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1.(MeSH)Relatively complete absence of oxygen in one or more tissues.;Relatively complete absence of oxygen in arterial blood.
Signs and Symptoms - Anoxia, Brain, Anoxia, Cerebral, Anoxic Brain Damage, Anoxic Encephalopathy, Brain Anoxia, Brain Hypoxia, Cerebral Anoxia, Cerebral Hypoxia, Encephalopathy, Hypoxic, Hypoxia, Brain, Hypoxia, Cerebral, Hypoxic Brain Damage, Hypoxic Encephalopathy - Brain Ischemia, Cerebral Ischemia, Encephalopathy, Ischemic, Ischemia, Cerebral, Ischemic Encephalopathy[Hyper.]
Signs and Symptoms, Respiratory[Hyper.]
Anoxia-Ischemia, Brain, Anoxia-Ischemia, Cerebral, Anoxic-Ischemic Encephalopathy, Brain Anoxia-Ischemia, Brain Hypoxia-Ischemia, Brain Ischemia-Anoxia, Brain Ischemia-Hypoxia, Cerebral Anoxia-Ischemia, Cerebral Hypoxia-Ischemia, Cerebral Ischemia-Anoxia, Cerebral Ischemia-Hypoxia, Encephalopathy, Anoxic-Ischemic, Encephalopathy, Hypoxic-Ischemic, Hypoxia-Ischemia, Brain, Hypoxia-Ischemia, Cerebral, Hypoxic-Ischemic Encephalopathy, Ischemia-Anoxia, Brain, Ischemia-Anoxia, Cerebral, Ischemia-Hypoxia, Brain, Ischemia-Hypoxia, Cerebral, Ischemic-Hypoxic Encephalopathy[Analogie]
Hypoxemia (n.) [MeSH]
Hypoxemia (or hypoxaemia) was originally defined as a deficiency of oxygen in arterial blood, and standard manuals take this to mean an abnormally low partial pressure of oxygen (mm Hg), content of oxygen (ml oxygen per dl blood) or percent saturation of hemoglobin with oxygen, either found singly or in combination. One simple rule is that hypoxemia becomes very serious when the decreased partial pressure of oxygen in blood is less than 60 mm Hg, because that point is the beginning of the steep portion of the hemoglobin dissociation curve, where a small decrease in the partial pressure of oxygen results in a large decrease in the oxygen content of the blood. or when hemoglobin oxygen saturation is less than 90%.
While the term hypoxemia is limited to low oxygen in the blood, the more general term is hypoxia, which is an abnormally low oxygen content in any tissue or organ. It will be seen that hypoxemia can cause hypoxia (the hypoxemic hypoxia) along with other mechanisms (e.g. anemic hypoxia or histotoxic hypoxia). Informally, hypoxemic hypoxia is sometimes informally given as hypoxic hypoxia.
Disagreements exist concerning the scope of the term hypoxemia. At one extreme, there is nearly universal agreement that a blood gas determination which shows that the partial pressure of oxygen in a good arterial sample of whole blood is lower than normal constitutes hypoxemia. One important condition that tests this rule is carbon monoxide poisoning, where the arterial partial pressure of oxygen is normal, but the content is much reduced. (The content is reduced because the hemoglobin is tightly bound by the carbon monoxide, which effectively excludes oxygen.)
There is also nearly universal agreement that an abnormally low percent saturation of arterial hemoglobin with oxygen constitutes hypoxemia. This concept has given rise to a ready measurement of percent saturation by pulse oximetry. However this measurement can be very misleading when blood flow is slowed or interrupted, leading to a local tissue hypoxia even though arterial blood in patent blood vessels is normal.
There is less agreement concerning whether the oxygen content of blood is relevant to hypoxemia, particularly because the measurement of oxygen content requires tonometry, a method that is not always available. Pulmonary medical specialists would say yes, as would the more technical dictionaries, but in so doing they include severe anemia as a cause of hypoxemia due to the disease's greatly reduced quantity of hemoglobin, the oxygen binding protein within the red blood cell. Trauma Critical Care specialists tend to say no, conforming to the simpler definition of hypoxemia being a low partial pressure of oxygen only, reserving the concept of oxygen content to discussions of oxygen delivery to the tissues..
Finally, the term was initially proposed to describe the low blood oxygen seen at high altitude and, had a general, non-technical definition - a defective oxygenation of the blood Current dictionaries and web sites track the original definition, generally defining the term as insufficient oxygenation of the (arterial) blood. Other sites speak of "level" of oxygen but this non-technical usage sidesteps the contentious details, and does not offer a definitive solution to the problem of whether to include anemia in the scope of hypoxemia. With this caveat, the following article will include low oxygen content as a cause of hypoxemia because it is a functionally and clinically important reason why tissues become hypoxic and must be considered when there are symptoms of tissue hypoxia.
Many of the causes for hypoxemia fall into the general category of problems that concern the lung and heart.The most common source of an individual's hypoxemia is the lung, where inspired air is wasted by going to regions that are poorly supplied with blood and (conversely) blood is perfusing areas of the lung that receive little of the inspired air; this is termed ventilation-perfusion mismatch. A second category includes those causes where the ventilatory drive is inadequate for some reason. Finally, the blood might be incapable of carrying sufficient oxygen for the body's needs for reasons such as anemia or carbon monoxide poisoning; remember, this final category is for those whose definition of hypoxemia includes situations where the quantity of oxygen is deficient, as well as when the partial pressure and/or the percent saturation is reduced.
In addition to the two common reasons (ventilation-perfusion mismatch and shunt), a diffusion limitation in the lung may also cause hypoxemia. (Incidentally, this order of presentation is different from that found in a textbook because - didactically - it is more effective for them to present the ideas in the reverse order of how common they really are.)
Key to understanding whether the lung is involved in a particular case of hypoxemia is the difference between the alveolar and the arterial oxygen levels; this A-a difference is often called the A-a gradient and is normally small. The arterial oxygen partial pressure is obtained directly from an arterial blood gas determination. The oxygen contained in the alveolar air can be calculated because it will be directly proportional to its fractional composition in air. Since the airways humidify (and so dilute) the inhaled air, the barometric pressure of the atmosphere is reduced by the vapor pressure of water.
When alveolar ventilation (in liters of air per minute) and alveolar capillary blood flow (in liters of blood per minute) are approximately equal, oxygen equilibrates across the alveolar-capillary membrane well before the blood has traversed the alveolus. This equilibration does not occur when the alveolus is insufficiently ventilated, and as a consequence the blood exiting that alveolus is relatively hypoxemic. When such blood is added to blood from well ventilated alveoli, the mix has a lower oxygen partial pressure than the alveolar air, and so the A-a difference develops.
Some venous blood never circulates by alveoli before returning to the arterial vasculature, thus diluting the freshly oxygenated blood and reducing its oxygen content. The shunt may be intracardiac or may be intrapulmonary, and cannot be corrected by administering 100% oxygen.
Impaired diffusion across the blood-gas barrier in the lung can cause hypoxemia. However this is a rare cause as it is a problem only in extremely unusual circumstances. Most of the past cases once thought to be due to a diffusion problem are now recognized as being due to ventilation-perfusion inequality.
If the alveolar ventilation is insufficient, there will not be enough oxygen delivered to the alveoli for the body's use. This can cause hypoxemia even if the lungs are normal, as the cause is in the brainstem's control of ventilation or in the body's inability to breathe effectively.
The rate of breathing and the depth of each breath is controlled by the brainstem and are generally controlled by the blood level of carbon dioxide, as determined by chemoreceptors in the aorta. Hypoxia occurs when the breathing center doesn't function correctly or when the signal is not appropriate.
A variety of conditions that physically limit airflow can lead to hypoxemia.
The oxygen contained in the alveolar air is directly proportional to its fractional composition in air. The oxygen content also depends on the barometric pressure, so altitude reduces alveolar oxygen. For calculations, the barometric pressure of the atmosphere is reduced by the vapor pressure of the water that comes to saturate the air in the lung, since the airways humidify the inhaled air:
Almost all the oxygen in the blood is bound to hemoglobin, so interfering with this carrier molecule limits oxygen delivery to the periphery.
In blood, most of the oxygen is carried by hemoglobin. The following calculations demonstrate the quantitative difference between the quantity of oxygen dissolved in the blood and that carried by oxygen. First, the dissolved oxygen is calculated as the product of the gas solubility times the partial pressure of the gas, and is given as ml of gas per 100 ml blood.
The partial pressure of oxygen in the alveoli is the product of its fractional composition (21% in air) times the barometric pressure (less the water vapor pressure):
The quantity of oxygen carried by hemoglobin depends on its concentration (Hb) and the fraction of the hemoglobin that is actually carrying oxygen (the percent saturation). If Hb is 15 g/dl and it is 98% saturated:
These two calculations illustrate that hemoglobin increases the amount of oxygen that the blood can carry by ~40-fold. Such a large improvement means that any substantial degree of anemia will have a large impact on the oxygen carrying capacity of the blood.
Carbon monoxide poisoning can occur acutely, as with smoke intoxication, or over a period of time, as with cigarette smoking. Hemoglobin binds CO hundreds of times tighter than oxygen, so very low concentrations of CO will occupy hemoglobin binding sites, excluding oxygen. The quantity of CO in the blood is never zero since metabolic processes have CO as a waste product, giving us 4 ppm to 6 ppm at all times. Urban dwellers have 7 ppm to 13 ppm, but smokers have 20 ppm to 40 ppm! To calculate some consequences of these values, we can calculate (for urban dwellers at 5 ppm CO in their blood):
For heavy smokers (40 ppm):
CO has a second toxic effect, namely removing the allosteric shift of the oxygen dissociation curve and shifting the foot of the curve to the left. In so doing, the hemoglobin is less likely to release its oxygens at the peripheral tissues. Certain abnormal hemoglobin variants also have higher than normal affinity for oxygen, and so are also poor at delivering oxygen to the periphery.
Hemoglobin's function can also be lost by chemically oxidizing its iron atom to its ferric form. This form of inactive hemoglobin is called methemoglobin and can be made by ingesting sodium nitrite as well as certain drugs and other chemicals.
The final word is that the body may be able to compensate for hypoxemia due to any of these causes, and so symptoms may be overlooked initially. However, further disease or a stress such as any increase in oxygen demand may finally unmask the existing hypoxemia.