A pulse oximeter is a small, non-invasive device commonly clipped onto a fingertip to monitor two important physiological measurements. The top number is the oxygen saturation level (SpO2), which indicates the percentage of oxygen-carrying hemoglobin in the blood. The device shines light through the tissue to assess oxygen delivery. The second measurement, typically appearing as the bottom number, offers insight into the circulatory system. This article clarifies what this second reading represents and how to interpret its meaning.
The Identity of the Bottom Number (Pulse Rate)
The bottom number displayed on a pulse oximeter is the Pulse Rate (PR), also known as the Heart Rate, and is measured in beats per minute (BPM). This reading reflects the number of times the heart contracts within sixty seconds, serving as a fundamental indicator of cardiovascular function. The device often uses the abbreviations “PR” or “BPM” next to this numerical value to identify the measurement clearly.
The pulse rate measures how frequently the heart is pumping blood, a process linked to oxygen delivery. A healthy heart rate ensures that oxygenated blood (measured by SpO2) is circulated efficiently to all tissues. While SpO2 provides the quality of the blood, the Pulse Rate provides the frequency of its circulation. Together, these two measurements offer a comprehensive picture of a person’s cardiopulmonary status.
Interpreting Pulse Rate Results (Normal Ranges and Context)
For a healthy adult at rest, the normal range for the resting heart rate is generally considered to be between 60 and 100 beats per minute (bpm). Readings within this range suggest the heart is working normally to meet the body’s baseline needs for circulation. A heart rate that consistently falls outside this typical range may warrant discussion with a healthcare provider.
A resting heart rate persistently above 100 bpm is defined as Tachycardia (too fast). Conversely, a resting rate below 60 bpm is called Bradycardia (slower than normal). A low heart rate is not always concerning, as highly conditioned athletes may have resting pulse rates well below 60 bpm because their hearts are exceptionally efficient.
Many temporary factors can cause a healthy person’s pulse rate to fluctuate. Emotional stress, anxiety, or recent physical activity naturally elevate the heart rate. Substances like caffeine or certain medications can also increase the pulse rate temporarily. Conversely, the pulse rate is typically lower during deep sleep or periods of relaxation.
Normal heart rate ranges also differ significantly across age groups. Children and infants have much higher typical rates than adults; for example, a newborn’s heart rate may normally range from 100 to 180 bpm. If an adult’s reading is consistently too high or too low, especially when accompanied by symptoms such as dizziness, chest discomfort, or shortness of breath, seeking medical advice is appropriate.
How the Pulse Oximeter Measures the Heartbeat
The pulse oximeter derives the pulse rate through a process called photoplethysmography (PPG), which uses light to detect blood volume changes. The device contains a light source, typically consisting of red and infrared light-emitting diodes, and a photodetector. It shines these wavelengths of light through the tissue, usually the fingertip, and measures the amount of light that passes through.
With every heartbeat, a surge of blood flows through the arteries and capillaries, briefly increasing the volume in the measured area. This temporary increase in blood volume changes the amount of light absorbed by the tissue. The device registers these rhythmic changes in light absorption as a distinct pulse wave. By counting the frequency of these pulse waves over time, the oximeter accurately calculates the number of beats per minute.
This measurement process is distinct from how the device calculates oxygen saturation (SpO2). The pulse rate is determined by the timing of the light absorption peaks, regardless of the oxygen content. This allows the device to simultaneously provide both the heart rate and the oxygen saturation from the same light source.