How Long After a Heart Attack Can an ECG detect it?
The factors that determine an ECG's ability to detect a heart attack are the timing of the test and any pre-existing medical conditions. Early testing within minutes to hours of a heart attack identifies acute changes, such as ST-segment elevation.
ECG must be performed as soon as possible after symptoms appear. The sooner it is done, the greater the possibility of revealing changes. Delayed testing may miss these initial signs as they subside over time.
When part of the heart is damaged, its functions are affected. ECG detects and records these changes. Do these changes appear instantaneously or take some time to appear? In this blog, we will discuss how long it takes after a heart attack for changes to be detected on an ECG.
What Causes a Heart Attack?
Coronary arteries supply blood to the heart. A heart attack occurs when the blood flow through the arteries is blocked or reduced. The blocked flow damages a part of the heart muscle, leading to changes within the heart that can be recorded on the ECG.
Stages of a heart attack
Based on the severity of damage to the tissues, heart attacks are classified into four stages.
Stage 1: Medically termed the aborted MI. It involves no death of heart tissue.
Stage 2 is when some cardiomyocyte cells, responsible for forming heart muscle, are killed. The tiny blood vessels around the heart are undamaged in this stage.
Stage 3 is when cardiomyocytes (cells that form heart muscle) are damaged, and a blockage occurs in the small blood vessels.
Stage 4: When maximum damage to the heart and blood vessels occurs.
Timely treatment restores the blood supply within an hour of the start of symptoms, aborting the myocardial infarction (MI). Early medical help reduces the extent of damage, resulting in normal heart function. There is no damage to the heart muscles; this is also the reason to seek medical help early if a person suspects a heart attack, even if the symptoms are not severe.
Symptoms
Heart attacks cannot always be recognised; some attacks present as severe chest pain, while others may go unnoticed. Symptoms may not always indicate a heart attack. Some common symptoms are:
- Pressure, tightness, pain, squeezing, or aching of the chest.
- Pain is referred to other regions of the body - the shoulder, arm, back, neck, jaw, and sometimes even to the belly.
- Cold sweat.
- Fatigue.
- Heartburn or indigestion.
- Lightheadedness or dizziness.
- Shortness of breath.
- Sudden development of loud snoring, choking, or gasping during sleep.
- A persistent cough.
- Swollen ankle, legs, and feet.
- Palpitations or irregular heartbeat.
Symptoms in women
In women, these symptoms are brief and less specific, such as nausea, vomiting, shortness of breath, back pain, and jaw pain.
Confirming a heart attack
Myocardial infarction is confirmed when a patient presents with any two of the following symptoms or signs.
- Clinical chest pain.
- Elevated cardiac markers in blood (troponin I, CK-MB, Myoglobin).
- Changes in serial ECG.
Diagnosing a Heart Attack
ECG is a primary tool to confirm a heart attack. Other tests used to diagnose a heart attack are blood tests and cardiac imaging.
Cardiac biomarkers
When the heart is damaged or stressed, cardiac biomarkers are released into the blood. The elevation of markers in blood confirms a heart attack.
They are
- The troponin test is the most sensitive blood test for detecting heart muscle damage 12 hours after the attack.
- Creatine kinase (CK-MB) measures enzymes specific to heart muscle; their level rises 10 to 24 hours after the heart muscle is damaged.
- The myoglobin test is a less specific protein test, but it tests positive within two hours.
Electrocardiogram
An ECG records the heart's electrical activity. It generates a waveform on the tracing sheet.
- ST-segment elevation indicates a blockage in the coronary artery.
- ST depression indicates partial obstruction or narrowing of the coronary artery.
- T wave inversion.
- Presence of pathological Q waves.
Pathological Q waves
A Q-wave more than .04 seconds in duration and more than 25% of the size of the following R waves
- Pathological Q Waves take hours to develop and last for a long time.
- New pathological waves on ECG indicate an acute myocardial infarction.
ST-segment changes
- ST-segment elevation occurs in myocardial infarction.
- The ST-segment is the end of the S Wave and ends at the beginning of the T wave.
- The point where the end of the Q Wave and the ST segment meet is the J Point. If the J Point is greater than 2 millimeters above the baseline, it is consistent with ST-segment elevation myocardial infarction.
- ST-segment depression occurs in non-ST-segment elevation myocardial infarction, NSTEMI.
Imaging
The common tests done to assess heart blockage and muscle damage are:
- An echocardiogram uses sound waves to create live images of the heart.
- Detailed images of the heart structure to pinpoint abnormalities are made using Cardiac CT.
- Cardiac MRI, or magnetic resonance imaging, uses magnetic fields and radio waves to create images of soft tissue damage and scarring in the heart.
- Coronary angiography, also known as cardiac catheterization, involves injecting a dye into the arteries through a catheter inserted into the groin to produce coloured images of the heart.
- Nuclear stress test: It uses a radioactive tracer injected in a small amount to track blood flow to the heart during physical activity or rest.
How Long Does it Take for a Heart Attack to Test Positive Through a Blood Test?
As the heart muscle is damaged, protein levels rise. A troponin blood test can detect a heart attack. The levels rise 3 to 6 hours after the heart attack and attain a maximum level 12 to 24 hours after it.
Its level remains elevated for up to one to two weeks after the attack. The creatine kinase (CK-MB) test is less specific than troponin; its level rises after a heart attack but returns to normal within three days.
How Long do Markers Stay in the Blood After a Heart Attack?
When the heart muscle is damaged, it releases proteins called cardiac enzymes, troponin, and creatine kinase, into the blood. These enzymes can be present for about 4 to 9 hours after a heart attack begins.
Troponin is the most important marker; it rises within two to three hours, peaks around 12 to 24 hours, and can remain high for up to two weeks. A troponin test measures the troponin T or I proteins in the blood. These proteins are released when the heart muscle is damaged during a heart attack. The more damage to the heart, the greater is the amount of troponin T and I in the blood.
Young and healthy adults have little or no troponin in their blood.
- The level of Troponin T is less than 0.01 nanograms per ml.
- The level of Troponin I is less than 0.12 nanograms per ml.
Creatine kinase (CK) also rises after a heart attack and peaks between 24 and 48 hours. It returns to its normal values within five to 14 days. A repeat blood test is done 4 to 6 hours after the first one to confirm the diagnosis. Normal troponin levels, twelve hours after the chest pain, make a heart attack unlikely.
Factors Influencing the ECG in Heart Attack Detection
- Individual differences, such as abnormalities in heart structure and valves, can affect the diagnosis.
- The timing of the test: the earlier a test is done, the greater the chances of detecting changes.
- Pre-existing conditions, such as diabetes and hypertension, alter the heart’s electrical activity.
Recovery time
It varies depending on the treatment and severity of the heart attack and whether a person has undergone any surgical procedure. Most people can return to work within two to three months; however, people with mild attacks or who received treatment can return earlier after recovery. Patients should follow the treatment plan and lead a healthy lifestyle.
Conclusion
A heart attack progresses from plaque buildup to atherosclerosis and ends with myocardial damage. The best time to detect a heart attack is within two to three hours after the symptoms appear. A delay in restoring the blood supply decreases the chance of survival due to the damage to the heart muscles.
Here to answer all your questions
Yes, an ECG can detect an old or a previous heart attack by identifying persistent changes in the heart’s electrical pattern.
An ECG can predict the future risk of a heart attack by revealing abnormalities in the heart’s electrical activity.
Yes, an ECG can detect a silent heart attack by revealing changes in the heart's electrical activity.
An ECG provides evidence for a heart attack, but is not definitive on its own. We need certain blood investigations and even imaging to confirm the diagnosis.
An ECG can predict the future risk of a heart attack by revealing abnormalities in the heart’s electrical activity.
Blood tests like troponin CK-MB and imaging tests such as echocardiograms or coronary angiograms are used along with an ECG to confirm a heart attack.
