How To Properly Read An ECG

How To Properly Read An ECG


Just like a magician performing the perfect illusion, ECG interpretation, when done right, is an art that anyone can learn. Unlike magic, though, there is no misdirection and everything is straight forward. Let’s review how to properly read an ECG.


Reading the ECG is easy, but it is imperative to follow a system. A systematic approach, such as the one I teach in my video course, ( assures that you will not miss a step nor a diagnosis. Through repetition and muscle memory, a system eventually becomes automatic.


The EEE-ECG Reading Approach


The EEE-ECG Reading Approach which is based on the ACC/AHA ECG reading guidelines is as follows. First, check the patient’s age and gender if available. Next look at the ECG scale. Now, read the heart rate and rhythm, followed by calculating the axes and intervals. Next, you look at the wave morphology: P waves, Q waves, QRS complexes, ST segments; and finally, T waves. All of the diagnoses go into the ECG diagnosis section. Finally, assign clinical diagnoses, if possible. This is no different than the way cardiologists read ECGs, but it is laid out simply. Let’s look at each step in a little more detail.


Diagram outlining the ECG reading approach described in Executive Electrocardiogram Education (
Diagram of a proper approach to reading an ECG


Age and Gender


The ECG technician often types the patient’s name and gender at the top of the ECG.


Rate and Rhythm




Always remember to check both the atrial heart rate (P wave rate) and the ventricular rate (QRS complex rate). Most of the time they are the same, but they can be different. For example, in sinus bradycardia and normal atrioventricular node function, both the atrial rate and the ventricular rate are less than 60 beats per minute. In atrial fibrillation with a controlled ventricular response, the atrial rate is greater than 350 beats per minute and the ventricular rate is between 60 and 100 beats per minute.




In order to accurately name the heart rhythm, you need to look at two factors. Impulse formation is the first and impulse conduction is the second.


Impulse Formation


Impulse formation describes where an impulse originates and the characteristics of the impulse. For example, does the impulse begin in the sinoatrial node, the atria, the atrioventricular node, or the ventricles?


Impulse Conduction


Impulse conduction describes how impulses travel through various cardiac tissue. For example, does the impulse travel normally from the sinus node, through the atria, through the atrioventricular node, into the bundle of His, through the right and left bundle branches, and into the Purkinje fibers?


Naming Rhythms


Ultimately, rhythms are named by where they originate and then something about their rate. For example, a rhythm that originates from the sinoatrial node at a rate greater than 100 beats per minute is called sinus tachycardia. A rhythm that originates from the ventricle at a rate between 40 and 100 beats per minute is called an accelerated idioventricular rhythm (AIVR).


Abnormal Rhythms (Arrhythmias)


Arrhythmias are abnormalities of impulse formation and/or conduction. For example, complete heart block (an impulse conduction abnormality), resulting in a ventricular escape rhythm (an impulse formation abnormality) may occur concomitantly with sinus tachycardia (another impulse formation abnormality). All of these factors go into making the final rhythm diagnoses.




The axis describes the overall direction of impulse conduction. This includes P waves, QRS complexes, and T waves. P waves axis and QRS axis relates to the depolarization of the atria and ventricles, respectively. The T wave axis relates to the repolarization of the ventricles. Understanding the axes helps determine whether ECG wave changes are primary abnormalities or secondary abnormalities.




Overall intervals tell about the rate of conduction through the cardiac tissue. The PR interval tells about conduction from the sinoatrial (SA) node, through the atria, and through the atrioventricular (AV) node. ECG readers should measure the PR interval, QRS interval, QT interval, and then calculate the corrected QT interval.


Wave Morphology


Looking at the waves and their morphology should follow a consistent and precise pattern.


An ECG waveform including a P wave, Q wave, R wave, S wave and T wave
ECG waveform



P Waves


Since heart rhythms generally begin in the sinoatrial (SA) node, P wave analysis is first. For example, is there a left atrial abnormality or right atrial abnormality?


Q Waves


Next, look for Q waves. Q waves often represent myocardial infarctions. If Q waves are found in contiguous leads, look at the ST segments and T waves in these leads to date the myocardial infarction. If Q waves are found in the inferior leads (suggesting inferior wall myocardial infarction), be sure to look at the posterior leads (V1 and V2) for concomitant posterior wall myocardial infarction.


QRS Complexes


Next, look at the QRS complexes. I typically ask and answer six basic QRS complex questions.

  1. Are the QRS complexes tall for left ventricular hypertrophy (LVH)?
  2. Are they small for low QRS voltage?
  3. Are they wide representing a bundle branch block (BBB) or nonspecific intraventricular conduction delay (IVCD)?
  4. Are there tall R waves in lead V1, which may suggest right ventricular hypertrophy (RVH), right ventricular conduction abnormalities, or posterior myocardial infarctions?
  5. Is there poor R wave progression (PRWP) that may suggest an anterior myocardial infarction?
  6. And lastly, is the transition point in the normal spot?


ST Segments


Now you evaluate the ST segments. Look for ST-segment elevation or depression. ST-segment elevation may indicate subepicardial injury whereas ST-segment depression may indicate a subendocardial injury. Be aware though, that these changes may be due to other conditions (e.g. digoxin toxicity) or even secondary changes due to depolarization problems.


T Waves


Evaluate T wave changes next. T wave inversion may indicate myocardial ischemia, however other conditions need to be considered. Examples include subdural hematomas, drug effects, and secondary changes from depolarization abnormalities. Peaked T waves may represent hyperkalemia or hyper-acute changes from an early myocardial infarction.


“Think About” Section


Frequently ECG abnormalities seen during the above process, may not have a diagnosis at that time. Sometimes you will want to come back to an abnormality later to determine an etiology. I put these conditions into a special section that I call the “Think About” section. An example may be left axis deviation. Is it due to left ventricular hypertrophy, an inferior wall myocardial infarction, or a left anterior fascicular block? Left axis deviation may be placed in the “Think About” section so that I do not forget about it later.


Final ECG Diagnoses


As I make diagnoses along the way, I list them in the ECG Diagnosis section. By the time you finish the ECG reading approach, then all of the diagnoses are listed.


Clinical Diagnoses


The purpose of any procedure in medicine is to accurately diagnose and treat your patient. Clinical diagnoses are sought with every ECG. For example, does the patient have ischemia, coronary artery disease, an electrolyte imbalance, a faulty pacemaker, cardiac tamponade, or other conditions? Always try to make a clinical diagnosis if possible.




EKG interpretation is easy when the reader knows the criteria and follows a systematic approach. ECG may seem intimidating at first, but with practice, you can properly read an ECG.