--- module: 014-01 language: en chapter: 14 title: "Respiratory Mechanics, Control, and Respiratory Failure" module_title: "Foundations" source_sha256: e87e06acb7f3b94dc3c5b3555c4a9ba4c621831c49b5fd12a54052d48054f3d8 --- # Respiratory mechanics, control, and respiratory failure ## Effort, reserve, and danger signs ### Drive, pump, airways, compliance, endurance ### Failure of oxygenation, ventilation, or both ### Compensation can precede abrupt deterioration ### Assess effort, trajectory, and reserve ### Danger: exhaustion, silent chest, falling rate ### Less wheeze may mean critical airflow loss ## Pressure and the respiratory pump ### Air moves on alveolar minus atmospheric pressure ### Diaphragm descends and external intercostals expand thorax ### Transpulmonary pressure keeps the lungs expanded #### Chest wall outward against inward lung recoil #### Balance sets functional residual capacity ### Inward route: parietal pleura, space, visceral pleura #### Pleural space and alveolus are separate compartments #### Flow stops when alveolar equals atmospheric pressure ### Pneumothorax lets the lung recoil inward #### Tension physiology impairs venous return ## Compliance and elastic recoil ### Volume change per unit pressure change ### Stiffness: fibrosis, oedema, acute respiratory distress syndrome ### Emphysema raises compliance, reduces elastic recoil ### Surfactant from type two cells lowers surface tension #### Deficiency central to neonatal respiratory distress ### Elastic work, resistive work, tissue resistance ### Stiff lungs favour rapid shallow breathing ### Obstruction favours slower deeper breathing ## Airway resistance and flow limitation ### Resistance depends strongly on airway radius ### Mediators, mucus, oedema, smooth-muscle contraction ### Loss of radial traction narrows airways ### Beta two relaxation via catecholamines and medicines ### Dynamic compression beyond the equal-pressure point ### More effort without proportionally more flow ### Air trapping raises end-expiratory lung volume #### Intrinsic positive end-expiratory pressure as threshold ## Lung volumes and spirometry ### Tidal volume and the reserve volumes ### Residual volume escapes simple spirometry ### Obstruction reduces the one-second ratio ### Restriction needs confirmed low total lung capacity ### Absent bronchodilator response does not exclude asthma ### Flow-volume loops suggest upper-airway lesions ## Neural control of breathing ### Brainstem networks generate respiratory rhythm ### Cortical, limbic, and hypothalamic influences ### Central chemoreceptors sense brain extracellular acidity #### Carbon dioxide crosses and forms acid ### Carotid and aortic bodies sense low arterial oxygen ### Chronic retention buffered by renal bicarbonate ### Never withhold oxygen from dangerous hypoxaemia #### Worsened matching, Haldane effect, altered drive ## Respiratory failure ### Type one is predominantly hypoxaemic #### Mismatch, shunt, diffusion limitation, low inspired oxygen ### Type two adds hypercapnia from low alveolar ventilation ### Central, nerve, junction, muscle, chest-wall causes ### Fever and seizures raise carbon dioxide production ### Near-normal pH with high carbon dioxide suggests chronicity ### Falling pH suggests acute deterioration ## Oxygen and ventilatory support ### Titrate oxygen to a context-appropriate target ### High-flow nasal therapy washes out dead space ### Continuous positive airway pressure recruits lung #### It adds no direct inspiratory assistance ### Bilevel support raises tidal volume, clears carbon dioxide ### Invasive ventilation when work is unsustainable ### Injury from volume, pressure, repeated opening, oxygen ## Sleep-disordered breathing ### Sleep lowers ventilatory response and airway tone ### Obstructive apnoea: pharyngeal collapse despite effort #### Intermittent hypoxaemia, arousal, sympathetic activation ### Body size alone neither confirms nor excludes ### Central apnoea: reduced or unstable drive ### Obesity hypoventilation: awake hypercapnia after exclusions ## Weaning and respiratory-muscle reserve ### Resolved disease does not guarantee weaning ### Readiness: oxygenation, secretions, stability, consciousness, load ### Spontaneous breathing trial tests integrated reserve ### Failure: cardiac, weakness, obstruction, anxiety, fever ### Diaphragm weakens with illness and controlled ventilation ### Cough strength and vital capacity in neuromuscular disease ### Deterioration can precede falling oxygen saturation