Editing Hypoxia (medicine) (section)

===By cause===
*[[Hypoxic hypoxia]], also referred to as [[generalised hypoxia]], may be caused by:
**[[Hypoventilation]],<ref name="Manninen and Unger 2016" /> which is insufficient ventilation of the lungs due to any cause (fatigue, excessive [[work of breathing]], [[Barbiturate overdose|barbiturate poisoning]], [[pneumothorax]], [[sleep apnea]], etc.).
**Low-inspired oxygen partial pressure, which may be caused by breathing normal air at low ambient pressures due to altitude,<ref name="Manninen and Unger 2016" /><ref name="Bhutta et al 2022" /> by breathing [[Breathing gas#Classification by oxygen fraction|hypoxic breathing gas]] at an unsuitable depth, by breathing inadequately re-oxygenated recycled breathing gas from a [[rebreather]],<ref name="Elliott 1997" /> [[life support system]], or [[anesthetic machine]].
**[[Hypoxia of ascent]] (latent hypoxia) in [[freediving]] and [[rebreather diving]].<ref name="Lindholm 2006" />
**[[Airway obstruction]], [[choking]],<ref name="Manninen and Unger 2016" /> [[drowning]].
***[[Chronic obstructive pulmonary disease]] (COPD)<ref name="Cleveland" />
***Neuromuscular diseases or [[interstitial lung disease]]<!-- plausible but no ref -->
**Malformed vascular system such as an [[Anomalous left coronary artery from the pulmonary artery|anomalous coronary artery]].{{citation needed|date=November 2022}}
*Hypoxemic hypoxia is a lack of oxygen caused by low oxygen tension in the arterial blood, due to the inability of the lungs to sufficiently oxygenate the blood. Causes include hypoventilation, impaired alveolar [[diffusion]], and pulmonary shunting.<ref name="Bhutta et al 2022" /> This definition overlaps considerably with that of hypoxic hypoxia.
*{{visible anchor|Pulmonary hypoxia}} is hypoxia from hypoxemia due to abnormal [[pulmonary]] function, and occurs when the lungs receive adequately oxygenated gas which does not oxygenate the blood sufficiently. It may be caused by:<ref name="Manninen and Unger 2016" />
**[[Ventilation perfusion mismatch]] (V/Q mismatch), which can be either low or high.<ref name="Bhutta et al 2022" /> A reduced V/Q ratio can be caused by impaired ventilation, which may be a consequence of conditions such as bronchitis, obstructive airway disease, mucus plugs, or pulmonary edema, which limit or obstruct the ventilation. In this situation there is not enough oxygen in the alveolar gas to fully oxygenate the blood volume passing through, and P<sub>a<sub>O<sub>2</sub></sub></sub> will be low. Conversely, an increased V/Q ratio tends to be a consequence of impaired perfusion, in which circumstances the blood supply is insufficient to carry the available oxygen, P<sub>a<sub>O<sub>2</sub></sub></sub> will be normal, but tissues will be insufficiently perfused to meet the oxygen demand. A V/Q mismatch can also occur when the surface area available for gas exchange in the lungs is decreased.<ref name="Bhutta et al 2022" />
**[[Pulmonary shunt]], in which blood passes from the right to the left side of the heart without being oxygenated. This may be due to anatomical shunts, in which the blood bypasses the alveoli, via [[wikt:intracardial|intracardiac]] [[Shunt (medical)|shunts]], [[pulmonary arteriovenous malformation]]s, [[fistula]]s, and [[hepatopulmonary syndrome]], or physiological shunting, in which blood passes through non-ventilated alveoli.<ref name="Bhutta et al 2022" />
**Impaired diffusion, a reduced capacity for gas molecules to move between the air in the alveoli and the blood, which occurs when alveolar–capillary membranes thicken. This can happen in [[interstitial lung disease]]s such as [[pulmonary fibrosis]], [[sarcoidosis]], [[hypersensitivity pneumonitis]], and [[connective tissue]] disorders.<ref name="Manninen and Unger 2016" />
*{{visible anchor|Circulatory hypoxia}},<ref name="Bhutta et al 2022" /> also known as ischemic hypoxia or stagnant hypoxia, is caused by abnormally low blood flow to the lungs, which can occur during [[shock (circulatory)|shock]], [[cardiac arrest]], severe [[congestive heart failure]], or [[abdominal compartment syndrome]], where the main dysfunction is in the cardiovascular system, causing a major reduction in perfusion. Arterial gas is adequately oygenated in the lungs, and the tissues are able to accept the oxygen available, but the flow rate to the tissues is insufficient. Venous oxygenation is particularly low.<ref name="Manninen and Unger 2016" /><ref name="Cleveland" />
*{{anchor|Anemic hypoxia}}[[Anemia|Anemic]] hypoxia or hypemic hypoxia is the lack of capacity of the blood to carry the normal level of oxygen.<ref name="Bhutta et al 2022" /> It can be caused by anemia or:<ref name="Manninen and Unger 2016" />
**[[Carbon monoxide poisoning]], in which carbon monoxide combines with the hemoglobin, to form [[carboxyhemoglobin]] (HbCO) preventing it from transporting oxygen.<ref name="Manninen and Unger 2016" /><ref name="Bleecker 2015" />
**[[Methemoglobinemia]], a change in the hemoglobin molecule from a ferrous ion (Fe<sub>2</sub><sup>+</sup>) to a ferric ion (Fe<sub>3</sub><sup>+</sup>), which has a lesser capacity to bind free oxygen molecules, and a greater affinity for bound oxygen. This causes a left shift in the O<sub>2</sub>–Hb curve. It can be congenital or caused by medications, food additives or toxins, including chloroquine, benzene, nitrites, benzocaine.<ref name="Manninen and Unger 2016" />
*[[Histotoxic hypoxia]] (Dysoxia)<ref name="Bhutta et al 2022" /> or {{visible anchor|Cellular hypoxia}} occurs when the cells of the affected tissues are unable to use oxygen provided by normally oxygenated hemoglobin.<ref name="Manninen and Unger 2016" /> Examples include [[cyanide poisoning]] which inhibits cytochrome c oxidase, an enzyme required for cellular respiration in mitochondria. [[Methanol poisoning]] has a similar effect, as the metabolism of methanol produces formic acid which inhibits mitochondrial cytochrome oxidase.<ref name="Manninen and Unger 2016" /><ref name="Mandal" />{{clarify|is there a difference between cellular and histotoxic hypoxia? if so, what exactly? both refer to cyanide poisoning as an example, but the explanations differ|date=December 2022}}

[[Intermittent hypoxic training]] induces mild generalized hypoxia for short periods as a training method to improve sporting performance. This is not considered a medical condition.<ref name="Levine 2002" /> Acute cerebral hypoxia leading to blackout can occur during [[freediving]]. This is a consequence of prolonged voluntary apnea underwater, and generally occurs in trained athletes in good health and good physical condition.<ref name="Pearn et al 2015" />