Varying advice means patients at high-risk of having a heart attack are unclear about when symptoms are potentially life threatening and when they should call an ambulance, argue a group of heart specialists in this week's BMJ.
At least 70% of people who die from coronary heart disease have had previous heart problems.
Yet recent data from the British Heart Foundation shows that 40% of the general population would not immediately call an ambulance during a suspected heart attack and the greatest delays in calling 999 are among the high-risk group.
This implies, say Dr Khavandi and colleagues, that high-risk patients do not seem to be receiving clear effective guidance from their doctors.
One of the symptoms of a heart attack is angina. Yet angina can also be a stable condition which lasts only a few minutes and can be controlled by medication. The writers say ideally they would like patients to be able to tell the difference between the two types but in reality they do not.
In particular, high-risk patients with ischaemic heart disease (IHD) - which causes a reduction in the blood supply to the heart, usually felt as angina - are given varying advice from doctors, health organisations and drug manufacturers about how to use the drugs (sublingual nitrate GTN sprays) prescribed to relieve the condition.
The advice also varies on when they should call an ambulance.
The British Heart Foundation advises patients with known IHD to call an ambulance if chest pains last longer than 15 minutes and to use their spray three times during that period.
The American College of Cardiology recommends one spray and 5 minutes before calling an ambulance.
Manufacturer's instructions are sometimes non-specific, for example, recommending no more than 3 doses and 15 minutes between treatments - leaving it to the prescribing doctor to guide the patient.
Waiting 15 minutes, say the writers, could be too long for some patients. One study has shown that the median time from onset of symptoms to cardiac arrest is ten minutes.
They recommend patients and their relatives should be explicitly primed to recognise high-risk features of chest pain. They advise patients at high-risk or with IHD to carry a GTN spray with them at all times, to take two metered doses immediately if they get chest pain and to wait 5 minutes before calling an ambulance. They should not waste time by calling a friend or relative first and should not drive themselves to hospital.