By Dr. Thomas Burnell and Dr. Bethany Turner
Next Lesson - Electrocardiogram Pathology
Core
Here is a brief recap of the conduction system, which is explained in more detail in the Cardiac Cycle article.
- Sinoatrial (SA) node (the heart's natural pacemaker, in the wall of the right atrium) sends an action potential (AP) which spreads across the atria, causing them to contract.
- The AP reaches the atrioventricular (AV) node, where it is briefly delayed, before it is transmitted to the Bundle of His (a bundle of specialised fibres that runs down the septum between the ventricles).
- The Bundle of His transmits APs down the left and right bundle branches (one for each ventricle) to the Purkinje fibres (specialised fibres that spread the impulse quickly through the ventricles).
- The impulse conducts across the ventricular myocardium causing contraction.
The signals sent throughout the heart by the conduction system can be picked up by an electrocardiograph (ECG machine) and recorded as an electrocardiogram (ECG). In this article the word ‘electrode’ will be used to describe the wires you place on the body, and the word ‘lead’ to describe the views of the heart, and the recordings on the ECG.
When recording an ECG, 10 electrodes are placed on the patient's body to give 12 views of the heart:
- 4 limb electrodes which are placed on the limbs, just above the wrists and ankles (never on the torso):
- Right arm - Red electrode
- Left arm - Yellow electrode
- Left leg - Green electrode
- Right leg - Black electrode (this is the earth electrode: it does not contribute to the recording, so only 9 of the 10 electrodes are used to make the 12 leads)
This placement can helpfully be remembered by the acronym Ride Your Green Bike when starting from the right wrist and working clockwise when looking at the front of the patient.
- 6 chest electrodes. Each of these gives its own lead/view of the heart. V1-V6 which are placed across the chest in specific positions:
- V1 - right sternal edge, 4th intercostal space (ICS, the gap between two ribs)
- V2 - left sternal edge, 4th ICS
- V3 - halfway between V2 and V4
- V4 - mid-clavicular line (a vertical line down from the middle of the collarbone), 5th ICS
- V5 - halfway between V4 and V6
- V6 - mid-axillary line (a vertical line down from the middle of the armpit), in line with V4
To find the 4th intercostal space (ICS), first find the sternal angle (a ridge of bone that can be felt where the manubrium, the upper part of the breastbone, meets the body of the sternum). The angle lies next to the 2nd ICS, so if you move your fingers across from the sternal angle, you should feel a dip which is the 2nd ICS. From there simply count down two spaces to reach the 4th ICS.
It is usually best to place electrodes V1, V2, V4 and V6 first to help with placement of V3 and V5.
Diagram - The location of where the chest electrodes should be placed
Creative commons source by JMarchN [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
The multiple leads give us different views of the heart and allow us to see where pathology is. Locating pathology in this way is important in myocardial infarctions, because it shows which coronary artery is affected.
Each lead/view of an ECG looks at a different part of the heart and is associated with a coronary artery:
Diagram - Coronary arteries and their corresponding ECG lead. Correction: the anterior leads V3 and V4 are supplied by the left anterior descending artery (LAD), not by the right coronary artery as this diagram labels them.
SimpleMed original by Dr. Thomas Burnell
Leads V1 to V6 are known as the chest (precordial) leads, because their electrodes sit on the chest wall in front of the heart. More specifically, V1 and V2 are the septal leads and look at the interventricular septum (the wall between the two ventricles), V3 and V4 are the anterior leads and look at the front of the heart, and V5 and V6 are lateral leads (see below). These views can be remembered by thinking about where the electrodes are placed on the patient. Infarction of the right ventricle is best detected with right-sided leads such as V4R (the V4 electrode placed on the right side of the chest).
Leads II, III and aVF are the inferior leads and look at the bottom part of the left ventricle. The augmented limb leads (aVR, aVL and aVF) have their positive electrodes on the right arm, left arm and left leg respectively.
Leads I, aVL, V5 and V6 are the lateral leads, and look at the side of the left ventricle.
Diagram - The view that each lead 'sees' the heart. With this diagram you can see how the leads can localise pathology to certain regions of the heart
Creative commons source by Npatchett [CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0)]
Electrodes on the body look at the flow of current in the heart, and whether the electrical activity of the heart is going towards or away from the electrode. This gives us either an upward or downward deflection. This concept is best thought about in the context of the cardiac conduction pathway.
- Depolarisation towards the electrode is seen as an upward deflection, e.g. the main wave of ventricular depolarisation normally spreads down and to the left, towards leads such as II and V6, giving the upward deflection (the R wave) of the QRS complex.
- Depolarisation away from the electrode is seen as a downward deflection, e.g. in a lateral lead such as V6, septal depolarisation spreads from left to right, away from the electrode, giving a small downward deflection (a q wave) at the start of the QRS complex.
- Repolarisation towards the electrode is seen as a downward deflection.
- Repolarisation away from the electrode is seen as an upward deflection.
How the Conduction System looks on an ECG
Lead II records the voltage difference between the right arm (negative) and left leg (positive) electrodes. Its electrical axis (the direction in which the lead views the heart) points inferiorly and to the left: the positive electrode is on the left leg, not at the apex of the heart. The deflection seen depends on the direction of the net electrical activity relative to the lead.
Diagram - A schematic of electrical activation and recovery during the cardiac cycle. The P wave represents atrial depolarisation, the QRS complex ventricular depolarisation, and the T wave ventricular repolarisation. This trace illustrates the sequence of electrical events; the direction and polarity of a deflection must be interpreted in the particular ECG lead, as explained below.
Creative commons source by OpenStax College [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]
- Before atrial depolarisation, the SA node depolarises but the signal is too small to register on an ECG, so it appears as an isoelectric segment (flat line).
- Then there is atrial depolarisation which appears as an upward deflection as depolarisation spreads towards the electrode. This is the P wave.
- The impulse reaches the AV node and is delayed after atrial depolarisation. This appears as another isoelectric segment.
- The impulse spreads to the ventricles, and ventricular depolarisation is now seen as the QRS complex.
- Normal septal depolarisation begins on the left side and spreads from left to right. It may produce small septal q waves in lateral leads such as I, aVL, V5 and V6. A q wave is not required in lead II; the initial deflection depends on the lead and electrical axis.
- Most of the QRS complex reflects depolarisation of ventricular myocardium after rapid delivery of the impulse through the His-Purkinje system (the Bundle of His, its branches and the Purkinje fibres). A net depolarisation vector (the overall direction in which depolarisation is spreading) towards the positive pole produces an upward deflection (R wave).
- An S wave is a negative deflection after an R wave. It reflects late ventricular depolarisation directed away from the positive pole of that lead, rather than a fixed sequence of upward conduction along the ventricular walls.
- There is then an isoelectric segment once ventricular depolarisation is complete.
- Finally ventricular repolarisation occurs which is seen as the T wave.
An additional U wave (a small deflection immediately after the T wave) can sometimes be seen in an ECG. It may be a normal finding, but prominent U waves can be associated with pathology, e.g. hypokalaemia (a low blood potassium level).
Diagram - A normal ECG
SimpleMed original by Dr. Bethany Turner
When reading an ECG, each small box is equal to 0.04 seconds while each large box is equal to 0.20 seconds (there are 5 small boxes in the length of one large box).
An easy way to calculate a patient's HR using an ECG is:
- Count the number of large boxes between two R waves (count any small boxes as 0.2 of a large box).
- Then calculate 300/(number of large boxes) which will give the heart rate.
If the patient has an irregular rhythm then the above method will not work as you will get different values depending on which heart beats you use. The following alternative method can be used to calculate the average heart rate:
- Look at the rhythm strip (the long recording along the bottom of the ECG, usually lead II).
- Count how many times a QRS complex occurs in 30 large squares - 30 large squares is 6 seconds.
- Multiply the number of complexes by 10 (6 seconds x 10 = 1 minute) to give the heart rate in beats per minute. You can also count the complexes along the whole 10-second rhythm strip (50 large squares) and multiply by 6.
PR interval - interval between the start of the P wave and the start of the QRS complex. It is normally 3-5 small boxes. >1 large box is prolonged.
- A prolonged PR interval suggests delayed conduction through the AV node. This can be seen in:
- First-degree heart block (a delay in conduction from the atria to the ventricles, defined as a PR interval of more than 5 small boxes)
- Ischaemic heart disease
- Hypokalaemia
- A shortened PR interval suggests that the delay at the AV node has not happened. This can be seen in:
- Wolff-Parkinson-White syndrome (there is a faster conduction pathway between the atria and ventricles)
QRS interval - length of the QRS complex (start of the Q wave to the end of the S). It is normally no more than 2.5 small boxes (0.10 seconds). A QRS of 3 small boxes (0.12 seconds) or more is described as widened.
- A widened QRS indicates prolonged ventricular depolarisation, not necessarily an impulse originating in the ventricles. Ventricular ectopy (an extra beat arising in the ventricles) commonly activates myocardium more slowly, but a supraventricular impulse (one arising above the ventricles) can also produce a broad QRS through bundle branch block or aberrant conduction (an abnormal spread of the impulse through the ventricles). Pre-excitation (early activation of the ventricles through an extra pathway between the atria and ventricles, called an accessory pathway) can broaden the QRS as well. Causes include:
- Bundle branch block (delayed conduction down the left or right bundle branch)
- Hyperkalaemia (a high blood potassium level)
- Ventricular ectopics
QT interval - interval between the start of the Q wave and the end of the T wave. The QT interval varies with heart rate, so it is corrected for heart rate to give the QTc (corrected QT interval), which estimates the QT interval at a heart rate of 60 beats per minute. A QTc over about 440 milliseconds in men or about 460 milliseconds in women is prolonged (the exact cut-offs vary a little between sources). As a rule of thumb, a normal QT interval is less than half of the preceding R-R interval (the time between two R waves).
- A prolonged QT interval suggests delayed ventricular repolarisation. A very long QTc (over 500 milliseconds) increases the risk of torsades de pointes, a dangerous abnormal heart rhythm. It can be seen in:
- Use of certain drugs, e.g. amiodarone (an antiarrhythmic) and tricyclic antidepressants (an older group of antidepressants, such as amitriptyline).
- Low blood levels of potassium (hypokalaemia), magnesium or calcium.
Diagram - The intervals seen in an ECG
Creative commons source by Agateller (Anthony Atkielski), converted to svg by atom. [Public domain]
Sinus rhythm is a heart rhythm that originates from the sinoatrial node. Normal sinus rhythm, the default heart rhythm, has all of the following features:
- A regular rhythm (small variations with breathing, called sinus arrhythmia, are normal, especially in young people)
- HR of 60-100 beats per minute (bpm)
- P waves seen
- P waves upright in leads I and II
- A constant PR interval (normally 3-5 small boxes)
- Every P wave followed by a QRS
- Every QRS preceded by a P wave
- Narrow QRS complexes, unless there is a bundle branch block or another delay in conduction through the ventricles
Patients can still have a sinus rhythm despite being bradycardic (a heart rate below 60 bpm) or tachycardic (a heart rate above 100 bpm). In these circumstances, they would have sinus bradycardia or sinus tachycardia.
When confronted with an ECG, it's good to have a system in your head that you can use to analyse the ECG and describe it. The following is a suggested order for looking at different aspects of the rhythm strip of an ECG.
- Calculate the heart rate.
- Is the rhythm regular or irregular? This can be hard to determine in faster heart rates, so a good technique is to mark the distance between two or three tips of QRS complexes on a piece of paper and slide it across the rhythm strip. If the marks align, then the rhythm is regular.
- Are there P waves? P waves show atrial activity.
- Are there QRS complexes? QRS complexes show ventricular activity.
- Is each P wave followed by a QRS complex, and each QRS complex preceded by a P wave?
- Measure the intervals: PR interval, QRS width, QT interval.
If the rate, rhythm, P waves, QRS complexes and intervals are all normal, this is a normal sinus rhythm. If one or more of these is abnormal, the ECG needs further assessment. This list covers the rhythm only: a full interpretation also checks the cardiac axis (the overall direction of ventricular depolarisation), Q waves, ST segments and T waves. The last three are covered in the Electrocardiogram Pathology article. A normal resting ECG does not exclude an acute coronary syndrome (for example, a heart attack).
Edited by: Dr. Ben Appleby
Reviewed by: Dr. Marcus Judge
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