sensor
Meaning of sensor: Every sensor answers exactly one question, so a reading that looks wrong is as likely to reflect the sensor as the patient.
Definition of sensor
A sensor is the part of a device that responds to one physical or chemical change, such as light, pressure, temperature or the concentration of a substance, and converts that response into an electrical signal a monitor can display, record or act on. Sensors are where a patient meets a machine, and they set the limits of what it can report.
What do medical sensors measure?
| Sensor | What it detects | What is displayed |
|---|---|---|
| Pulse oximeter probe | Light absorption through a finger or earlobe at two wavelengths | Oxygen saturation and pulse rate |
| Continuous glucose monitor | Glucose in the fluid between cells under the skin | An estimated glucose level and its trend |
| Electrocardiogram electrode | Electrical activity reaching the skin from the heart | The cardiac rhythm trace |
| Ventilator flow and pressure sensor | Air moving in and out, and pressure within the circuit | Breath volumes, rates and airway pressures |
| Temperature probe | Temperature at the skin, in the ear, or inside the body | A temperature reading |
How does a sensor turn a physical change into a number?
Three steps sit between the body and the display. The sensing element itself changes an electrical property, such as resistance or voltage, in proportion to whatever it is exposed to. That small change is amplified and converted into digits. Software then applies a calibration, a stored relationship between raw signal and the quantity of interest, and usually a smoothing step that averages several readings together. The number on the screen is therefore an interpretation of a signal rather than a direct observation, and every stage can introduce error. This is why a medical device carries a stated accuracy range instead of a promise of a correct figure.
What makes a reading unreliable?
A sensor reports only the one thing it was built to detect, and it reports that thing wherever it happens to be sitting. Movement, a loose or displaced probe, poor circulation to a cold finger, bright ambient light, electrical interference and dried gel under an electrode all corrupt the signal. Some errors are systematic rather than random. A pulse oximeter measures how much light hemoglobin absorbs and cannot tell oxygen bound to hemoglobin apart from carbon monoxide bound to it, so it reads falsely high in carbon monoxide poisoning. Skin pigmentation can also shift oximeter readings, with a tendency to overestimate saturation in people with darker skin. A continuous glucose monitor samples fluid between cells rather than blood, so its reading lags several minutes behind a blood level that is rising or falling quickly.
Used in a sentence
The oxygen saturation sensor was repositioned after the monitor displayed an inconsistent reading.
Good to know
A sensor reports only the specific signal it was built to detect, so a value inconsistent with the rest of the clinical picture can reflect the sensor's placement or condition rather than a change in the patient.