VENTILATION

Minute Ventilation (VE)

• What it is: Total air moving in and out of lungs per minute
• Formula: VE = VT × RR
• Normal values to memorize:
  • RR = 12-20 breaths/min
  • VT (tidal volume) = 500 mL/breath
  • VD (dead space) = 150 mL/breath
• Example: 500 × 15 = 7,500 mL/min

Alveolar Ventilation (VA)

• What it is: Air that actually reaches the alveoli and participates in gas exchange (more important than minute ventilation)
• Formula: VA = (VT - VD) × RR
• Why subtract VD? The 150 mL dead space (trachea, bronchi) never reaches alveoli - it just sits in the airway doing nothing
• Example: (500 - 150) × 15 = 5,250 mL/min

FCPS tip: If a patient takes slow, deep breaths vs. fast, shallow breaths, the VA is very different even if VE is the same. Deep breaths = better alveolar ventilation.

LUNG AND CHEST WALL PROPERTIES

Hysteresis

• Simple concept: The pressure-volume curve during inflation is DIFFERENT from deflation
• Why? During inflation you must overcome surface tension (like blowing up a new balloon - harder at first)
• The graph (top right): Notice the lung volume and alveolar/intrapleural pressure waves during breathing
  • Intrapleural pressure swings between -6 and -10 cm H2O
  • It is always negative (keeps lungs from collapsing)

Elastic Recoil

• Lungs want to collapse inward (like a stretched rubber band wanting to snap back)
• Chest wall wants to spring outward
• At FRC (Functional Residual Capacity) these two forces balance each other perfectly
• At FRC: alveolar pressure = atmospheric (zero), intrapleural pressure is negative (-5 cm H2O)
• This negative intrapleural pressure is what prevents atelectasis (lung collapse)

Think of it like this: Lungs are a balloon inside a box. The balloon wants to shrink. The box wants to expand. FRC is where they reach a truce.

COMPLIANCE

• Definition: How easily the lung stretches - change in volume per change in pressure (ΔV/ΔP)
• Inversely proportional to stiffness - stiff lung = low compliance
• Surfactant increases compliance (reduces surface tension, makes inflation easier)

The Compliance Graph (bottom right)

• X-axis: Airway pressure (cm H2O)
• Y-axis: Volume (L)
• Green curve (normal lung-chest wall): Sits between the individual curves
• Emphysema curve: Shifted UP and LEFT = high compliance (fills easily at low pressure)
• Fibrosis curve: Shifted DOWN and RIGHT = low compliance (needs more pressure to fill)
• FRC is marked on the Y-axis - this is where breathing starts at rest

HIGH-YIELD EXAM MNEMONICS

• "ARDS, Fibrosis, Edema, Pneumonia" = all decrease compliance (stiff lungs)
• "Emphysema = Easy to fill, hard to empty" = increased compliance + air trapping
• Dead space = 150 mL = roughly 1 mL/lb body weight
• Negative intrapleural pressure = keeps lungs open at all times