The ST segment is a short, flat stretch on an electrocardiogram (EKG) that sits between the spike of the heartbeat (the QRS complex) and the rounded T wave that follows it. It represents the brief pause when the heart’s lower chambers are fully contracted and holding steady before they begin to relax. In a healthy heart, the ST segment runs along the baseline, neither rising nor falling. When it shifts up or down, it can signal problems ranging from a heart attack to inflammation to an electrolyte imbalance.
Where the ST Segment Sits on an EKG
An EKG tracing breaks each heartbeat into a series of waves and segments. The ST segment begins at a specific landmark called the J-point, which is the exact junction where the QRS complex ends. It extends as a mostly flat line until the T wave begins to rise. In a normal reading, the ST segment should be isoelectric, meaning it sits right at the same level as the baseline of the tracing.
To measure whether the ST segment is elevated or depressed, clinicians compare it to a reference line. That reference is typically the T-P segment (the flat stretch between the end of one T wave and the start of the next P wave, when the heart is electrically quiet). Some practitioners use the PR segment instead. Either way, the goal is the same: establish a flat baseline, then see if the ST segment deviates from it.
What’s Happening Inside the Heart
The ST segment corresponds to phase 2 of the cardiac action potential, often called the plateau phase. During this moment, the muscle cells of the ventricles have already fired and contracted, and their electrical charge is holding nearly steady. This stability comes from a precise balance of ions flowing in and out of heart cells: potassium drifts outward while smaller amounts of sodium and calcium shift inward. Because these currents essentially cancel each other out, the voltage across the cell membrane barely changes, and the EKG line stays flat.
When something disrupts that ion balance, whether it’s blocked blood flow, inflammation, or abnormal potassium levels in the blood, the plateau phase becomes uneven across different regions of the heart. That electrical mismatch shows up on the EKG as the ST segment rising above or dropping below the baseline.
ST Elevation: What It Can Mean
ST-segment elevation is the hallmark of the most dangerous type of heart attack, known as a STEMI (ST-elevation myocardial infarction). When a coronary artery is completely blocked, the starved muscle produces a characteristic upward bulge in the ST segment. Specific thresholds help distinguish a true STEMI from normal variation: the elevation must reach at least 1 mm (measured at the J-point) in two or more adjacent leads. In certain chest leads (V2 and V3), the bar is higher: at least 2 mm for men 40 and older, 2.5 mm for men under 40, and 1.5 mm for women of any age.
But a heart attack is not the only cause. Several other conditions push the ST segment upward, and telling them apart matters enormously because the treatments are completely different.
Pericarditis
Inflammation of the sac surrounding the heart produces widespread ST elevation that rarely exceeds 4 to 5 mm. Unlike a heart attack, the elevation appears across many leads rather than clustering in one region, and there is little or no reciprocal depression in other leads (except commonly in aVR and V1). A useful distinguishing feature is the ratio of ST height to T-wave height in lead V6: a ratio of 25% or more points toward pericarditis rather than a normal variant. Transient PR depression, especially in leads II, aVF, and V4 through V6, is another clue.
Early Repolarization
This is a normal variant seen most often in younger, healthy individuals. The ST segment is gently concave (curving upward like a shallow bowl), and the J-point may show a small notch or slur. Elevation is typically no more than 3 mm, and the T waves can be tall, sometimes exceeding 10 mm in the chest leads, but they stay narrow and smaller than the QRS complex. The ST-to-T-wave ratio stays below 25%, which helps separate it from pericarditis.
Brugada Syndrome
This inherited electrical disorder produces a distinctive pattern in leads V1 through V3: ST elevation paired with a shape resembling a right bundle branch block. The most dangerous form, type 1, shows a coved, downward-sloping ST elevation of 2 mm or more followed by T-wave inversion. The pattern can come and go, sometimes triggered by fever, cocaine, or certain heart rhythm medications. Unlike a heart attack, where the ST segment tends to bulge upward in a convex shape, Brugada produces a downward slope after the initial rise.
ST Depression: What It Can Mean
When the ST segment drops below the baseline, the most common concern is reduced blood flow to the heart muscle, a condition called myocardial ischemia. ST depression is the classic finding during a stress test when the heart is working hard but not getting enough oxygen. Horizontal or downsloping depression of 1 mm or more is a reliable predictor of coronary events, even in people with no symptoms. Upsloping depression (where the segment dips but quickly angles back up) is considered less specific and harder to interpret in someone who feels fine.
The shape of the depression matters. A flat, horizontal drop is more concerning than a gentle upslope. And downsloping depression, where the segment continues to fall as it approaches the T wave, carries the strongest association with significant coronary artery disease. These morphology differences help clinicians gauge severity when interpreting a stress test or monitoring someone with chest pain.
How Electrolyte Imbalances Affect the ST Segment
Because the ST segment depends on a delicate balance of potassium, sodium, and calcium flowing through heart cells, shifts in blood electrolyte levels can distort it in ways that mimic more serious conditions. High potassium (hyperkalemia) is the most dramatic example. As potassium levels climb, the EKG changes progressively: first the T waves become tall, peaked, and tent-shaped. Then the QRS complex widens. At dangerously high levels, the P wave shrinks and disappears, ST elevation can appear in a pattern that mimics a heart attack (sometimes called a “pseudoinfarction” pattern), and the tracing eventually degenerates into a sine wave before the heart stops.
Low potassium and calcium abnormalities can also flatten or prolong the ST segment, though these changes tend to be subtler. The key point is that an abnormal ST segment does not always mean a blocked artery. Blood work to check electrolytes is a routine part of evaluating any unexpected ST-segment change.
How ST-Segment Changes Are Measured
All ST-segment measurements are taken at the J-point, right where the QRS complex hands off to the ST segment. The amount of elevation or depression is expressed in millimeters, where each small box on standard EKG paper equals 1 mm. A shift has to appear in at least two anatomically related leads to be considered significant, which helps rule out electrical noise or lead placement errors.
Context shapes interpretation. A young athlete with 2 mm of concave ST elevation in the chest leads likely has early repolarization. The same finding in a 60-year-old with crushing chest pain is treated as a heart attack until proven otherwise. Age, sex, symptoms, and the overall shape of the EKG tracing all factor into what an ST-segment shift actually means.