2015A09 What are the physiological consequences of decreasing
functional residual capacity by one litre in an adult?

 

List:

·         Intro: definition and significance

·         Determinants of FRC

·         Effects of 1L reduction in FRC

 

Intro: functional residual capacity (FRC)

Definitions

·   Volume of air in the lungs at the end of tidal expiration

·   Point at which lung inward recoil = chest wall outward recoil

·   Sum of residual volume + expiratory reserve volume

Normal

·   Upright: 40mL/kg (~2,800mL)

·   Supine: 30mL/kg (~2,100mL)

Significance of 1L decrease in FRC

·   Large reduction (30-50%)

·   From upright FRC: well tolerated

·   From supine FRC: poorly tolerated, especially if obese, pregnant

 

Determinants of FRC:

Absolute lung size

·   Age: adult FRC > child

·   Sex: male FRC > female

·   Height: taller FRC > shorter

Alveolar patency

·   Atelectasis -> ↓FRC:

o Obstructive: e.g. mucus plugging

o Compressive: e.g. pregnant -> supine compression by gravid uterus

o Absorptive: e.g. 100% FiO2 (especially if narrowed airway)

·   Positive airway pressure -> ↑FRC

o PEEP -> prevent collapse, distend alveoli

o Recruitment manoeuvres -> re-open collapsed alveoli

Lung inward recoil

·   Intrinsic elasticity: e.g. emphysema -> ↑FRC (e.g. smoking, age ↑1% per year)

·   Surfactant: e.g. premature neonate -> ↑surface tension -> ↓FRC

Chest wall outward recoil

·   Intrinsic elasticity: e.g. neonate (cartilaginous ribs) -> ↓FRC

·   Distortion of chest wall: e.g. pregnancy, kyphoscoliosis -> ↓FRC

·   Diaphragm and intercostal tone: ↓FRC 15% under GA (paralysed or not)

·   Chest wall compression: e.g. ↓FRC supine, pregnant, obese

 

Effects of 1L reduction in FRC:

(1)↓O2 Reservoir

·   ↓O2 for gas exchange when apnoeic (and between breaths)

·   ↑Speed of desaturation

·   At supine FRC 1.1L:

·   -Room air: reservoir 145mL, desaturation <1 minute

·   -Pre-oxygenated: reservoir 955mL, desaturation <4 minutes

(2)Impaired gas exchange

·   When closing capacity > lung volume:

·   ↓Distending transpulmonary pressure

·   ↑Airway closure and alveolar collapse

·   ↑Physiological shunt

·   ↓PaO2

(3)↓Compliance

·   ↓Static compliance: due to ↓Lung volume -> ↓alveolar radius

o LaPlace’s law: tension = pressure x radius / 4

·   ↓Dynamic compliance: due to re-opening of collapsed alveoli

o Starling resistor effect

·   Overall ↓compliance -> ↑work of breathing

(4)↑Airway resistance

·   Laminar flow: R = (8 x length x viscosity) / (π x radius4)

·   Turbulent flow: (P1-P2) ∝ (length x density) / (radius5)

·   ↓Lung volume -> ↓airway radius -> ↑resistance

·   Radius major factor, since power 4 or 5

(5)↑Pulmonary vascular resistance

·   Low volume: compress extra-alveolar vessel

·   High volume: compress alveolar vessels

 

With 1L reduction in FRC:

·   New equilibrium at reduced FRC

·   But ↓FRC -> ↓all vessel radii -> ↑resistance

 

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