Pulse oximetry 101: what the number and the waveform mean

The pulse oximeter is one of the most familiar devices in medicine, a clip on a fingertip with a glowing red light. Most people, and many clinicians, watch only the number. The waveform underneath it is doing more work than it gets credit for.

The number: SpO2

The big number on the display is SpO2, an estimate of the percentage of hemoglobin in the blood that is carrying oxygen. The device shines two wavelengths of light through the finger and infers saturation from how each is absorbed as the blood pulses. It is a genuinely useful vital sign, but it is narrow: it measures oxygen saturation, not blood pressure, not how hard the heart is working, and not the effort of breathing.

The waveform: the PPG trace

To produce that number, the oximeter has to detect each heartbeat as a pulse of blood. That pulsation, drawn out over time, is the photoplethysmography or PPG waveform. Its shape, and crucially how its size changes with each breath, carry information the single number throws away. A small, steady respiratory variation is normal. A large one is not. For the clinical sign behind that, see our guide to pulsus paradoxus.

Common pitfalls

A few things are worth knowing about pulse oximeter readings:

• Motion and cold can distort the signal; a shaking or cold hand often reads poorly.
• Poor perfusion, nail polish, and very bright ambient light can interfere.
• A normal number is not an all-clear. SpO2 can stay normal while circulation or breathing effort is changing. The number alone is not designed to catch every problem.

Why the waveform is underused

Most monitors display the waveform but never analyze it. The information about respiratory variation is right there in the trace, every second, and it is usually discarded. That is the gap PulSentry is built around: reading the waveform continuously and summarizing its respiratory variation as a single number over time. The mechanics are in FFT and power spectral density of the PPG waveform.

PulSentry is investigational and not FDA-cleared. And if you are a patient or family member rather than a clinician, we have a plain-language version of all of this in the patient guide, including what the finger clip senses about breathing.