= Solution
\b[Sustained failure of respiratory gas exchange progressively depletes usable <oxygen> and impairs elimination of <carbon dioxide>; its rate and visible effects depend strongly on the cause, initial stores, <metabolism> and remaining circulation.] Here the historical \i[apnæa] is best read in its broad sense of suspended respiration and resulting asphyxial danger, rather than as one precisely defined modern sleep disorder.
Contemporary usage supports that reading: an 1858 review of <Marshall Hall>'s work explicitly joins drowning with other forms of <apnoea> or suspended respiration. An airway obstruction, drowning, absence of respiratory drive and failure of the respiratory apparatus can all interrupt effective ventilation, although they add different injuries. The historical label does not prove a single mechanism in every case.<Prone and Postural Respiration in Drowning, and Other Forms of Apnoea (review, 1858)>[Opening title and pp. 289–293: suspended respiration, airway obstruction and postural experiments]{ref}
When ventilation stops while ordinary <blood circulation> and <metabolism> continue, alveolar <oxygen> is consumed and arterial oxygenation falls. <Carbon dioxide> usually accumulates, with rising acidity; falling <oxygen> delivery then impairs <ATP> production and especially threatens the <brain> and <heart>. A conscious person may develop air hunger and vigorous respiratory efforts, followed in severe progression by impaired consciousness, gasping and loss of effective circulation. The sequence and colour changes are variable: cyanosis, convulsions or a particular pulse pattern are not individually necessary in every cause of <oxygen> deprivation. If circulation has already stopped, simple arterial-gas descriptions no longer capture tissue delivery.
The distinction between <hypoxia> and <hypercapnia> is important. <Oxygen>-free inspired gas can cause profound <hypoxia> while <carbon dioxide> is still exhaled; complete airway obstruction commonly produces both. Modern experimental comparisons show that <oxygen> deficiency and raised <carbon dioxide> interact in stimulating ventilation. One cannot assign all consequences to a single poisonous property of retained <carbon dioxide>.<The Respiratory System: Basic Science and Clinical Conditions/Chemical Control of Breathing>[Asphyxia; interactions of oxygen, carbon dioxide and acidity in ventilatory drive]{ref}
The time course changes with the initial <oxygen> store in the <lungs> and <blood>, the amount of oxygenated <blood> still reaching tissues, and the rate at which tissues consume it. Exercise, fever and struggling can accelerate depletion; a lower metabolic rate can slow it. <Temperature> has several competing effects, so cold exposure is not a general guarantee of protection. Pulmonary volume, <haemoglobin> concentration, existing <lung> disease, airway patency and circulatory failure all change the relation between an interruption and tissue injury. Duration matters, but the same duration is not physiologically equivalent in every person and every mechanism.
A rough explanatory relation is
$$
\boxed{\text{time scale of oxygen depletion}\sim\frac{\text{usable oxygen store}}{\text{oxygen consumption rate}}}.
$$
It is an accounting guide, not a safe breath-holding limit or a formula for predicting irreversible injury. A modern laboratory analysis could record gas concentrations, respiratory movement, flow and circulation separately; visible chest movement alone does not demonstrate airflow through an obstructed airway.
Modern \i[<apnoea>] can also mean a temporary absence of respiratory effort, for example after a fall in arterial <carbon dioxide> following hyperventilation. That narrower phenomenon must not be confused with every use of the historical word. Hyperventilation does not reliably create a large extra <oxygen> reserve in an already oxygenated person, and delayed air hunger is not proof that <oxygen> supply remains adequate. Historical postural resuscitation experiments illustrate why airway patency and actual ventilation were distinguished; their methods should not be presented as current clinical instructions.
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