Flattened T wave on Apple Watch ECG — what it means

If ECG+ flagged a flattened T wave in your recording, it means the T wave in your ECG has lost its usual height and appears diminished or nearly flat. A normal T wave has a gentle rounded peak — it represents your heart muscle recovering between beats. When it flattens, that recovery signal is weaker than expected.

A flattened T wave is a mild waveform change. It is often caused by something correctable, and in many cases it is a normal variant. It is worth knowing about, but it is rarely a reason for immediate concern.

What does the T wave do?

Each heartbeat on an ECG has several distinct waves. The large spike — the QRS complex — represents the main contraction. The T wave that follows represents ventricular repolarisation: the heart muscle recharging for the next beat.

In a healthy ECG, the T wave is a smooth, rounded peak with a clear upward deflection. When it flattens, the amplitude decreases — the peak shrinks toward the baseline, sometimes becoming barely visible.

What causes a flattened T wave?

The most common cause is an electrolyte imbalance, particularly low potassium. Other causes include:

• Low potassium (hypokalaemia) — the most frequent cause; potassium is essential for normal cardiac repolarisation
• Low magnesium — often accompanies low potassium and has a similar effect on the T wave
• Certain medications — diuretics, some antiarrhythmics, and other drugs that affect electrolyte balance
• Mild ischaemia — reduced blood flow to the heart can produce T wave flattening before more pronounced changes appear
• Normal variation — a low-amplitude T wave can be normal, particularly in certain leads or in some individuals

Will I feel anything?

Usually not. A flattened T wave is a quiet finding. If it is being caused by low potassium, you might notice muscle weakness, fatigue, or occasional cramping — but most people with mild electrolyte shifts feel nothing out of the ordinary.

Any symptoms you are experiencing come from the underlying cause, not from the T wave change itself.

Should I be concerned?

A flattened T wave by itself is a mild finding, and in many cases it resolves on its own or with simple dietary changes. It is worth mentioning to your doctor if:

• It appears consistently across multiple recordings
• You are on diuretics or other medications that affect electrolytes
• You have had recent illness with vomiting, diarrhoea, or poor fluid intake
• You notice muscle weakness, unusual fatigue, or cramps alongside it
• You have a known heart condition

In many cases, your doctor may simply check your potassium level with a blood test. If it is low, correcting it often resolves the T wave finding entirely.

A good next step

1. Take a few more recordings to see if the finding is consistent or appears only occasionally.
2. Consider your diet and medications — low potassium can come from diuretics, poor diet, or recent illness. Bananas, leafy greens, and legumes are good potassium sources.
3. Share with your doctor — ECG+ lets you export a PDF report so your doctor can see the recording clearly and decide whether a blood test or further review is needed.

Frequently asked questions