ECG criteria
- S wave in lead I — prominent/deep; there is no standardized quantitative depth threshold of S for S1Q3T3
- Q wave in lead III — typically small and may be narrow; an isolated QIII without pathological Q waves in adjacent inferior leads does not equal infarction. There is no unified threshold for QIII specifically in S1Q3T3
- Negative T wave in lead III
- Pathological Q wave in myocardial infarction (Universal Definition of MI): Q duration ≥0.03 s and depth ≥1 mm, or QS, in any two contiguous leads (I, II, aVL, aVF, V4–V6); separate criteria apply for V2–V3. In S1Q3T3, such a combination is absent in adjacent inferior leads — this helps distinguish the pattern from a true inferior infarction, but the presence or absence of a pathological Q wave by itself neither diagnoses nor excludes acute MI without clinical presentation, ST-T dynamics, and troponin
- Sinus tachycardia is frequently concomitant
- Signs of acute RV overload (strain) may be present: incomplete or complete right bundle branch block, ST elevation in V1–V3 or aVR, T wave inversion in V1–V4 (sometimes wider) — these signs support the presence of RV overload, but do not diagnose PE
- The electrical axis can be normal, deviated to the right, or occupy an intermediate position
What it looks like on ECG
Classic triad: in lead I — a relatively deep S wave, in lead III — a Q wave and a negative T wave. Outwardly, the complex in lead III resembles an inferior myocardial infarction, but the Q wave in lead III in S1Q3T3 is typically narrow and shallow.
A pathological Q wave in infarction according to standardized criteria is a Q wave with a duration ≥0.03 s and depth ≥1 mm or QS, detected in any two contiguous leads (I, II, aVL, aVF, V4–V6); separate thresholds are applied for V2–V3. For the inferior localization, this means evaluating II, III, and aVF as an adjacent group. An isolated Q wave in lead III by itself does not prove infarction and does not exclude it: the differentiation is based not on a single wave, but on the combination of Q-wave width and depth in multiple leads, ST-T dynamics, clinical presentation, and troponin. It is also important that pathological Q waves are not mandatory for diagnosing acute myocardial infarction and are not an independent sign of the acuity of the process — the diagnosis of acute MI requires clinical presentation, ECG dynamics, and troponin elevation.
Concomitant findings that may occur due to the same cause (e.g., in acute RV overload), but do not increase the specificity of S1Q3T3 itself and require separate clinical evaluation:
| Finding | Leads | What it reflects |
|---|---|---|
| Incomplete/complete RBBB | V1–V2, as well as I, aVL, V5–V6 | conduction slowing through the overloaded RV |
| ST elevation | sometimes V1–V3 or aVR | possible acute RV overload/strain, the finding is nonspecific |
| T wave inversion | V1–V4 (sometimes wider, up to V5–V6), III | possible RV strain, but nonspecific: may also reflect ischemia and other causes |
| Sinus tachycardia | all leads, by heart rate | compensatory reaction, nonspecific |
| S1S2S3 pattern | frontal plane | nonspecific variant of a vertical axis, does not prove RV overload |
The pattern is nonspecific and inconstant: it may be absent even in massive PE and, conversely, is found in healthy individuals as a variant of the norm (especially with a vertical heart or axis shift).
Mechanism
In acute right ventricular overload (most commonly due to massive or submassive PE), pulmonary artery pressure rises sharply. The RV can acutely dilate and dysfunction, although the degree of dilation is variable; this can alter the position of the heart, the frontal depolarization vector, and the sequence of ventricular activation. The appearance of a deep S in I and a Q wave in III is associated with this, while the negative T in III reflects impaired repolarization of the overloaded RV myocardium. The S1Q3T3 combination itself does not prove RV dilation or electrical axis deviation — the exact electrophysiological mechanism of the pattern is not definitively established: it is likely caused by a combination of changes in heart position, right ventricular overload, and repolarization abnormalities, rather than a single strictly directed vector loop.
The S1S2S3 pattern is described separately, when the S wave is visible in all three standard leads. This is a descriptive finding that often represents a variant of a normal vertical cardiac axis and is not a universally accepted "syndrome"; it should not be automatically linked to more pronounced axis rotation or RV overload.
Causes
- Pulmonary embolism (acute, classic association)
- Other causes of acute right ventricular overload: severe pulmonary hypertension, cardiac tamponade, tension pneumothorax, acute respiratory distress syndrome
- Severe exacerbation of COPD or bronchial asthma — a possible but nonspecific association via hypoxia and hyperinflation, rather than a typical direct sign of acute RV overload
- Variant of normal — vertical heart position, deep breathing, lean physique, young age
- Chronic cor pulmonale (as a background finding)
The list is not exhaustive; interpretation is always complemented by the clinical context.
Differential diagnosis
- Inferior myocardial infarction: according to standardized criteria, a pathological Q wave is a Q wave with a duration ≥0.03 s and depth ≥1 mm or QS in two contiguous leads (for the inferior wall — II, III, aVF), with corresponding ST-T dynamics; an isolated narrow Q wave in lead III alone without changes in neighboring leads does not indicate infarction. At the same time, the mere fact of a pathological Q wave is not mandatory for diagnosing acute MI and does not prove the acuity of the process — the entire complex must be compared: Q-wave width and depth, ST-T dynamics, clinical presentation, troponin, and imaging data.
- Variant of normal in healthy individuals: isolated S1Q3T3 without tachycardia, without RBBB, without T wave inversion in right precordial leads, and without clinical symptoms — more often normal rather than PE.
- WPW syndrome: can mimic infarction due to pseudopathologic Q waves and alterations of the initial part of the QRS complex in various leads; differentiated by shortening of the PQ interval, presence of a delta wave, and widening of QRS, rather than by an isolated sign in a single lead.
- Right ventricular hypertrophy / chronic cor pulmonale: distinguished by the chronic rather than acute nature of the changes and the corresponding clinical context.
Clinical significance and management
S1Q3T3 syndrome is an auxiliary, nonspecific, and insensitive sign: it occurs only in a minority of patients with PE, has low sensitivity and low specificity, and can be observed in other conditions and occasionally in healthy individuals. By itself, it does not allow confirming or excluding PE and does not prove RV overload or dilation. Clinical significance increases when combined with sinus tachycardia, RBBB, T-wave inversion in right precordial leads, and a corresponding clinical presentation (sudden dyspnea, chest pain, risk factors for venous thromboembolism) — then the signs of right ventricular strain support the presence of RV overload, but do not diagnose PE and require clinical correlation.
The physician should evaluate the finding in conjunction with the clinical probability of PE (Wells, Geneva scores). In low or intermediate probability, D-dimer is used (with age-adjustment in patients over 50 years); in high probability, proceeding directly to imaging, usually CT pulmonary angiography if available and without contraindications, without waiting for the D-dimer result. In hemodynamic instability, bedside emergency echocardiography and management according to the massive PE algorithm take precedence. ECG does not replace imaging, but helps in triage and raises suspicion for acute RV overload, especially when time is limited.
Common interpretation errors
- Interpreting a Q wave in lead III in isolation as a sign of inferior infarction without assessing the duration and depth of Q in adjacent inferior leads and ST-T dynamics
- Considering S1Q3T3 a diagnostic sign of PE by itself or proof of electrical axis deviation
- Ignoring the pattern in a patient with clinical presentation of PE if it is weakly expressed
- Failing to account for the fact that the pattern is often absent even in massive PE
- Confusing it with the S1S2S3 pattern in young and lean patients without any clinical symptoms, automatically considering it a more severe stage of RV overload
S1Q3T3 — "S in the first, Q and T in the third": look for a deep S in I and Q with a negative T in III
1. Which ECG sign distinguishes a true inferior myocardial infarction from S1Q3T3 syndrome?
In inferior myocardial infarction, a pathological Q wave in terms of duration and depth is detected in at least two adjacent inferior leads, rather than isolated in lead III as seen in S1Q3T3.
2. Which condition is classically associated with the S1Q3T3 syndrome?
Classically, the pattern is associated with acute RV overload in pulmonary embolism, but it is neither specific nor an indicator of PE severity, nor definitive proof of this overload.
3. Which characteristic of the S1Q3T3 syndrome is correct?
The pattern is unspecific and insensitive: it is frequently found in healthy individuals and may be absent in confirmed pulmonary embolism.
Frequently asked questions
Is S1Q3T3 syndrome dangerous?
The sign itself is not dangerous and does not establish a diagnosis. It merely increases suspicion of acute right ventricular overload, most commonly in pulmonary embolism, and requires clinical evaluation and further workup.
How to distinguish S1Q3T3 from inferior myocardial infarction?
In inferior infarction, a pathological Q wave has a duration of ≥0.03 s and depth of ≥1 mm or a QS pattern in two adjacent leads (II, III, aVF), accompanied by ST-T dynamics. Isolated QIII without such changes in neighboring leads is usually non-specific, but final differentiation requires clinical context, serial ECGs, and troponins rather than assessing a single wave; Q waves alone do not diagnose acute MI.
Does S1Q3T3 always indicate pulmonary embolism?
No. The pattern is unspecific: it occurs in healthy individuals as a normal variant and may be completely absent in confirmed massive pulmonary embolism.
What other ECG changes accompany S1Q3T3 in pulmonary embolism?
Sinus tachycardia, incomplete or complete right bundle branch block, sometimes ST elevation in V1–V3 or aVR, and T-wave inversion in the right precordial leads V1–V4. These are signs of possible RV strain that require clinical correlation rather than diagnosing pulmonary embolism on their own.
Is CT pulmonary angiography immediately required in case of S1Q3T3?
Decisions are made based on the overall clinical probability of pulmonary embolism and other data, rather than a single ECG finding. When clinical probability is high, imaging is indicated regardless of the presence of S1Q3T3.
ECG Course
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Related terms
More in the section "Pericardium, Lungs, Brain and Others"
Sources
- Goldberger A. Clinical Electrocardiography: A Simplified Approach
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- ESC Guidelines on acute pulmonary embolism (diagnosis and ECG findings)
- Universal Definition of Myocardial Infarction (pathological Q-wave criteria)
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.