A pulse oximeter is a small medical device that estimates how much oxygen your blood is carrying. It is often clipped onto a fingertip because the skin is thin and has many small blood vessels. The device matters because oxygen saturation helps doctors, athletes, pilots, and patients monitor breathing and circulation.
A normal reading for many healthy people at sea level is usually about 95% to 100%, but readings must be interpreted with the person's condition in mind.
The oximeter shines red light and infrared light through the finger and measures how much light reaches a sensor on the other side. Oxygen-rich hemoglobin and oxygen-poor hemoglobin absorb these two colors differently, so the device compares the signals to estimate arterial oxygen saturation, called SpO2. It also detects the pulsing change in blood volume caused by each heartbeat, which helps separate arterial blood from skin, bone, and steady venous blood.
Motion, poor circulation, nail polish, and some lighting conditions can reduce accuracy because they affect the light signal.
Understanding Medical Technology: Pulse Oximeters
The calculation inside a pulse oximeter is not a direct measurement of oxygen molecules. It is an estimate based on a pattern of changing light absorption. With every heartbeat, a small extra amount of arterial blood enters the finger.
The device examines the part of the light signal that rises and falls in time with that pulse. It forms a ratio from the red signal and the infrared signal, then uses a built-in calibration curve to convert that ratio into a saturation estimate.
Manufacturers create these curves by comparing sensor readings with carefully measured blood samples from volunteers. This is why two devices can give slightly different readings.
Oxygen saturation is useful, but it does not tell the whole story about breathing. Blood can be nearly fully saturated while the total amount of hemoglobin is low. This happens in anemia.
In that case, each hemoglobin molecule may carry oxygen, yet there are fewer hemoglobin molecules available to transport it. A pulse oximeter cannot detect this problem by itself.
It does not measure how much oxygen reaches body tissues either. Doctors combine the reading with breathing rate, skin color, alertness, chest symptoms, medical history, and sometimes a blood test called an arterial blood gas test.
The relationship between oxygen pressure in the lungs and saturation in blood is not a straight line. Hemoglobin holds oxygen strongly over a broad range of normal lung oxygen levels. This creates a fairly flat part of the oxygen hemoglobin curve.
A small change in lung oxygen on this flat part may barely change the oximeter reading. At lower oxygen levels, the curve becomes steep. Then a small further decrease can cause saturation to fall quickly.
This is one reason a falling reading needs attention, especially when it is paired with signs of difficult breathing. At high altitude, lower air pressure means less oxygen enters the lungs, so healthy people may have readings lower than the usual sea level range.
Students should treat a displayed number as evidence with limits, not as a final diagnosis. Cold hands, low blood flow, shivering, movement, or an incorrectly sized sensor can make the pulse signal weak. Strong external light can enter the sensor, while nail products or artificial nails may change the light path.
Some blood conditions create more serious errors. Carbon monoxide binds to hemoglobin and can make a reading appear reassuringly high even when oxygen delivery is dangerous. Methemoglobin can push readings toward about 85 percent regardless of the true oxygen state.
A good measurement is taken when the person is still, the hand is warm, and the value has settled for several seconds. Repeated trends are often more informative than one isolated reading.
Key Facts
- SpO2 is the estimated percent of hemoglobin in arterial blood that is carrying oxygen.
- Pulse oximeters commonly use red light near 660 nm and infrared light near 940 nm.
- Oxygenated hemoglobin absorbs more infrared light, while deoxygenated hemoglobin absorbs more red light.
- Beer-Lambert law idea: A = log10(I0/I), where A is absorbance and I is transmitted light intensity.
- Percent saturation can be written as SpO2 = oxygenated hemoglobin / total hemoglobin x 100%.
- The pulse signal lets the device focus on arterial blood because arterial volume changes with each heartbeat.
Vocabulary
- Pulse oximeter
- A device that estimates blood oxygen saturation and pulse rate by shining light through tissue.
- SpO2
- The pulse oximeter estimate of arterial oxygen saturation expressed as a percentage.
- Hemoglobin
- A protein in red blood cells that binds oxygen and carries it through the bloodstream.
- Absorbance
- A measure of how much light is taken in by a material instead of passing through it.
- Photodetector
- A sensor that converts received light into an electrical signal for the device to analyze.
Common Mistakes to Avoid
- Treating SpO2 as the exact oxygen level is wrong because a pulse oximeter gives an estimate based on light absorption and calibration.
- Ignoring poor finger placement is wrong because gaps, loose contact, or misalignment can let extra light reach the sensor and distort the reading.
- Taking a reading during motion is wrong because movement changes the light path and can be mistaken for the pulse signal.
- Assuming a normal SpO2 always means normal breathing is wrong because oxygen saturation does not directly measure carbon dioxide level, ventilation, or all circulation problems.
Practice Questions
- 1 A pulse oximeter reports SpO2 = 97%. If total hemoglobin is modeled as 150 arbitrary units, how many units are oxygenated?
- 2 Light entering a fingertip has intensity I0 = 100 units. The detector receives I = 25 units. Using A = log10(I0/I), calculate the absorbance.
- 3 Explain why a pulse oximeter needs to detect the heartbeat pulse instead of only measuring the total light passing through the finger.