--- module: 014-02 language: en chapter: 14 title: "Respiratory Mechanics, Control, and Respiratory Failure" module_title: "Time constants, respiratory load, and ventilatory-support reasoning" source_sha256: 9a32ee726559d71e34bcc79e16e0dd5932733df6cdbb65573950f60b91a8c090 --- # Time constants, load, and ventilatory-support reasoning ## Time constants and uneven emptying ### Time constant is resistance times compliance ### High resistance or compliance empties slowly ### Stiff units accept little volume but fill quickly ### Disease broadens the distribution of time constants ### Short expiratory time traps gas in slow units ### The next breath starts above resting volume #### Dynamic hyperinflation and intrinsic positive pressure ### Longer expiratory time reduces trapping ## Reading airway pressure ### Stiff lungs need higher pressure per volume ### Peak pressure holds resistive and elastic components ### An inspiratory pause estimates plateau pressure #### Plateau reflects alveolar and chest-wall elastic pressure ### High peak with lower plateau: airway resistance ### Both high: low compliance or excessive volume ### Chest-wall load raises pressure without lung stress #### Oesophageal pressure estimates pleural pressure ## Load, power, and muscle endurance ### Recoil, resistance, intrinsic pressure, external apparatus ### Power also depends on rate and flow ### Hyperinflation shortens and flattens the diaphragm ### Malnutrition, sepsis, steroids, neuropathy weaken force ### Fever, acidosis, dead space raise demand ### Failure when load exceeds neuromuscular capacity ### Slowing rate in an exhausted patient is ominous ### Great effort with small movement needs urgent support ## Flow limitation and bronchodilator logic ### Medium bronchi carry most normal resistance ### Parallel small airways matter once widely narrowed ### Small-airway disease precedes obvious spirometry change ### Bronchodilators do not clear mucus or fibrosis ### Limited response does not exclude benefit or asthma ### Delivery depends on particle size and inspiratory flow #### Spacers cut coordination demand, dry powder needs flow ## Control of breathing and compensation ### Central receptors sense carbon dioxide derived acidity ### Metabolic hydrogen ions cross the barrier poorly #### Peripheral receptors give the rapid metabolic response ### Drive blunted by opioids, sedation, sleep, adaptation ### Drive raised by pain, fever, pregnancy, acidosis ### Judge carbon dioxide against the expected drive ### High bicarbonate may hide a metabolic alkalosis ## Non-invasive support ### Continuous pressure raises pressure through the cycle #### Splints airway, recruits alveoli, raises residual capacity #### Reduces left-ventricular afterload #### No separate inspiratory pressure boost ### Bilevel difference supports volume and carbon dioxide clearance ### Expiratory pressure offsets intrinsic positive pressure #### Excess worsens hyperinflation and venous return ### Failure: acidosis, drowsiness, refractory hypoxaemia, exhaustion ## Invasive ventilation and lung stress ### Volume control fixes volume, pressure control limits pressure ### Neither mode is protective by itself ### Volutrauma, barotrauma, atelectrauma, biotrauma ### Extreme drive causes self-inflicted lung injury ### Excess positive pressure overdistends and raises dead space #### It impairs venous return and right-ventricular afterload ### Not every opaque region is safely recruitable ### Dyssynchrony: check trigger, flow, intrinsic pressure first ## Weaning, secretions, and extubation ### The breathing trial tests several systems together ### Negative pressure raises venous return and afterload #### Oedema despite an improving lung infection ### Extubation needs patency, cough, alertness, a plan ### Passing a trial does not guarantee airway protection ### Prolonged intubation brings pneumonia and weakness ### Humidification, mobilisation, suction, physiotherapy move secretions ### Adequate tidal volume with weak expiratory force ## Naming the failure and reviewing settings ### Name the failing component explicitly #### Drive, pump, chest wall, airway, parenchyma, circulation ### State whether support reduces load and protects tissue ### Escalation without a causal statement treats numbers ### Reducing sedation watches pain, delirium, drive, dyssynchrony #### Over-sedation blocks assessment, cough, mobilisation, weaning #### Inadequate analgesia generates dangerous effort and demand ### State target depth, accumulation, withdrawal risk ### Support sleep, orientation, family contact, early movement ### After a change check the effect and new harm #### Plateau, driving pressure, end-expiratory flow, right heart ### Higher saturation can accompany overall worsening