Pulse Oximeters for CPAP: Why Track SpO2

What an Overnight Pulse Oximeter Measures

A pulse oximeter shines two wavelengths of light through a fingertip or earlobe and reads how much each wavelength is absorbed. Oxygenated and deoxygenated hemoglobin absorb the two wavelengths differently, so the device can estimate the percentage of your hemoglobin that is currently carrying oxygen. That percentage is your peripheral oxygen saturation, written as SpO2.

A consumer overnight oximeter, like the Wellue O2 Ring, samples your SpO2 and pulse every few seconds for the entire night and writes the result to a CSV. You charge it once, slip it on a finger before bed, and have a continuous oxygen recording in the morning. The hardware costs roughly the price of two months of CPAP supplies. The data costs nothing because it lives on your phone.

Healthy adults sit at 95 to 99 percent SpO2 during sleep, with brief dips of a few percent that are unremarkable. A real respiratory event, the kind a CPAP is designed to prevent, drops your SpO2 by 3 percent or more for at least 10 seconds. If you have sleep apnea and your CPAP is working, those drops are rare. If you have sleep apnea and something is leaking through, those drops are everywhere.

Why CPAP Users Should Care

Your CPAP scores AHI by counting events the machine itself detects. That number is the standard measure of therapy effectiveness, and CPAP Clarity surfaces it on every dashboard. AHI under 5 is the clinical target.

The catch: not every event your body experiences shows up in the AHI count. The CPAP only sees what the pressure sensor and flow sensor can resolve. Subtle flow limitation, central events that slip through the device's classifier, and arousals from non-respiratory causes can all leave your blood oxygen lower than your AHI would suggest. Your body knows. Your CPAP does not.

This is the case for adding an overnight oximeter to a CPAP user's data set. The oximeter watches the outcome (your blood oxygen) instead of the input (the pressure delivered to your airway). When AHI is low and SpO2 stays in the 95 to 97 percent band, the CPAP and the oximeter are telling the same story. When AHI is low but SpO2 keeps dipping, that disagreement is the signal worth investigating with your sleep physician.

SpO2, ODI, and Time Below 88 Percent

Three numbers do most of the work when reading an overnight oximetry recording.

SpO2 (mean and minimum). The mean tells you whether your overall oxygenation looked normal. The minimum tells you whether anything brief but severe happened. A healthy adult typically averages 95 to 97 percent with a minimum that does not drop below 90 percent. CPAP users with controlled apnea look the same. If your average is solid but your minimum is 80 percent, something brief and severe happened during the night and is worth a closer look at the timeline.

ODI (Oxygen Desaturation Index). ODI counts how many times per hour your SpO2 dropped more than 3 percent below your stable overnight level and then recovered. It is the oximetry analogue of AHI: a per-hour event count, and it uses the same severity bands. ODI under 5 is the target band. From 5 to 14 is mild, 15 to 29 is moderate, and 30 or more is severe. The clinical sources behind these bands include the American Academy of Sleep Medicine scoring manual and a growing body of home-sleep-study research that uses ODI as a screening signal. CPAP Clarity is built to line up with the drop count your oximeter's own app shows: your typical stable SpO2 for the night is the baseline, any reading more than 3 percent (or more than 4 percent) below it counts as a drop, and motion-flagged readings are set aside. On the nights we have checked against a Wellue device report, our drop counts match the number on the device screen. On an unusually busy night our count can read lower than your device's, so treat the report on your monitor as the higher-count reference.

Time below 88 percent. Time spent under 88 percent SpO2 is the threshold most clinicians flag as clinically significant hypoxemia. Healthy nights show zero. Even a few minutes is worth bringing to your physician. Twenty minutes or more is the kind of number that sometimes triggers a follow-up sleep study or supplemental oxygen prescription.

These three numbers, taken together, paint a picture your AHI alone cannot. If your CPAP says AHI 2 and your O2 Ring says ODI 1 with mean SpO2 96 and zero minutes below 88 percent, you have a clean night by every available measure. If your CPAP says AHI 2 but your O2 Ring says ODI 8 with five minutes below 88 percent, the AHI is missing something. Add a wearable and the picture widens again: here is how to see your CPAP, oximeter, and watch data in one place.

How CPAP Clarity Joins the Two Streams

CPAP Clarity supports two Wellue import formats on the same dropzone that handles your CPAP SD card: the CSV the ViHealth phone app exports, and the raw session files that Wellue's O2 Insight desktop program pulls off the ring over USB (files named like 20260705220726, with no extension). The two data sources sit in separate IndexedDB tables and join by date. Your CPAP nights and your oximetry nights never overwrite each other.

When both streams exist for the same date, the Therapy Outcomes section on the dashboard surfaces the two numbers side by side: your AHI and your minimum SpO2 with time below 88 percent. The History page adds an Oxygen overlay toggle that drops an ODI trend line next to your AHI and Usage trends. The standalone /oximeter route shows the full SpO2 timeline for any single night, plus a desaturation timeline that highlights every continuous run below 88 percent.

The dashboard's insight rules read both streams. Six oxygen-specific rules fire on top of the CPAP rule set. Three are severity bands on ODI plus time-below-88 plus minimum SpO2. One flags an elevated overnight pulse on a low-event night. Two are correlation rules: when your CPAP AHI dropped this week but your ODI did not, you see a "the events the CPAP scores are improving but your oxygen is not" insight. That is exactly the conversation worth having with your physician.

Common Questions

Is a fingertip oximeter accurate enough for overnight use?

A consumer fingertip oximeter is not a hospital pulse oximeter. Overnight rings like the Wellue O2 Ring, whose manufacturer (Viatom) holds FDA clearances for its pulse oximeters, are designed for at-home tracking. Manufacturer specs put their accuracy within a few percent of a reference instrument under good conditions, but the FDA cautions that real-world readings can vary more than that, and that motion, cold fingers, dark nail polish, and darker skin tones all widen the error band. Treat the trend as more reliable than any individual sample, and treat persistent low readings at rest as a signal to consult a clinician rather than to self-diagnose.

Will my oximetry data ever leave the browser?

CPAP Clarity parses the Wellue CSV in your browser using the same client-side architecture as every other parser on the site. The samples and per-night summary go straight into IndexedDB on your device. Nothing about your oximetry recording touches CPAP Clarity servers, ever. The same privacy model as your CPAP data.

Can the O2 Ring replace a sleep study?

No. An overnight oximeter is a screening and tracking tool. A diagnostic sleep study includes EEG, EMG, eye movement, airflow, effort, and oximetry, scored by a registered technologist and read by a board-certified sleep physician. If your oximetry numbers are concerning, your next step is a clinician, not a different device.

What if my AHI is 2 but my ODI is 8?

That mismatch is the entire point of this feature. It usually means residual flow limitation or central events that the CPAP did not classify as full apneas or hypopneas, but that were severe enough to drop your blood oxygen. Bring the actual numbers to your sleep physician. Possible next steps include a pressure adjustment, an ASV trial, or a follow-up sleep study to characterize the residual events. Our dedicated walkthrough on why your AHI and ODI diverge covers the three most common causes and what to bring to the conversation.

Is the O2 Ring worth it for someone whose CPAP numbers look good?

If your AHI is consistently under 2 and you feel well-rested, you are probably not the audience. If your AHI is borderline, if you feel tired despite a clean AHI, or if you are in the first year of therapy and want a second source of evidence that the prescription is working, an overnight oximeter is among the cheapest pieces of CPAP-adjacent gear you can buy.

Why Your Drop Count Can Differ From the ViHealth App

If you compare a drop count computed from the CSV against the number on your ViHealth screen, calm nights line up and busy nights sometimes read lower in the CSV. The reason is where each number comes from. The ring computes its official drop counts and O2 Score on the device itself, from its own high-rate internal readings, and writes those results into the raw session file. The exported CSV keeps only a thinned-out sample of the recording (every second to every fourth second, depending on the model). On a calm night that loses nothing. On a busy night with fast, repeated dips, the thinned-out readings can merge two neighboring drops into one, so a count recomputed from the export lands a little below the ring's own tally. This is a property of the export, not a scoring disagreement: no calculation can recover events the file no longer contains.

CPAP Clarity handles this two ways. Counts computed from a CSV use the same recipe the app uses (a stable-level baseline, drops of more than 3 or 4 percent, checked against real Wellue device reports), so they match the app on typical nights and can read lower on heavy ones; when the two disagree, your device's app is the reference. Or you can close the gap by importing the raw O2 Insight session files: for the file format current rings produce, CPAP Clarity reads the ring's own counts and O2 Score straight from the file and marks the night with a note saying the numbers came from your ring. If a future firmware changes that format, the site falls back to computing from the readings rather than guessing at the ring's numbers, so you are never shown a made-up count under a "from your ring" label.

Try It

Once your O2 Ring CSV is exported from the ViHealth app, drop it into the import zone on the dashboard the same way you import your CPAP SD card. The site detects it automatically and routes it to the oximeter pipeline. You can then visit /oximeter to see the night, / to see the joined Therapy Outcomes section, and /history to toggle the Oxygen overlay on your trend charts. If you use Wellue's O2 Insight program on a computer, you can drop its raw session files into the same import zone; for the file format current rings produce, that reads your ring's own counts directly.

If you want the device-specific overview before importing, the Wellue O2 Ring landing page walks through what CPAP Clarity reads from the ViHealth CSV, how ODI3 and ODI4 are scored, and how the ring compares to the Apple Watch SpO2 sensor and a home sleep study.

If you want a backgrounder on how AHI itself is calculated before stacking ODI on top of it, What Is AHI? covers it. If you want the full feature inventory, How to Use CPAP Clarity walks every page.

Primary Sources

• U.S. Food & Drug Administration. Pulse Oximeter Basics (Consumer Updates). Explains the difference between medical, over-the-counter, and general-wellness oximeters, and that circulation, skin pigmentation, temperature, tobacco use, and nail polish all affect accuracy. fda.gov Pulse Oximeter Basics (opens in new tab)
• U.S. Food & Drug Administration. Pulse Oximeters (Products and Medical Procedures). Covers the FDA's ongoing concern that readings can be less accurate in people with darker skin pigmentation. fda.gov Pulse Oximeters (opens in new tab)
• U.S. Food & Drug Administration. 510(k) Premarket Notification K191088 (Checkme O2 Pulse Oximeter, Shenzhen Viatom). An FDA clearance held by the O2 Ring's manufacturer. FDA 510(k) K191088 (opens in new tab)