--- module: 013-02 language: en chapter: 13 title: "Ventilation, Perfusion, Diffusion, and Gas Transport" module_title: "Alveolar gas reasoning, shunt behaviour, and oxygen-delivery traps" source_sha256: 28db2195aa76b456df8e467ce36aff479f5540d17db4c7e2d9af907b4001a4d3 --- # Alveolar gas reasoning, shunt behaviour, and oxygen-delivery traps ## Using the alveolar gas relationship ### Alveolar oxygen from inspired gas and carbon dioxide ### Equation compares calculated alveolar with measured arterial ### Difference small in hypoventilation or low inspired oxygen ### Widens with diffusion limitation, inequality, shunt ### Exchange ratio, pressure, water vapour assumed ### High inspired fraction widens the numerical gradient #### Devices not comparable without inspired oxygen #### Variable-performance devices follow inspiratory flow ### Altitude: pressure falls, fraction unchanged ## Ventilation-perfusion distributions ### Lungs hold a distribution, not one ratio ### Low-ratio blood mixes with better-oxygenated blood ### High-ratio units are already near saturation ### Carbon dioxide compensates through increased ventilation #### Normal value despite intense respiratory work #### A rising value signals failing ventilatory reserve ### True shunt bypasses ventilated alveoli #### Limited but rarely absent oxygen response ### Dead space: embolism, emphysema, low output ## Diffusion and capillary transit ### Transfer needs equilibration during capillary transit ### Resting reserve: equilibrium before transit ends ### Exercise shortens transit but recruits capillaries ### Lost reserve reveals exertional desaturation ### Transfer reflects membrane and capillary blood volume #### Anaemia lowers, polycythaemia and haemorrhage raise #### Read with spirometry, volumes, haemoglobin, imaging ## Oxygen content, delivery, and extraction ### Tension measures dissolved-gas pressure ### Saturation measures occupied binding sites ### Content measures oxygen carried per volume ### Delivery multiplies content by cardiac output ### Severe anaemia: normal saturation, dangerous content ### Carbon monoxide: false reading, impaired unloading ### Shock: adequate content, inadequate flow ### Mixed venous oxygen reflects residual oxygen #### High value: high flow, low demand, impaired extraction ## Pulse oximetry and co-oximetry ### Pulsatile absorption separated from surrounding tissue ### Assumes only oxyhaemoglobin and deoxyhaemoglobin ### Co-oximetry estimates carboxyhaemoglobin and methaemoglobin ### Perfusion, movement, venous pulsation, ambient light ### Displayed pulse should match the patient ### Oximetry does not assess ventilation #### Saturation held while carbon dioxide rises ## Carbon dioxide as flow and buffer ### Production follows metabolism and substrate use ### Carried dissolved, protein-bound, as bicarbonate ### Deoxygenated haemoglobin accepts hydrogen and carbon dioxide ### Haldane effect: oxygenation promotes unloading ### Oxygen in chronic hypercapnic disease #### Reversed vasoconstriction feeds poorly ventilated units #### Reduced drive contributes but does not explain all ### Normal value inappropriate during metabolic acidosis ## Applying mechanism at the bedside ### Start with inspired oxygen, ventilation, the difference ### Then low-ratio units, shunt, diffusion, mixing ### Pair saturation with haemoglobin and perfusion ### Pair carbon dioxide with pH and expected compensation ### Oxygen-responsive hypoxaemia identifies no single disease ### Persistent hypoxaemia suggests shunt or technical failure ### Rising saturation with falling consciousness hides retention ### Posture redistributes ventilation and perfusion #### Healthier lung dependent may oxygenate better #### Prone position recruits dorsal regions ### Final statement: mechanism, adequacy, content, work, reserve ## Oxygen prescribing and exercise testing ### Specify the target range and device ### Specify flow rate or inspired fraction ### Specify the point of reassessment ### A rising requirement signals deterioration #### Reconsider device fit, airway, secretions #### Reconsider collapse, pneumothorax, oedema, embolism #### Reconsider low cardiac output ### Oxygen treats hypoxaemia, not the lesion ### Exercise reveals limitation hidden at rest #### Record saturation, rate, symptoms, distance, recovery #### Breathlessness without any fall in saturation #### Check artefact when the value does fall #### Standard conditions allow comparison between occasions