Hello everyone, I'm Olivia from Geeky Medics. Today I'm going to take you through how to interpret an ECG. If you haven't already, make sure you check out our video about how to record an ECG and subscribe to this channel so you can be notified about new videos. In this video, we're going to work through a structured approach to ECG interpretation. At the end of the video, we'll work through a case for you to test your ECG interpretation skills.
Let's start by discussing the basic principles of an ECG. An ECG is used to record the electrical activity of the heart from different angles to identify and locate pathology. ECGs are recorded by placing electrodes on the patient. These are conductive pads that record the electrical activity of the heart. These electrodes create a graphical representation of the heart's electrical activity, which we refer to as the ECG leads.
It's worth noting that a 12-lead ECG produces 12 separate graphs on a piece of ECG paper. However, only 10 physical electrodes are actually attached to the patient to generate the 12 leads. Here we can see the different components of an ECG cycle.
P waves represent atrial depolarization, or contraction. In healthy individuals, there should be a P wave preceding each QRS complex. The PR interval begins at the start of the P wave and ends at the beginning of the Q wave.
It represents the time for electrical activity to move between the atria and the ventricles. The QRS complex represents the depolarization or contraction of the ventricles. It appears as three closely related waves on the ECG, the Q, R and S wave.
The ST segment is an isoelectric line representing the time between depolarization and repolarization of the ventricles. The T wave represents ventricular repolarization. It appears as a small upwards deflection after the QRS complex. The QT interval begins at the start of the QRS complex and finishes at the end of the T wave. It represents the time taken for the ventricles to depolarize and then repolarize.
Before beginning ECG interpretation, you should always check the following details. Firstly, confirm the name and date of birth of the patient and check this matches the details on the ECG. Check the date and time that the ECG was performed. Check the calibration of the ECG.
You should also understand the clinical context and why the ECG is being performed. For example, does the patient have any symptoms such as chest pain or breathlessness? Knowing the clinical context and the indication for performing the ECG will help you in your overall interpretation.
The first step to our structured approach to ECG interpretation is to calculate the patient's heart rate. As a reminder, a normal heart rate is between 60 to 100 beats per minute. Tachycardia, or a fast heart rate, refers to a heart rate that is greater than 100 beats per minute.
Bradycardia, or a slow heart rate, refers to a heart rate that is greater than 100 beats per minute. that is less than 60 beats per minute. There are two methods for calculating the heart rate and the choice of these depends on whether the patient has a regular or irregular heart rhythm. Let's look at the first method using the RR interval. This method is suitable for regular rhythms.
To use this method, count the number of large squares within one RR interval. Next, divide 300 by this number to calculate the heart rate. In this example, there are 7 large squares, therefore the calculation is 300 divided by 7 which equals 48. This second method is useful when the heart rhythm is irregular.
In this situation, we can't use the RR interval method as the RR interval will vary. To use this method, you need to make sure you're dealing with a standard ECG strip, which is typically 10 seconds long or 50 large squares. Count the number of QRS complexes on the rhythm strip.
After you've done this, multiply the number of QRS complexes by 6, giving you the number of QRS complexes within 1 minute. In this example, we can see we have 11 QRS complexes. Therefore, when we times this by 6, we get a heart rate of 66 beats per minute. As we've mentioned, some patients can have regular or irregular heart rhythms.
Irregular rhythms can be either regularly irregular, i.e. a recurrent pattern of irregularity, or irregularly irregular, i.e. completely disorganised. An example of this is atrial fibrillation. To assess whether a rhythm is regular, mark out several consecutive RR intervals on a piece of paper. Then, move them along the rhythm strip to check if the subsequent intervals are similar. Here are two examples of irregular heart rhythms.
Let's look first at the top ECG. You can see the irregularly irregular rhythm is very disorganised with variable RR intervals. This strip shows atrial fibrillation.
The irregularly irregular rhythm is different. and there is a recurrent pattern of irregularity. These rhythms can be seen with atrioventricular blocks, which we will discuss later in the video.
Let's look now at the cardiac axis. The cardiac axis represents the overall direction of electrical activity as it spreads through the cardiac conduction system. In healthy individuals, you would expect the cardiac axis to lie between minus 30 degrees and plus 90 degrees. Before we continue, There are some important concepts to understand. Firstly, whenever the net direction of electrical activity is towards a particular ECG lead, you should see a positive deflection in that lead of the ECG.
Whenever the net direction of electrical activity is away from a particular ECG lead, you should see a negative deflection in that lead on the ECG. So, let's apply these concepts to a normal cardiac axis, which is seen in the diagram on this slide. In the context of a normal cardiac axis, the axis normally lies between minus 30 degrees and plus 90 degrees, which you'll see represented by the yellow arrow on the diagram. As a result, you'll see a positive deflection in lead 1 and lead 2, with the most positive deflection being in lead 2, as it is most closely aligned to the overall direction of electrical spread. These are the leads you should pay the closest attention to, as if these are both positive, the cardiac axis is normal.
Lead III may be slightly positive, isoelectric, or rarely ever so slightly negative. You would expect to see the most negative deflection in AVR due to AVR looking at the heart in the opposite direction. So now we understand the normal cardiac axis, let's move on and look at when this is abnormal.
Right axis deviation involves the direction of depolarization being distorted to the right, meaning it ends up between plus 90 degrees and plus 180 degrees on the cardiac axis. The most common cause of right axis deviation is right ventricular hypertrophy. Extra right ventricular tissue results in a stronger electrical signal being generated by the right side of the heart.
This causes the deflection in lead 1 to become negative and the deflection in leads AVF and lead 3 to be more positive. Right axis deviation is commonly associated with conditions which result in the development of right ventricular hypertrophy such as pulmonary hypertension. Right axis deviation can, however, be a normal finding in very tall individuals. Left axis deviation involves the direction of depolarization being distorted to the left, meaning the electrical signal travels between minus 30 degrees and minus 90 degrees. Lead 1 becomes positive, whilst in lead 3 there is a negative deflection.
Do know, however, that this is only considered significant if the deflection of lead 2 also becomes negative. Left axis deviation is usually caused by left ventricular hypertrophy or conduction abnormalities. The next step in our ECG interpretation process is to look at the P waves, which represent atrial depolarization. We need to ask the following questions. Firstly, are P waves present?
If so, is each P wave followed by a QRS complex? Next, do the P waves look normal? Check the duration, direction, and shape of the P wave.
Finally, if P waves are absent, is there any atrial activity? As a reminder, a sawtooth baseline represents flutter waves, a chaotic baseline represents fibrillation waves, and finally a flat line represents no atrial activity at all. Let's now discuss the PR interval in more detail.
The PR interval should be between 120 and 200 milliseconds, that is three to five small squares. A prolonged PR interval suggests the presence of atrioventricular delay, or an AV block. Let's look at some examples of AV blocks.
First degree AV blocks involves the consistent prolongation of the PR interval, defined as being greater than 200 milliseconds. This is due to delayed conduction via the atrioventricular node. If we look at the ECG, we can see that every P wave is followed by a QRS complex and there are no dropped complexes. This is unlike some other forms of AV block, which we'll discuss later.
First-degree AV block is common and can often be an incidental finding, with patients usually being asymptomatic. Second-degree AV block type 1 is also known as Mobitz type 1 AV block, or Wenkeback phenomenon. Typical ECG findings in Mobitz type 1 AV block include progressive prolongation of the PR interval until eventually the atrial impulse is not conducted and the QRS complex is dropped. AV nodal conduction resumes within the next beat.
and the sequence of progressive PR interval prolongation and the eventual dropping of a QRS complex repeats itself. Second-degree AV block type 1 is usually benign and rarely causes hemodynamic compromise. Usually, no intervention is required if the patient is asymptomatic. Second-degree AV block type 2 is also known as Mobitz type 2 AV block.
Typical ECG findings in Mobitz type 2 AV block Include a consistent PR interval duration with intermittently dropped QRS complexes due to a failure of conduction. The intermittent dropping of the QRS complexes typically follows a repeating cycle after every third P wave in a 3-to-1 block or after every fourth P wave in a 4-to-1 block. Mobitz type 2 AV block is always pathological, with the block typically occurring at either the bundle of His, which occurs in 20% of cases, or the bundle branches in 80% of cases.
Patients are at risk of progressing to complete AV block. The underlying cause of the AV block therefore should always be investigated. Third degree or complete AV block occurs when there is no electrical communication between the atria and ventricles.
In other words, the atria and the ventricles are functioning independently. If the PR interval is shortened this can mean one of two things. Simply, The P wave originates from somewhere closer to the AV node, so the conduction takes less time. Remember the sinoatrial node is not in a fixed place and some other people's atria are smaller than others. The other reason is that the atrial impulse is getting to the ventricle by a faster shortcut, instead of conducting slowly across the atrial wall.
This accessory pathway can be associated with a delta wave, which we will discuss next. This ECG shows a Wolff-Parkinson white pattern. which is typically associated with Wolff-Parkinson-White syndrome. In Wolff-Parkinson-White, an accessory pathway leads to stimulation of the ventricles. This accessory pathway enables electrical conduction to bypass the AV node and stimulate the proximal ventricles prematurely.
We call this pre-excitation. This, in addition to normal electrical conduction through the AV node, leads to double excitation of the ventricles. On this ECG, we can see a shortened PR interval, a delta wave and a widened QRS complex. A delta wave is a slurred upstroke of the QRS.
Patients with Wolff-Parkinson-White syndrome are at a risk of developing tachyarrhythmias, which are abnormal heart rhythms with a ventricular rate of 100 or more beats per minute. Let's move on and look at QRS complexes in more detail. The QRS complex represents the depolarization of the ventricles.
When assessing a QRS complex, you need to pay attention to the following characteristics. Firstly, look at the width. A normal QRS complex should be less than 0.12 seconds, or three small squares. Look at the height to see if there are small complexes or tall complexes. Small complexes are defined as being less than 5 millimetres in the limb leads or less than 10 millimetres in the chest leads.
Tall complexes imply ventricular hypertrophy. although this can be due to body habitus, such as in tall, slim people. Look at the morphology, see if you can see a delta wave.
Broad QRS complexes occur if there is an abnormal depolarisation sequence, for example a ventricular ectopic, where the impulse spreads slowly across the myocardium from the focus in the ventricle. Similarly, a bundle branch block results in a broad QRS complex because the impulse gets to one ventricle rapidly down the intrinsic conduction system, and then spreads slowly across the myocardium to the other ventricle. Let's look at the ECG features of bundle branch block in more detail. In both forms, the hallmark feature is broad QRS complexes. The William-Marrow mnemonic can be used to quickly recognise left and right bundle branch blocks by looking at lead V1 and V6.
The middle letters of the names help you remember which bundle branch block each name is referring to. Note the two Ls in William in the left bundle branch block. and the two Rs in marrow in the right bundle branch block. Here we can see the characteristic features of right bundle branch block, with an RSR prime pattern in V1, seen as an M shape, and a broad S wave in V6, seen as a W shape. In contrast, in left bundle branch block, there is a deep S wave in V1, which may be notched and seen as a W, and a broad M-shaped R wave in V6.
The ST segment is the part of the ECG between the end of the S wave and the end of the R wave. and the start of the T wave. In a healthy individual, it should be an isoelectric line that is neither elevated nor depressed.
Abnormalities of the ST segment indicate ischemia or infarction of the myocardium, seen in acute coronary syndromes. High take-off or benign early repolarization is a normal variant that causes a lot of angst and confusion, as it looks like ST elevation. Here we can see an example of ST elevation.
ST elevation is significant when it is greater than 1 mm or 1 small square in 2 or more contiguous limb leads or greater than 2 mm in 2 or more chest leads. It is most commonly caused by acute full thickness myocardial infarction, also known as a STEMI. ST depression that is 0.5 mm or greater in 2 or more contiguous leads indicates myocardial ischemia.
This may be seen in an N STEMI. It's important to understand which leads represent which anatomical territory of the heart, as this allows you to localise pathology to a particular heart region. For example, if there is ST elevation in leads V3 and V4, it suggests an anterior myocardial infarction. You can then combine this with your anatomical knowledge of the heart's blood supply to determine which artery is likely to be affected. T waves represent the repolarisation of the ventricles.
Tall T waves can be associated with hyperkalemia or a hyperacute STEMI. T wave inversion can represent normal physiological variation or underlying pathology. Isolated T wave inversion in leads AVR and V1 is normal.
Other normal variants include isolated T wave inversion in leads III, V2 and V3. New T wave inversion as compared to a patient's prior ECGs should always be treated as pathological. Pathological T wave inversion is often a non-specific sign in the context of acute illness. However, it can be a more specific sign of conditions such as myocardial ischemia or myocarditis. U waves are not a common finding.
The U wave is a greater than 0.5 mm deflection which is seen after the T wave and best visualised in lead V2 or V3. These become larger the slower the bradycardia. Classically, U waves are seen in various electrolyte imbalances, hypothermia, and secondary to antiarrhythmic therapy, such as digoxin or amiodarone.
So this concludes our structured approach to interpreting an ECG. Once you've interpreted an ECG, it's important to document your interpretation in the notes. You should include the patient's details, the time and date the ECG was performed, your interpretation of the ECG.
and your overall impression and plan. Let's have a look at a case study to put our ECG interpretation skills to the test. Take a moment now to pause the video and we'll go through the answers afterwards. Of course remember first of all you'd always be checking the patient details, the clinical context as to why you're performing the ECG, and making sure that the ECG is calibrated. If we look at the rate there are 17 QRS complexes.
Therefore, when we times this by 6, it gives a rate of 102 beats per minute. If we look at the rhythm, it is clearly irregular, and there aren't any clear patterns between the RR intervals. Therefore, it's irregularly irregular. If we look at the cardiac axis, we can see that lead 1 and lead 2 are positive.
This means it is a normal cardiac axis. If we look at calculating the PR interval, You'll soon realise that there aren't actually any P waves on this ECG, therefore we can't calculate this. The QRS is of a normal width and height and there is no abnormal morphology, therefore this is normal. There is no evidence of ST elevation on this ECG.
The T waves are also normal, with no evidence of tall tented T waves. When we put all of this together, the characteristic irregularly irregular rhythm in combination with the lack of P waves, indicates a diagnosis of atrial fibrillation. Congratulations!
You've come to the end of our guide to ECG interpretation. Learning to interpret an ECG can be challenging, but following a structured approach can help you to identify pathology. For further ECG guidance, head over to the GeekyMedics website or practice your interpretation skills using our OSCE station bank. If you liked this video, you'll love our textbook.
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