ECG Basics

The electrocardiogram (ECG) is one of the most common, and important, investigations interpreted by doctors. In order to help the interpretation process, it is necessary to understand the physiology behind the ECG. In this article, we will explore the basics of ECG physiology, and build on this knowledge for interpretation in later articles. In a nutshell, however, the ECG is used to assess the magnitude, timing, and direction of electrical flow in the heart.

The ECG Strip

The paper used for ECGs contains a series of squares. Five small squares in the x-and y-axis make a large square. The size of each small square is 1 mm x 1 mm, and therefore a large square is 5 mm x 5 mm. In normal calibration (as we will see below), the values of each square can be seen.

ECG Paper Dimensions

ECG Calibration

The ECG looks at electrical rhythm over time, meaning how we calibrate speed and voltage will affect the tracing.

Speed calibration

The normal calibration speed of a standard ECG in the UK is 25 mm/s. Therefore, the trace is produced at 5 big squares per second, which means each large square is 0.2s on the x-axis. Consequently, if a beat were to occur every large square (0.2 s), then there would be 5 beats per second and 300 beats per minute. As we will see in later articles, this explains why the heart rate per minute can be calculated using 300 divided by the number of large squares between beats.

Voltage calibration

The normal voltage calibration is where 10 small squares represents 1 mV on the y-axis, or one large square as 0.5 mV. This means two large squares represents 1mV. The standard calibration is shown as an n-shaped signal at the start of a trace, where the height represents 1 mV (two large squares).
(Sometimes the voltage calibration may be changed if the waveforms are too small or large. However, this is unlikely to occur in a general ward setting).

ECG Calibration Picture 1

Waveforms and Intervals


In the normal heart, electrical activity is initiated in the sinoatrial node (SAN) in the right atrium, and moves through a series of structures in the following way:

SAN → Atrioventricular node (AVN) → Bundle of His → L. and R. Bundles → Purkinje Fibres

Cardiac Electrical Anatomy

The ECG detects this flow, and it is shown as different deflections or ‘waveforms’ on the trace. Each waveform shows something specific. During depolarisation, when the mean direction of electrical flow is in the same direction as a lead (or within 90’ of that direction), it will produce a positive deflection on the ECG. If it is in the opposite direction, it will produce a negative deflection. The height of the wave measures the magnitude of electricity, and the width measures the time.

The above image (right) shows the normal direction of electrical flow. Lead II measures flow in this direction / at this angle. Lead aVR, however, measures flow in the opposite direction. Therefore, you would see positive and negative deflections in these respective leads, as below. This touches on the cardiac axis, and is explained in more detail later on.

Positive and Negative ECG deflections 1

Important waves to be aware of include:
P-wave: small deflection, before the QRS complex, that represents atrial depolarisation
• QRS-complex: high amplitude deflections representing ventricular depolarisation. Strictly, not all ‘QRS-complexes’ have a Q, R and S wave. A Q-wave is the first negative deflection; the R-wave is the positive deflection; and the S-wave is the negative deflection after an R-wave. Nonetheless, the term is commonly used to describe ventricular depolarisation.
T-wave: small deflection following the QRS-complex that represents ventricular repolarisation.

ECG Waves and Intervals Annotated
Intervals (and segments)

As with waveforms, there are different intervals (and segments) to be aware of.
• PR-interval: the time interval between the P-wave and the QRS-complex that represents the electrical delay in the AV node.
• QT-interval: the time between the start of the QRS-complex and the end of the T-wave, representing the time for ventricular depolarisation and repolarisation.
• ST-segment: the isoelectric line between the QRS-complex and the T-wave, representing the time between depolarisation and repolarisation.

ECG Electrodes and Leads

To produce a 12-lead ECG, we need 10 electrodes placed in pre-determined positions on the body; a ‘lead’ and an ‘electrode’ are not the same things – specifically, a ‘lead’ tracing is produced from how the electrodes communicate with each other. This is why there is a difference in the number of leads and electrodes. There are 6 electrodes on the chest that produce 6 ‘precordial leads’, and 4 electrodes on the limbs that produce 6 ‘limb leads’.

‘Precordial lead’ electrode placement:
• V1 – 4th intercostal space (ICS), right sternal edge
• V2 – 4th ICS, left sternal edge
• V3 – halfway between V2 and V4
V4 – 5th ICS, mid-clavicular line
• V5 – halfway between V4 and V6
• V6 – 5th ICS, mid-auxiliary line

*N.B. Sometimes, V4-V6 may be placed on the patient’s back if there is a concern for an ongoing pathology in the posterior heart (i.e., posterior myocardial infarction). This is not part of the normal 12-lead ECG, and is only performed in specific circumstances. In this case, they become known as V7-V9, and should be labelled on the ECG trace.

• V7 – left posterior auxiliary line, same horizontal level as V6
• V8 – inferior tip of left scapula, same level as V6
• V9 – left paraspinal zone, same level as V6

‘Limb lead’ electrode placement
• Right: right wrist (or upper right chest, inferior to clavicle)
• Left: left wrist (or upper left chest, inferior to clavicle)
• Foot: Left ankle (or left iliac fossa)
• Neutral: Right ankle (or right iliac fossa)

(In case a patient is an amputee, these are alternative locations)

Chest external landmarks in ECG placament

Precordial leads

The precordial leads, or chest leads, are produced from six electrodes across the chest labelled V1-V6. The position of the leads, especially along the horizontal, is important, as this is what helps with assessing different cardiac territories. If we were to look at how these leads are orientated on the chest from a birds-eye view, we would see the following:

Precordial leads axis transverse view 1

Remember that, from a birds-eye view, the mean direction of electrical flow would be from right to left, towards leads V4-V6, and away from V1-V2. Therefore, a normal ECG strip would produce a negative QRS deflection in V1, and progressively become more positive through the leads up to V5/6. This is called R-wave progression (as the R-wave is getting larger).

The Cardiac Axis and the Limb Leads

The cardiac axis refers to the direction of mean electrical flow through the heart. When the sinoatrial node (SAN) fires a depolarisation wave, remember that it moves in the following way:

SAN → Atrioventricular node (AVN) → Bundle of His → L. and R. Bundles → Purkinje Fibres

Cardiac Electrical Anatomy 1

The above highlights the basic anatomy of the electrical conduction system of the heart (left image), and the general direction of the mean electrical flow through the system (right image). The mean direction in this view (frontal plane) is from top-to-bottom, and right-to-left. This is a normal axis. When the mean direction of electrical flow does not move from top-to-bottom, and right-to-left, there is axis deviation. This is most commonly due to some form of pathology. How we produce the axis is described below.

Einthoven’s Triangle

Einthoven’s triangle is an abstract triangle formed by three limb leads: leads I, II, and III. These leads are formed by electrodes on the patient’s right arm, left arm, and left leg as follows:

Einthovens Triangle

The image on the left shows the electrodes on the right arm, left arm, and left leg communicate with each other directly. Two electrodes communicate with one another by creating a bipolar lead (vector) to measure the direction of flow of mean electrical activity in the heart. This lead is created by each electrode, with respect to one other, acting as if with different charges. Remember, electrical flow (electrons) moves from a negative to positive charge.

The image on the right shows Einthoven’s triangle with three more leads superimposed (aVR, aVL, and aVF). These leads are augmented (i.e., aVR = augmented vector right) where the midpoint of two electrodes is used as a ‘potential’ starting point for a lead toward the positively charged third. This defines them as unipolar.   

Taken together, the three bipolar leads and three unipolar leads create the six limb leads, each with its own vector to measure electrical flow. The angle of these vectors (where the horizontal is 0’ degrees) can be used to create an axis, as shown. This ‘axis’ is the cardiac axis.

Cardiac Axis Two Pictures 2

Cardiac Territories

Cardiac territories refer to which part of the heart each lead is ‘looking at’. By having an understanding of the cardiac axis, it makes for easy understanding. For example, as leads II, III, and aVF are pointing inferiorly, they assess the inferior part of the heart. By extension, leads I, aVL, V5 and V6 are the lateral leads, V1-V2 the septal leads, and V3-V4 the anterior leads.

• Inferior – II, III, aVF
Lateral – I, aVL, V5, V6
• Septal – V1, V2
• Anterior – V3, V4

Each territory is characteristically supplied by a coronary vessel. This knowledge helps dictate the location of a coronary lesion in the event of ischaemia and thus planning if re-perfusion therapy is required.

• Inferior – Right coronary artery
• Posterior – Right coronary artery
• Anterior – Left anterior descending
• Septal – Left anterior descending
• Lateral – Left circumflex artery



Useful links

Author: Dr Steven Scholfield (FY2)

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