An Electrocardiogram (EKG or ECG) is a non-invasive diagnostic tool that records the heart’s electrical impulses. Healthcare providers use it to evaluate heart health, offering insights into its rate, rhythm, and overall function. The EKG translates these electrical signals into a visual representation, helping identify potential heart conditions.
Understanding How an EKG Works
The heart generates its own electrical signals, originating from the sinoatrial node in the right atrium. These impulses spread throughout the heart muscle, causing the atria and then the ventricles to contract. An EKG machine detects these electrical changes on the skin’s surface through electrode patches placed on the chest, arms, and legs.
The machine translates these signals into a graph. The P wave represents electrical activity spreading through the upper heart chambers (atria), initiating their contraction. The QRS complex shows impulses moving through the lower heart chambers (ventricles) as they contract. The T wave signifies the electrical recovery or “resetting” of the ventricles, preparing them for the next beat.
Identifying Signs of a Past Heart Attack
An EKG can provide evidence of a past heart attack, also known as a myocardial infarction, by detecting lasting changes in the heart’s electrical activity. When heart muscle is damaged or scarred due to a lack of blood flow, it becomes electrically inactive. This “electrically silent” tissue alters electrical conduction, which can be reflected on the EKG tracing.
The most significant indicator of a previous heart attack on an EKG is the presence of “pathological Q waves.” While small Q waves can be normal, pathological Q waves are deeper and wider, measuring more than 0.04 seconds in duration or over 25% of the height of the subsequent R wave. These abnormal Q waves indicate an area of electrically dead or scarred tissue, emerging several hours to days after the event and persisting indefinitely. Pathological Q waves in at least two anatomically contiguous leads help pinpoint the location of the past damage within the heart.
When an EKG Might Not Show a Past Heart Attack
Despite its utility, an EKG has limitations in detecting all past heart attacks. Not every myocardial infarction leaves clear, permanent changes visible on an EKG. Factors such as the size and specific location of the heart attack can influence whether it registers on the test. For instance, smaller heart attacks or those affecting certain areas may not produce the characteristic pathological Q waves.
Some heart attacks are classified as “non-Q wave” infarctions, meaning they do not result in pathological Q waves on the EKG, even though heart muscle damage has occurred. The time elapsed since the event can also play a role, as some EKG changes might diminish over extended periods, though pathological Q waves persist. Therefore, a seemingly normal EKG does not definitively rule out an undiagnosed heart attack, and EKG interpretations can be “false positives” due to normal variations in heart position or electrode placement.
Additional Tests to Detect Past Heart Damage
When an EKG is inconclusive or further confirmation of past heart damage is needed, healthcare professionals employ other diagnostic tools. An echocardiogram, which uses sound waves to create moving pictures of the heart, provides detailed information about its structure, function, and blood flow through its chambers. This test can identify areas of weakened heart muscle or scarring that might indicate a past heart attack, even if not evident on an EKG.
Cardiac MRI, or magnetic resonance imaging, offers detailed images of the heart muscle and surrounding tissues. This advanced imaging technique can identify areas of damaged or scarred heart tissue, assessing any previous injury. Stress tests, which involve monitoring the heart’s activity during physical exertion or with medication, can reveal how well blood flows through the heart arteries and detect any reduced blood flow to the heart muscle, potentially indicating prior damage or blockages.