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Occupational lung disease illustrates how a brief encounter with an inhaled particle can initiate a biological response that persists for decades. When an asbestos fibre, fine enough to reach the lower lobes, is engulfed by an alveolar macrophage, the cell can neither digest nor release it; instead it coats the fibre in iron-rich protein, producing the beaded asbestos body. The trapped fibre keeps releasing reactive oxygen species and inflammatory cytokines, and over decades this smouldering injury is written into three different lesions: calcified pleural plaques, diffuse interstitial fibrosis and malignant mesothelioma.

Asbestos bodies lie most readily in the seemingly normal parenchyma beside a tumour rather than within it; they prove exposure, never diagnosis, and mesothelioma is confirmed by thoracoscopic biopsy. Asbestos favours the lower zones and the pleura, and its malignant companions are mesothelioma and bronchogenic carcinoma, the latter multiplied by cigarette smoke, whereas pulmonary lymphoma is not an asbestos disease. Silica, by contrast, settles in the upper lobes with eggshell calcification and a raised risk of tuberculosis, while plaques are inert scars that never transform.

A peripheral nodule invites a second line of reasoning, this time about the cell of origin. Tumours arising from small airways sit at the periphery and are usually adenocarcinomas, the histology of never-smokers and of the EGFR and ALK alterations that respond to oral tyrosine-kinase inhibitors. Tumours arising from the central bronchi are either squamous, with keratin pearls, cavitation and parathyroid hormone-related peptide that raises calcium without bone metastases, or small cell carcinomas of neuroendocrine lineage, which declare themselves through SIADH, ectopic ACTH or Lambert-Eaton syndrome, and are treated with chemotherapy rather than surgery.

Mechanism likewise separates the two causes of trapped air. Emphysema is an imbalance of proteases and their inhibitors: neutrophils and macrophages recruited by smoke release elastase and matrix metalloproteinase-12, alpha-1 antitrypsin is overwhelmed or, in the young non-smoker with lower-lobe panacinar disease, congenitally deficient, and elastic fibres are digested until the acinus dilates and recoil is lost. Asthma travels the opposite road: smooth muscle that contracts daily against histamine and leukotrienes is trained rather than tired, and consequently it hypertrophies.

Spirometry asks three questions in sequence. Is expiration too slow? A post-bronchodilator FEV1 to FVC ratio below 0.70 establishes obstruction. Is the lung too small? Restriction is confirmed only by a total lung capacity below 80 per cent of predicted, because a reduced FVC may merely reflect gas trapped behind narrowed airways. Is gas exchange impaired? A reduced diffusing capacity localises the injury to the alveolar wall, so that obstruction with a low DLCO suggests emphysema whereas a preserved DLCO suggests asthma, and restriction with a low DLCO suggests fibrosis whereas a normal DLCO with weak inspiratory pressures points to neuromuscular disease. Reversibility demands both a rise of at least 12 per cent and at least 200 millilitres; one criterion without the other proves nothing, and even a positive test cannot exclude COPD.

The central distinctions can be recalled as follows.

All that asbestos bodies prove is past exposure; mesothelioma is diagnosed on thoracoscopic tissue, not on counted fibres.
Is the post-bronchodilator ratio below 0.70? Then the defect is obstructive, and restriction needs a total lung capacity below 80 per cent.
On the DLCO the localisation turns: low with obstruction means emphysema, low with restriction means fibrosis, normal with restriction means chest wall or muscle.
Lung cancers announce their origin by position, peripheral adenocarcinoma carrying EGFR and central squamous tumours raising calcium through PTHrP.

Similar symptoms of expiratory airflow limitation can arise from distinct mechanisms, making the clinical context as important as the symptom itself. COPD is the slowest story: decades of smoke recruit neutrophils, macrophages and CD8-positive T lymphocytes, tip the balance between proteases and antiproteases, and scar the small airways until airflow limitation becomes persistent. The diagnosis is a definition: a post-bronchodilator FEV1 to FVC ratio below 0.70. Because resistance sits in the small airways, early disease is often silent on auscultation, and chronic cough rather than wheeze is the usual first complaint.

Since 2023 initial treatment has followed the ABE scheme rather than FEV1 alone. Group A receives a single bronchodilator, group B a LABA with a LAMA, and group E, defined by two moderate exacerbations or one admission in a year, begins on LABA and LAMA with an inhaled corticosteroid added once blood eosinophils reach 300 per microlitre. Pulmonary rehabilitation carries strong evidence, intravenous theophylline almost none. In an exacerbation the bundle is short-acting bronchodilators, five days of systemic corticosteroid, antibiotics for purulent sputum, non-invasive ventilation when the pH falls below 7.35 with a rising carbon dioxide, and oxygen titrated to 88 to 92 per cent.

That last figure matters because more oxygen is not always better. High inspired oxygen blunts the hypoxic drive on which they depend, displaces carbon dioxide from haemoglobin through the Haldane effect, and by releasing hypoxic vasoconstriction sends blood to poorly ventilated units. Carbon dioxide climbs and consciousness falls, while a bicarbonate in the mid-thirties reveals a retention that began days earlier, since kidneys compensate slowly. A high carbon dioxide is therefore an indication for non-invasive ventilation, never a contraindication; coma, shock and secretions the patient cannot clear are the true barriers.

Asthma tells the opposite story, and its defining word is reversibility. Injured epithelium releases the alarmins TSLP, IL-25 and IL-33, which activate ILC2 and Th2 cells; IL-5 recruits eosinophils, while IL-4 and IL-13 switch B cells to IgE, drive mucus and raise FeNO. Confirmation requires FEV1 to rise by at least 12 per cent and 200 millilitres after a bronchodilator, or a methacholine PC20 below 8 milligrams per millilitre when spirometry is normal. Every adult receives an inhaled corticosteroid, never a reliever alone; rising reliever use signals lost control, the step-up is a regular LABA in combination, and step-down waits for three stable months.

The third story is nocturnal, and its discriminating question is whether respiratory effort persists. In obstructive sleep apnoea the upper airway collapses while the chest keeps working; in central apnoea the drive disappears; in obesity hypoventilation the patient is hypercapnic awake, with a daytime PaCO2 of 45 mmHg or more. Polysomnography counts events per hour: a symptomatic patient qualifies at five, an asymptomatic one at fifteen, and severity is cut at fifteen and thirty. Body mass index is a risk factor, not a grade. CPAP is first-line for moderate to severe disease, and an obese, hypercapnic patient should raise hypoventilation before COPD is assumed.

Four sentences gather the night together.

All chronic retainers are titrated to 88 to 92 per cent, because excess oxygen removes hypoxic drive and unloads carbon dioxide from haemoglobin.
Is the pH below 7.35 with a rising carbon dioxide? Then non-invasive ventilation is indicated, and only coma, shock or unmanageable secretions forbid it.
On reversibility asthma is confirmed, FEV1 rising at least 12 per cent and 200 millilitres, and every adult is treated with an inhaled corticosteroid.
Lumen collapse of the upper airway in sleep is graded by the apnoea-hypopnoea index, symptomatic at five and asymptomatic at fifteen, never by body mass index.

In respiratory emergencies, identifying the mechanism of failure helps the clinician select support that addresses the physiological problem. Acute respiratory failure divides on one value, the arterial carbon dioxide: type one is failed gas exchange, hypoxaemia with a normal or low PaCO2, as in pneumonia, oedema, ARDS and embolism; type two is a failed pump, hypoxaemia with a rising PaCO2, as in COPD, neuromuscular disease and overdose. Non-invasive ventilation suits hypercapnic COPD and cardiogenic oedema; a high carbon dioxide is its indication, and only coma, shock or unmanageable secretions should stop it.

ARDS is a flood through a broken wall. Pneumonia, aspiration, sepsis or pancreatitis breach the alveolar-capillary barrier, protein-rich fluid floods the air spaces, hyaline membranes form, and perfused, unventilated alveoli create a shunt. The Berlin definition requires onset within a week, bilateral opacities, no cardiac cause (wedge pressure at or below 18 mmHg), and grading by the PaO2 to FiO2 ratio on at least 5 cmH2O of PEEP: mild between 200 and 300, moderate between 100 and 200, severe at 100 or below. Ventilation is lung-protective: 6 millilitres per kilogram of predicted, not actual, body weight, a plateau pressure below 30 cmH2O and early proning in severe disease.

The pleura is read by pairing fremitus with percussion: reduced fremitus with dullness means effusion, reduced fremitus with hyperresonance means pneumothorax, and increased fremitus with dullness means consolidation, because fluid-filled alveoli conduct bronchial breath sounds. Fluid is an exudate if any Light criterion is met: a protein ratio above 0.5, an LDH ratio above 0.6, or a pleural LDH above two-thirds of the serum upper limit. A pH below 7.2, a glucose below 3.3 millimoles per litre and frank pus each demand drainage, glucose below 2.2 being the strongest signal; triglyceride above 1.24 millimoles per litre marks chylothorax, a haematocrit above half that of blood marks haemothorax, and a lymphocytic exudate with adenosine deaminase above 40 units per litre points to tuberculosis.

Spontaneous pneumothorax favours the tall, thin, young male smoker whose apical bleb has burst. When the leak acts as a one-way valve, pressure rises until the mediastinum shifts and venous return fails; tension is therefore a clinical diagnosis, recognised by hypotension, tracheal deviation and a hyperresonant, silent hemithorax, needled before any image and then drained to an underwater seal, since the needle buys minutes while the lung still leaks.

Pulmonary embolism is not a drowning but a strangulation of the right heart. A dislodged deep vein thrombus narrows the pulmonary bed, resistance soars, the right ventricle dilates and pushes the septum leftward, and death comes from obstructive shock rather than hypoxaemia. Dyspnoea, pleuritic pain and tachycardia form the triad, with sinus tachycardia and hypocapnia. Wells stratifies probability, D-dimer excludes disease only in the unlikely group, CT pulmonary angiography confirms it, and bedside echocardiography serves the unstable. Sustained hypotension defines massive embolism and calls for thrombolysis; normotensive patients are anticoagulated, three months after a transient trigger and indefinitely when none is found.

The central distinctions can be recalled as follows.

All tension pneumothoraces are diagnosed clinically and needled before any X-ray, then drained through an intercostal tube.
Is the pleural pH below 7.2, the glucose below 3.3 millimoles per litre, or the fluid frank pus? Then the effusion is an empyema that needs a drain tonight.
On the Berlin definition ARDS means bilateral opacities within a week, no cardiac cause and a PaO2 to FiO2 ratio below 300 on 5 cmH2O of PEEP, ventilated at 6 millilitres per kilogram of predicted weight.
Lung emboli kill through right ventricular failure, so sustained hypotension means thrombolysis, while a negative D-dimer excludes disease only when probability is low.

A pulmonary opacity is a finding to interpret, not a diagnosis in itself; its location, associated features and histology guide the distinction between small-cell and non-small-cell carcinoma. Small cell carcinoma arises centrally in heavy smokers, has usually spread when it declares itself, and is treated with chemotherapy rather than resection; its sensitivity to platinum and etoposide makes drugs the fastest remedy even for a compressed vena cava. Non-small cell carcinoma is cured only by surgery in early disease; adenocarcinoma sits peripherally with EGFR and ALK alterations, whereas squamous carcinoma sits centrally, cavitates and secretes parathyroid hormone-related peptide.

Local spread is read from anatomy: a tumour steals the function of whatever it touches. An apical Pancoast tumour invades the brachial plexus and wastes the hand; slightly higher it reaches the cervical sympathetic chain and produces Horner's ptosis, miosis and anhidrosis, not the raised hemidiaphragm of phrenic invasion. The left recurrent laryngeal nerve gives hoarseness; the encased superior vena cava gives the swollen face and arms. Apical lesions hide behind the clavicles and need a lordotic view; a monophonic wheeze of fixed pitch signals a fixed lesion.

Hormones complete the translation. Small cell tumours of neuroendocrine lineage make antidiuretic hormone, causing euvolaemic hyponatraemia with concentrated urine, and ectopic ACTH, whereas hypercalcaemia belongs to squamous carcinoma. SIADH is managed with fluid restriction and a correction no faster than about 8 millimoles per litre a day, since faster rises risk osmotic demyelination. Mild SVC compression in small cell disease is treated with prompt chemo-immunotherapy, stenting being reserved for a threatened airway.

An upper-zone cavity in a thin, sweating non-smoker tells another story. Bacilli seeded during a forgotten primary infection reactivate where oxygen tension is highest, in the apical and posterior segments, when immunity wanes. Latent infection, detected by tuberculin skin test or interferon-gamma release assay, is neither symptomatic nor infectious, carries roughly a ten per cent lifetime risk of progression, and cannot be told from active disease by either test. Active disease needs six months of therapy, two of isoniazid, rifampicin, pyrazinamide and ethambutol and four of isoniazid and rifampicin, with pyridoxine, monthly liver function rather than creatine kinase, and interruption only when transaminases exceed three times normal with symptoms or five times without. Every case is notified and its contacts screened.

In children the sound locates the lesion before the film names it. Inspiratory stridor arises from the extrathoracic airway, which collapses under negative inspiratory pressure; expiratory wheeze arises from intrathoracic airways compressed on expiration. Croup is subglottic oedema from parainfluenza virus, with a barking cough, a steeple sign and a single dose of dexamethasone for every severity, adding nebulised adrenaline only for stridor at rest. Epiglottitis is Haemophilus influenzae type b swelling the supraglottis into a thumb sign, with drooling and tripod posture, and the tongue depressor stays in the drawer. Bronchiolitis is RSV disease of infants under two, in whom salbutamol does little.

Four sentences gather the ward round together.

All small cell carcinomas are treated with chemotherapy rather than surgery, and their hormones are antidiuretic hormone and ACTH, whereas hypercalcaemia belongs to squamous tumours.
Is the eyelid drooping with a small pupil beside an apical mass? Then the cervical sympathetic chain, not the phrenic nerve, has been invaded.
On tuberculosis the regimen is two months of four drugs and four months of two, monitored by liver function and never by creatine kinase, with contacts screened and the case notified.
Lung sounds in children localise the lesion: inspiratory stridor with a steeple sign is croup, treated with dexamethasone, whereas a thumb sign with drooling is epiglottitis.

Sleep reduces pharyngeal muscle tone; whether that normal change obstructs breathing depends on the interaction between airway anatomy and neuromuscular control. Where the lumen is already crowded by parapharyngeal fat, a thick neck, enlarged tonsils or a receding mandible, however, the slackened walls are drawn inwards with each inspiratory effort until airflow ceases. An apnoea is an interruption of airflow lasting at least ten seconds; in the obstructive form the effort to breathe persists against a closed pharynx, whereas in central apnoea, seen in heart failure, stroke and opioid use, the brainstem drive falls silent. Alcohol, sedatives and the supine position deepen the loss of tone, and events cluster in rapid-eye-movement sleep, when tone is lowest and the lumen most likely to close.

Diagnosis rests on overnight polysomnography, since home testing merely screens. The apnoea–hypopnoea index grades severity: five to fifteen events an hour is mild, fifteen to thirty moderate and more than thirty severe, a hypopnoea being a fall in flow of at least thirty per cent with a three per cent desaturation or an arousal. An Epworth score above ten signals excessive sleepiness.

The harm arises less from the pause than from what ends it. Each obstruction is terminated by a brief arousal and a surge of sympathetic activity; hundreds of such surges a night, superimposed on intermittent hypoxia, raise morning blood pressure, promote atrial fibrillation and pulmonary hypertension, and fragment cognition. Hypertension that resists two agents should therefore prompt a search for apnoea, which European guidance lists among the secondary causes; hypertension is a consequence of the disorder, not its cause. By the same logic a protruding mandible enlarges the retroglossal space and is not a risk factor, whereas a retruded one is. Sleepiness at the wheel multiplies crash risk, making the condition a matter for the licence as much as for the lungs.

Treatment follows the mechanism. Weight loss, avoiding alcohol before bed and lateral sleeping widen the airway, but for severe disease continuous positive airway pressure is first-line, splinting the pharynx open pneumatically; adherence is benchmarked at four or more hours a night, with a residual index below five. A mandibular advancement splint suits mild to moderate disease, and adenotonsillectomy is first-line in children. Under Australian fitness-to-drive standards a commercial driver holds a conditional licence only once a sleep physician confirms effective treatment, reviewed at least annually; until then, the truck stays parked.

A different disorder of timing belongs to the adolescent who cannot fall asleep before three in the morning yet sleeps normally once asleep. The suprachiasmatic pacemaker is entrained chiefly by light, and the phase response curve dictates the remedy: morning light, after the core temperature minimum, advances the clock, evening light delays it, and melatonin acts in mirror image. Delayed sleep phase therefore responds to morning light with early-evening melatonin; evening light would only push the clock later, and the boy later still.

All obstructive apnoeas share one signature: airflow stops for at least ten seconds while the effort to breathe persists.
Is the index above thirty events an hour? Then the disease is severe and CPAP, at least four hours every night, is first-line.
On the causal chain, hypertension lies downstream of apnoea, whereas a receding jaw, not a protruding one, lies upstream.
Lucid clock work follows the phase response curve: morning light and evening melatonin advance a delayed rhythm; evening light delays it.

The traditional six Ps provide a memorable framework for recognising acute limb ischaemia: pain, pallor, pulselessness, paraesthesia, paralysis and poikilothermia, a limb that takes the temperature of the room. The first three describe the blocked artery; the last three describe tissue beginning to fail, and these set the clock. Rutherford's categories formalise the point: a viable limb keeps sensation and Doppler signals, a threatened limb loses sensation and then movement, and an irreversible limb is anaesthetic, paralysed and rigid. Skeletal muscle tolerates roughly six hours of warm ischaemia, so a foot that is numb yet still moving is not a reprieve but a countdown.

The source is usually the heart. Atrial fibrillation lets blood stagnate in the left atrial appendage, and the thrombus formed there travels the aorta to lodge at a bifurcation, most often the common femoral. Embolism therefore strikes suddenly, in a limb without claudication, while the contralateral pulses stay normal; thrombosis on an atherosclerotic plaque evolves more slowly, cushioned by collaterals grown over years. Both receive an immediate heparin bolus, typically five thousand units, then an infusion, but the embolus is removed by balloon catheter embolectomy through a groin incision, whereas a thrombosed plaque needs angiography and often catheter-directed lysis or bypass.

Restoring flow brings its own hazard. Ischaemic muscle accumulates potassium, lactate, creatine kinase and myoglobin, and reperfusion flushes them into the circulation: hyperkalaemia provokes arrhythmia, acidosis deepens and myoglobin obstructs the renal tubules. Calcium moves the other way, precipitating into necrotic fat, so early hypocalcaemia rather than hypercalcaemia is the rule. Swelling within unyielding fascial compartments then throttles perfusion from inside, and a compartment pressure within thirty millimetres of mercury of the diastolic pressure calls for fasciotomy, which is why the leg is watched as closely after the operation as before.

The chronic river tells a slower story. The ankle-brachial index is normal between 0.9 and 1.3; below 0.9 defines peripheral arterial disease and below 0.4 critical ischaemia, whereas a value above 1.3 betrays calcified, incompressible vessels and demands a toe-brachial index. Fontaine's stages run from asymptomatic disease through claudication to rest pain and tissue loss, and only the last two justify revascularisation; claudication is treated with smoking cessation, risk-factor control, antiplatelet therapy, supervised exercise and cilostazol. Below the knee, autologous great saphenous vein outperforms any prosthetic conduit.

On the venous side Virchow's triad of stasis, endothelial injury and hypercoagulability explains every deep vein thrombosis, and one variant deserves a name: in May–Thurner syndrome the right common iliac artery compresses the left common iliac vein, so an isolated left iliofemoral thrombosis in a young woman points to a cause, not a complication. Diagnosis proceeds from a Wells score to a D-dimer, whose negative predictive value approaches ninety-nine per cent yet whose positive result proves nothing, and then to compression ultrasound. Anticoagulation, not surgery, is the treatment; a filter is reserved for those who cannot be anticoagulated, and is useless once the cava is chronically occluded.

All six Ps matter, but numbness and weakness are the ones that start the six-hour clock.
Is the onset sudden, the other leg normal and the rhythm fibrillating? Then the clot came from the heart, and embolectomy follows heparin.
On reperfusion, potassium leaks out and calcium moves in, so hypercalcaemia is the one finding you will not see.
Lumen calcification pushes the ankle-brachial index above 1.3, a false reassurance that a toe pressure corrects.

The timing of mechanical complications after myocardial infarction reflects the changing strength of injured tissue, with particular vulnerability during the early days of repair. Infarcted muscle undergoes coagulative necrosis and is then invaded by neutrophils that digest the dead fibres, so tensile strength reaches its nadir before fibroblasts have laid down collagen; the softened scar tears under the pressure it once withstood. Rupture favours a first, single-vessel infarction with delayed or absent reperfusion, older age, female sex and hypertension, and in the reperfusion era it complicates only a few in every thousand infarctions, though each remains a race against the clock.

Three structures give way. Free wall rupture floods the pericardium, producing tamponade, pulseless electrical activity and, usually, death within minutes; a contained rupture forms a pseudoaneurysm. Septal rupture follows an anterior infarct at the apex and an inferior infarct at the base, and announces itself with a harsh pansystolic murmur and thrill at the lower left sternal edge, a left-to-right shunt that overloads the lungs, and an oxygen step-up at ventricular level. Papillary muscle rupture, most often posteromedial because that muscle depends on the posterior descending artery alone, causes torrential mitral regurgitation whose apical murmur may be surprisingly soft. Acute aortic regurgitation belongs to dissection and endocarditis, never to this triad.

Counterpulsation buys time. The intra-aortic balloon inflates in diastole, raising coronary perfusion pressure, and deflates just before systole, lowering afterload; its tip sits two centimetres distal to the left subclavian artery, above the renal origins. Aortic regurgitation, dissection and severe peripheral arterial disease are contraindications. The IABP-SHOCK II trial found no thirty-day survival benefit in cardiogenic shock after infarction, yet in septal or papillary rupture the pump remains the standard bridge to surgery. Noradrenaline is the first-line vasopressor, and falling lactate signals returning perfusion.

Definitive treatment is patch repair on cardiopulmonary bypass, which in shock is safer than off-pump surgery because the circuit guarantees perfusion while the heart is stopped. Ticagrelor is ordinarily withheld for three to five days before cardiac surgery, but necrotic myocardium cannot wait, so platelets are held on standby instead. Without repair almost no patient survives; with it, roughly half do. Where the patient lacks capacity and no relative can be reached, an immediately life-threatening emergency permits treatment under presumed consent.

The same haemodynamic signs recur in trauma, where distended neck veins with hypotension may mean tension pneumothorax or tamponade. Both share the neck veins and the low pressure, so neither distinguishes them; the discriminator is breath sounds, absent on the affected side in tension pneumothorax and symmetrical in tamponade, whose heart sounds are muffled. Tension pneumothorax is decompressed by needle at the fourth or fifth intercostal space between the anterior and mid-axillary lines, without waiting for an X-ray, and a chest tube follows; massive haemothorax draining more than fifteen hundred millilitres, or over two hundred an hour for three to four hours, needs a thoracotomy.

All three mechanical complications, free wall, septum and papillary muscle, strike between days two and seven, when necrotic muscle is softest.
Is the murmur pansystolic with a thrill at the lower left sternal edge? Then the septum has ruptured, and echocardiography confirms the left-to-right shunt.
On the balloon pump, inflation in diastole feeds the coronaries and deflation before systole unloads the ventricle, so aortic regurgitation forbids it.
Lumen-splitting dissection and endocarditis cause acute aortic regurgitation, which is not a mechanical complication of infarction.