The heartbeat in the Primus airway signals¶
A pilot in 30 recordings for a heartbeat-locked route from the Primus to the SNUADC. Airway pressure shows the heartbeat in 9% of windows only. CO₂ shows a heartbeat-locked trace in 60% of windows, tight to 20 ms, but its lag does not follow the Primus drift, so it cannot time the Primus. Closed as a clock route.
| Date | 2026-10-19 |
| Status | current |
| Methods | One event, two clocks |
| Techniques | Template beat timing, Null controls |
| Clocks | Primus clock, SNUADC sample clock |
| Devices | Primus anaesthesia machine, SNUADC |
| Variables | Airway pressure (Primus), Capnogram (Primus), ECG lead II (SNUADC) |
Data¶
30 ventilated recordings with AWP, CO₂, ECG and CVP, chosen across setups; 1,608 (AWP) and 1,604 (CO₂) windows.
Method¶
R peaks on the ECG's stamp-model timeline. AWP and CO₂ high-passed at 0.6 Hz, the breath-locked shape subtracted and expiration masked; an R-triggered average per window, its lag tracked from window to window. A control triggers on R times jittered by up to half a beat; the peak-to-noise ratio (PNR) is against it. The breath-matched (CVP) lag of the same windows gives the Primus drift to compare with.
Results¶
| Signal | Windows with PNR ≥ 5 | Lag scatter (MAD) | Slope against breath slope |
|---|---|---|---|
| AWP | 9% | 22 ms | r = 0.02 (9 stretches) |
| CO₂ | 60% | 20 ms | r = −0.02 (56 stretches); median difference −114 ppm |
The CO₂ trace is flat on the SNUADC timeline, while the Primus drifts by about +104 ppm against it.
Discussion¶
The CO₂ waveform carries a clear heartbeat-locked signal, but it does not move with the Primus clock, so it is not a Primus timing route (A heartbeat-locked trace in the CO₂ waveform that ignores the Primus clock). Airway pressure carries too little cardiac signal once the breath shape is removed.
Observations it supports¶
| Observation | Status | Summary |
|---|---|---|
| A heartbeat-locked trace in the CO₂ waveform that ignores the Primus clock | open | Averaged on the ECG's R peaks, the Primus CO₂ waveform shows a heartbeat-locked signal in 60% of 10-minute windows (30-recording pilot), tight to about 20 ms. Unlike the breath, its lag does not drift with the Primus clock: it stays fixed on the SNUADC timeline. |
Limitations¶
A 30-recording pilot; the jitter control guards against breath-heart coupling but not every artefact.