--- module: 062-02 language: en chapter: 62 title: "Critical Care Physiology, Oxygen Support, Ventilation, and Organ Support" module_title: "Oxygen transport, ventilator mechanics, haemodynamic support, and intensive-care recovery" source_sha256: f2f89591cedfe294454aea46b02020b1234a7c517df9d28f46e79636f57ff36a --- # Oxygen, ventilation, circulation, and recovery ## Support has a cost ### Temporary substitute while recovery occurs ### Oxygen injures lung, pressure lowers output ### Sedation prolongs ventilation, devices infect ### Define target, least harmful support, remove early ## Oxygen transport and monitoring ### Delivery is cardiac output times content #### Haemoglobin times saturation dominates #### Dissolved oxygen contributes little ### Full saturation cannot offset anaemia or low output ### Venous saturation and lactate are indirect clues ### Oximetry biased by perfusion, pigment, probe ### Oxygen can conceal hypoventilation #### Saturation stays high while carbon dioxide rises ### Co-oximetry detects abnormal haemoglobins ### Persistent hypoxaemia: five mechanisms #### Low inspired oxygen, hypoventilation, diffusion #### Ventilation-perfusion mismatch or shunt ## Non-invasive support ### High flow meets demand, washes dead space ### Continuous pressure recruits, lowers afterload ### Bilevel adds inspiratory assistance ### Needs cooperation, mask fit, rapid improvement ### Failure triggers intubation, not mask escalation #### Rising rate, acidosis, falling consciousness #### Delay until collapse is harmful ## Intubation physiology ### Induction lowers sympathetic tone and resistance ### Paralysis removes respiratory muscle pump ### Positive pressure cuts right-heart venous return ### High arrest risk in hypovolaemia, right failure ### Prepare vasopressor, minimise apnoea ### Confirm with waveform carbon dioxide ## Ventilator mechanics ### Alveolar ventilation is minute minus dead space #### Rises with less alveolar ventilation or more production ### Oxygenation follows oxygen, end-expiratory pressure, shunt ### Peak includes resistive and elastic load ### Plateau approximates alveolar pressure ### Rising peak, stable plateau: resistance #### Secretions, kink, biting, bronchospasm ### Rising peak and plateau: lower compliance #### Oedema, pneumothorax, atelectasis, abdomen ### Driving pressure: plateau minus end-expiratory pressure #### Tidal strain relative to aerated lung ## Lung injury and distress syndrome ### Overdistension, pressure, cyclic opening, oxygen ### Tidal volume from height-based predicted weight #### Aerated lung does not scale with fat ### Sudden deterioration: disconnect, assess manually ### Inflammatory permeability oedema #### Dependent collapse, small lung takes tidal volume ### Prone positioning redistributes stress #### Reduces mortality if early and prolonged ### Conservative fluid after shock reduces lung water ## Synchrony and liberation ### Dyssynchrony is a sign, not a sedation order ### Pain, acidosis, intrinsic pressure, trigger mismatch ### Waveforms show missed and double triggers ### Liberation tests integrated reserve ### Trial fails from load, diaphragm, heart, fluid ### Laryngeal oedema can defeat extubation ## Shock and vasoactive support ### Blood pressure is a means, not the target ### Fluid challenge has endpoint and stopping rule #### Responsiveness does not mean benefit ### Sepsis needs source control and narrowing ### Slow lactate fall: adrenergic, liver, seizure ### Norepinephrine for distributive shock ### Epinephrine adds inotropy, arrhythmia, lactate ### Right-heart failure worsened by excess fluid #### Septal shift reduces left filling ## Kidney, nutrition, and brain ### Creatinine lags injury and is diluted ### Replacement for complications, not a number ### Overfeeding raises carbon dioxide and liver fat ### Early mobilisation is organ support ### Prevent secondary brain injury ### Hyperosmolar therapy and hyperventilation are bridges ## Devices, delirium, and recovery ### Devices must justify risk daily ### Pneumonia prevention: head up, oral care ### Fever and opacity alone are not pneumonia ### Iatrogenic withdrawal mimics delirium or sepsis #### Taper by exposure and half-life ### Time-limited trials specify review date ### Recovery planning starts before discharge ### Handover states indication for new medicines