The ECG trace displayed during echocardiography is more than a decorative timeline. It helps identify rhythm, orient the operator to the cardiac cycle, select comparable beats, and relate electrical events to valve motion and chamber mechanics.
However, electrical and mechanical timing are not identical. The ECG should guide frame selection, while valve opening and closure and chamber size confirm the actual mechanical event.
Electrical sequence
| ECG component | Electrical event | Mechanical relationship |
|---|---|---|
| P wave | Atrial depolarization | Atrial contraction follows; contributes to late diastolic filling |
| PR interval | AV conduction from atria to ventricles | Allows ventricular filling before systole |
| QRS complex | Ventricular depolarization | Mechanical systole begins shortly afterward |
| ST segment | Ventricles remain depolarized | Corresponds broadly to ventricular ejection phase |
| T wave | Ventricular repolarization | Mechanical relaxation and valve transitions follow |
End-diastole
The onset of the QRS complex is often used as a practical timing marker for end-diastole, especially for linear measurements and M-mode. The preferred mechanical definition is the first frame after mitral-valve closure or the frame in which the relevant LV dimension or volume is largest.
In conduction delay, pacing, or ectopy, the QRS onset may not correspond neatly to the largest cavity frame. Review the cine loop and valve motion.
End-systole
End-systole is commonly identified as the frame immediately before mitral-valve opening, after aortic-valve closure, or when the LV cavity is smallest. The end of the T wave is only an approximation and may not coincide exactly with mechanical end-systole.
Atrial measurements
LA and RA size are measured at maximal atrial volume, usually near ventricular end-systole before AV-valve opening. The ECG helps locate the phase, but the actual maximal chamber frame should be selected.
Doppler timing
Mitral inflow
The E wave occurs in early diastole after mitral-valve opening. The A wave follows atrial contraction after the P wave. In atrial fibrillation, organized A waves are absent.
Pulmonary venous flow
Systolic and diastolic forward waves relate to atrial pressure and ventricular filling. Atrial reversal follows atrial contraction and is absent or altered when atrial activity is not organized.
LVOT and aortic flow
Systolic flow begins after ventricular depolarization and is bounded by aortic-valve opening and closure. Ejection time should be measured from the Doppler envelope rather than estimated from the ECG alone.
Tissue Doppler
S′ occurs during systole; e′ and a′ occur during early and late diastole. The ECG helps identify the waves when velocities are fused or rhythm is irregular.
Beat selection in sinus rhythm
Select a representative beat with a stable baseline, complete cardiac cycle, and good image quality. Avoid post-ectopic beats and cycles affected by respiratory or translational motion when possible.
Atrial fibrillation
Beat-to-beat filling and stroke volume vary with the preceding and pre-preceding R–R intervals. Measurements should use representative cycles, and several beats may need to be averaged depending on the parameter and guideline.
- Avoid very short or very long cycles when they are not representative.
- Prefer cycles in which the preceding and pre-preceding intervals are similar.
- Record enough beats to permit review.
- State rhythm and averaging method when relevant.
Ectopy
Premature beats alter filling time, contractility, and loading. The post-ectopic beat may show increased stroke volume and should not be used as a routine representative beat unless the clinical question specifically concerns post-ectopic physiology.
Pacing and conduction delay
Pacing spikes, broad QRS complexes, left bundle-branch block, and ventricular pre-excitation can create electrical–mechanical dyssynchrony. Wall motion, valve timing, and chamber dimensions should be assessed from the images rather than assuming normal mechanical timing from the QRS.
Respiration and timing
Respiratory variation can be as important as ECG timing for right-heart Doppler, IVC assessment, and constrictive or tamponade physiology. Save enough cycles to show both electrical and respiratory context.
Use the ECG to find the neighbourhood; use valve events and chamber size to choose the exact frame.
In atrial fibrillation, select representative beats with similar preceding and pre-preceding R–R intervals when possible and average multiple measurements according to the parameter and laboratory protocol.
Common timing errors
- Measuring LV wall thickness after systolic thickening has begun.
- Tracing LA volume after mitral opening, when the atrium has started to empty.
- Selecting the smallest-looking LV frame from a foreshortened or translational beat.
- Using a post-PVC beat for EF or Doppler.
- Confusing fused E/A or e′/a′ velocities at high heart rate.
- Using one irregular beat without documenting rhythm or averaging.
Knowledge check
1. What is the preferred definition of LV end-diastole?
Answer: The first frame after mitral-valve closure or the frame in which the LV is largest, with QRS onset used as a practical guide.
2. Why should a post-PVC beat generally be avoided?
Answer: Compensatory filling and post-extrasystolic potentiation can produce non-representative dimensions and stroke volume.
3. What happens to the mitral A wave in atrial fibrillation?
Answer: Organized atrial contraction is absent, so a consistent A wave is not present.