Seismocardiography for Primary Care (pocket-guide)

A practical visual guide to SCG waveforms and what they may suggest

What SCG measures

• SCG is a chest vibration signal from cardiac mechanics recorded by an accelerometer.
• In routine traces you usually see two dominant systolic landmarks:
  • AO - aortic valve opening, the main ejection “thump.”
  • AC - aortic valve closure, the second prominent peak.
• Timing tracks ECG rhythm: mechanical events follow the electrical R wave by a short electromechanical delay. You read SCG the same way you read a pulse strip: rate, regularity, and beat-to-beat strength.

Acquisition basics that matter

• Phone or sensor on the chest with firm contact. Two useful spots: mid-sternum or left lower ribs near the apex.

• 60-120 s recording, minimal movement, normal breathing. Note coughs, sighs, or posture changes.

How to read an SCG quickly

1. Rate and regularity - are intervals even or not.
2. Amplitude - does AO height stay stable or alternate.
3. AO-AC timing - is the systolic interval stable or drifting.
4. Pauses - any long gap, and what happens after it.

Visual patterns you can trust

Below, related patterns are grouped. For each group: what you see, how to tell look-alikes apart, and what it may suggest. This is pattern recognition, not a diagnosis.

Group 1 - Early beats and pauses

PAC vs PVC on sight

• Shared visual core - an early beat with smaller AO than neighbors.
• PAC hint - the following pause is shorter than a full compensatory pause. Rhythm feels “nudged,” then snaps back.
• PVC hints
  • Beat can be smaller OR paradoxically larger/deformed compared to neighbors.
  • Compensatory pause longer – often looks like a whole cycle missing.
  • The next AO is larger (postextrasystolic potentiation).
  • Patient often describes: “thump → pause → strong thump.”

• Clinical idea - PAC is supraventricular, PVC is ventricular. This split is useful triage, but ECG decides.

Postextrasystolic potentiation

• What you see - after a longer pause, the next AO is clearly taller.
• Meaning - more filling, stronger beat. Common after a PVC. Not a diagnosis on its own, but confirms the pause was real.

Sinus pause / SA exit block

• What you see - a single long flat gap with no SCG peaks, then the old rhythm resumes at the same rate.
• How to separate from AV block - in AV block you may see periodic dropped mechanical beats forming a repeating group pattern. A lone long gap that resets cleanly suggests sinus pause.
• Meaning - missed sinus impulse or exit block. Note duration and symptoms.

Group 2 - Alternating strong and weak beats

Bigeminy / trigeminy vs pulsus alternans

• Bigeminy - strict alternation strong–weak–strong–weak with regular timing. Often the weak beat is an ectopic with a shorter coupling interval. Can present as 2 normal + 1 weak (trigeminy).
• Pulsus alternans - alternation of AO height at a steady rate without obvious premature coupling. Looks like strong–weak–strong–weak with equal spacing.
• Quick split
  • If every weak beat is early and followed by a longer pause → think bigeminy.
  • If timing stays even and only amplitude alternates → think pulsus alternans.
• Meaning
  • Bigeminy - ectopy in a regular pattern.
  • Pulsus alternans - reduced contractile reserve, consider LV dysfunction context.

Group 3 - Regular fast runs

Sinus tachycardia vs short monomorphic SVT (PSVT)

• Shared - a block of densely packed, similar-amplitude beats.
• Sinus tachycardia - gradual speed-up and slow-down, amplitude tracks physiology. Usually context driven (stress, exertion, fever).
• PSVT - abrupt start and abrupt stop, rate is very regular and often faster than typical sinus. Appears as a compact burst then clean return to baseline.

• Meaning - sinus is expected response, PSVT is a paroxysm worth documenting and correlating with symptoms.

Group 4 - Irregular rhythms

Atrial fibrillation

• What you see - irregularly irregular intervals and variable AO heights beat to beat. No repeating cadence.
• How to separate from artifact - AF keeps a pulse cadence despite variability. Motion artifact creates noncardiac deflections and off-rhythm chunks.
• Meaning - AF pattern on SCG is a red flag for ECG confirmation and risk assessment.

Group 5 - AV conduction phenomena

Wenckebach (Mobitz I) vs Mobitz II vs AV dissociation

• Wenckebach (Mobitz I)

• What you see - a grouped pattern: cycles where AO-AC timing or beat spacing drifts, culminating in a dropped mechanical beat, then the group repeats.
• Meaning - progressive conduction delay with periodic non-conducted beat.
• Mobitz II
  
  • What you see - mostly stable timing with intermittent dropped beats without preceding drift. Example 2:1 or 3:1 mechanical pattern.
  • Meaning - higher-grade block. Red flag for escalation.
• AV dissociation
  • What you see - AO-AC relationship is unstable. The systolic interval wanders, AC can appear “out of place,” and large beats may show up at odd times as atria and ventricles march independently.
  • Meaning - complete AV block or ventricular rhythm. Urgent ECG correlation.

Group 6 - Sinus bradycardia with periodic pauses

• What you see - slow, regular sequence with occasional longer gaps, AO height otherwise stable.
• Meaning - often physiologic during sleep or in trained individuals. Context and symptoms decide if this is benign or needs ECG.

Group 7 - Respiratory modulation

• Physiologic respiratory sinus arrhythmia
  
  • What you see - cyclic small changes in intervals and AO height in phase with breathing.
• Possible pulsus paradoxus
  • What you see - consistent AO reduction on inspiration that is larger than usual and repeats with each breath.
• Caution - strong respiratory effects are a clue, not a diagnosis. Correlate with clinical picture and blood pressure if concerned.

Pitfalls that masquerade as pathology

• Motion and posture - turning the torso, talking, coughing, chuckling. These inject noncardiac wiggles. Real beats keep a coherent cadence.
• Contact and pressure - loose sensor or changing hand pressure flattens AO randomly across several seconds. True alternans keeps a beat-to-beat pattern.
• Breath holds and sighs - a big sigh can look like a pause plus amplified next beat. Check for concurrent respiratory deflection.
• Orientation - SCG amplitude can vary with angle. Rate and the presence or absence of a beat are more reliable than raw height.

Minimal workflow a non-cardiologist can follow

1. Scan 60–120 s with good contact and quiet breathing.
2. Mark the cadence - regular, grouped, irregularly irregular, or fast bursts.
3. Inspect AO height dynamics - stable, alternating, intermittently weak, post-pause boosted.
4. Map to this guide and write a plain hypothesis:
   • Early small beat + long pause + big next beat → likely PVC with compensatory pause and postextrasystolic potentiation.
   • Strict alternation at even timing → likely pulsus alternans.
   • Grouped pattern ending in a missed beat → likely Wenckebach.
   • Irregularly irregular intervals with variable heights → likely AF.
   • Abrupt fast burst → likely PSVT run.
   • Lone long gap with clean resumption → likely sinus pause.
   • Unstable AO-AC relationship and odd “big beats” → possible AV dissociation.

When to escalate

• Syncope, chest pain, dyspnea, or presyncope with any abnormal pattern.
• Suspected Mobitz II, AV dissociation, sustained fast tachycardia, or AF in a new patient.
• Frequent PVCs in runs or bigeminy with symptoms.

Final notes

• SCG is a mechanical lens on rhythm. It is excellent for spotting patterns you can also feel at the wrist.
• Use SCG to form a working hypothesis and to time ECG capture or referral.
• Document context: symptoms, activity, medications, and any triggers during the recording.