Heart Attack ECG vs Normal

What is an Electrocardiogram (ECG)?

A measure of the electrical activity of the heart is called an electrocardiogram (ECG). Each time the heart beats, it produces electrical signals that are detected by sensors attached to the skin. These signals are displayed by a machine (an electrocardiograph) as an electrocardiogram. An electrocardiogram is done to investigate or monitor conditions affecting the heart, such as chest pains, sudden noticeable irregular heartbeats (palpitations), dizziness, and shortness of breath, and to check the heart’s overall health. Therefore, an ECG can detect arrhythmias (a condition where the heart beats too slowly, too quickly, or irregularly), coronary heart disease (the blockage of the supply of blood to the heart or an interruption by the buildup of fatty substances), heart attack (blood supply to the heart is blocked), and cardiomyopathy (the heart becomes thickened or enlarged).1 

How does an ECG detect a heart attack?

The principle behind the ECG is that during every heartbeat, depolarisation of the heart occurs, which triggers its contraction. This depolarisation results in electrical activity that is transmitted throughout the body and can be detected on the skin. The ECG machine has electrodes (leads) that are attached to the skin, which record this electrical activity and display them graphically. An ECG has ten electrodes, one attached to each limb and six across the chest. The ECG machine then processes the electrical signals received from the different parts of the body through its electrodes and produces a graphical representation of the patient’s heart rhythm.2 The graphic pattern of ECG is in the form of waves, and the logic behind it is that:  

  • Towards a lead (the electrical image of the heart derived from any one electrode combination), electrical activity results in an upward deflection.
  • A downward deflection is produced by electrical activity away from a lead.
  • Depolarization and repolarization deflections occur in opposite directions.

The basic pattern of the electrical activity of the heart comprises the following waves: 2 

  • P wave: This is a small deflection wave that represents atrial depolarization. The P wave is usually very small and barely detectable on an ECG graph. 
  • PR interval: The interval between the P wave's first deflection and the QRS complex's first deflection.
  • QRS wave complex: The QRS complex comprises three (Q, R, and S waves) waves representing ventricular depolarization. The Q waves are tiny waves that correspond to the depolarization of the interventricular septum. These waves can either relate to breathing or an old myocardial infarction. On the other hand, the R wave is the largest wave that reflects the depolarization of the main mass of the ventricles. Finally, the S wave denotes the final depolarization of the ventricles at the base of the heart. A short QRS complex indicates that the conduction system of the heart functions properly, while a broad QRS complex indicates some type of dysfunction in the conduction system. 
  • ST segment: Also known as the ST interval, is the time between the end of the QRS complex and the start of the T wave. This signifies the time of zero potential between ventricular depolarization and repolarization.
  • T wave: This represents ventricular repolarization.
  • QT interval: The time between the star of the Q wave and the end of the T wave which represents the time taken for depolarization and repolarization. 

Source: https://www.aclsmedicaltraining.com/basics-of-ecg/ 

If there is a change in the regular electrical activity of the heart, ECG can detect it by producing an abnormal wave pattern on the recording. 

Normal ECG Pattern

A normal ECG shows the pattern of the wave produced by a heart that is beating in a regular sinus rhythm, which is between 60 to 100 beats per minute. All the waves produced by a normal ECG are at intervals that are within normal ranges. These ranges are as follows: 3

1.  P waves:

  • Leads I, aVF, and V3–V6 are upright.
  • Last for less than or equal to 0.11 seconds.
  • In leads I, II, aVF, and V4 - V6 polarity is positive; in leads V1 and V3 it is diphasic and negative in aVR.
  • The shape is generally smooth, not notched, or peaked.

2.  PR interval normally lasts between 0.12 and 0.20 seconds.

3.  QRS Complex:

  • Lasts less than or equal to 0.12 seconds, amplitude in one standard lead is greater than 0.5mV and greater than 1.0mV in at least one precordial lead. 2.5 to 3.0 mV is the upper limit of the typical amplitude. 
  • Small septal Q waves (amplitude less than 1/3 of the R wave's amplitude in the same lead; duration less than or equal to 0.04 seconds) can be seen in leads I, aVL, V5, and V6.
  • In leads I, II, and V4-V6, the QRS complex is represented by a positive deflection with a large upright R. At the same time, there is a large deep S in aVR, V1, and V2 which proceeds from V1 to V6. 
  • While the R wave gets taller, the S wave gets smaller. At V3 or V4 these waves are usually equal, a situation known as a transitional zone.

4.  ST segment:

  • Isoelectric, slanting upwards to the T wave.
  • Can be slightly elevated (up to 2.0 mm in some precordial leads).
  • Never normally depressed greater than 0.5 mm in any lead.

5.  T wave

  • T wave and QRS complex deflection should be in the same direction. 
  • Symmetrical, typically circular, and with an ascent that is more gradual than a fall.
  • It should be upright in leads V2 - V6 and inverted in aVR.
  • In leads V3 and V4, the amplitude is at least 0.2mV.
  • Isolated T wave inversion in an asymptomatic adult is generally a normal variant.

6.  QT interval: Lasts less than or equal to 0.40 seconds in males and 0.44 seconds in females.

ECG pattern after a heart attack

During a heart attack (myocardial infarction), the pattern of waves produced by ECG changes as follows: 4 

  • Pathological Q waves: This is a Q wave that is longer than 0.4 seconds in duration and is 25% larger than the size of the following R waves leads other than III and Avr. The presence of new pathological Q waves is indicative of an acute myocardial infarction because pathological Q waves may take hours to develop and can last for a long period of time. On the other hand, the simple presence of Q waves doesn’t necessarily signify a myocardial infarction.
  • ST segment changes: The presence of an elevated ST-segment is one of the most significant indicators of myocardial infarction. During a myocardial infarction, the J point (the point where the end of the Q wave and the ST segment meet) is consistently greater than 2mm above the baseline. In leads that face the damaged area of the heart, the ST segment may be depressed during the acute phase of non-ST segment elevation myocardial infarction (NSTEMI). An ECG alone cannot identify an NSTEMI. 
  • At the anterior wall, myocardial infarction is characterised by poor R-wave progression, ST-segment elevation, and T-wave inversion.
  • Myocardial infarction at the septal wall is characterised by the disappearance of the R-wave, a rise in the ST-segment, and inversion of the T-wave.
  • The ST-segment elevation occurs at the lateral wall during a myocardial infarction.
  • T-inversion and ST-segment elevation signify myocardial infarction at the inferior wall.
  • Myocardial infarction at the posterior wall is characterised by tall R-waves, ST-segment depression, and upright T-waves.

Differences between patterns of a normal ECG and an ECG after a heart attack

Can an ECG detect a previous heart attack or a future heart attack?

Considering that one may have a heart attack without experiencing any signs and symptoms, it is possible that you might have had a heart attack without knowing. An ECG has the potential to detect whether you suffered from a heart attack in the past or not. This is shown in abnormal patterns during the test, suggesting that a part of the heart may have been damaged due to a lack of oxygen. A study revealed that when compared to cardiac magnetic resonance imaging (MRI), an ECG has the following characteristics in the identification of previous heart attacks: 5

  • Poor sensitivity: Identifies that there was a previous heart attack 48.4% of the time.
  • Good sensitivity: Identifies that there was no previous heart attack 83.5% of the time.
  • Positive predictive accuracy: There was a 72% chance that an ECG results will show that someone actually had a heart attack.
  • Negative predictive accuracy: ECG results suggested that people who did not previously have a known heart attack had a 64.2% chance of not having actually had a heart attack in the past.

Due to poor sensitivity and modest negative predictive accuracy, ECG cannot be used alone to detect a previous heart attack.

Again, an ECG can be used to detect future heart attacks by uncovering some abnormalities in the electrical activity of the heart. Studies have shown that the risk scores for cardiovascular disease based on ECG are as good or better than those obtained from patients’ histories.6 Another study showed that people with arteriosclerotic diseases or multiple risk factors for heart attack usually have abnormal ECG readings.7

Can you have an undetected heart attack with a normal ECG reading?

It should be noted that an ECG has limited ability to detect previous as well as predict future heart attacks. Therefore, it is recommended that other tests be performed alongside an ECG for proper diagnosis. In addition, an ECG cannot show asymptomatic blockage in the arteries that may put you at risk for a future heart attack. This, therefore, suggests that people can have a heart attack with a normal ECG reading. 6

Other tests for the diagnosis of a heart attack

Other tests that could be used in the diagnosis of a heart attack, in addition to an ECG, include:

  • Holter monitor: An ECG that tracks your heart's electrical activity for 24 hours or longer as you go about your regular daily activities.
  • Blood tests: Blood markers such as troponin are measured to determine if you’ve had a heart attack. After a heart attack, this protein can remain in the blood for about 2 weeks.
  • Coronary CT angiogram: X-rays are used in a coronary CT angiography to create an image of the arteries supplying blood to your heart. An injection of a particular dye into your bloodstream enables a physician to observe the dye as it travels through your arteries.
  • Cardiac catheterization: Through a skin incision, a long tube known as a catheter is placed into the artery that supplies blood to your heart. Your blood is infused with a contrast dye so the doctor can inspect your heart.
  • Echocardiogram: An echocardiogram shows a real-time image of your heart using ultrasound waves. If one area of your heart isn't pumping blood as well as it should, the image can inform your doctor.
  • Magnetic resonance imaging (MRI): A 3-D image of your heart is created by a cardiac MRI using radio waves and powerful magnetic fields. An MRI enables the doctor to determine whether there is inadequate blood flow to a certain place or whether your heart may be injured in some way.


An ECG is a measure of the electrical activity of the heart. This electrical activity is shown graphically in the ECG machine as a pattern of waves which can be interpreted to determine the health of the heart. Through an ECG recording of the pattern of waves, one can determine whether the heart is normal or whether one is experiencing a heart attack. Moreover, an ECG can be a useful addition to other tests to detect previous and/or future heart attacks.


  1. NHS. Electrocardiogram (ECG). [Online].; 2021 [cited 2022 June 30. Available from: https://www.nhs.uk/conditions/electrocardiogram/.
  2. Ashley EA NJ. Cardiology Explained: London: Remedica; 2004.
  3. Queens University, School of Medicine. Analysis and Interpretation of the Electrocardiogram. [Online]. [cited 2022 June 30. Available from: https://elentra.healthsci.queensu.ca/assets/modules/ts-ecg/normal_ecg.html.
  4. ACLS Medical Training. ECGs in Acute Myocardial Infarction. [Online]. [cited 2022 June 30. Available from: https://www.aclsmedicaltraining.com/ecg-in-acute-myocardial-infarction/.
  5. Paul TK, Mukherjee D. Silent myocardial infarction and risk of heart failure. Ann Transl Med. 2018 Nov;6(Suppl 1):S35.
  6. Miller AC,OZ,&MS. A Comparison of Patient History- and EKG-based Cardiac Risk Scores. AMIA Joint Summits on Translational Science. 2019;82–91.
  7. Krittayaphong R, Muenkaew M, Chiewvit P, Ratanasit N, Kaolawanich Y, Phrommintikul A; CORE Investigators. Electrocardiographic predictors of cardiovascular events in patients at high cardiovascular risk: a multicenter study. J Geriatr Cardiol. 2019 Aug;16(8):630-638.
This content is purely informational and isn’t medical guidance. It shouldn’t replace professional medical counsel. Always consult your physician regarding treatment risks and benefits. See our editorial standards for more details.

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Fuanyi Awatboh

M.Sc. in Epidemiology and Control of Infectious Diseases, University of Buea, Cameroon

I am a professional Quality Assurance Mentor with Global Health Systems Solutions. My job is to enhance HIV testing, Care, and treatment by making sure that all entry points in the three hospitals under me follow standard testing algorithms, have all necessary tools and equipment needed for testing, train testers if need be, and monitor that all positive cases are under treatment among other duties. I have also mentored laboratories to increase their quality of service and standards of operations.

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