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Syncope (pronounced /’s??k?pi/) is the medical term for fainting, a sudden, usually temporary, loss of consciousness generally caused by insufficient oxygen in the brain either through cerebral hypoxia or through hypotension, but possibly for other reasons. Typical symptoms progress through dizziness, clamminess of the skin, a dimming of vision or brownout, possibly tinnitus, complete loss of vision, weakness of limbs to physical collapse. These symptoms falling short of complete collapse, or a fall down, may be referred to as a syncoptic episode.
Syncope due to hypoxia will normally occur if the air breathed contains less than 16% oxygen at atmospheric pressure. Less than 11% oxygen in the air at this pressure can lead to death by suffocation. However, the amount of oxygen in the air is dependent on the partial pressure of oxygen, meaning that, if a person inhales pressurized gas, e.g., while scuba diving, a breathing gas containing less than 16% oxygen can still contain enough to prevent hypoxia. On the other hand, mountaineers, pilots, and astronauts breathe oxygen-enriched gas because the partial pressure of oxygen in normal air mixture is not enough to prevent hypoxia, since the total pressure is reduced at high altitude. Syncope due to hypoxia can also occur because the lungs are not working properly, because a person is not breathing, because the blood is not circulating, or because the blood’s ability to transport oxygen is destroyed or blocked, e.g., by carbon monoxide, which, if present, binds itself to the blood’s hemoglobin.
The central ischaemic response is the brain’s response to a chronic lack of blood available to the brain. The brain attempts to prioritise its own needs above any others by commandeering all available blood flow at the expense of all other bodily functions and increasing the rate of blood oxygenation.
It diverts blood to itself by reducing the supply to most of the rest of the body through vasoconstriction and increasing the pulse rate, tachycardia. It attempts to increase oxygenation of the blood by accelerating the breathing rate hyperventilation. This gives rise to the typical symptoms of fainting: pale skin, notably a white face, rapid breathing, and weakness of the limbs, particularly the legs, progressing to collapse, which, in many, is felt as weakness and nausea.
Weakness of the legs tends to cause victims to lay themselves down or fall down involuntarily, thereby countering low blood pressure, hypotension, in the brain. It is unclear whether this is an evolved response or merely a serendipitous result of collapsing.
Factors that influence fainting are fasting long hours, taking in too little food and fluids, low blood pressure, hypoglycemia, growth spurts,[citation needed] physical exercise in excess of the energy reserve of the body, emotional distress, and lack of sleep. Orthostatic hypotension caused by standing up too quickly or being in a very hot room can also cause fainting.
More serious causes of fainting include cardiac (heart-related) conditions such as an abnormal heart rhythm (an arrhythmia), wherein the heart beats too slowly, too rapidly, or too irregularly to pump enough blood to the brain. Some arrhythmias can be life-threatening. Other important cardio-vascular conditions that can be manifested by syncope include subclavian steal syndrome and aortic stenosis.
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