--- module: 016-02 language: en chapter: 16 title: "Respiratory History, Examination, Blood Gases, and Investigations" module_title: "Pattern localisation, blood-gas compensation, and investigation quality" source_sha256: f50de6f3d5fdfaeb8cd8d891ac6cc2761bf1658a69e19ffa9f31ccdd708b37d9 --- # Localisation, compensation and investigation quality ## Localising the respiratory problem ### Upper airway: stridor, voice, swallowing, position ### Conducting airways: wheeze, cough, mucus, variable flow ### Alveolar: hypoxaemia, crackles, opacities ### Interstitial: reduced compliance and diffusion ### Vascular: dead space and right-heart load ### Pleural: pain and mechanical restriction ### Pump failure despite normal parenchyma #### Central, spinal, nerve, junction, muscle, wall, obesity #### Weak cough and bulbar dysfunction raise aspiration risk #### Normal early saturation does not exclude failure ### Tempo refines localisation #### Abrupt: pneumothorax, embolism, pleural irritation #### Months to years: chronic airway, interstitial, vascular ## Examination as transmission tests ### Percussion asks how tissue transmits sound #### Air resonant, fluid dull, excess gas hyperresonant ### Fremitus and vocal resonance follow voice vibration #### Airless lung with an open airway transmits strongly #### Pleural fluid or gas separates lung from chest wall #### Bronchial breathing above a large effusion ### Crackles are brief discontinuous sounds #### Fine late: sudden opening of small stiff airways #### Coarse: larger-airway secretions, change after cough ### Wheeze needs airflow, so severe obstruction goes quiet ### Asymmetry has high localising value #### Dullness suggests fluid, hyperresonance pleural gas #### Confirm anatomy with ultrasound or radiography ## Blood-gas compensation step by step ### A normal pH can hide two opposing disorders ### Compensation never overshoots the opposite state ### Respiratory acidosis: buffers first, kidneys over days ### Metabolic acidosis should lower carbon dioxide ### Carbon dioxide above expectation: added respiratory acidosis ### Carbon dioxide below expectation: added respiratory alkalosis ### Metabolic alkalosis: hypoxaemia limits hypoventilation ### Anion gap estimates unmeasured anions ### Hypoalbuminaemia conceals an important gap ## Oxygenation and sampling reliability ### A sample after a device change is not at equilibrium ### Device and flow are part of the result ### Air bubbles raise low oxygen, lower high carbon dioxide ### Delayed analysis consumes oxygen, makes carbon dioxide ### Venous contamination lowers measured oxygen ### An inconsistent result needs a sample-quality review ### Oximetry gives trend, not pH or carbon dioxide ## Pulmonary-function quality ### Early termination lowers capacity and raises the ratio ### Cough, leak, glottic closure and pain leave signatures ### Lower limit of normal beats one fixed ratio ### A low value is not automatically important ### Total lung capacity establishes restriction ### Diffusing capacity needs haemoglobin interpretation ### Transfer coefficient is not a hidden normal lung ## Imaging patterns and pre-test probability ### Technical adequacy first: rotation, inspiration, supine ### Air-space opacity: infection, oedema, haemorrhage, tumour ### Interstitial pattern: oedema, fibrosis, lymphatic spread ### Distribution, air bronchograms and time course narrow it ### Nodules: size, morphology, risk and prior imaging ### Pleural sliding argues against pneumothorax at that point ## Microbiology and invasive sampling ### Squamous cells suggest oral contamination ### Lavage samples a defined region but adds risk ### Tissue is needed for architecture, invasion, malignancy ### Molecular assays stay positive after viability falls ### Culture is slower but gives susceptibility ### Pleural fluid: systemic pressure or local pleural disease ## Closing the diagnostic loop ### Reconcile every result with the physiological model ### Normal spirometry with episodic wheeze: look further ### Severe imaging, mild symptoms: assess reserve ### Treatment failure: adherence, technique, resistance, second pathology ### Document oxygen, rate, work, speech, gas trajectory ### Safety comes from convergence, not one image ### State the decision an invasive test will change ### Weigh procedural risk against what can be established ### Prepare reserve, urgency, alternatives, specimen handling ### Serial observation needs constant measurement conditions ### Record rest or walking, position, oxygen, inhaler, fever ### Same saturation on more oxygen is deterioration ### Falling rate with falling volume means exhaustion