Limitations of ECG /EKG Machines
ECG or EKG machine is a very powerful and common screening tool for heart diseases. It is relatively inexpensive, non-invasive and easy to use. It does have some limitations. Understanding those limitations is important to put things in proper perspectives.
The ECG monitor displays the electric activities generated by the heart and gives a snapshot of the heart rate and rhythm during the test; however it does not reflect many underlying problems. Cardiac abnormalities may occur only intermittently and ECG needs to be performed at the "right" moment to capture the episodes. At other times, a patient may have entirely normal ECG.
To compensate for this, many ECG are performed when the patient is exercising, which increases heart rate and put the heart under stress. The exercise stress ECG test, in many situations, can reveal many hidden conditions which otherwise would not be detected.
When the ECG machine displays an unusual pattern, there can be multiple unrelated reasons, including a normal variant. A doctor needs to do more detailed investigation, including other tests (e.g. echocardiogram), to sort things out.
False negative is probably the biggest concern with ECG. For some heart patients, the EKG may be entirely normal and yet their conditions should be reflected in the ECG. The reasoning behind this is not well understood. A good ECG reading does not preclude having the underlying heart disease and other symptoms, such as chest pain, must be taken into account and further evaluation may be required.
Not all heart problems will show up on an ECG. A prime example is vulnerable plaque (a form of atheroma). Vulnerable plaque is a fast growing deposit or degenerative accumulation of lipid-containing plaques on the innermost layer of the wall of an artery. Because artery walls typically enlarge in response to enlarging plaques, they do not affect blood flow and cannot be detected even in a cardiac stress ECG test. Yet vulnerable plaque is a major cause of heart attacks.
A stress ECG test requires high-grade stenosis to show positive reading. High-grade stenosis is a good indicator of advanced heart disease; however it is not the major cause of heart attack.
False positives and false negatives are common among clinical tests. A doctor has to weight all the evidence prior to making a diagnosis.