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Thursday, November 20, 2008

Oxygen saturation

Oxygen saturation 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.

Medical science

In medicine, oxygen saturation (SO2), commonly abbreviated 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% is termed hypoxemia. This may be due to various medical conditions.

The SvO2 (venous oxygen saturation) 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 (Extra Corporal 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.

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 litre (ml/L), millimoles O2 per liter (mmol/L), milligrams O2 per liter (mg/L) and moles O2 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 log 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.

1 comment:

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