Sheesh, if we queried every errant looking ECG, we'd have the cardiologists here flat out 24/7, but saying that, are a great diagnostic tool. They are able to pick up ischemia (dead and scarred tissue), abherrant activity or lack of electrial activity to the heart muscle.
Quick overview of an EGC....
It is a picture of the hearts electrical activity and can be "observed" from a number of different angles. Something like looking around the exterior of an aircraft on preflight. If you stand and look at the front, you cant see the tail, so to see it, you walk around the back, and so it is with reading ECG's. Each lead has a specific placment and in reference to other leads creates a line of electrical activity on the portion of cardiac muscle it is "looking" at.
Thus the multiple lines on the printout.
Hearts have four chambers, two atria up the top and two ventricals on the bottom. The left side is much thicker and larger than the right, as this is the pump that supplies the aorta and thus the blood supply to the body. The whole electrical supply to the heart tissue depends on an effective chemical reaction to take place. Left side thickening is common in athletes.
The electrial heart function is a chemical reaction and very basically involves Calcium, Potassium and Sodium. Any abnormality such as dehydration, too much or too little potassium or changes in calcium levels will affect the conductivity of the heart. This is where the thyroid changes or kidney disease can affect heart activity.
Other major and common factors involve lack of effective oxygen to the cells, and either too high or too low blood glucose levels. Oxygen and glucose supply the energy that the cell needs to function. Water is a mediator - a transport agent if you will.
The initial electrical activity seen in the wave form is the Sino-atrial node activating all the excitable cardiac cells which in turn contracts the atria. There is then a short pause while the atria fill the ventricals. This is then followed by a triggering of the Atrio-ventricular node a short time later as the electrical wave hits it. This in turn fires down the septum of the heart (the layer of muscle between the ventrials) creating the QRS complex, then after the electrical activity has gone down the septum and continued around the outside of the ventrical thus contracting them, they repolarise creating the T wave, after another short pause as the ventricals empty and the atria refil. This is then followed by a longer pause, and the process starts again.
Any one of the cells has the capability of self firing if required, without the stimulation of the node initialising an electrical "wave". If the cells bellow a damaged area dont get a stimuli, they fire off by themselves, usually at a different rate than the nodes firing rate, thus the electrical activity looks different to the standard norm.
For example if the sino atrial node does fire but the area around the atrio-ventricular node is damaged, the ventricals will fire off after waiting for the message and without the required stimulation, thus creating a lag between electrical activity and hopefully an effective contraction of the ventricals. If the damage is extensive enough large portions may fail to fire, or fire at abnormal rates causing abnormal rhythms. This is one of the results of heart disease leading to heart failure.
This is great in one respect, our pump has three ways of continuing operation if one of the nodes or bundles fail. Good for human survival, bad when the doc sees the ECG. Some misfiring can be caused by chemical imbalances, some by heart muscle damage, others can be genetic and so on. Some folks will be completely assymptomatic. (no obvious symptoms)
The normal wave structure the Docs are looking for is a nice P wave (atrial contraction), QRS complex (the ventricals contracting and the atria repolarise at this point as well, but cannot usually be seen as the electrial conductivity of the ventricals is much stronger) and a T wave (the ventricular repolarisation phase). Each segment is broken down into time lapses for which a standard has been developed.
One good indicator of heart disease is abnormalities within the various segments of the wave form.
Some anormalities occur simply with dehydration, stress, weight if the patient is carrying heaps up top, any sweating, deviation in the placement of the leads, electrical activity within the vicinity, movement etc and the list goes on...
The diagnostic facility in most 12 lead ECG's is brilliant and is designed to bring attention to what may be entirely normal for that patient. Any changes to ECGs from the patients norm are usually investigated fairly thoroughly, especially if associated with chest or thorasic back pain, left shoulder pain, or shortness of breath either during exercise or at rest.
Hope I havent bored everyone, apart from flying, ECG's are a pet topic.....