ECG criteria
- The negative QRS component following the R wave is the S wave; the negative component preceding R is the Q wave; if no R wave follows the initial negative deflection, the entire complex is designated as QS
- Measure the depth from the isoelectric line to the lowest point of the wave, in mm (1 mm = 0.1 mV at standard calibration)
- In V1–V2, the S wave is typically deep (the net depolarization vector is directed leftward and posteriorly toward the massive LV, i.e., away from these leads)
- In V5–V6, the S wave is typically small or absent; a deep S in V5–V6 is abnormal
- The R/S ratio in the chest leads is used to assess the transition zone (typically between V3 and V4)
- Disappearance of S in V5–V6 is usually a normal pattern of R-wave progression; a persistent R/S<1 or R≤S in later chest leads is evaluated as a late transition zone, and insufficient R-wave growth from V1 to V4 as poor R-wave progression
- When assessing LV hypertrophy, the amplitude of S in V1 is added to the amplitude of R in V5 or V6 (Sokolow-Lyon index, threshold ≥35 mm)
What it looks like on ECG
The S wave is the third component of the classic QRS complex (after Q and R), appearing as a negative deflection below the isoelectric line. Its shape and depth vary significantly across leads.
In the right chest leads (V1–V2), the S wave is typically deep: after initial septal activation, the net depolarization vector is directed predominantly leftward and posteriorly due to the greater mass of the LV. Therefore, in V1–V2, it moves away from the electrode, forming the S wave. As one moves toward the left chest leads (V5–V6), the R-wave amplitude increases while the S wave decreases or disappears—this forms the normal pattern of R-wave growth and S-wave reduction from right to left.
In limb leads, the shape of the S wave depends on the position of the electrical axis of the heart (EAH). With a vertical EAH, a deep S is typical for lead I. When the EAH deviates to the left, a predominantly negative QRS is typical for lead III, and with more pronounced deviation, also for II and aVF; aVF is not considered an obligatory sign of a horizontal heart position, as these are different concepts.
Approximate pattern by zones:
| Lead | Typical S depth |
|---|---|
| V1–V2 | deep |
| V3–V4 | transition zone, R≈S |
| V5–V6 | small or absent |
If a second positive deflection occurs after the S wave, it is designated as R' (e.g., an rSR' complex in right bundle branch block).
Mechanism
The S wave is formed by the terminal portion of ventricular depolarization—the excitation wave moves away from the recording lead electrode, creating a negative deflection. Normally, interventricular septal depolarization proceeds from left to right (yielding a small initial r in V1 and q in V5–V6), and then the bulk of excitation spreads through both ventricles with predominance of the left ventricle due to its greater mass. The resultant vector is directed leftward and posteriorly (toward the left ventricle), so the right chest leads record this receding vector as a deep S, and the left leads as a tall R with a small or absent S.
Normal Variants
- Deep S in V1–V2 in young and asthenic individuals
- Persistence of S in V5–V6 (late S disappearance / poor leftward R-wave progression)—observed in some healthy individuals, especially the young, and evaluated in the context of the rest of the ECG
- S wave in lead III as part of normal EAH variation
- Absence of S in left chest leads in some healthy individuals with a horizontal heart position
Pathological changes
- A deep S wave persisting across all chest leads up to V5–V6 (poor R progression / late transition zone)—a finding evaluated in conjunction with other data (can occur in right bundle branch block, right ventricular hypertrophy, chronic obstructive pulmonary disease, prior anterior myocardial infarction)
- Absence of the normal decrease in S from V1 to V6 (persistence of a deep dominant S on the left with insufficient R growth)—evaluated along with R-wave amplitude
- Predominantly negative QRS in I, often also in aVL, with rightward EAH deviation
- Deep S in III, aVF with leftward EAH deviation
- Inclusion of S in formulas for voltage criteria of ventricular hypertrophy (Sokolow-Lyon index: S in V1 + R in V5 or V6)
Differential diagnosis
The S wave is often confused with the Q wave—the difference lies in the order: Q is always the first component of QRS and precedes R, whereas S is a negative deflection following R. If no R wave follows the initial negative deflection, the entire complex is designated as QS, not Q. Right bundle branch block is recognized not by a deep S in V1, but by a terminal positive R' wave in leads V1–V2 (rsR'/rSR' pattern) combined with a wide terminal S in I, aVL, V5–V6; complete block is QRS ≥120 ms, incomplete is typically 110–119 ms in adults. The SI SII SIII pattern consists of prominent or prevailing S waves in leads I, II, and III; this is a non-specific pattern evaluated alongside the EAH, QRS morphology, and clinical presentation, and it should not be confused with a simply deep S in a single lead.
Clinical significance and management
The S wave alone is not a diagnosis—it is evaluated only as part of the QRS complex and in correlation with the clinical picture. The physician must:
- Measure S depth in chest leads and determine the transition zone
- Correlate S with R-wave amplitude for voltage criteria of ventricular hypertrophy
- Assess the cardiac axis primarily by the polarity of QRS in I and aVF; use II and III when necessary, along with the isoelectric lead and its perpendicular
- Assess QRS width and morphology in the presence of a deep terminal S in lateral leads—rule out right bundle branch block
- When pathology is suspected (PE, COPD, hypertrophy), interpret the S finding only in combination with other criteria and the clinical picture, not in isolation
Common interpretation errors
- Confusing the S wave with the Q wave due to inattention to the order of QRS components, and referring to a totally negative complex without an R wave as Q instead of QS
- Evaluating S depth without reference to a specific lead or comparison with adjacent complexes
- Drawing diagnostic conclusions (hypertrophy, block) based on a single deep S without considering other criteria
- Diagnosing right bundle branch block based on a deep S in V1 while ignoring the terminal R' in V1–V2 and QRS width
- Failing to account for age-related and constitutional normal variants (persistence of S in V5–V6 in youth) and mistaking them for pathological R-wave progression abnormalities
1. How to correctly identify the S wave in the QRS complex?
By definition, the S wave follows the R wave. The deflection preceding R is the Q wave, and if there is no R wave at all, the entire negative complex is called QS.
2. What S-wave pattern is normally typical for chest leads from V1 to V6?
Normally, the terminal depolarization vector is directed toward the left ventricle, so S is maximal in the right chest leads and decreases toward the left leads.
3. Which voltage criterion utilizes the amplitude of the S wave?
The Sokolov–Lyon index sums the S-wave amplitude in V1 with the R-wave amplitude in V5 or V6 to assess left ventricular hypertrophy (threshold ≥35 mm).
Frequently asked questions
What is the S wave on an ECG in simple terms?
This is a negative deflection of the QRS complex that occurs immediately after the R wave. It reflects the final stage of electrical impulse propagation through the ventricles.
Is a deep S wave dangerous?
A deep S wave on its own is not dangerous and does not constitute a diagnosis. It is evaluated alongside other QRS elements, voltage criteria, and the clinical picture — only then may it indicate hypertrophy, bundle branch block, or other conditions.
How to distinguish an S wave from a Q wave?
The Q wave is the initial deflection of the QRS complex, occurring before the R wave, and is also negative. The S wave is the negative deflection that follows the R wave. If there is no R wave at all following the initial negative deflection, the entire complex is designated as QS rather than Q.
Why is the S wave deep in V1 and disappears in V6?
This is a normal pattern: the terminal vector of ventricular depolarization is directed toward the left ventricle, moving away from the right precordial leads (deep S in V1) and toward the left precordial leads (small or absent S in V6).
What does a persistent S wave in V5–V6 mean?
This is a descriptive finding — the persistence of a prominent S wave in the left precordial leads, where it is normally small or absent. By itself, it is not equivalent to a delayed transition zone (determined by persistent R/S<1 or R≤S in more lateral precordial leads) nor is it equivalent to poor R-wave progression (evaluated by the R-wave trajectory from V1 to V4); both of these signs are nonspecific, occurring even in healthy individuals, especially young adults, and the finding should be interpreted in the context of the entire ECG.
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Related terms
More in the section «Basics: waves, intervals, axes»
Sources
- Goldberger A. Clinical Electrocardiography, latest edition
- Hampton J. The ECG Made Easy
- Orlov V.N. Guide to Electrocardiography
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 2009
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.