What is the significance of an S1Q3T3 pattern on an electrocardiogram (ECG) in a patient with suspected pulmonary embolism?

S1Q3T3 Pattern in Pulmonary Embolism

Direct Answer

The S1Q3T3 pattern is a classic but insensitive ECG finding in pulmonary embolism that indicates right ventricular strain and correlates with more severe disease, but it has limited standalone diagnostic value and must be integrated with clinical prediction scores and definitive imaging rather than used in isolation. 1, 2

Diagnostic Accuracy and Limitations

The S1Q3T3 pattern has poor diagnostic performance when used alone:

• Sensitivity and specificity are limited, with recent prospective data showing a positive likelihood ratio of only 2.07 (95% CI 1.27-3.39), meaning it modestly increases post-test probability but lacks sufficient accuracy to rule in or rule out PE. 3
• The pattern is present in many PE cases but is neither sensitive nor specific enough for standalone diagnosis. 4
• The ECG cannot diagnose or exclude pulmonary embolism—its primary role is to exclude alternative diagnoses (acute MI, pericarditis) and assess disease severity through detection of right ventricular strain patterns. 2, 4

Clinical Significance and Prognostic Value

When S1Q3T3 is present, it carries important prognostic implications:

• The pattern indicates acute right ventricular strain and correlates with more severe PE, particularly when part of the broader acute cor pulmonale ECG pattern. 1
• In the Pisa prediction model, acute cor pulmonale findings (including S1Q3T3, S1S2S3, negative T waves in right precordial leads, transient RBBB, or pseudoinfarction) have a coefficient of 1.96 and odds ratio of 7.11 for PE diagnosis. 1
• The presence of ≥1 classic RV strain sign is associated with higher adverse event rates and worse short-term prognosis. 1, 2
• S1Q3T3 combined with other RV strain patterns correlates with severity of pulmonary hypertension and RV dysfunction. 1

Critical Clinical Context

When Hemodynamic Instability is Present

• Ischemic ECG patterns combined with S1Q3 and/or abnormal QRS morphology in V1 are much more common (90%) during hemodynamic instability than at baseline (5%), reflecting myocardial ischemia combined with right ventricular strain. 5
• In patients with shock or hypotension showing S1Q3T3, urgent echocardiography should be obtained to assess RV function and guide reperfusion therapy decisions. 2, 4
• The absence of echocardiographic signs of RV overload or dysfunction virtually excludes PE as the cause of hemodynamic instability, even if S1Q3T3 is present. 2

More Specific ECG Findings

While S1Q3T3 has limited accuracy, other ECG findings are more clinically useful:

• T-wave inversions in right precordial leads (V1-V4) have the highest specificity (97.4%) for RV strain and suggest more severe PE. 2, 4
• In multivariable analysis, only sinus tachycardia (OR 1.93) and inversion/flattening of inferior T-waves (OR 1.82) remained independently significant for PE diagnosis. 3
• Liberal RV strain criteria (inverted/flattened T-waves in ≥2 inferior + ≥2 anterior leads) yielded a positive likelihood ratio of 4.75 (95% CI 2.3-9.8), superior to S1Q3T3 alone. 3

Algorithmic Approach to ECG Interpretation in Suspected PE

Step 1: Exclude Alternative Diagnoses

• Use the ECG immediately to rule out acute MI, pericarditis, and other cardiac emergencies that may present similarly. 2, 4

Step 2: Assess for RV Strain Patterns

• Look specifically for T-wave inversions in V1-V4 (highest specificity at 97.4%). 2, 4
• Note presence of S1Q3T3, but recognize its limited diagnostic accuracy. 3
• Check for right bundle branch block (present in 4.8-9% of PE cases, associated with RV strain). 2, 4

Step 3: Integrate with Clinical Prediction Scores

• ECG findings must be used in conjunction with Wells or revised Geneva scores to determine pre-test probability, never in isolation. 1, 2, 4
• The Pisa model formally incorporates ECG findings of acute cor pulmonale with clinical variables for probability estimation. 1

Step 4: Risk Stratification Based on Hemodynamic Status

• If hemodynamically stable with S1Q3T3: proceed with standard diagnostic algorithm based on clinical probability. 1, 2
• If hemodynamically unstable with S1Q3T3 or other RV strain: obtain urgent echocardiography and consider immediate anticoagulation or reperfusion therapy. 2, 5

Step 5: Continuous Monitoring

• Implement continuous ECG monitoring during transport/transfer for suspected PE, as the S1Q3T3 pattern can evolve and arrhythmias may develop. 2, 4

Common Pitfalls to Avoid

• Never use S1Q3T3 alone to diagnose or exclude PE—it has insufficient accuracy and must be part of a comprehensive assessment. 2, 3
• Do not mistake ST-segment elevation in anterolateral leads for acute MI; this can represent reciprocal changes from RV/septal strain in massive PE. 6
• Recognize that up to 20% of PE patients have completely normal ECGs, so absence of S1Q3T3 does not exclude PE. 1
• The S1Q3T3 pattern can resolve spontaneously with treatment, so serial ECGs may show dynamic changes. 7
• Many ECG abnormalities in PE are non-specific and can occur in numerous other conditions causing RV strain. 1