Paced ECG as an independent clock¶
Fixed-rate pacing gives a clock the recording chain cannot touch. Timing paced beats on three timelines shows that each SNUADC step-mode correction loses about 18 ms of signal, and that nothing else breaks the sample stream.
| Date | 2026-10-17 |
| Status | current |
| Methods | Clocks carried in the signal |
| Techniques | Template beat timing, Phase-step detection |
| Clocks | Pacemaker clock, SNUADC sample clock, Recorder PC clock |
| Devices | Implanted pacemaker, SNUADC, Vital Recorder |
| Variables | ECG lead II (SNUADC) |
Background¶
Every clock in the recording chain is either corrected by the recorder or measured against a clock that is. A pacemaker at a fixed rate is different: beat n falls at t₀ + n·P on its own crystal, whatever the recorder does. Any break in a timeline shows as a phase step in the pacing train on that timeline.
Data¶
All 6,388 cases were screened for runs of beats at one constant interval. Seven pass, paced at about 1,204, 1,000 and 860 ms (50, 60 and 70 bpm); only two of them list a pacemaker in the pre-operative ECG field. About 40 more cases have long regular runs at 370–920 ms, but their interval wanders by about 1 ms between stretches: regular sinus rhythm, not pacing.
Method¶
Each beat was timed by template matching (fit residual about 1 ms) and placed on three timelines: the sample count, the record stamps, and the stamp-model line. Between stretches paced at the same interval P, a phase step on a timeline marks a break in that timeline. Steps of 6 ms or more were examined one by one, and an intervening poorly matched beat (an intrinsic or fusion beat, which resets a demand pacemaker) was taken as an explanation.
Results¶
- The pacing intervals agree between stretches to 0.05–0.1 ms on the sample count.
- Each step-mode correction loses about 18 ms of signal. At the three corrections inside paced rhythm (two cases, both leads where available), the pacing train steps on the sample count by −17.2 to −18.2 ms. The record stamps step by −480.6 to −481.0 ms, and the train on stamp time by −498.1 to −498.8 ms. So the true correction is about 0.498 s, close to a 0.5 s threshold, and the stamps show only 0.48 s of it because about 9 samples are never taken. Every record still holds 100 samples, so nothing in the file shows the loss.
- Recording gaps of 10–18 s shift the stamps by 127–149 ms against the pacing train (four gaps, two cases).
- No other breaks. Apart from corrections and gaps, no phase step of 6 ms or more went without an intervening beat or a poorly matched beat. No hidden concatenation was found in the paced stretches.
| Parameter | Value | Status | From |
|---|---|---|---|
| Pacing interval agreement between stretches | 0.05–0.1 ms | measured | Paced ECG as an independent clock |
| Stamp shift across a recording gap | 127–149 ms early (4 gaps, 2 cases; modulo one pacing period) | measured | Paced ECG as an independent clock |
| Signal lost at a SNUADC step-mode correction | 17.7 ms (range 17.2 to 18.2; n = 3; 3 corrections in 2 paced cases; ECG–EEG bursts give 13.0 ± 1.7 ms) | measured | Paced ECG as an independent clock |
| SNUADC step-mode correction size | about 0.498 s true (paced: 0.4981–0.4988 s); −0.480 s on the stamps | measured | Paced ECG as an independent clock |
Discussion¶
The −0.48 s correction quoted for step mode is the visible part of a 0.5 s resynchronisation. Without the loss, every sample after a correction sits about 18 ms early, adding up over a case's one to three corrections. The ECG–EEG burst lag confirms the sign without pacing: it steps by +13.0 ± 1.7 ms at each correction (93 cases), a little under the direct measurement.
The stamp model now adds the loss back at each correction, which removes the burst-lag step (−3.3 ± 1.8 ms after the fix) but barely moves the overall error, because the loss is small beside the window-to-window spread of the burst lag. The slight overshoot hints that the average loss is nearer 15 ms.
Observations it supports¶
| Observation | Status | Summary |
|---|---|---|
| About 20 ms of signal lost at every SNUADC reset | established | At each step-mode reset the acquisition pauses for about 20 ms inside the last record before the new timing, and that time is never sampled: 17.7 ms on the sample count (Signal lost at a SNUADC step-mode correction), once the filler sample is counted. Every record still holds 100 samples, so nothing in the file shows it. |
| SNUADC stamps run −0.95 ppm against the sample count | established | Between corrections each 0.2 s record stamp increment is about 0.19 µs short: stamps run −0.95 ppm from the sample count in 88% of files (both SNUADC modes). It is arithmetic, not clock drift. The other 12% sit between −0.90 and −0.55 ppm. |
Limitations¶
Three paced corrections in two cases. The gap shift is measured modulo one pacing period. The location of the lost time was not resolved here; Records out of order at SNUADC resets, and where the lost time goes later placed it inside the last old-grid record.
Open questions¶
- Long regular runs that may be paced (unexplained phenomenon). About 40 cases have long regular runs (370–920 ms) whose interval wanders by about 1 ms between stretches. Whether they are paced is not settled.