In an adult with systolic heart failure (left‑ventricular ejection fraction ≤35 %) and a QRS duration ≥120 ms in sinus rhythm who is being considered for cardiac resynchronisation therapy, how should mechanical dyssynchrony be assessed using echocardiography?

Echocardiographic Assessment of Mechanical Dyssynchrony for CRT Candidacy

Echocardiographic assessment of mechanical dyssynchrony should NOT be routinely performed for CRT candidate selection in patients with QRS ≥150 ms, but may be considered only in the narrow subset with QRS 120-149 ms and LBBB morphology to support the decision for CRT implantation.

Primary Selection Criteria: Electrical Dyssynchrony Takes Precedence

Current guidelines prioritize electrical dyssynchrony (QRS duration and morphology) over mechanical dyssynchrony for CRT selection. 1

Class I Indications (No Echocardiographic Dyssynchrony Required)

• Patients with LBBB and QRS ≥150 ms should receive CRT based on electrical criteria alone, without requiring echocardiographic dyssynchrony assessment. 1
• The strongest evidence for CRT benefit exists in patients with QRS ≥150 ms, where mechanical dyssynchrony assessment adds no incremental value to decision-making. 1

Class IIa Indications (Echocardiography May Add Value)

• For patients with QRS 120-149 ms and LBBB morphology, echocardiographic evidence of mechanical dyssynchrony may be considered to strengthen the indication for CRT. 1
• NICE guidelines specifically recommend considering mechanical dyssynchrony on echocardiography for patients with QRS 120-149 ms to support CRT candidacy. 1

Evidence Against Routine Dyssynchrony Assessment

Failed Trials Demonstrate Limited Clinical Utility

Multiple randomized controlled trials have definitively shown that CRT in patients with narrow QRS (<120 ms) and echocardiographic dyssynchrony is not beneficial and may cause harm. 1

• The EchoCRT trial demonstrated that patients with QRS <130 ms and mechanical dyssynchrony on echocardiography showed no benefit from CRT, establishing that mechanical dyssynchrony alone is insufficient justification for CRT. 1
• The LESSER-EARTH study was prematurely terminated due to futility and safety concerns when CRT was applied to patients with narrow QRS regardless of dyssynchrony status. 1
• The RethinQ trial showed no significant improvement in peak oxygen consumption or reverse remodeling in patients with QRS <130 ms despite mechanical dyssynchrony. 1

Guideline Evolution Away from Mechanical Dyssynchrony

The 2012 ESC guidelines explicitly state that the possibility of mechanical dyssynchrony in patients with QRS <120 ms benefiting from CRT "remains to be proven" and is an area of research interest only. 1

Current ACC/AHA/HRS guidelines give a Class III (No Benefit) recommendation against CRT in patients with NYHA class I-II symptoms and non-LBBB pattern with QRS <150 ms, regardless of mechanical dyssynchrony. 1

When Mechanical Dyssynchrony Assessment May Be Considered

Specific Clinical Scenario: Intermediate QRS Duration

The only guideline-supported role for echocardiographic dyssynchrony assessment is in patients with:

• LVEF ≤35% 1
• QRS duration 120-149 ms 1
• Sinus rhythm 1
• NYHA class II-IV symptoms on optimal medical therapy 1

In this narrow population, demonstrating mechanical dyssynchrony may help tip the decision toward CRT implantation, particularly when other factors are equivocal. 1

Echocardiographic Techniques (If Assessment Is Performed)

Available Methods

If mechanical dyssynchrony assessment is undertaken, multiple echocardiographic modalities exist, though none has emerged as a clear standard: 2, 3

• Tissue Doppler imaging (TDI) measuring septal-to-lateral wall delay 2
• Speckle-tracking strain imaging to assess regional contraction patterns 2, 4
• M-mode measurement of septal-to-posterior wall motion delay 2
• Standard deviation of time-to-peak contraction across 12 LV segments 2
• Three-dimensional echocardiography for comprehensive dyssynchrony assessment 2, 5

Technical Limitations

Echocardiographic dyssynchrony parameters suffer from significant measurement variability, lack of standardization, and poor reproducibility across centers. 3, 6, 5

The PROSPECT trial demonstrated that no single echocardiographic dyssynchrony parameter could accurately predict CRT response, with high interobserver variability and low sensitivity/specificity. 3, 6

Critical Pitfalls to Avoid

Do Not Delay CRT for Dyssynchrony Assessment

In patients who meet electrical criteria for CRT (LVEF ≤35%, QRS ≥150 ms, LBBB, NYHA II-IV), do not delay device implantation to perform echocardiographic dyssynchrony studies—electrical criteria are sufficient. 1

Do Not Use Dyssynchrony to Justify CRT in Narrow QRS

Never implant CRT in patients with QRS <120 ms based solely on echocardiographic mechanical dyssynchrony, as this approach has been proven harmful in randomized trials. 1

The aggregate clinical experience consistently demonstrates that significant CRT benefit requires QRS ≥150 ms, and mechanical dyssynchrony prevalence increases with QRS duration (40% at QRS ≥120 ms vs. 70% at QRS ≥150 ms). 1

Recognize the Disconnect Between Mechanical and Electrical Dyssynchrony

Mechanical dyssynchrony detected by echocardiography does not reliably correlate with CRT response unless accompanied by significant electrical dyssynchrony (prolonged QRS). 1, 3, 6

Practical Algorithm for Clinical Decision-Making

Step 1: Assess Electrical Criteria First

• Measure QRS duration and morphology on 12-lead ECG 1
• Confirm LVEF ≤35% and NYHA class II-IV symptoms on guideline-directed medical therapy 1

Step 2: Apply Guideline-Based Recommendations

• If QRS ≥150 ms with LBBB: Proceed to CRT without echocardiographic dyssynchrony assessment (Class I indication) 1
• If QRS 120-149 ms with LBBB: Consider echocardiographic dyssynchrony to support decision (Class IIa indication) 1
• If QRS <120 ms: Do not perform CRT regardless of mechanical dyssynchrony (Class III: No Benefit) 1

Step 3: If Dyssynchrony Assessment Is Performed (QRS 120-149 ms Only)

• Use standardized techniques with experienced operators to minimize measurement error 3, 5
• Recognize that positive findings support but do not mandate CRT, while negative findings do not exclude benefit 3, 6
• Integrate dyssynchrony findings with other clinical factors (QRS morphology, etiology, comorbidities) rather than using as sole criterion 1, 6