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Oxygen Saturation

Oxygen Saturation

Saturation of oxygen is a comparative measurement of the amount of oxygen in a given medium. The following paragraphs will discuss more about this measurement parameter.
Narayani Karthik
Oxygen is vital for every living entity. But how do you go about measuring how much of it is contained in your blood, or in the water you drink, or in the air you breathe? The relative measurement of oxygen content, which is carried or dissolved in a given medium is termed as oxygen saturation. In humans, it is a measure of oxygen that is dissolved in blood. For example, one hemoglobin molecule contains four molecules of oxygen. Thus, how much oxygen would be contained by 100 hemoglobin molecules? Suppose, the 100 molecules of hemoglobin are carrying 380 molecules of oxygen. Thus, the oxygenation of blood can be calculated as (380/400) x 100, which is equal to 95%. This measurement is often referred to as SpO2.

Implications of Saturated Oxygen Levels
A normal human body needs a constant oxygen supply for healthy functioning. Low oxygen levels identified in blood can lead to serious and life-threatening medical conditions. Normal oxygen levels in body range between 95 - 100% at sea level. If the percentage goes below the oxygen saturation normal range, or drops even less than 80%, it indicates that the blood contains very low level of oxygen. Some facts to remember on this are:
  • If the dissolved oxygen falls below normal levels (i.e., less than 92%), there is a strong probability of the body succumbing to respiratory ailments such as hypoxemia, which shows the symptoms of body ache, fatigue, shortness of breath, and confusion.
  • As the blood oxygen levels are low, this gas is unable to penetrate the walls of red blood cells. In cases of low levels of this gas in the body, people are afflicted with weakening eyesight, memory loss, weakening heart muscles, tingling fingers, chronic cough, and water retention at feet and ankles.
  • Low saturation of oxygen in blood is mostly possible in cases of already existing respiratory illnesses, retention of excess fluid in body, sleep apnea, and inhaling extremely polluted air.
  • If the oxygen level is below 88%, no insurance company will pay for your medication.
How is it Measured?
Oximeter is a small device that is used to measure the oxygen levels in blood. The precision and accuracy of the measurement is based on various factors like hemoglobin level, percentage of inhaled oxygen, atmospheric temperature, arterial blood flow to the vascular tissues in body, venous return of blood, and the oxygenation capacity of the person. With the above factors and recordings of an oximeter, an oxygen saturation curve can be achieved. This curve plots the oxygen saturation against its partial pressure, thereby giving the p50 value, which indicates the point where the hemoglobin has a 50% saturation of oxygen value. With the help of this value, one can assess the index of oxygen affinity in blood. For a healthy human, the clinically accepted oxygen level is 95%, although the saturation value may vary between 97% - 99%.

Pulse oximeters are used for measuring oxygen levels, but they cannot be used in certain medical conditions like carbon monoxide poisoning and cardiac arrest situations. This is because they fail to detect any difference between the oxygen and carbon monoxide bound to the hemoglobin in blood.

How to Test Oxygen Saturation?
Testing can be done by two methods. One of them is by fitting a finger clip on your finger, which shines a light through a side of the finger, and the light is measured from the other end by a detector. Blood cells with different levels of oxygen exhibit a different color. The cells show a bright red color if they are loaded with oxygen. Otherwise, the color is a bluish tinge indicating that there is not enough oxygen in the blood cells. The finger clip gives a rough estimate value of the saturation level. The better alternative is to go for an arterial blood gas test. Usually, for blood tests, blood is drawn from veins. However, in this test, it is drawn from arteries (the artery for blood test is mostly in the wrist). This test provides with the exact value of the oxygen saturation. Both these procedures yield fast results with the former being available immediately, and the later being available in a span of 15 minutes.