there are many facors which can influence a 12 lead ecg readout.
Person -
Age of the candidate
Current stress (and most people sweat a bit on their medical)
Caffine and chemical intake
Dehydration
Body Weight
Lead placement (they have to be specifically placed for an accurate readout)
Sweating
Breathing abnormally thus moving the leads irregularly
Some atrial ectopic beats produce a very distinctive pattern as does a ventricular ectopic beat. Unless more than a given number are present per minute, it can be classed as normal. Both can be duplicated to some extent by various mechanical and electrical factors.
Machine -
Leads not on properly or breaking down
Movement on the candidate
Operator Misreading the data
Older machines more prone to outside fielding (one of my tests showed a major silent - almost pain free - heart attack in progress, turns out the machine was copping electrical current spiking
so if in doubt, do a second one. Chest problems can be assymptomatic (no signs or symptoms in a few rare cases) but classically heart related chest pain can be exacerbated by exercise, brings on shortness of breath, tingling in the extremities and pain is usually centrally located in the chest although is common for patients in the throws of an angina or heart attack to have the pain radiate up the neck, down arms and shoulder, and to stomach level.
Other pains are often musculoskeletal in origin. If in doubt, particularly if you are having problems catching a breath with associated unrelieved chest pain, for no apparent reason, get a paramedic straight away. Dont drive yourself to the doctors. Early intervention is paramount.
As stated earlier, most people have various arrhthmias normally...