Volumes and capacities

\(N_2\) wash-out method

Subject breathes 100% FIO2

Exhaled nitrogen concentration over time is collected, and integrated to find total exhaled \(N_2\) volume

Dividing by 0.79 (initial \(N_2\_ concentration) gives TLV

Drawbacks: Some \(N_2\) is washed out from blood and tissues (~250mL), only measures ventilated lung volume, leaks will befoul measurement

Helium wash-in method

Inhale gas bolus of known concentration and volume

Hold breath and wait to equilibrate throughout lung volime

Exhale gas

$$C_1 V_{\text{bolus}} = C_2 (\text{FRC} + V_{\text{bolus}})$$

Drawbacks: Helium will dissolve into tissues, only measures ventilated lung

Body Plethysmography

Subject confined in a closed box; airway pressure + box pressure measured

Exhales against closed airway \( \to \ \downarrow V_{\text{Chest}} \ \uparrow V_{box} \ \to \uparrow P_{\text{mouth}} \ \downarrow P_{\text{box}} \)

Measures whole intrathoracic gas volume

Drawbacks: Expensive, immobile equiptment

$$PV = nRT = \text{constant}$$

$$P1_{mouth} = P1_{box} = P_{\text{barometric}}$$

$$P1 V_{box} = P2_{box} (V_{box} + \Delta V)$$

$$\Delta V = V_{box} \frac{P1 - P2_{box}}{P2_{box}}$$

$$P1 FRC = P2_{mouth} (FRC - \Delta V)$$

$$\Delta V = FRC \frac{P1 - P2_{mouth}}{P2_{mouth}}$$

$$FRC \frac{P1 - P2_{mouth}}{P2_{mouth}} = V_{box} \frac{P1 - P2_{box}}{P2_{box}} $$

$$FRC = V_{box} \frac{P1 - P2_{box}}{P1 - P2_{mouth}} \frac{P2_{mouth}}{P2_{box}} $$

$$FRC = V_{box} \frac{\Delta P_{box}}{\Delta P_{mouth}} \frac{P2_{mouth}}{P2_{box}} $$

Because the pressure swings are small compared to barometric pressure, \(\frac{P2_{mouth}}{P2_{box}} \approx 1\) and

$$FRC = V_{box} \frac{\Delta P_{box}}{\Delta P_{mouth}}$$

The maximal lung volume at which airway closure occurs.

Equals RV + closing volume.

Effect of increased closing capacity

• Impairs denitrogenation (collapsed airways when FRC < CC are not preoxygenated)
• Causes atelectasis
• Causes age-related decrease in sats by creating shunt
• Aggrevates lung injury

Increased by:

1. Higher gas flow rate
2. Higher expiratory effort
3. Small airways disease
4. Increased pulmonary blood volume
5. Decreased surfactant
6. Age (at 44, supine FRC = CC; at 66, erect FRC = CC)

Measured by nitrogen washout

• Subject exhales to RV (apical alveoli open, dependant alveoli collapsed
• Inhales 100% FiO2 (apical alveoli filled with mixed gas, dependant filled with pure O2)
• Exhalation has four stages - dead space is pure O2, then N2 rises with mixed alveolar/dead space gas, plateaus with alveolar gas, then sharply rises again as dependant alveoli collapse and more nitrogenated apical alveoli continue to exhale

FRC occurs at balance of outwards chest wall recoil and inwards lung recoil, i.e. point of no potential energy. Therefore it is altered by...

Factors affecting lung recoil

1. Elastic tissue of lung (\uparrow\) in fibrosis, \(\downarrow\) in emphysema; age \(\to\) FRC increases with age
2. Surface tension forces (\uparrow\) in infantile ARDS

Factors affecting chest wall recoil

1. Skin; FRC \(\downarrow\) with circumferential burns
2. Fat; FRC \(\downarrow\) with obesity
3. Bones; FRC \(\uparrow\) with chest fractures, open chest, Ehlers-Danlos
4. External compression; FRC \(\downarrow\) with \(\uparrow\)IAP, bindings