Pulse oximetry

Pulse oximetry

Pulse oximetry is the measurement of oxygen saturation in arterial blood by using a non-invasive device, usually connected to one of the fingers. A wave form and percentage saturation display are usually given. Pulse oximeters are routinely used in operating rooms, emergency rooms and intensive  care units. Outside of the medical setting, they can be used by mountain climbers and pilots of un-pressurized aircrafts to decide whether supplemental oxygen is needed at high altitudes. Usual normal range for pulse oximetry readings would be 95 to 99 percent. But in patients with chronic obstructive pulmonary disease, slightly lower levels may be acceptable.

Principle of pulse oximetry

The technique depends on the absorption of two wave lengths of light transmitted through a thin region of the body like finger or ear lobe. Typically the device transmits infra red and red light and receives the transmitted light in a sensor. The attached microprocessor calculates the amount of light absorbed in each wave length and determines the oxygen saturation. Oxygen rich hemoglobin absorbs more of infra red light and hemoglobin devoid of oxygen absorbs more of red light. Thus it is able to compare between arterial and venous blood and display the arterial oxygen saturation which has a pulsatile flow.

Limitations of pulse oximetry

The techniques gives only the percentage oxygen saturation of hemoglobin. In severe anemia, even if the oxygen saturation is good, actual tissue delivery of oxygen may be poor. Erroneous values may be obtained in hypoperfusion, during use of vasoconstrictor drugs and with movement artefacts. Wrong values can also occur in carbon monoxide and cyanide poisoning. Methemoglobinemia can also lead to erroneous values of pulse oximetry.