Early warning signs may have been missed in one in six heart attack deaths in England, a study suggests.
Researchers looked at all heart attack hospital admissions and deaths between 2006 and 2010.
The team, from Imperial College London, found 16% who died of a heart attack within 28 days of an admission had not been diagnosed - despite symptoms like chest pain most likely being present.
The British Heart Foundation has called the research "concerning".
The study authors from the School of Public Health at Imperial College say more research is "urgently needed".
The research, published in the Lancet, looked at the hospital records of all 135,950 deaths in England due to heart attacks over the four-year period.
The records showed whether the person had been admitted to hospital in the previous four weeks and whether signs of a heart attack were recorded as the primary reason for the hospital admission, a secondary reason or not recorded at all.
The data showed 21,677 of the patients had no mention of heart attack symptoms in their hospital records.
Lead author Dr Perviz Asaria said: "Doctors are very good at treating heart attacks when they are the main cause of admission, but we don't do very well treating secondary heart attacks or at picking up subtle signs which might point to a heart attack death in the near future."
Heart attack symptoms
- Chest pain - a sensation of pressure, tightness or squeezing in the centre of the chest
- Pain in other parts of the body - it can feel as if the pain is travelling from the chest to the arms (usually the left arm is affected, but it can affect both arms), jaw, neck, back and abdomen
- Feeling lightheaded or dizzy
- Shortness of breath
- Feeling sick (nausea) or being sick (vomiting)
- Overwhelming sense of anxiety (similar to having a panic attack)
- Coughing or wheezing
Although the chest pain is often severe, some people may only experience minor pain, similar to indigestion. In some cases, there may not be any chest pain at all, especially in women, elderly people and people with diabetes.
The report authors say symptoms, such as fainting, shortness of breath and chest pain, were apparent up to a month before death in some patients.
But they point out doctors may not have been alert to the possibility that these signalled an approaching fatal heart attack because there was no obvious damage to the heart at the time.
Prof Majid Ezzati, who also worked on the study, said: "We cannot yet say why these signs are being missed, which is why more detailed research must be conducted to make recommendations for change.
"This might include updated guidance for healthcare professionals, changes in clinical culture, or allowing doctors more time to examine patients and look at their previous records."
Prof Jeremy Pearson, associate medical director at the British Heart Foundation, said: "This study shows that large numbers of people who die of a heart attack have visited hospital in the month before, but have not been diagnosed with heart disease.
"This failure to detect warning signs is concerning and these results should prompt doctors to be more vigilant, reducing the chance that symptoms are missed, ultimately saving more lives."
A spokesman for the Royal College of Physicians said: "The treatment of heart attacks is one of the success stories of modern medicine but this paper is an important reminder that there are still areas where we can improve care.
"While many heart attacks present with classical pain in the chest in people who smoke and have other risk factors for heart disease, many heart attacks don't present this way and in people not obviously at high risk.
"The challenge is to accurately and speedily diagnose all these patients so that they can be offered best care. Education of the public, of GPs, paramedics and Emergency Department doctors is essential if we are to improve even further the care we offer to patients having a heart attack."