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Electrocardiogram (ECG)

Author: Alexandra Juhász

An electrocardiogram (ECG) is a simple, painless, and non-invasive diagnostic test.

What is an electrocardiogram?

An ECG is a simple test that can be used to check the heart’s rhythm and electrical activity. During electrocardiography (the process of recording the heart's electrical activity), sensors attached to the skin detect the electrical signals produced by your heart each time it beats.

Electrocardiography can be used to diagnose various heart conditions, such as arrhythmias, heart attacks, and heart disease. To apply ECG as a routine test, we first need to learn the basics of electrocardiography.

Below is an ECG paper (graph paper, also called ECG recording paper) that records the electrical activity of all myocardial fibres.

Electrocardiogram paper

Time

Calibration signal

Amplitude

10 mm = 1mV

1mm

1 sec

1 sec = 1000ms (25mm)

5 mm

0.04 sec
/40ms

0.20 sec
/200ms

On a standard ECG, the paper speed is 25 mm/s. Occasionally, the paper speed can be increased to 50 mm/s to better define waveforms.

The vertical axis of the ECG represents the amplitude of the electrical signal generated by the heart in millivolts (mV), while the horizontal axis represents time in milliseconds (ms).

On the ECG paper, one SMALL square is 1 mm on the horizontal axis and is 40 ms (0.04 sec). 1 SMALL square = 1 mm = 0.1 mV. 5 SMALL squares (5 mm) or 1 LARGE square is 200 ms, which is 0.5 mV. 5 LARGE squares are 1 second.

On the ECG paper, the standard calibration is 10 mm (10 small squares), equal to 1 mV.

The ECG leads

Ten electrodes are required to produce a 12-lead ECG. Four electrodes on all four limbs (RA, LL, LA, RL) and six electrodes on the chest (V1, V2, V3, V4, V5, V6).

Placement of the limb and chest electrodes

R (- -)

F (+ +)

L (+ -)

N

Placement of the limb and chest electrodes

There are three bipolar limb leads (I, II, III); the Einthoven's leads, which roughly form an equilateral triangle (with the heart at the centre), are called the Einthoven triangle. In these leads, one lead carries a positive electrode and the other a negative. Lead I has the positive electrode on the left arm and the negative electrode on the right arm; thus, it measures the potential difference between the two arms. In the lead II configuration, the positive electrode is on the left leg, and the negative electrode is on the right arm. Lead III has a positive electrode on the left leg and a negative electrode on the left arm.

Einthoven’s and Goldberger’s leads

R

F

L

I.

II.

III.

ground

WCT

avR

avL

-

+

-

The Goldberger's leads are aVR (augmented Voltage Right), aVL (augmented Voltage Left), and aVF (augmented Voltage Foot). These unipolar leads require an artificially constructed reference for surface electrocardiography, assumed to be near zero and steady, the Wilson Central Terminal (WCT).

The aV stands for augmented vector. Lead aVR is on the right wrist (red), lead avL is placed on the left wrist (yellow), and lead aVF has to be put on the left foot (green). “Ride Your Green Bike” is a helpful mnemonic for remembering the limb lead placement, starting at the right wrist and proceeding clockwise. The right leg electrode (black) functions as an electrical ground.

Limb leads

Wilson introduced the precordial leads (chest leads), which are: V1, V2, V3, V4, V5, V6.

Placement of the chest electrodes

V₁: Fourth intercostal space to the right of the sternum.
V₂: Fourth intercostal space to the right of the sternum.
V₃: placed diagonally between V2 and V4 on the left side.
V₄: Fifth intercostal space in line with the middle of the clavicle (mid-clavicular).
V₅: placed on the same level as V4 but in the anterior axillary line.
V₆: placed on the same level as V4 but in the midaxillary line.

Chest electrodes

The three standard limb leads (I, II, III) and the three augmented leads (aVR, aVL, aVF) view the heart's electrical activity from the frontal plane, while the six precordial leads (V1, V2, V3, V4, V5, V6) examine the heart in the horizontal plane, resulting in the 12-lead ECG.

The planes of the leads

+120°

+90°

+60°

0°

-30°

-90°

avR

avF

avL

I.

III.

II.

180°

-150°

The planes of the leads

If necessary, additional chest leads can be positioned on the dorsal side. These are V7, V8, and V9.
Leads V7–V9 are placed on the posterior chest wall in the fifth intercostal space in the following positions:

V7: Left posterior axillary line, same horizontal plane as V6.
V8: Left mid-scapular (scapular) line, same horizontal plane as V6.
V9: Left paravertebral line, same horizontal plane as V6.

Placement of the chest leads

Specific ECG leads are the paravertebral V7, V8, V9 electrodes.

Leads V7–V9 are placed on the posterior chest wall in the following positions:

V7 – Left posterior axillary line, in the same horizontal plane as V6.
V8 – Tip of the left scapula, in the same horizontal plane as V6.
V9 – Left paraspinal region, in the same horizontal plane as V6.

The specific leads are the right-sided leads (V1R, V2R, V3R, V4R, V5R, V6R), which are positioned in a mirror-image fashion to the standard precordial leads on the right side of the chest.

Placement of the right-sided chest leads

Contiguous leads

Anatomically contiguous leads are two or more leads that look at adjoining areas and provide information about adjacent regions. The leads of the inferior regions are II, III, and aVF. Leads of the lateral region: I, avL, and V6. V1 and V2 represent the heart's septum, while leads V3 and V4 represent the anterior region of the heart. The standard 12-lead ECG does not directly visualise the posterior wall.

Contiguous leads

Technical errors during electrocardiogram acquisition

Various errors during electrocardiography that can create artifacts are the following:

Patient/limb movement
Rest tremor or shivering
Coughs, hiccups, increased work of breathing
Displacement of electrodes
Calibration error or incorrect paper speed
Cable problems, e.g., broken or faulty electrodes
Loose electrodes

External factors – as extra-corporeal interference – cause often artifacts on the ECG.

It is important to place the surface electrodes correctly, as mispositioning the leads may yield false ECG results. For example, reversal of the arm electrode results in negative P waves and QRS complexes. A common chest lead placement error also results in poor R-wave progression.

If there is any suspected error during the electrocardiogram, retest the patient. Adequate attention to the possibility of recording errors during an electrocardiogram usually results in recognition of the technical defect and, thereby, the possible serious misdiagnoses can be avoided.

ECG Waveforms

Let's have look of the detailed description of the ECG waves.

Waveforms

Isoelectric line

J

ST segment

PR interval

QRS complex

QT
interval

P wave
duration

ST
interval

PR
interval

TP
interval

T

U

R

P

Q

S

RR interval

0,04 s = 40 ms

0,20 s = 200 ms

A horizontal line between the U and P waves, or between the T and P waves if the U wave is absent, is called the isoelectric line (baseline) of the ECG. Waveforms above the isoelectric line are positive, and waves below the isoelectric line are negative. If the electrical current flows toward the lead, it is a positive deflection; if it flows away from the lead, it is a negative deflection. Waves of the ECG represent the sequence of depolarisation and repolarisation of the atria and ventricles.

P wave axis

aVR

aVL

aVF

III.

II.

I.

-150°

-30°

0°

60°

90°

120°

Normal waveforms

I.

II.

III.

aVR

aVL

aVF

V1

V2

V3

V4

V5

V6

The duration of a segment or interval (e.g. QT) depends on the patient’s heart rate. Normal ECG values when the heart is beating in a regular sinus rhythm between 60–100 beats per minute (specifically 82 bpm).

P wave

The P wave represents atrial depolarisation. The normal duration of the P wave is 50–100 ms. Knowing that a P wave must be upright in leads II and aVF and inverted in lead aVR is essential to designate a cardiac rhythm as a normal sinus rhythm. During atrial depolarisation, the electrical impulse travels to lead II. Thus, the P wave has a positive deflection. However, it has to be noted that the P waves in the inferior leads are inverted, so the P wave tends to be upright in lead II (positive) and inverted (negative) in lead aVR.

PQ or PR interval

The normal PQ or PR interval is between 120–200 ms. This represents when the impulse travels through the atria, atrioventricular node (AVN), bundle of His, and Tawara branches to reach the ventricles. The PR interval is measured from the onset of the P wave to the onset of the QRS complex. The PQ segment also serves as the ECG curve's baseline (reference line or isoelectric line) and helps identify ST segment abnormalities. The term “PQ interval” is preferred by some electrocardiograph technicians because it is the period measured unless the Q wave is absent.

QRS complex

The QRS complex represents ventricular depolarisation. The normal physiologic duration of the QRS complex spans 60 to 120 ms. The Q wave is the first downward deflection (negative) and the first element in the QRS complex. The R wave is the first upward deflection (positive), while the second negative wave is the S wave.

There are several morphological variations of the QRS complex. The complex consists of a Q, R, and S wave, but additional waves, marked by a ’, might occur. For example, rSR’, if the R wave is not present on the ECG, small waves are marked with lower case letters, while large waves are marked with capital letters.

Morphological variations of the QRS complex

rSR'

QS

rS

qRs

ST segment

The ST segment is the flat, isoelectric section between the S wave's end and the T wave's beginning. This segment is the isoelectric period when the ventricles are between depolarisation and repolarisation. The ST segment begins at the J point, where the QRS complex and ST segment meet. If no S wave is present on the ECG, the ST segment is between the end of the R wave and the beginning of the T wave.

T wave

The T wave is the first deflection following the QRS complex and represents ventricular repolarisation.

QT interval

This is the time from the start of the Q wave to the end of the T wave, the time taken for ventricular depolarisation and repolarisation. As the QT interval depends on heart rate (the higher the heart rate is, the shorter the QT interval gets), the corrected QT interval is calculated as follows:

QT_c = QT / √RR, where QT_c = corrected QT; QT = duration of QT interval; and RR = duration of RR interval. The normal QTc interval ranges from 350 to 450 ms.

U wave

It follows the T wave of ventricular repolarisation and may not always be observed; if present, it is best seen in leads V2–V4. The exact source of the U wave is unclear.

ECG lead placement instructions and analysis

ECG step-by-step

Before the patient arrives, ensure your equipment is properly prepared and ready for the test you will perform.

ECG recording

After patient identification

Check the patient’s ID card and Social Security Card and

Check the patient’s identification and Social Security card, and complete the necessary ECG documentation (including patient details, date, and time of recording).
Prepare the patient by briefly explaining the procedure and obtaining verbal consent for the ECG. It is essential that the patient is comfortable and relaxed to ensure an accurate recording with minimal interference or artefact.

The acquisition of a 12-lead ECG is a relatively intimate procedure. Nurses should make every effort to preserve the patient’s dignity, using gowns or towels whenever possible.

Ask the patient to remove their upper clothing so that electrodes can be attached to their chest, arms, and legs. Wearing a separate top with trousers or a skirt allows easy access to the chest. As bras may interfere with ECG leads, patients should remove them before the test.
The patient should be lying on a bed (wide enough to support the patient’s arms at their sides so they aren’t left dangling), with the shoulders relaxed and the legs uncrossed. The head of the bed should be elevated at a 15-degree angle.
Wash your hands and put on non-sterile gloves.
Assemble the required equipment: Connect the electrodes to the lead wires before placing them on the patient, and connect the cables to the monitor. Ensure the wires are not tangled.
An ECG works best on clean, dry skin free of oils and lotions. However, if hair prevents the electrodes from sticking correctly, the skin may need to be shaved.
Place the limb leads in the appropriate locations. RA and LA leads can be placed on the deltoids or wrists, while RL and LL should be near the ankles (or on the lower left leg). In all cases, ensure the leads are not positioned over bone.
Peel the electrodes from the plastic liner. Gently press around the adhesive edge in a circular motion to place them on the chest. If available, use hydrogel-based disposable electrodes. When using this type of electrode, avoid pressing down on the centre.
Connect the cable. Electrodes with offset connectors should be positioned so that the connectors face in the same direction as the cable.
Ask the patient to remain still, relax, not talk, and breathe calmly.
Ensure the power cord is connected correctly, turn on the machine, check its performance, and ensure the ECG paper is loaded correctly. Then, accurately enter the patient's information.
Wait until the ECG waveform is stable and free of interference, then record it and press the "START" button to print the ECG graph.
After printing, turn off the machine and verify the patient's information again.
Remove the electrodes from the patient's body and wipe the skin.
Allow the patient to dress and thank them.
Clean and disinfect the ECG cables and accessories after each use.
Remove gloves and perform hand hygiene.
Write the patient's data on the ECG paper and hand the record to the clinician.

It is imperative that any deviations from the standard recording procedure be noted on the ECG to prevent misinterpretation (e.g., if the patient is sitting upright in a wheelchair).

Specific ECG machines can automatically record and store patient data, which is printed on ECG paper.

ECG machine

The ECG interpretation

Rhythm

A rhythm strip is best used to analyse the rhythm. On a 12-lead ECG, a continuous strip recording is found at the bottom of the page from lead II. Most modern monitor recording systems print the date, time, and the patient’s name and identification number; however, if the monitor you are using cannot do this, label the rhythm strip with the date, time, patient’s name, identification number, and rhythm interpretation.

Normal sinus rhythm

In a normal sinus rhythm, pacemaking impulses arise from the sinus node and progress to the ventricles through a normal conduction pathway—from the sinus node to the atria and AV node, through the bundle of His, to the bundle branches, and on to the Purkinje fibres. In adults, a normal sinus rhythm usually accompanies a heart rate of 60 to 100 beats per minute. Normal sinus rhythm is another name for the normal heart rhythm. Characteristics of normal sinus rhythm:

Heart rate of 60‒100 bpm
A P wave for every QRS complex
P waves are rounded, smooth, and upright in lead II, signalling that a sinus impulse has reached the atria.

Heart rate

There are different ways to calculate the heart rate. One easy method is to count the number of large squares between R waves and divide 300 by this number.
Example: heart rate = 300/4 = 75 bpm.

Calculating rate

1

2

1

2

3

1

2

3

4

1

2

3

4

5

300

300
bpm

300 ÷ 2

150
bpm

300 ÷ 3

100
bpm

300 ÷ 4

75
bpm

300 ÷ 5

60
bpm

Speed: 25mm/sec

*Rate =

300

R-R interval in large squares

To calculate heart rate when the rhythms are irregular, locate and mark the starting and ending points of 30 squares, count the number of QRS complexes between these two points, and multiply this number by 10.
Example: heart rate = 11 × 10 = 110 bpm.

The normal heart rate is between 60 and 100 bpm, which is known as a normofrequent sinus rhythm. Sinus bradycardia occurs when the heart beats slower than 60 bpm, and sinus tachycardia occurs when it beats faster than 100 bpm.

Cardiac axis

After checking the rhythm and calculating the heart rate, the cardiac axis has to be determined. The cardiac axis represents the overall direction of electrical activity as it spreads through the cardiac conduction system. In other words, it represents the net effect of all the action potentials generated by the heart.

The green arrow represents the direction of electrical conduction flow in the heart. As shown in the figure, 0 degrees is represented by the horizontal line parallel to the axis of lead I. Degrees above this horizontal line, 180–0 degrees, will be marked as negative, while degrees below the horizontal line are positive.

The cardiac axis can be easily determined by assessing whether the QRS complex is positive in leads I and aVF. This means that if in the lead I, the R wave of the QRS complex is 5 small cubes (positive deflection), and its negative deflection is 2 small cubes, then +5-2=3, which means in the lead I, the deflection is positive. Following the same method, if in aVF the positive deflection of the QRS complex is 4 small squares, while the negative deflection is 2 small squares, then +4-2=+2. If both lead I and aVF are positive, the cardiac axis is normal.

Normal cardiac axis

Normal cardiac
axis

0°

-90°

+90°

180°

I.

aVF

The electrical axis of the heart

The table below shows the possible cardiac axis variations.

Cardiac axis variations

I.	aVF	Quadrant	Cardiac axis
POSITIVE	POSITIVE		Normal cardiac axis
POSITIVE	NEGATIVE		Left axis deviation
NEGATIVE	POSITIVE		Right axis deviation
NEGATIVE	NEGATIVE		Extreme right axis deviation

Determine the cardiac axis with the help of the animation below.

_{https://david-shrk.github.io/ecgaxistrainer/}

After the assessment of rhythm, heart rate, and cardiac axis, the evaluation of the waveforms completes the ECG reading.

Pacemakers

After discussing the basics of the ECG, it is also important to mention pacemakers, as these can alter the ECG tracing.

In case of a slow heart rate, implantation of a pacemaker is a standard treatment. For people with bradycardia (heart rate below 60 bpm), this small device can help restore the heart rhythm. Pacemakers deliver electrical stimuli via pacing leads to the heart, which causes the heart muscle to contract.

There are several methods for temporary heart pacing (transcutaneous, transvenous, transesophageal, transthoracic, and epicardial), but transvenous and transcutaneous cardiac pacing are the most commonly used.

In the case of a permanent pacemaker (PPM), a small device is implanted under the skin of the chest to help the heart beat in a regular rhythm.

NBG^* Pacemaker Code

(*NBG is a blend of the British Pacing and Electrophysiology Group (BPEG) and the North American Society of Pacing and Electrophysiology (NASPE)

The letter/number stands for
Position 1	Chamber(s) paced	0=none
			A= atrium
			V= ventricula
			D= both (A+V)
Position 2	Chamber(s) sensed	0=none
			A= atrium
			V= ventricula
			D= both (A+V)
Position 3	Response to sensing	0=none
			I= inhibition
			T= triggered
			D= dual (T+I)
Position 4	Rate modulation	0= none
Position 4	Rate modulation		R= rate modulation
Position 5	Multisite pacing	0=none
			A= atrium
			V= ventricula
			D= both (A+V)

Pacing spikes are a key feature of the paced ECG.

If the pacing is atrial, the spike is followed by a P wave; if ventricular, the spike is followed by a wide QRS complex. In atrial and ventricular dual chamber pacing, pacing spikes may precede a negative P wave and a broad QRS complex.

Ventricular paced rhythm

Pacing spike

Rhythm analysis during Cardiopulmonary Resuscitation (CPR)

Chain of survival

Early recognition
and call for help

Early CPR

Early defibrillation

Post resuscitation
care

To prevent
cardiac arrest

To buy time

To restart
the heart

To restrore quality
of life

The basis of the chain of survival is early defibrillation. To decide whether defibrillation is necessary, we need to know whether, based on the observed heart rhythm, an electrical shock is needed. For this reason, it is clear that in cardiopulmonary resuscitation (CPR), reading an ECG is crucial. Early CPR and defibrillation provide the best chance of survival.

Basic Life Support (BLS)

During Basic Life Support (BLS), if a defibrillator is automatic or semi-automatic, the device performs its own rhythm analysis. If the device is automatic and detects a shockable rhythm, it delivers the shock itself after a countdown and warning. Semi-automatic models evaluate the heart rhythm and require the rescuer to press the shock button if a shockable rhythm is detected.

When performing Advanced Life Support (ALS), you need to know which rhythms require a shock.
There are two categories of rhythms: shockable and non-shockable.

Shockable rhythms:

Ventricular fibrillation (VF)
Pulseless ventricular tachycardia (pnVT)

Non-shockable rhythms:

Asystole
Pulseless electrical activity (PEA) or Electromechanical Dissociation (EMD)

Ventricular fibrillation (VF)

VF is an arrhythmia, or irregular heartbeat, which affects your heart's ventricles. The ECG shows irregular waves with varying rates, morphology, and amplitude. Ventricular fibrillation is a dysrhythmia that does not produce a pulse.

Ventricular fibrillation (VF)

Pulseless ventricular tachycardia (pnVT)

Ventricular tachycardia is not always pulseless; thus, it must be checked whether the patient has a palpable central pulse. If there is no palpable central pulse in VT (i.e., pulseless VT), delivering a shock is necessary as soon as possible.

Ventricular tachycardia with broad QRS complex and with uniform morphology

Asystole

Asystole, colloquially referred to as flatline, represents the cessation of electrical and mechanical activity of the heart, meaning that there is no heartbeat, no heart contractions, and no pulse. P waves may be present if an AV block exists, but no QRS complexes are observed. During asystole, the ECG usually shows slight baseline undulation.

Asystole

Pulseless Electrical Activity (PEA) or Electromechanical Dissociation (EMD)

In cases of a non-shockable rhythm (such as PEA), in addition to reading the ECG, it is important to palpate the patient's central pulse to determine whether a detectable heartbeat is present. If a central pulse cannot be palpated, it is called a pulseless electrical activity.

Pulseless electrical activity is a type of cardiac arrest in which the electrocardiogram shows a heart rhythm that should produce a pulse but does not. It means that the electrical activity is a pertinent, but not sufficient, condition for contraction.

Kardiovaskulárny systém

-