.
    • Causes: Atelectasis, pneumonia, ARDS. Leads to ↓$PaO_2$.

    ⭐ True shunt (V/Q=0) causes hypoxemia refractory to 100% O2 administration.

• Regional Distribution (Upright Lung):
  • Apex: ↑V/Q (↓$Q$ more than ↓$V$)
  • Base: ↓V/Q (↑$Q$ more than ↑$V$)

Respiratory Control & Failure - Breathless Alarms

• Neural Control:
  • Medulla: DRG (Dorsal - Inspiration 📌 "DI"), VRG (Ventral - Expiration 📌 "VE", forced).
  • Pons: Pneumotaxic (↑Rate, ↓Depth 📌 "Taxes breath"), Apneustic (↑Depth, ↓Rate 📌 "Aids inspiration").
• Chemical Control:
  • Central Chemoreceptors: Medulla, sense CSF $H^+$ (from $CO_2$).
  • Peripheral Chemoreceptors: Carotid/Aortic bodies, sense $↓PaO_2 (< \textbf{60} mmHg)$, $↑PaCO_2$, $↑H^+$.
• Respiratory Failure (RF): Inability to maintain $O_2$ or eliminate $CO_2$.
  • Type I (Hypoxemic): $PaO_2 < \textbf{60} mmHg$, normal/↓$PaCO_2$. (e.g., ARDS, Pneumonia).
  • Type II (Hypercapnic): $PaCO_2 > \textbf{45-50} mmHg$. (e.g., COPD, NMD).
• Alarms: Dyspnea, tachypnea, cyanosis, altered sensorium, ABG.

⭐ ARDS Berlin Criteria: Acute onset, bilateral opacities (not fully cardiac), $PaO_2/FiO_2 \le \textbf{300} mmHg$. Severity: Mild ($\textbf{201-300}$), Mod ($\textbf{101-200}$), Severe ($\le \textbf{100}$).

High‑Yield Points - ⚡ Biggest Takeaways

• ARDS: Bilateral opacities, PaO2/FiO2 < 300 mmHg, non-cardiogenic edema.
• Lung Protective Ventilation: Low tidal volumes (4-6 mL/kg PBW), Pplat < 30 cmH2O.
• PEEP: Improves oxygenation, recruits alveoli; risks barotrauma, hemodynamic compromise.
• Driving Pressure (Pplat - PEEP): Target < 15 cmH2O in ARDS for improved outcomes.
• Permissive Hypercapnia: Minimizes Ventilator-Induced Lung Injury (VILI).
• Prone Positioning: Improves V/Q matching and oxygenation in severe ARDS.