Oxygen saturation
 |
It has been suggested that Oxygenation (medical) be merged into this article or section. (Discuss) |
Oxygen saturation or dissolved oxygen (DO) is a relative measure of the amount of oxygen that is dissolved or carried in a given medium. It can be measured with a dissolved oxygen probe such as an oxygen sensor or an optode in liquid media, usually water.
In medicine, oxygen saturation (SO2), commonly referred to as "sats", measures the percentage of hemoglobin binding sites in the bloodstream occupied by oxygen. At low partial pressures of oxygen, most hemoglobin is deoxygenated. At around 90% (the value varies according to the clinical context) oxygen saturation increases according to an oxygen-hemoglobin dissociation curve and approaches 100% at partial oxygen pressures of >10 kPa. A pulse oximeter relies on the light absorption characteristics of saturated hemoglobin to give an indication of oxygen saturation. An SaO2 (arterial oxygen saturation) value below 90% causes hypoxemia (which can also be caused by anemia). Hypoxemia due to low SaO2 is indicated by cyanosis.
Venous oxygen saturation (SvO2) is measured to see how much oxygen the body consumes. Under clinical treatment, a SvO2 below 60%, indicates that the body is in lack of oxygen, and ischemic diseases occur. This measurement is often used under treatment with a heart-lung machine (Extracorporeal Circulation), and can give the perfusionist an idea of how much flow the patient needs to stay healthy.
Tissue oxygen saturation (StO2) can be measured by near infrared spectroscopy. Although the measurements are still widely discussed, they give an idea of tissue oxygenation in various conditions.
Saturation of peripheral oxygen (SpO2) is an estimation of the oxygen saturation level usually measured with a pulse oximeter device. It can be calculated with the pulse oximetry according to the following formula:
Medical Significance: Healthy individuals usually exhibit oxygen saturation values between 97% and 99%. An SaO2 (arterial oxygen saturation) value below 90% causes hypoxemia (which can also be caused by anemia). Hypoxemia due to low SaO2 is indicated by cyanosis. But it must be noted that oxygen saturation does not directly reflect tissue oxygenation. The affinity of hemoglobin to oxygen may impair or enhance oxygen release at the tissue level. Oxygen is more readily released to the tissues when pH is decreased, body temperature is increased, arterial partial pressure of carbon dioxide (PaCO2,) is increased, and 2,3-DPG levels (a byproduct of glucose metabolism also found in stored blood products) are increased. When the hemoglobin has greater affinity for oxygen, less is available to the tissues. Conditions such as increased pH, decreased temperature, decreased PaCO2, and decreased 2,3-DPG will increase oxygen binding to the hemoglobin and limit its release to the tissue.[1]
[edit] Environmental sciences
In aquatic environments, oxygen saturation is a relative measure of the amount of oxygen (O2) dissolved in the water. Supersaturation can sometimes be harmful for organisms and cause decompression sickness. Dissolved oxygen (DO) is measured in standard solution units such as millilitres O2 per liter (ml/L), millimoles O2 per liter (mmol/L), milligrams O2 per liter (mg/L) and moles O2 per cubic meter (mol/m3). For example, in freshwater under atmospheric pressure at 20°C, O2 saturation is 9.1 mg/L.
Solubility tables (based upon temperature) and corrections for different salinities and pressures can be found at the USGS web site. Tables such as these of DO in milliliters per liter (ml/L) are based upon empirical equations that have been worked out and tested (e.g. Weiss, 1970):
ln(DO) = A1 + A2 * 100 / T + A3 * ln(T / 100) + A4 * T / 100 + S * [B1 + B2 * T / 100 + B3 * (T / 100)2]
Where ln is the natural logarithm and the other variables take the following values:
| A1 | = | -173.4292 | B1 | = | -0.033096 | |
| A2 | = | 249.6339 | B2 | = | 0.014259 | |
| A3 | = | 143.3483 | B3 | = | -0.001700 | |
| A4 | = | -21.8492 | ||||
| T | = | temperature (kelvins) | S | = | salinity (g/kg) |
To convert the calculated DO above from ml/L to mg/L, multiply the answer by 1.4276.
Oxygen content in water can be measured by adding equal quantities of Manganese and Iodine ions in an alkaline solution to a sample of the water. This is then titrated against sodium thiosulfate with a starch indicator and the oxygen concentration determined. One such test is the Winkler test for dissolved oxygen.
[edit] References
- ^ Schutz 2001
- Graßhoff, Klaus.; Anderson, Leif. (1999). Methods of Seawater Analysis. Whiley-VHC. ISBN 3-527-29589-5.
- Weiss, R. (1970). "The solubility of nitrogen, oxygen, and argon in water and seawater". Deep-Sea Res. 17: 721–35.
