Each waveform component on an electrocardiogram (ECG) represents a specific electrical event in the heart. The P wave captures the electrical activation of the upper chambers, the QRS complex captures activation of the lower chambers, and the T wave captures the electrical recovery that resets the heart for its next beat. The segments and intervals between these waves carry clinical meaning too, reflecting how quickly electrical signals travel through the heart’s conduction system.
P Wave: Activation of the Atria
The P wave is the first small upward deflection on the ECG tracing. It represents the electrical signal that spreads across both upper chambers (atria), triggering them to contract and push blood into the ventricles below. That signal originates at the heart’s natural pacemaker, the sinoatrial (SA) node, located in the right atrium.
The early portion of the P wave reflects electrical activation of the right atrium, while the middle and tail end reflect the left atrium firing slightly afterward. A normal P wave is small and rounded, lasting roughly 80 to 120 milliseconds. When the P wave appears unusually tall, wide, or notched, it can indicate that one or both atria are enlarged or under strain.
PR Interval: The Pause Before the Ventricles Fire
The PR interval spans from the start of the P wave to the start of the QRS complex. It measures the total time it takes for the electrical impulse to travel from the atria, through the atrioventricular (AV) node, and into the ventricles. The normal range is 120 to 200 milliseconds.
The AV node acts as a deliberate bottleneck. It slows conduction just enough to let the atria finish contracting before the ventricles begin. When the PR interval stretches beyond 200 milliseconds, it means conduction through this pathway is sluggish, a pattern called first-degree heart block. The delay can occur at the AV node itself, or less commonly in the electrical wiring below it. A PR interval that’s too short, on the other hand, may suggest an accessory pathway is bypassing the AV node entirely.
QRS Complex: Activation of the Ventricles
The QRS complex is the tallest, sharpest set of deflections on the tracing and represents the electrical activation of both ventricles, the heart’s main pumping chambers. It normally lasts 60 to 100 milliseconds. A duration over 120 milliseconds suggests the electrical signal is taking an abnormal route through the ventricles.
Each letter within the complex corresponds to a slightly different phase of ventricular activation:
- Q wave: A small initial downward dip caused by the interventricular septum (the wall between the two ventricles) activating first. Normal Q waves are thin and shallow. Large, wide Q waves can be a sign of a previous heart attack, where dead tissue no longer conducts electricity.
- R wave: The tall upward spike that follows. It reflects the bulk of the ventricular muscle mass contracting and is the largest wave on the entire ECG for that reason.
- S wave: A small downward deflection after the R wave, representing the final regions of the ventricles to be activated, typically near the base of the heart.
The overall shape and height of the QRS complex vary depending on which ECG lead you’re looking at, since each lead “views” the heart from a different angle. Body type matters too. QRS width tends to be slightly greater in males and in taller individuals, and the electrical axis can shift leftward in people who are obese or older.
ST Segment: The Plateau Between Beats
The ST segment is the flat stretch between the end of the QRS complex and the beginning of the T wave. It represents the brief period when the ventricles are fully activated and holding their contraction to eject blood. At the cellular level, this corresponds to a plateau phase where voltage across heart muscle cells stays nearly constant, producing minimal electrical activity. That’s why the ST segment normally sits flat on the baseline.
This flatness is exactly what makes the ST segment so useful clinically. Any deviation from the baseline signals a voltage imbalance in the heart muscle. During a heart attack, injured tissue creates an “injury current” between damaged and healthy zones, pulling the ST segment upward (elevation) or downward (depression) depending on the type and location of the damage. ST elevation is one of the most time-sensitive findings on an ECG because it typically indicates an artery is completely blocked and heart muscle is actively dying.
The J Point
The J point is the exact spot where the QRS complex ends and the ST segment begins. It marks the junction between ventricular activation and the plateau phase. Slight J point elevation (under 0.1 millivolts) with a rapidly rising ST segment is common and generally harmless, especially in younger adults and athletes. When J point elevation exceeds 0.2 millivolts or the ST segment slopes downward or stays flat after it, the pattern carries a higher risk of dangerous heart rhythms.
T Wave: Electrical Recovery of the Ventricles
The T wave represents ventricular repolarization, the process by which heart muscle cells reset their electrical charge in preparation for the next beat. It’s a broader, more rounded wave than the QRS complex because repolarization happens more gradually than the rapid-fire activation that produces the QRS.
One detail that surprises many people: the T wave normally points in the same direction as the QRS complex, even though repolarization is the electrical reverse of depolarization. This happens because the regions of the ventricle that activate first actually recover last. Since both the direction of the wave and the electrical polarity are reversed, the two negatives cancel out, producing a T wave that looks concordant (same direction) with the QRS. When the T wave flips and points opposite to the QRS, it can indicate ischemia, strain, or other abnormalities disrupting the normal recovery sequence.
Tall, peaked T waves may reflect high potassium levels, while flattened or inverted T waves can appear in conditions ranging from heart strain to electrolyte imbalances.
U Wave: A Subtle, Debated Signal
The U wave is a small, low-amplitude bump that occasionally appears after the T wave. It is most visible at slower heart rates and in certain chest leads. Not every ECG shows a U wave, and its exact origin has been debated for decades.
The leading theory, first proposed over 50 years ago by researcher Lepeschkin and supported by more recent studies, is that the U wave reflects delayed electrical activity in ventricular muscle cells triggered by mechanical stretching of the heart wall. Prominent U waves can show up with low potassium or low magnesium levels, and in some cases with certain medications that affect the heart’s electrical cycle.
QT Interval: Total Ventricular Electrical Activity
The QT interval is measured from the very start of the QRS complex to the end of the T wave. It captures the entire duration of ventricular electrical activity, both activation and recovery. Because the QT interval naturally shortens when the heart beats faster and lengthens when it beats slower, clinicians use a corrected version (QTc) that adjusts for heart rate.
Normal QTc values fall below 440 milliseconds in males and below 460 milliseconds in females. A prolonged QT interval (above 450 ms in males, above 470 ms in females) means the ventricles are taking too long to reset, which creates a window of vulnerability for dangerous irregular rhythms. QT prolongation can be inherited or triggered by a surprisingly long list of common medications, from certain antibiotics to anti-nausea drugs. This is why QT monitoring is routine during hospital stays and when starting new prescriptions known to affect the heart’s electrical timing.
How These Components Work as a Sequence
Reading an ECG becomes more intuitive when you see the waveforms as a narrative. The P wave is the opening act: the atria activate and squeeze. The PR interval is the controlled delay at the AV node, giving the atria time to finish emptying. The QRS complex is the main event: the ventricles fire and contract with force. The ST segment is the sustained contraction, holding pressure long enough to push blood into the arteries. The T wave is the reset, restoring each cell’s electrical charge. And if a U wave appears, it’s a quiet postscript before the next cycle begins.
Each component has a normal shape, duration, and direction. Deviations from those norms point to specific problems, whether it’s a conduction delay, muscle damage, electrolyte imbalance, or structural change. That specificity is what makes the ECG, a test that takes seconds to perform, one of the most information-dense tools in medicine.