Why a Normal ECG Can Miss an Irregular Heartbeat

A standard ECG records about ten seconds of the heart's activity. An irregular rhythm only has to be absent for those ten seconds to be missed.

An ECG, or electrocardiogram, records the heart's electrical signals and shows whether it is beating in a normal rhythm. In hospital, it is one of the most common tests there is: quick, painless and very informative. It is also a snapshot. Many rhythm problems come and go, so a patient can have an irregular heartbeat for twenty minutes in the afternoon and a perfectly normal ECG that evening.

Heart rhythms that come and go

Several common rhythm problems, which doctors call arrhythmias, appear and disappear on their own. Paroxysmal atrial fibrillation is an irregular, often fast heartbeat that starts and stops by itself. Supraventricular tachycardia causes sudden bursts of a very fast heart rate. Short runs of ventricular tachycardia may last only seconds.

These episodes matter. A fast or irregular rhythm can make the heart pump less well and lower blood pressure, and in some patients it leads to more serious problems. Catching an episode early gives the team a chance to treat it before that happens.

The longer you look, the more you find

Research outside hospital shows how much depends on how long you watch. In the EMBRACE trial, published in the New England Journal of Medicine in 2014, people aged 55 or over who had recently had a stroke with no clear cause were checked for atrial fibrillation. It was found in 16.1 per cent of those who wore a heart monitor for 30 days, compared with 3.2 per cent of those monitored for the standard 24 hours.

Those were stroke patients monitored at home, not people in hospital. The principle still holds: when a rhythm comes and goes, a longer watch finds more of it.

Two patients, two unscheduled moments

On hospital wards, Lifesigns Smart Monitoring uses wireless, wearable ECG devices to record patients' heart rhythm continuously. The readings go to Lifesigns Nexus, where nurses and doctors can see each patient's rhythm and how it has changed, and where hospitals choose, to a Central Monitoring Team of trained staff who watch the data around the clock and alert the ward. Lifesigns' case reports include many rhythm changes caught this way. Two show the pattern.

A 55-year-old man was admitted under cardiology with chest pain and palpitations. He had triple-vessel coronary artery disease, diabetes and high blood pressure, so continuous remote monitoring was part of his care. At 3:20 PM on 6 January 2026, monitoring detected atrial fibrillation with a fast ventricular response, his heart rate fluctuating up to 168 beats per minute. The event was escalated, and the medical team gave a beta-blocker and intravenous magnesium and corrected his potassium. According to the case report, he was stabilised without needing to move to the ICU.

In another case, a man in his eighties admitted with sepsis, a serious body-wide reaction to infection, was placed on continuous monitoring on the night he arrived. Three days later, at 7:38 AM, monitoring recorded supraventricular tachycardia with his heart rate reaching 196 beats per minute and his oxygen level falling to 88 per cent. He was escalated to the attending physician and moved to the medical ICU for further care.

In both cases, the rhythm changed at a moment when no test was planned.

Not only a heart-ward issue

Rhythm problems are not confined to cardiology. Infection, surgery, low potassium or magnesium and many other conditions can trigger them, as the sepsis case shows. That is why continuous rhythm monitoring can matter for patients on almost any ward.

The ten-second ECG will stay an essential test. Continuous ECG adds a record of the heart at the moment its rhythm changes, which is often the moment that matters.