The S wave is the small downward dip that closes the tall spike of each heartbeat on an ECG.1PubMed Central. Ventricular Depolarization Abnormalities and Their Role in Cardiac Risk Stratification — A Narrative Review It marks the last moment of the electrical wave that makes the lower chambers squeeze. On its own, it is usually just a normal part of every heartbeat. Doctors read its depth, width and shape as clues, and those clues only mean something alongside the rest of the tracing and the person’s health.
Where the S Wave Sits on Your ECG
Each heartbeat on an ECG has three main parts: a small bump called P, a sharp group of strokes called the QRS complex, and a rounder bump called T. Within the QRS complex, Q is a small dip before the main stroke, R is the tall upward stroke, and S is the dip after it. The S wave forms as the signal travels toward the base, the top part of the lower chambers.2PubMed Central. Ventricular Depolarization Abnormalities and Their Role in Cardiac Risk Stratification — A Narrative Review The last areas to switch on, including the upper wall of the left ventricle, send their signal away from some recording points, and that draws the downward line.3CV Physiology. Ventricular Depolarization and the Mean Electrical Axis
One ECG shows the same heartbeat from twelve angles. A standard tracing records 12 leads, each a different viewing angle, labeled I, II, III, aVR, aVL, aVF and V1 to V6. A wave pointing away from one lead’s view shows as a deep dip there and may barely show in another. Exactly where the signal starts inside the heart also varies between people, which changes how deep the S waves look.4PubMed Central. Human ventricular activation sequence and the simulation of the electrocardiographic QRS complex and its variability in healthy and intraventricular block conditions
To find the S wave on your own printout, look for the tallest sharp spike in a beat. The S wave is the downstroke that drops below the baseline right after it. Not every lead shows a clear S wave, and that is normal. The grid helps with size: each small square is 1 mm, which is how depths like “10 mm” in reports are measured.
The S wave is only the tail of the same electrical wave, not a second beat. Q, R and S together last about a tenth of a second, and what matters is whether the whole spike looks right, not the S wave alone.
The lettering follows simple rules. A first downward stroke is a Q, an upward stroke is an R, and a downward stroke after an R is an S. Some leads show only an R with no dip, and some show a downward spike with no R at all. A beat that is entirely downward is called a QS wave, so a purely downward spike is not labeled an S wave by itself. A beat with two upward strokes and a dip between them has an R, then an S, then a second small upward stroke called R-prime, a clue to a wiring delay. Because this is a naming convention, the same electrical event can carry different letters in different leads.
What a Normal S Wave Looks Like
The chest leads V1 to V6 run from the right of the breastbone around to the left side under the armpit. Across them, S waves are small or absent in V5 and V6 and grow larger toward V2.5ecg.utah. – / -. 3. Characteristics of the Normal ECG The S wave in V1 is usually a little smaller than in V2.6ecg.utah. – / -. 3. Characteristics of the Normal ECG The point where the downward S stops being larger than the upward R usually falls at V3 or V4.7ecg.utah. – / -. 3. Characteristics of the Normal ECG
On a standard printout the six chest leads usually appear in order, so you can follow the S wave from left to right as it shrinks. In V1 and V2 it should be a large downward stroke with a small R. By V5 and V6 the R should dominate with little or no S. Somewhere around V3 or V4 the beat looks about equally up and down, and that is the normal turning point.
In young adults there are typical upper limits. The S wave in V1 averages about 10 mm, with up to about 23 mm still within normal.8Royal Children’s Hospital Melbourne. Paediatric ECG interpretation In V2 it averages about 14 mm, with up to about 36 mm still normal.9Royal Children’s Hospital Melbourne. Paediatric ECG interpretation
On a report, phrases like “normal R-wave progression” cover the S wave pattern too, and a normal report often does not mention the S wave at all. Two ECGs taken on different days can also differ because the stickers sat in slightly different places.
Deep S Waves and a Thickened Heart Wall
A thicker heart wall produces stronger electrical signals, so the waves get taller and deeper. A common check adds the S wave depth in V1 to the tallest R wave in V5 or V6, and 35 mm or more meets the criteria for a thickened left ventricle.10PubMed Central. Validity of electrocardiographic criteria for increased left ventricular mass in young patients in the general population This sum, called the Sokolow-Lyon voltage, often appears by name on reports.11PubMed Central. Indices of left ventricular voltage on electrocardiogram are closely associated with serum cardiac troponin I levels in normotensive Japanese individuals
You can try the sum yourself. Find the deepest S wave in V1 and count how many small squares it drops below the baseline. Then find the tallest R wave in V5 or V6, count its squares upward, and add the two numbers. A total of 35 or more meets the criteria, but that only means the voltage is high, not that the heart is thickened. A printed comment such as “LVH by voltage” means the machine found the total crossed the threshold.
Deep S waves on the left side can also point to a strained right ventricle, though they are poor at catching it. An S wave deeper than about 3 mm in V6 is one sign doctors use for a thickened right ventricle.12Juvenis Scientia. The Diagnostic Value of Electrocardiographic Criteria for Right Ventricular Hypertrophy Even the best of these right-side voltage signs picks up only about 1 in 5 people who actually have the thickening.13Juvenis Scientia. The Diagnostic Value of Electrocardiographic Criteria for Right Ventricular Hypertrophy In people with a weakened, enlarged heart, deep S waves in V5 or V6 caught only about a third to half of those with high lung blood pressure.14PubMed Central. Electrocardiogram signs of right ventricular hypertrophy may help identify pulmonary hypertension in patients with dilated cardiomyopathy When these signs were present, though, they were rarely wrong.15PubMed Central. Electrocardiogram signs of right ventricular hypertrophy may help identify pulmonary hypertension in patients with dilated cardiomyopathy
An ECG measures electricity, not wall thickness directly. An echocardiogram, an ultrasound of the heart, measures the wall itself and is the usual next test. A thin chest wall or slim build can make waves larger without any thickening. And because these signs miss many cases, a normal ECG does not rule thickening out.
Wide or Slurred S Waves From a Wiring Delay
The signal reaches the lower chambers through a right and a left branch, like two wires. When one conducts slowly, it is called a bundle branch block. The signal then spreads more slowly and the spike widens.
In a complete right bundle branch block the whole spike lasts 120 milliseconds or more, with an M-shaped pattern in V1 and V2.16PubMed Central. Right Bundle Branch Block: Current Considerations The S wave in leads I and V6 becomes longer than the R wave or longer than 40 milliseconds.17PubMed Central. Right Bundle Branch Block: Current Considerations Reports often describe this as a wide, slurred S wave in leads I, V5 and V6.18The Cardiology Advisor. Right Bundle Branch Block (RBBB)
The M shape is what some reports call RSR-prime or “rabbit ears.” V1 often shows a small R, a dip, then a second taller R. The S wave is the dip in the middle, and the second upward stroke is the late signal reaching the right ventricle. On the left side, in leads such as I and V6, that late signal draws a wide S wave, mirroring V1.
A slow left branch shows up on the opposite side. In left bundle branch block the spike also lasts 0.12 seconds or longer.19The Cardiovascular. Left bundle branch block (LBBB): ECG criteria, causes, management V1 and V2 show a deep, broad S wave, while V5 and V6 show a wide, notched R wave.20The Cardiovascular. Left bundle branch block (LBBB): ECG criteria, causes, management Because a left bundle branch block can be linked to a weaker pumping heart, the deep S wave is one of the features doctors look at closely.21PubMed Central. Prediction of left ventricular systolic dysfunction in left bundle branch block using a fine-tuned ECG foundation model
The two blocks carry different weight. A right bundle branch block is often found in healthy people and may need no treatment. A left one more often comes with underlying heart disease and usually prompts an echocardiogram, and a new one with chest pain needs urgent care.
“Incomplete” on a report means the same pattern with a spike that is not as wide. It is a common finding and less significant than a complete block.
The S1Q3T3 Pattern and Lung Clots
S1Q3T3 means a deep S wave in lead I, a Q wave in lead III and an upside-down T wave in lead III.22PubMed Central. Massive pulmonary embolism presenting with hemoptysis and S1Q3T3 ECG findings It is listed in European guidelines as one ECG sign of strain on the right ventricle in a lung clot.23PubMed Central. S1Q3T3 electrocardiographic pattern in saddle pulmonary embolism In people with a lung clot, the S1Q3T3 pattern is among the signs linked with a higher risk of getting worse or dying.24PubMed Central. S1Q3T3 electrocardiographic pattern in saddle pulmonary embolism Many people with a lung clot do not show it, so a normal ECG does not rule a clot out.
A clot blocking a lung artery makes the right ventricle push against sudden high resistance, so it stretches and strains. That stretch changes the direction and timing of the right ventricle’s electrical signal, which draws the deep S in lead I and the Q and flipped T in lead III. The ECG is reflecting strain on the right heart, not the clot itself, which sits in the lung where the ECG cannot see it. The pattern is most likely with larger clots and is often absent with smaller ones.
Sudden breathlessness, chest pain when breathing in, coughing blood or fainting need emergency care. A clot is confirmed with a CT scan of the chest and blood tests, not the ECG alone. Recent surgery, long travel or a swollen leg raise the chance of a clot. If the S1Q3T3 pattern appears on a routine ECG and you feel well, ask your doctor promptly rather than waiting.
Notched S Waves, Scarring and Other Shapes
A notch at the lowest point of the S wave in two neighboring leads is called fragmented QRS.25Cardiology Journal. QRS fragmentation: Diagnostic and prognostic significance Scar or fibrous tissue, for example after a heart attack, forces the signal onto a slower, uneven path.26Cardiology Journal. QRS fragmentation: Diagnostic and prognostic significance Some notched patterns are harmless normal variants, so doctors judge them in context.27PubMed Central. QRS Fragmentation Patterns Representing Myocardial Scar Need to Be Separated from Benign Normal Variants: Hypotheses and Proposal for Morphology based Classification
On a report, fragmented QRS may be abbreviated fQRS. It means the spike has extra notches or splits instead of a smooth line. Requiring two neighboring leads makes it more likely to be a real pattern and not a stray bump. The notches show the signal took an uneven, slower route through damaged or patchy tissue, so it is a clue to look for scarring, not proof of it.
A doctor will check your history for a past heart attack or chest pain, and compare with older ECGs to see whether the notch is new. An echocardiogram or heart MRI can show scarring directly. A notch alone in a healthy person is often left alone.
Two other patterns name the S wave directly. An S1S2S3 pattern means S waves in leads I, II and III together.28PubMed. The S1S2S3 electrocardiographic pattern – Prevalence and relation to cardiovascular and pulmonary diseases in the general population In the general population one form appeared in about 3 in 100 people and was linked to a higher chance of heart failure.29PubMed. The S1S2S3 electrocardiographic pattern – Prevalence and relation to cardiovascular and pulmonary diseases in the general population And in people with an extra electrical pathway (Wolff-Parkinson-White), a dominant S in V1 suggests the pathway is on the right side.30Life in the Fast Lane. Pre-excitation syndromes
Athletes and Normal Hearts With Big S Waves
Voltage criteria for a thickened left ventricle are commonly met on athletes’ ECGs because training increases heart muscle.31British Journal of Sports Medicine. International criteria for electrocardiographic interpretation in athletes: consensus statement Right-side voltage criteria and incomplete right bundle branch block are also common, from a larger right ventricle.32British Journal of Sports Medicine. International criteria for electrocardiographic interpretation in athletes: consensus statement High voltages on their own, with no other abnormal findings or symptoms, are considered normal training changes that need no further testing.33British Journal of Sports Medicine. International criteria for electrocardiographic interpretation in athletes: consensus statement That is reasonable because in a serious inherited thickening of the heart, voltage alone without other ECG changes appears in fewer than 2 in 100 cases.34British Journal of Sports Medicine. International criteria for electrocardiographic interpretation in athletes: consensus statement Isolated right-side voltage findings in athletes have not been linked to disease.35PubMed Central. Interpreting the Athlete’s ECG: Current State and Future Perspectives
Regular endurance or strength training enlarges the heart’s chambers and thickens the wall slightly, which produces stronger electrical signals. Athletes also tend to have slow resting heart rates and strong nerve influence on the heart, which changes the tracing in other benign ways. These changes usually reverse partly if training stops for a long period, which can help doctors tell training effect from disease. The finding is more common in people who train many hours per week, in men, and in Black athletes.
Some situations still call for follow-up: fainting during exercise, chest pain, or a family history of sudden death. Inverted T waves or other abnormal findings alongside the voltage also need checking. Tell the person reading the ECG that you train regularly so they use athlete standards. A pre-participation screen flagged only for voltage, with no symptoms or other abnormal findings, usually clears.
When the Sticker Placement Changes the S Wave
Chest stickers placed too high, low, far in or far out can change the height of R waves and the depth of S waves in V1 to V6.36PubMed Central. Electrocardiographic lead reversals Swapping the V1 and V4 stickers can give a deeper S wave in V4 and a tall R in V1 that looks out of place.37PubMed Central. Electrocardiographic lead reversals Such errors can wrongly suggest a thickened right ventricle or an extra pathway.38PubMed Central. Electrocardiographic lead reversals
V1 and V2 go on either side of the breastbone in the fourth space between the ribs, and V4 goes in the fifth space under the middle of the collarbone. V3 sits between V2 and V4, and V5 and V6 continue in line around the left side of the chest toward the armpit. Even one rib space off can change S wave depth in V1 and V2, which is why technicians feel for the rib spaces.
A sudden change from a previous ECG, with no symptoms, raises the question of a recording error. Ask whether the ECG can simply be repeated, which is quick and painless. Large breasts, chest hair or body shape make placement harder and are worth mentioning.
How S Waves Differ in Children
S wave depth increases through infancy and early childhood, levels off by mid-childhood, and stays fairly stable through the teen years.39PubMed Central. Pediatric Electrocardiogram in Preparticipation Screening: Narrative Review of Normal Values in Key Features In V1 the upper normal limit is about 18 mm in newborns but about 25 mm at ages 8 to 12.40Royal Children’s Hospital Melbourne. Paediatric ECG interpretation In V6 the pattern runs the other way: the S wave is larger in babies, with an upper limit about 10 mm in the first month falling to about 4 mm by age 8.41Royal Children’s Hospital Melbourne. Paediatric ECG interpretation Normal values also vary between studies and machines more for S waves than R waves.42PubMed Central. Pediatric Electrocardiogram in Preparticipation Screening: Narrative Review of Normal Values in Key Features
Before birth the right ventricle does most of the work, and in newborns it is still dominant, so the signal directed to the right chest leads is stronger. Over the first months the left ventricle takes over and thickens, and the pattern gradually shifts toward the adult one. A newborn ECG that would look like right ventricle enlargement in an adult can be completely normal.
For parents, a value that looks high by adult standards can be normal for a child’s age, so it is fair to ask whether the reader used age-specific ranges. Children’s sports screening ECGs are best read by someone experienced with young hearts. A flagged result usually leads to an echocardiogram rather than treatment.