Cross-node synthesis, Tepna physiological-signal suite
Motivation. A suite of physiological-signal analyses accumulates as much knowledge in what it has ruled out as in what it has confirmed, yet negative results are almost never collected into a citable artifact. This paper is that artifact for the Tepna suite: a structured map of the walls — the analyses that looked promising and did not survive scrutiny, each with the evidence that felled it and its current disposition (fixed, flagged, or fundamental). Contents. Seven results: (1) a single-value optical PRV is not a drop-in for ECG/RR HRV — unbiased in the mean but carrying ~22× wider limits of agreement; (2) a per-beat signal-quality index stays green while optical beat-yield fails, inflating rMSSD a median +83%; (3) daily glycemic variability (CGM-CV) has near-zero test-retest reliability (ICC₁≈0) — a state, not a one-number trait; (4) a glucose↔HRV "coupling" collapses from −0.11 to +0.01 once a shared apnea driver is partialled out; (5) a rolling-baseline ODI-4 systematically under-counts events in severe apnea (slope 0.23 vs truth) — since traced to a trailing-mean baseline artifact and corrected at the detector level; (6) black-box vendor wellness composites emit a fabricated 0 when their inputs are absent; (7) cross-device wearable PAT (PPG foot − ECG R-peak) is drift-dominated — two BLE wearables logged by one phone share a clock only at the ~1 s start, then their device crystals drift ~48 ppm (~1147 ms/night, ~24× the PAT signal), so beat-level cross-device timing needs a hardware sync, not the phone stamp. Value. The contribution is framing and honesty, not new compute — a public account of where the field's intuitions break, so others need not re-walk into the same walls.
Keywords: negative results · reproducibility · consumer wearables · heart-rate variability · pulse oximetry · continuous glucose monitoring · signal quality · confounding · shared-driver effects · pulse arrival time · cross-device clock drift · scientific honesty
Most research papers tell you what worked. This one tells you what didn't — on purpose, because the failures are just as useful and almost nobody writes them down. Seven of them, from analyzing everyday health-gadget data: the wrist blood-flow sensor can't stand in for a chest strap for the fine heart-rhythm score (it's right on average but wildly variable); a "signal looks good" light stays green even while that same sensor is quietly losing heartbeats and inflating the score; a day's blood-sugar-variability number tells you almost nothing stable about a person; an apparent glucose–heart link turns out to be a third thing (breathing pauses) driving both; a cheap sleep-apnea count under-reports the worst cases (we found why and fixed it); and some gadgets' “wellness scores” silently print a zero when they actually have no data; and two health wearables recording the same night through one phone can't be lined up finely enough to time the tiny delay between heartbeat and pulse — their internal clocks drift about a second apart over the night (fixable later with a proper sync, but not from the phone's timestamps). None of this is a discovery in the usual sense — it's a map of the dead ends, so the next person doesn't waste the trip.
The Tepna suite's central discipline is to distrust its own outputs until a deterministic test earns them. That discipline produces a steady stream of negative results — analyses that a reasonable person would expect to work and that, on inspection, do not. Scattered across a dozen preprints, each negative result is a footnote to a positive one; collected, they form something more useful: a map of where consumer physiological-signal analysis breaks, which is exactly the knowledge a practitioner needs before trusting any of these signals. This paper assembles seven such walls. Each entry states the intuition, the evidence against it, and — critically — its current disposition, because a wall that has since been patched is a different object from one that is fundamental. We distinguish three dispositions: fundamental (a real property of the physics or statistics, not fixable), flagged (surfaced honestly to the consumer rather than corrected), and fixed (traced to a mechanism and repaired). No number here is new; each regenerates from the cited source paper's own named analysis tool.
The intuition is that a wrist/arm optical pulse gives you the same heart-rate-variability number as a chest ECG. On shared synthetic beats scored by the real detectors (rmssd-equivalence), ECG- and RR-derived rMSSD are genuinely interchangeable (bias −0.02 ms, Pearson r = 0.9999), but the optical arm is unbiased in the mean (+0.3 ms, +0.7%) while carrying ~22× wider limits of agreement (r = 0.93). The honest failure is therefore not a bias to subtract but a precision penalty: a single optical rMSSD reading cannot be trusted as a point estimate the way an ECG reading can, even though a large pooled average of optical readings converges to the truth. (Note: an earlier draft's headline "+32% optical bias" did not survive re-running on a broadband RR texture — the divergence was a texture artifact, not a real bias; correcting that is itself a negative result about the first analysis.)
The intuition is that a signal-quality index guards the beat train — if SQI is high, the beats are good. In the FULL-lane yield harness (qrs-yield, ≈363 windows/arm, ≈188k true beats), electrical QRS recall was ≈100% in both clean and apnea segments, but the optical arm recovered only ~96% and over-detected, and its mean SQI fell only from 0.90 to 0.78 under apnea — not low enough to mark the bad beats unusable. The downstream cost is large: rMSSD reconstructed identically from detected vs true beat-times was inflated a median +83% (+34 ms). SQI reports morphology plausibility, not beat completeness, so it cannot be the sole gate on HRV; the suite's response is to down-weight optical HRV on the event channel rather than trust SQI alone.
The intuition is that a person's CGM coefficient-of-variation is a stable personal number. In a 6,000-subject stable cohort (nights-icc), daily CGM-CV had a single-occasion ICC₁ ≈ 0.00 — essentially no between-subject variance — against rMSSD's 0.93 and ODI-4's 0.75 on the same harness. Stable people's days look alike in CV while each person's day-to-day swings dominate, so no number of averaged days makes a single CGM-CV a reliable individual trait (Spearman–Brown minimum occasions → ∞). Reporting it as a personal metric is the wall; it is a state descriptor.
The intuition is that a measured within-person glucose–HRV correlation reflects a direct physiological link. In the coupling analysis (cgm-hrv-coupling), partialling out the planted apnea burden collapsed the within-patient coupling from −0.11 to +0.01 — effectively zero — while the two driver legs (apnea→glucose, apnea→HRV) were strong and opposite-signed. The apparent coupling was almost entirely a third variable acting on both. The lesson generalizes: any cross-signal correlation in this domain must be tested against the obvious shared drivers before it is called a link.
The intuition is that an oxygen-desaturation-index event counter is severity-neutral. It is not: with a trailing-mean SpO₂ baseline (odi4-ahi-bias) ODI-4 was strongly linear in reference AHI but with slope 0.23 (R² 0.93) — recovering only ~a quarter of scored events, worst in severe OSA. The mechanism was self-suppression: dense event clusters drag a trailing-mean baseline down into the desaturations, so subsequent drops no longer clear the 4% threshold. This one is fixed — a ceiling (rolling-max) baseline raised the cohort slope from 0.42 to 0.69 without inflating the no-apnea stratum. It earns its place here as a documented wall and its repair — the model of what to do with a negative result.
The intuition is that a vendor "wellness/readiness" composite degrades gracefully when an input is missing. Some do not: when their black-box constituent inputs are absent they emit a fabricated 0 that is indistinguishable, downstream, from a true zero score. This is the same class of error as the timestamp parsers that return now() on a miss — a fabricated value where a null belongs. The suite's disposition is to treat such composites as untrusted at ingest and reject the sentinel 0, consistent with the never-fabricate-on-miss rule established in the Clock Contract benchmark.
Two wearables logged by one phone (a chest H10 ECG + an ankle Verity PPG, Polar Sensor Logger) are the same heartbeats — 89% beat coupling, 48 ms beat-to-beat — yet the R→foot lag drifts ~1.1 s/night at a fixed ~48 ppm across 11 nights (145k beats): ordinary quartz-crystal tolerance. Same-phone logging does not fix it — the "phone timestamp" equals each device's own sensor clock to within ms over 6.6 h (the logger writes start + device-elapsed), so each stream rides its own crystal. Passive re-sync from the two accelerometers fails by both windowed and event-matched cross-correlation (recovered offset noisier than the drift), because chest and ankle motion are decorrelated in sleep. So beat-level PAT is not recoverable from app-timestamped dual-BLE logs; it needs a single acquisition clock (host-side SDK capture). The suite's first cross-device-fusion wall now has a standalone account — One phone is not one clock.
Three patterns run through the six walls. First, unbiased is not the same as usable: the optical-PRV and reference-free-σ results both show a metric can be centered on truth yet too imprecise to act on from a single reading (2.1). Second, a quality signal must measure the failure that actually occurs: SQI guards morphology but not completeness, so it is silent on the yield failure that dominates the HRV error (2.2). Third, a number is only a trait if its variance separates people, and correlation in this domain is routinely a shared-driver artifact (2.3, 2.4). The two remaining walls are about honesty of missingness — a fabricated 0 or a fabricated now() is worse than an explicit null (2.5's mechanism, 2.6) — the through-line to the suite's timestamp and provenance discipline. A fourth pattern arrives with the newest wall: cross-device timing is not a shared clock — two streams logged by one phone agree at the second (their start) but not the sample, because each rides its own device crystal, so beat-level fusion across devices needs an explicit hardware sync, not a common host timestamp (2.7). The dispositions matter as much as the findings: one wall is fixed, two are flagged to the consumer, and four are fundamental (one, 2.7's cross-device PAT drift, fixable at capture time with a hardware sync). Publishing the fundamental ones is the point — they are the boundary of what consumer signals can honestly claim.
qrs-equiv-analysis.html (2.1), qrs-yield-analysis.html (2.2), nights-icc-analysis.html (2.3), the coupling analysis (2.4), odi-bias-analysis.html (2.5). Every number above is a direct quote of a value those tools produce live.synth-gen 2.1 / cohort-gen 1.9; the ODI-4 result (2.5) reflects the v22.36 detector correction. Where a figure changed across generator versions, both the old and current values are stated in-text rather than silently updated.PAT Feasibility.html / pat-feasibility.js) on a REAL single-night simultaneous capture (Polar H10 + Verity Sense), not synthetic ground truth — so its magnitudes (48 ppm drift, 1147 ms/night, 89.5% coupling, 48 ms beat-to-beat) are empirical. It is the one wall explicitly awaiting a positive re-run: after SDK / ACC-tap synchronization it graduates from dead-end to method (see PAT-FEASIBILITY-2026-07-08-BRIEF.md).papers/rmssd-equivalence.html — optical rMSSD: unbiased mean, ~22× wider LoA (wall 2.1).papers/qrs-yield.html — modality-asymmetric beat yield; SQI vs recall (wall 2.2).papers/nights-icc.html — test-retest reliability; CGM-CV ICC₁≈0 (wall 2.3).papers/cgm-hrv-coupling.html — shared-driver collapse of the glucose↔HRV coupling (wall 2.4).papers/odi4-ahi-bias.html — rolling-baseline ODI-4 under-count and its detector-level fix (wall 2.5).papers/timestamp-pathology.html — the never-fabricate-on-miss rule (wall 2.6's disposition).PAT Feasibility.html + PAT-FEASIBILITY-2026-07-08-BRIEF.md — cross-device PAT drift, real single-night H10+Verity (wall 2.7); unblock via POLAR-SDK-CAPTURE-2026-07-07-BRIEF.md.CLAUDE.md, SIGNAL-ADAPTER-AND-FRONTIER-2026-06-23-BRIEF.md §8 (the machine-readable graveyard registry), Tepna suite.