S1, S2, S3 Pattern on EKG
The S1S2S3 pattern refers to the presence of S waves in leads I, II, and III on a standard 12-lead electrocardiogram, and while historically associated with pulmonary embolism and right ventricular strain, it is actually a common finding in the general population (present in 37% of adults) and has limited diagnostic specificity.
What the Pattern Represents
The S1S2S3 pattern occurs when S waves are present in all three limb leads (I, II, and III). The clinical significance depends on the specific criteria used:
Two Distinct Patterns
- S1S2S3-I pattern: S waves present in leads I, II, and III, where the S-wave amplitude exceeds the R-wave amplitude in one of these leads 1
- S1S2S3-II pattern: S waves present in leads I, II, and III, where the S-wave amplitude exceeds the R-wave amplitude in two of these leads 1
Electrophysiologic Mechanism
The pattern results from an anomalous wavefront oriented rightward and superiorly, with delayed activation of the right ventricular inflow tract (56 ± 21 ms vs. 36 ± 9 ms in controls) and outflow tract (79 ± 13 ms vs. 39 ± 8 ms in controls), producing a QRS duration that is typically longer than normal (94 ± 14 ms vs. 84 ± 14 ms) 2.
Clinical Associations
Common Pattern (S1S2S3-I)
- Found in 36.9% of the general adult population 1
- Associated with hypertension (OR 1.25), particularly in men (OR 1.37) 1
- Not specific for pulmonary embolism or acute coronary syndrome 3
Less Common Pattern (S1S2S3-II)
- Found in only 3.1% of the general population 1
- Associated with heart failure (OR 1.85), particularly in men (OR 2.30) 1
- When combined with right-dominant forces (tall R in V1, deep S in V5-V6, right atrial enlargement), it becomes highly specific for right ventricular dysfunction with pulmonary hypertension 4
Critical Diagnostic Pitfall
The S1S2S3 pattern alone cannot differentiate between acute pulmonary embolism and non-ST elevation acute coronary syndrome, as both conditions show similar frequencies of this pattern (28% in PE vs. 16% in NSTE-ACS, not statistically significant) 3.
When Evaluating for Pulmonary Embolism
The S1S2S3 pattern is included in the Pisa model for PE probability assessment 5, but look for the complete constellation:
- S1Q3T3 pattern (more specific than S1S2S3 alone)
- Acute cor pulmonale signs on ECG (OR 7.11 for PE)
- Negative T waves in both V1-V3 AND inferior leads (II, III, aVF) has 85% positive predictive value and 87% specificity for PE over NSTE-ACS 3
When Evaluating for Acute Coronary Syndrome
Features that favor NSTE-ACS over PE include:
- Counterclockwise axis rotation (OR 4.57)
- ST depression in V1-V3 (OR 2.25)
- Negative T waves in V5-V6 (OR 2.08)
- Ventricular premature beats (OR 2.60) 3
Bottom Line for Clinical Practice
Do not rely on the S1S2S3 pattern in isolation. It is a nonspecific finding that requires integration with the complete clinical picture, additional ECG features, and objective testing (D-dimer, imaging) to establish diagnoses of pulmonary embolism, right ventricular dysfunction, or other cardiopulmonary conditions.