The QT interval is the portion of an electrocardiogram tracing that measures the time from the beginning of ventricular depolarization to the end of ventricular repolarization. It runs from the first deflection of the QRS complex to the point where the T wave returns to baseline. Because it captures both electrical activation and recovery of the ventricles, the QT interval is the standard surface marker of ventricular repolarization time, and clinicians correct it for heart rate as the QTc.
The interval begins where the QRS complex departs from the isoelectric line. That first deflection may be a Q wave, an R wave, or occasionally an rSR pattern. The endpoint is the finish of the T wave, where the tracing rejoins the baseline. Everything between those two landmarks—the QRS, the ST segment, and the T wave—is included in the measurement.
This matters because the interval bundles two physiologically distinct events. Depolarization consumes only a small fraction of it; repolarization occupies most of the span. The QT interval is therefore best understood as a repolarization measure that happens to include the depolarization that precedes it. Readings that seem to blend activation and recovery are capturing exactly what the interval was designed to record.
What the interval captures electrically
Ventricular depolarization begins when the electrical impulse reaches the ventricular myocardium and spreads from cell to cell. On the surface ECG this produces the QRS complex, normally under 120 milliseconds. The QT clock starts at the earliest ventricular activation visible on that lead and does not stop when the QRS ends.
The ST segment that follows represents the plateau phase, when inward calcium current balances outward potassium current and most ventricular cells sit near the same voltage. Because neighboring cells are at similar potentials, the surface tracing stays nearly flat. The T wave then reflects the end of that plateau and the rapid phase of repolarization, as potassium currents return cells toward resting voltage.
The T wave ends when the last ventricular region finishes repolarizing. That region can be a small patch of myocardium, so the terminal portion of the T wave is often low-amplitude and gradual. The imprecision of that endpoint is the main reason QT measurement is harder than it looks. A small error at the tail of the T wave translates into a meaningful error in the total interval.
Two ion channels dominate the physiology behind the interval. The rapid delayed rectifier potassium current, known as IKr, and the slow delayed rectifier current, IKs, govern the repolarization phase. Drugs or mutations that block IKr prolong the plateau and stretch the QT interval. That single mechanism explains most acquired long QT cases.
The numbers that define normal and abnormal
A normal QT interval at a heart rate near 60 beats per minute is roughly 350 to 440 milliseconds in men and 350 to 460 milliseconds in women. Because the interval shortens as rate rises, raw values must be adjusted. The corrected value, QTc, is what most thresholds refer to.
Bazett's formula divides the measured QT by the square root of the RR interval in seconds. It is the most widely used correction, though it overcorrects at fast rates and undercorrects at slow ones. Fridericia's cube-root formula behaves better at extremes. For most routine screening the specific formula matters less than using one consistently.
A QTc below 440 milliseconds is generally considered normal in men and women. Values from 440 to 460 milliseconds sit in a borderline zone. A QTc above 500 milliseconds carries substantially elevated risk of torsades de pointes, a polymorphic ventricular tachycardia. Some risk persists even between 460 and 500 milliseconds, which is why the 500 mark is a red flag rather than the boundary of safety.
The QRS complex itself normally stays under 120 milliseconds. In bundle branch block or ventricular pacing it widens beyond that, sometimes to 160 milliseconds or more. Because the QRS is embedded in the QT interval, a wide QRS inflates the measured QT without necessarily reflecting delayed repolarization.
A quick plain-language anchor: the QT interval is the whole ventricular electrical cycle, and QTc is that same span rescaled to a standard heart rate.
Rate correction and the measurement problem
Heart rate and QT move in opposite directions. At 100 beats per minute a normal QT may be around 300 milliseconds; at 50 beats per minute it may exceed 460 milliseconds. Without correction, a slow heart rate alone can mimic long QT syndrome. Correction formulas exist to strip out that rate dependence.
No formula is perfect. Bazett's method is standard in most automated ECG software and clinical trials, which keeps comparisons consistent. Fridericia's method is preferred by some electrophysiologists when rates fall outside 60 to 90 beats per minute. Neither converts a borderline raw value into certainty.
Measurement technique introduces its own error. Intervals are best read in the lead with the clearest T wave endpoint, often lead II or V5. When the T wave is biphasic or has a low tail, the tangent method draws a line along the steepest terminal slope and marks where it crosses baseline. Automated algorithms can differ from expert overreads by 10 to 20 milliseconds or more.
The number on the printout is a starting point, not a diagnosis. Automated QTc values should be confirmed by a trained reader whenever a drug decision or a family screening depends on them. That is the professional caveat most thin explainers omit entirely.
Edge cases that break the simple definition
Several common patterns make the interval hard to measure or misinterpret. Each requires a different approach, and some change the clinical meaning of the number.
Atrial fibrillation: absent P waves and variable RR intervals make rate correction unreliable, so multiple beats should be averaged. Bundle branch block and pacing: the widened QRS inflates QT; a separate measure such as the JT interval may better reflect repolarization. Prominent U waves: if the U wave fuses with the T wave, the interval may be undercalled; if it is separate, it is excluded. Early repolarization pattern: benign ST elevation can blend into the T wave and distort automated readings. Biphasic or notched T waves: the terminal deflection is the true endpoint, and misreading the first hump underestimates the QT. Very fast or very slow rates: correction formulas become less accurate outside roughly 50 to 100 beats per minute.
The counterintuitive detail sits in that list. A normal QTc does not exclude risk. Some patients with genetically confirmed long QT syndrome have resting QTc values in the 440 to 460 millisecond range, and provocation—exercise or standing—can expose prolongation that a resting tracing misses. The interval is a probabilistic marker, not a binary switch.
Congenital long QT syndrome types illustrate how mechanism maps to measurement. Type 1 involves loss of function in the slow delayed rectifier current and typically prolongs at fast rates. Type 2 involves the rapid delayed rectifier current and often shows a longer QT at rest. Type 3 involves the late sodium current and can produce a long ST segment with a late-appearing T wave. The shape of the tracing carries diagnostic weight beyond the number.
How the QT interval compares with related measures
The tracing offers several time-based markers, and confusing them is a common source of error. The table below separates the QT interval from its neighbors on the same strip.
| Measure | What it spans | Typical normal value | What it primarily reflects |
|---|---|---|---|
| PR interval | P wave onset to QRS onset | 120 to 200 ms | Atrial depolarization and AV node conduction |
| QRS duration | First to last QRS deflection | Under 120 ms | Ventricular depolarization |
| QT interval | QRS onset to T wave end | 350 to 440 ms at 60 bpm | Depolarization plus repolarization |
| QTc | QT corrected for rate | Under 440 ms | Repolarization at a standardized rate |
| JT interval | QRS end to T wave end | Varies with age and rate | Repolarization with depolarization excluded |
| RR interval | R wave to next R wave | Varies with rate | Ventricular cycle length |
The JT interval deserves a note. When the QRS is wide, subtracting it from the QT isolates repolarization more cleanly. Some clinicians use the JT interval in bundle branch block and pacing, though it is not as standardized as QTc.
The relationship among these measures is sequential. Atrial activation precedes ventricular activation, the QRS marks the start of ventricular activity, and the ST-T portion marks its recovery. The QT interval is the only standard measure that spans both ventricular phases.
What a prolonged or shortened interval signals
Prolongation of the QT interval reflects delayed repolarization, which extends the vulnerable window during which a premature beat can trigger torsades de pointes. That arrhythmia can degenerate into ventricular fibrillation. Causes include inherited channelopathies, electrolyte disturbances such as low potassium or low magnesium, and a long list of medications.
The medication list is long and clinically important. Some antiarrhythmics, certain antibiotics, some antipsychotics, and several antifungals block the rapid delayed rectifier current and prolong the QT. Risk rises with higher doses, with combinations of such drugs, and with kidney or liver impairment that raises drug levels. A QTc above 500 milliseconds on therapy is a recognized trigger for dose review.
Shortening carries its own meaning. A QTc below roughly 340 to 360 milliseconds is unusual and can appear in short QT syndrome, hypercalcemia, or digoxin effect. Short QT syndrome is rare but carries arrhythmic risk, so a very short interval should not be dismissed as a good sign.
Symptoms that should prompt urgent assessment include fainting, unexplained collapse, and palpitations with near-syncope, especially during exercise or loud startle. Family history of sudden cardiac death at a young age changes the meaning of a borderline QTc. These red flags belong in any serious explanation of the interval.
Practical steps for reading and acting on the interval
The following steps reflect how clinicians and trained technicians approach the measurement. They convert the definition into usable action.
Confirm the identifying details: lead, rate, and rhythm, because the interval is only interpretable in context. Identify QRS onset in the lead with the clearest initial deflection, and mark the first departure from baseline. Locate the T wave endpoint using the tangent method when the tail is gradual, and exclude a clearly separate U wave. Measure the raw QT in milliseconds, then apply the same correction formula for every comparison, usually Bazett. Average three to five beats in sinus rhythm; average more in atrial fibrillation and document the method. Flag a QTc of 500 milliseconds or more, or an increase of 60 milliseconds or more from a prior baseline, for clinician review. Check potassium and magnesium when the interval is prolonged, and review the medication list for QT-prolonging agents.
These numbers reconcile across the common thresholds. A QTc under 440 milliseconds is generally normal. Values from 440 to 460 milliseconds warrant attention and repeat measurement. Values from 460 to 500 milliseconds are prolonged and call for cause review. A QTc at or above 500 milliseconds is a high-risk finding. A change of 60 milliseconds or more from baseline carries similar weight regardless of the absolute value.
Limitations of the interval as a measure
The QT interval is a surface approximation of a cellular process. It cannot separate transmural from regional repolarization differences, and it cannot see what a single lead misses. The T wave endpoint is inherently subjective, which is why interreader agreement is moderate even among experts.
Automated algorithms reduce reader burden but can be confidently wrong. They may miss a fused U wave or misfire on a noisy baseline. Any high-stakes decision should rest on a manually confirmed measurement read in a consistent lead.
Rate correction formulas are models, not measurements. Bazett's formula distorts at high and low rates, and no formula accounts for individual repolarization reserve. A patient with a normal QTc can still have a channelopathy, and a patient with a long QTc may never have an arrhythmia.
The interval also changes with time of day, body position, and autonomic state. A single resting value is a snapshot. Serial tracings and provocation testing add information that one measurement cannot provide.
For general educational information only. This article is not a diagnosis or treatment plan and does not replace evaluation by a qualified clinician. Anyone with fainting, unexplained collapse, palpitations with near-syncope, or a family history of sudden cardiac death should seek prompt medical assessment through local emergency or clinical services.