Setting Tidal Volume and Rate by Physiology

1. Oxygenation — Getting O₂ In

• Primary levers: FiO₂ (fraction of inspired oxygen) and PEEP (positive end-expiratory pressure).
• Real driver: Mean Airway Pressure (MAP) :  the average pressure applied to the lungs across the entire respiratory cycle.
• Key physiology:
  • Oxygen enters blood by diffusion down a concentration gradient.
  • Adequate alveolar surface area is critical → PEEP keeps alveoli open, prevents collapse/reopen injury, and ensures FiO₂ delivery actually translates into effective oxygenation.
• MAP analogy: Just as mean arterial pressure drives perfusion, mean airway pressure drives oxygenation. Prolonged inspiratory time or sustained pressure (e.g., APRV, inverse I:E) can raise MAP.
• Risks: Excessive pressure/volume can cause barotrauma or volutrauma. 

2. Ventilation — Getting CO₂ Out

• Primary levers: Tidal Volume (TV) and Respiratory Rate (RR).
• Minute Ventilation = RR × TV.
• Mechanism: Ventilation removes CO₂ through bulk convection (movement of air in and out).

Disease-specific strategies:

• Obstructive Disease (COPD / Asthma)
  • RR ↓ to allow more time for exhalation.
  • Ensure expiratory flow = inspiratory flow → prevents air trapping.
  • If not equal → auto-PEEP → increased intrathoracic pressure → ↓ preload, risk of hypotension, cardiac arrest, or pneumothorax.
• Metabolic Acidosis
  • RR ↑ to blow off CO₂ and buffer acidosis.
• ARDS
  • Tidal volume limited to 4–6 mL/kg IBW to minimize ventilator-induced lung injury.
  • RR becomes the main adjustment knob.
  • Exception: in obstructive lung disease, patients need extra time to exhale (I:E may be 1:4–1:6).