� Intro: definition and significance
� Determinants of FRC
� Effects of 1L reduction in 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%), close to residual volume � From upright FRC: well tolerated � From supine FRC: poorly tolerated, especially if obese, pregnant |
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 |
(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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