Every normal heartbeat starts with a small, rounded bump. That is the P wave, and it is easy to miss next to the tall QRS spike that follows it. It is also one of the first things a clinician looks for, because it shows where the beat started.
What the P wave is
The P wave is the electrical signal of the upper chambers, the atria, being activated. A normal beat starts in the sinus node in the right atrium, and the signal spreads across both atria before it reaches the lower chambers. That spread is what draws the P wave.1
A regular P wave in front of every QRS is one of the signs of sinus rhythm. When the P wave is missing, for example in atrial fibrillation, the upper chambers are not being activated in an organized way.
P-wave duration on a real beat
P-wave duration runs from the first rise of the P wave off the baseline to the point where it returns. In ECG Tracker+, the Beat Morphology view shows it on a representative beat, with the start and end marked so you can see exactly what was measured.
Typical range
In adults, the P wave usually lasts less than about 120 ms.2 A longer P wave can reflect slower spread of the signal through the atria, for example when the left atrium is enlarged. On its own it does not tell you why, and it is a finding for a clinician to interpret with the whole ECG.
Why the P wave is hard to measure on a watch
The P wave is small, usually a fraction of a millivolt, and an Apple Watch records a single lead. A little noise from movement or poor contact can hide it or blur where it starts and ends. That is why P-wave duration is often unavailable on a watch recording, and why ECG Tracker+ shows it only when the wave stands out clearly. A measurement marked unavailable is not a problem with your heart.
What affects it
- Recording quality. Keep your arm still and resting on a table while you record.
- Lead. The P wave is smaller in lead I, the lead closest to an Apple Watch recording, than in some other leads.
- Heart rhythm. In atrial fibrillation there is no organized P wave to measure.
A value outside a typical range is not a diagnosis. If a measurement concerns you, a clinician can check it on a 12-lead ECG.
Frequently asked questions
What is a normal P-wave duration?
In adults, the P wave usually lasts less than about 120 ms.
Why is my P-wave duration unavailable?
The P wave is small on a single-lead recording, and noise can hide it. ECG Tracker+ leaves it unavailable rather than guess.
Does Apple Watch show the P wave?
The P wave is visible in the waveform of many Apple Watch recordings, but Apple's ECG app does not measure it. ECG Tracker+ estimates its duration when the wave is clear.
