2005B16 Explain the changes in functional residual capacity that take place with administration of anaesthesia.

Definition

·   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)

·   (↑16mL per year in adult life)

Determinants

·   Absolute lung size

·   Relative lung volume: inward vs outward recoil

·   Alveolar patency

Significance

·   O₂ reservoir during expiration, between breaths, during apnoea

·   Prevents alveolar collapse

·   Point of maximal compliance

·   Point of moderate airway resistance

·   Point of minimal PVR

Summary: effect of anesthesia

·   General anaesthesia reduces FRC

·   Poorly tolerated if FRC already reduced (e.g. pregnancy, obesity, lung disease)

·   Partly offset by PEEP

Pre-oxygenation

·   Loss of nitrogen splinting -> absorption atelectasis -> ↓FRC (worse if airway collapse)

General anaesthesia

·   ↓15% (30 -> 25mL/kg)

·   Immediate fall; nadir within minutes; normalizes hours after extubation

·   ? Due to relaxation of diaphragm, intercostals, with loss of end-expiratory tone

·   ? Due to shape change alone (cephalad shift at dependent regions)

·   Pulmonary vasodilatation -> ↑pulmonary blood volume (minor factor)

·   Exaggerated fall if obese

Neuraxial anaesthesia

·   Intercostal relaxation -> mild ↓FRC

·   No effect on diaphragm unless total spinal

Paralysis

·   Further relaxation of diaphragm, intercostals

·   Minimal or no further ↓ FRC

PEEP

·   ↑Airway pressure -> alveolar distension -> ↑FRC

·   Important for dependent areas

Extubation

·   Coughing, loss of positive airway pressure -> atelectasis -> ↓FRC

·   Attenuated by PEEP

Posture

·   Diaphragm compression: supine > lateral decubitus > beach chair

o (not sure where prone fits)

·   Pulmonary blood volume: 9% total blood volume upright, 15% recumbent (minor factor)

Surgery

·   Pneumoperitoneum -> ↑intra-abdominal pressure -> compression atelectasis, ↓FRC