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  <head>
    <title>013-02 Alveolar gas reasoning, shunt behaviour, and oxygen-delivery traps</title>
    <ownerName>Integrated Medical Foundations</ownerName>
  </head>
  <body>
    <outline text="Alveolar gas reasoning, shunt behaviour, and oxygen-delivery traps">
      <outline text="Using the alveolar gas relationship">
        <outline text="Alveolar oxygen from inspired gas and carbon dioxide"/>
        <outline text="Equation compares calculated alveolar with measured arterial"/>
        <outline text="Difference small in hypoventilation or low inspired oxygen"/>
        <outline text="Widens with diffusion limitation, inequality, shunt"/>
        <outline text="Exchange ratio, pressure, water vapour assumed"/>
        <outline text="High inspired fraction widens the numerical gradient">
          <outline text="Devices not comparable without inspired oxygen"/>
          <outline text="Variable-performance devices follow inspiratory flow"/>
        </outline>
        <outline text="Altitude: pressure falls, fraction unchanged"/>
      </outline>
      <outline text="Ventilation-perfusion distributions">
        <outline text="Lungs hold a distribution, not one ratio"/>
        <outline text="Low-ratio blood mixes with better-oxygenated blood"/>
        <outline text="High-ratio units are already near saturation"/>
        <outline text="Carbon dioxide compensates through increased ventilation">
          <outline text="Normal value despite intense respiratory work"/>
          <outline text="A rising value signals failing ventilatory reserve"/>
        </outline>
        <outline text="True shunt bypasses ventilated alveoli">
          <outline text="Limited but rarely absent oxygen response"/>
        </outline>
        <outline text="Dead space: embolism, emphysema, low output"/>
      </outline>
      <outline text="Diffusion and capillary transit">
        <outline text="Transfer needs equilibration during capillary transit"/>
        <outline text="Resting reserve: equilibrium before transit ends"/>
        <outline text="Exercise shortens transit but recruits capillaries"/>
        <outline text="Lost reserve reveals exertional desaturation"/>
        <outline text="Transfer reflects membrane and capillary blood volume">
          <outline text="Anaemia lowers, polycythaemia and haemorrhage raise"/>
          <outline text="Read with spirometry, volumes, haemoglobin, imaging"/>
        </outline>
      </outline>
      <outline text="Oxygen content, delivery, and extraction">
        <outline text="Tension measures dissolved-gas pressure"/>
        <outline text="Saturation measures occupied binding sites"/>
        <outline text="Content measures oxygen carried per volume"/>
        <outline text="Delivery multiplies content by cardiac output"/>
        <outline text="Severe anaemia: normal saturation, dangerous content"/>
        <outline text="Carbon monoxide: false reading, impaired unloading"/>
        <outline text="Shock: adequate content, inadequate flow"/>
        <outline text="Mixed venous oxygen reflects residual oxygen">
          <outline text="High value: high flow, low demand, impaired extraction"/>
        </outline>
      </outline>
      <outline text="Pulse oximetry and co-oximetry">
        <outline text="Pulsatile absorption separated from surrounding tissue"/>
        <outline text="Assumes only oxyhaemoglobin and deoxyhaemoglobin"/>
        <outline text="Co-oximetry estimates carboxyhaemoglobin and methaemoglobin"/>
        <outline text="Perfusion, movement, venous pulsation, ambient light"/>
        <outline text="Displayed pulse should match the patient"/>
        <outline text="Oximetry does not assess ventilation">
          <outline text="Saturation held while carbon dioxide rises"/>
        </outline>
      </outline>
      <outline text="Carbon dioxide as flow and buffer">
        <outline text="Production follows metabolism and substrate use"/>
        <outline text="Carried dissolved, protein-bound, as bicarbonate"/>
        <outline text="Deoxygenated haemoglobin accepts hydrogen and carbon dioxide"/>
        <outline text="Haldane effect: oxygenation promotes unloading"/>
        <outline text="Oxygen in chronic hypercapnic disease">
          <outline text="Reversed vasoconstriction feeds poorly ventilated units"/>
          <outline text="Reduced drive contributes but does not explain all"/>
        </outline>
        <outline text="Normal value inappropriate during metabolic acidosis"/>
      </outline>
      <outline text="Applying mechanism at the bedside">
        <outline text="Start with inspired oxygen, ventilation, the difference"/>
        <outline text="Then low-ratio units, shunt, diffusion, mixing"/>
        <outline text="Pair saturation with haemoglobin and perfusion"/>
        <outline text="Pair carbon dioxide with pH and expected compensation"/>
        <outline text="Oxygen-responsive hypoxaemia identifies no single disease"/>
        <outline text="Persistent hypoxaemia suggests shunt or technical failure"/>
        <outline text="Rising saturation with falling consciousness hides retention"/>
        <outline text="Posture redistributes ventilation and perfusion">
          <outline text="Healthier lung dependent may oxygenate better"/>
          <outline text="Prone position recruits dorsal regions"/>
        </outline>
        <outline text="Final statement: mechanism, adequacy, content, work, reserve"/>
      </outline>
      <outline text="Oxygen prescribing and exercise testing">
        <outline text="Specify the target range and device"/>
        <outline text="Specify flow rate or inspired fraction"/>
        <outline text="Specify the point of reassessment"/>
        <outline text="A rising requirement signals deterioration">
          <outline text="Reconsider device fit, airway, secretions"/>
          <outline text="Reconsider collapse, pneumothorax, oedema, embolism"/>
          <outline text="Reconsider low cardiac output"/>
        </outline>
        <outline text="Oxygen treats hypoxaemia, not the lesion"/>
        <outline text="Exercise reveals limitation hidden at rest">
          <outline text="Record saturation, rate, symptoms, distance, recovery"/>
          <outline text="Breathlessness without any fall in saturation"/>
          <outline text="Check artefact when the value does fall"/>
          <outline text="Standard conditions allow comparison between occasions"/>
        </outline>
      </outline>
    </outline>
  </body>
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