· Intro: FRC
· Effect of anaesthesia
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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 |
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Normal |
· Upright: 40mL/kg (~2,800mL) · Supine: 30mL/kg (~2,100mL) · (↑16mL per year in adult life) |
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Determinants |
· Absolute lung size · Relative lung volume: inward vs outward recoil · Alveolar patency |
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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 |
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Summary: effect of anesthesia |
· General anaesthesia reduces FRC · Poorly tolerated if FRC already reduced (e.g. pregnancy, obesity, lung disease) · Partly offset by PEEP |
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Pre-oxygenation |
· Loss of nitrogen splinting -> absorption atelectasis -> ↓FRC (worse if airway collapse) |
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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 |
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Neuraxial anaesthesia |
· Intercostal relaxation -> mild ↓FRC · No effect on diaphragm unless total spinal |
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Paralysis |
· Further relaxation of diaphragm, intercostals · Minimal or no further ↓ FRC |
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PEEP |
· ↑Airway pressure -> alveolar distension -> ↑FRC · Important for dependent areas |
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Extubation |
· Coughing, loss of positive airway pressure -> atelectasis -> ↓FRC · Attenuated by PEEP |
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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) |
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Surgery |
· Pneumoperitoneum -> ↑intra-abdominal pressure -> compression atelectasis, ↓FRC |