= Solution
\b[The chief physiological routes are abrupt failure of <blood circulation>, failure of <pulmonary ventilation> or <gas exchange>, or catastrophic failure of the central control of those functions.] Historical clinical accounts used labels such as <syncope>, <asphyxia> and <medical coma> in explaining deaths; these terms describe broad physiological failures rather than identifying the underlying <diseases>.<Hospital reports and lectures on modes of death in the British Medical Journal (1860)>[February 25, p. 142; March 10, pp. 181–183, contrasting cardiac failure, obstructed circulation and respiratory failure]{ref} A brief faint or ordinary <medical coma> is not itself necessarily fatal.
In primary circulatory failure, effective delivery of <blood> to the <brain> stops. Modern examples include <ventricular fibrillation>, pulseless <ventricular tachycardia>, severe <bradyarrhythmia> and <asystole>. They may arise in <coronary artery disease>, after <myocardial infarction>, in <cardiomyopathy>, or from inherited <cardiac ion channel disorders>. An electrical cause may leave little distinctive gross evidence at <autopsy>. These modern mechanisms refine the broad historical description of cardiac <syncope>.<What Causes Cardiac Arrest? by the American Heart Association>[Dangerous heart rhythms, scarred or thickened myocardium, and inherited electrical abnormalities]{ref}
Mechanical interruption of <blood circulation> gives another group: massive <haemorrhage>, rupture of an <aortic aneurysm>, <cardiac tamponade>, and large <pulmonary embolism>. The immediate deficit differs in each. <Haemorrhage> removes circulating volume; <cardiac tamponade> prevents adequate filling; <pulmonary embolism> obstructs flow through the <pulmonary circulation>. All can end with critically low <cardiac output> even if some electrical activity persists.<Cardiac Arrest by Merck Manual Professional Edition>[Etiology, noncardiac shock including pulmonary embolism and hemorrhage; Pathophysiology and reversible causes]{ref}
Respiratory failure includes obstruction of the <airway> by a foreign body, <laryngeal oedema> or <aspiration>; drowning; and depression of <breathing> by drugs or major <brainstem> injury. <Oxygen> delivery then fails and secondary <cardiac arrest> follows. <Carbon monoxide poisoning> and <cyanide poisoning> illustrate related failure of <oxygen> transport or use: not every hypoxic death is caused by physical obstruction of an <airway>.<Clinical Guidance for Carbon Monoxide Poisoning by CDC>[Overview, impaired oxygen delivery and cellular oxygen utilization]{ref}<Cyanide chemical fact sheet by CDC>[Background, effects on the body's ability to use oxygen]{ref}<Cardiac Arrest: Causes and Risk Factors by the National Heart, Lung, and Blood Institute>[Other medical conditions and events, respiratory arrest, poisoning and drug overdose]{ref}
A major <intracranial haemorrhage> or rapidly developing <brain herniation> can destroy or compress the pathways maintaining <consciousness> and <breathing>. A severe disturbance of <electrolytes>, especially <potassium>, may instead act directly on the <heart>'s electrical function. A fatal <seizure> can involve respiratory and circulatory mechanisms; <sudden unexpected death in epilepsy> remains a diagnosis requiring exclusion of other causes, not merely recognition of a history of <epilepsy>.<Cardiac Arrest: Causes and Risk Factors by the National Heart, Lung, and Blood Institute>[Heart problems; Other medical conditions and events, electrolytes and seizure disorders]{ref}
The practical distinction is between \b[mode, immediate mechanism and underlying cause]. For example, rupture of an <aneurysm> can cause <haemorrhage>, then circulatory failure, then irreversible <hypoxic-ischaemic brain injury>. A complete explanation identifies that chain. Inspection, <histology>, examination of the <airway> and <heart>, and appropriately interpreted <toxicology> may establish it; an unexplained sudden death cannot be assigned a precise cause from its suddenness alone.
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