What is Oxygen Saturation (SpO2)

Oxygen saturation is defined as the ratio of oxy-hemoglobin to the total concentration of hemoglobin present in the blood (ie Oxy-hemoglobin + reduced hemoglobin).

Hemoglobin is an iron-containing protein bound to red blood cells and makes up nearly all the oxygen presence (there is a minute amount dissolved in the plasma). Hemoglobin is responsible for transporting oxygen from lungs to other parts of the body, where the oxygen can be used by other cells. Oxy-hemoglobin (HbO2) is the bright red hemoglobin that is a combination of hemoglobin and oxygen from the lungs.

A hemoglobin molecule can carry a maximum of four oxygen molecules. 1000 hemoglobin molecules can carry a maximum of 4000 oxygen molecules; if they together were carrying 3600 oxygen molecules, then the oxygen saturation level would be (3600/4000)*100 or 90%.

Here is an informational video by the New England Journal of Medicine that explains in detail what is oxygen saturation levels or SpO2 and the technology behind pulse oximetry.

When arterial oxy-hemoglobin saturation is measured by an arterial blood gas it is called SaO2. When arterial oxy-hemoglobin saturation is measured non-invasively by a finger pulse oximeter or handheld pulse oximeter, it is called SpO2.

Methods for measuring oxygen saturation level

Pulse Oximeter: A pulse oximeter is a device intended for the non-invasive measurement of arterial blood oxygen saturation and pulse rate. Typically it uses two LEDs (light-emitting diodes) generating red and infrared lights through a translucent part of the body. Bone, tissue, pigmentation, and venous vessels normally absorb a constant amount of light over time. Oxy-hemoglobin and its deoxygenated form have significantly different absorption pattern. The arteriolar bed normally pulsates and absorbs variable amounts of light during systole and diastole, as blood volume increases and decreases. The ratio of light absorbed at systole and diastole is translated into an oxygen saturation measurement.

CO-oximeter: A CO-oximeter is a device for detecting hypoxia and works similar to a pulse oximeter. CO-oximeter measures absorption at several wavelengths to distinguish oxy-hemoglobin from carboxyhemoglobin and determine the oxy-hemoglobin saturation even when the patient has carbon monoxide poisoning.

Capnometer:A capnometer is an instrument for monitoring breathing rate and adequacy of ventilation. It attaches to the endotracheal tube and measure the carbon dioxide content in the inspired and expired air. It uses an infrared light to measure the amount of light absorbed by carbon dioxide molecules during breathing. It detects changes in carbon dioxide concentrations in patients who are hemodynamically stable, but not critically ill.

Arterial blood gas (ABG) analysis: This is a blood test using samples extracted from an artery. The test determines the pH of the blood, the partial pressure of carbon dioxide and oxygen, and the bicarbonate level. Many blood gas analyzers will also report concentrations of lactate, hemoglobin, several electrolytes, oxy-hemoglobin, carboxyhemoglobin and methemoglobin. The arterial blood gas analysis determines gas exchange levels in the blood related to lung function.

Reference

1. Oxygen saturation
2. Definition and notation of hemoglobin oxygen saturation