The electrocardiogram (ECG) is a diagnostic tool that … · · 2014-05-19An ECG can not only tell you if an MI is present but can also show the approximate ... hypoglycaemia and

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ECG Basics

The electrocardiogram (ECG) is a diagnostic tool that measures and records the electrical activity of the heart in detail. Being able to interpret these details allows diagnosis of a wide range of heart problems.

ECG Electrodes

ECG standard leads Lead I: is between the right arm and left arm electrodes, the left arm being positive. Lead II: is between the right arm and left leg electrodes, the left leg being positive. Lead III: is between the left arm and left leg electrodes, the left leg again being positive.

Chest Electrode Placementhttp://www.ambulancetechnicianstudy.co.uk/images/electrodes_position.gif

V1: Fourth intercostal space to the right of the sternum. V2: Fourth intercostal space to the Left of the sternum. V3: Directly between leads V2 and V4. V4: Fifth intercostal space at midclavicular line. V5: Level with V4 at left anterior axillary line. V6: Level with V5 at left mid-axillary line.

ECG Leads - Views of the Heart

Chest Leads V1 & V2 V3 & V4 V5 & V6

View Right Ventricle Septum/Lateral Left Ventricle Anterior/Lateral Left Ventricle

The ECG records the electrical activity that results when the heart muscle cells in the atria and ventricles contract.

Atrial contractions show up as the P wave. Ventricular contractions show as a series known as the QRS complex. The third and last common wave in an ECG is the T wave. This is the electrical

activity produced when the ventricles are recharging for the next contraction (repolarizing).

http://www.ambulancetechnicianstudy.co.uk/images/electrodes_position.gif


ECG Interpretation

The graph paper that the ECG records on is standardized to run at 25mm/second, and is marked at 1 second intervals on the top and bottom. The horizontal axis correlates the length of each electrical event with its duration in time. Each small block (defined by lighter lines) on the horizontal axis represents 0.04 seconds. Five small blocks (shown by heavy lines) is a large block, and represents 0.20 seconds.

Duration of a waveform, segment, or interval is determined by counting the blocks from the beginning to the end of the wave, segment, or interval.

P-Wave: represents atrial depolarization - the time necessary for an electrical impulse from the sinoatrial (SA) node to spread throughout the atrial musculature. Location: Precedes QRS complex Amplitude: Should not exceed 2 to 2.5 mm in height Duration: 0.06 to 0.11 seconds P-R Interval: represents the time it takes an impulse to travel from the atria through the AV node, bundle of His, and bundle branches to the Purkinje fibres. Location: Extends from the beginning of the P wave to the beginning of the QRS

complex Duration: 0.12 to 0.20 seconds.


QRS Complex: represents ventricular depolarization. The QRS complex consists of 3 waves: the Q wave, the R wave, and the S wave. The Q wave: is always located at the beginning of the QRS complex.

It may or may not always be present. The R wave: is always the first positive deflection. The S wave: the negative deflection, follows the R wave Location: Follows the P-R interval Amplitude: Normal values vary with age and sex Duration: No longer than 0.10 seconds

Q-T Interval: represents the time necessary for ventricular depolarization and repolarization.

Location: Extends from the beginning of the QRS complex to the end of the T wave (includes the QRS complex, S-T segment, and the T wave)

Duration: Varies according to age, sex, and heart rate T Wave: represents the repolarization of the ventricles. On rare occasions, a U wave can be seen following the T wave. The U wave reflects the repolarization of the His-Purkinje fibres.

Location: Follows the S wave and the S-T segment

Amplitude: 5mm or less in standard leads I, II, and III; 10mm or less in precordial leads V1-V6.

Duration: Not usually measured


S-T Segment: represents the end of the ventricular depolarization and the beginning of ventricular repolarization.

Location: Extends from the end of the S wave to the beginning of the T wave

Duration: Not usually measured

The ECG and Myocardial Infarction

During an MI, the ECG goes through a series of abnormalities. The initial abnormality is called a hyper-acute T wave. This is a T wave that is taller and more pointed than the normal T wave.

Hyper-acute T Wave

The abnormality lasts for a very short time, and then elevation of the ST segment occurs. This is the hallmark abnormality of an acute MI. It occurs when the heart muscle is being injured by a lack of blood flow and oxygen and is also called a current of injury.

ST Elevation

An ECG can not only tell you if an MI is present but can also show the approximate location of the heart attack, and often which artery is involved. When the ECG


abnormalities mentioned above occur, then the MI can be localized to a certain region of the heart. For example, see the table below:

ECG leads Location of MI Coronary Artery

II, III, aVF Inferior MI Right Coronary Artery

V1-V4 Anterior or Anteroseptal MI

Left Anterior Descending Artery

V5-V6, I,aVL Lateral MI Left Circumflex Artery

ST depression in V1, V2

Posterior MI Left Circumflex Artery or Right

Coronary Artery

NORMAL SINUS RHYTHM

Rhythm Regular

Rate 60-100 beats per minute

QRS Duration Normal

P Wave Visible before each QRS complex

P-R Interval Normal (<5 small Squares)

Sinus Bradycardia


A heart rate less than 60 beats per minute (BPM). This in a healthy athletic person may be 'normal', but other causes may be due to increased vagal tone from drug abuse, hypoglycaemia and brain injury with increase intracranial pressure (ICP) as examples

Rhythm Regular

Rate Less than 60 beats per minute

QRS Duration Normal




Sinus Tachycardia

An excessive heart rate above 100 beats per minute (BPM) which originates from the SA node. Causes include stress, fright, illness and exercise. Not usually a surprise if it is triggered in response to regulatory changes e.g. shock. But if their is no apparent trigger then medications may be required to suppress the rhythm

Rhythm Regular

Rate More than 100 beats per minute

QRS Duration Normal



Supraventricular Tachycardia (SVT) Abnormal

A narrow complex tachycardia or atrial tachycardia which originates in the 'atria' but is not under direct control from the SA node. SVT can occur in all age groups

Rhythm Regular


QRS Duration Usually normal

P Wave Often buried in preceding T wave

P-R Interval Depends on site of supraventricular pacemaker


Impulses stimulating the heart are not being generated by the sinus node, but instead are coming from a collection of tissue around and involving the atrioventricular (AV) node

Atrial Fibrillation

Many sites within the atria are generating their own electrical impulses, leading to irregular conduction of impulses to the ventricles that generate the heartbeat. This irregular rhythm can be felt when palpating a pulse

Rhythm Irregularly irregular

Rate 100-160 beats per minute but may be slow if on medication


P Wave Not distinguishable as the atria are firing off all over

P-R Interval Not measurable

Atrial Flutter

Rhythm Regular

Rate Around 110 beats per minute


P Wave Replaced with multiple F (flutter) waves, usually at a ratio of 2:1 (2F - 1QRS) but sometimes 3:1

P-R Interval Not measurable


1st Degree AV Block

1st Degree AV block is caused by a conduction delay through the AV node but all electrical signals reach the ventricles. This rarely causes any problems by itself and often trained athletes can be seen to have it. The normal P-R interval is between 0.12s to 0.20s in length, or 3-5 small squares on the ECG.

Rhythm Regular

Rate Normal

QRS Duration Normal

P Wave Ratio 1:1

P-R Interval Prolonged (>5 small squares)

2nd Degree Block Type 1 (Wenckebach)

Another condition whereby a conduction block of some, but not all atrial beats getting through to the ventricles. There is progressive lengthening of the PR interval and then failure of conduction of an atrial beat, this is seen by a dropped QRS complex.

Rhythm Regularly irregular

Rate Normal or Slow

QRS Duration Normal

P Wave Ratio 1:1 for 2, 3 or 4 cycles then 1:0.

P-R Interval Progressive lengthening of P-R interval until a QRS complex is dropped


2nd Degree Block Type 2

When electrical excitation sometimes fails to pass through the A-V node or bundle of His, this intermittent occurrence is said to be called second degree heart block. Electrical conduction usually has a constant P-R interval, in the case of type 2 block atrial contractions are not regularly followed by ventricular contraction

Rhythm Regular

Rate Normal or Slow

QRS Duration Prolonged

P Wave Ratio 2:1, 3: 1

P-R Interval Normal or prolonged but constant

Looking at the ECG you'll see that:

3rd Degree Block

3rd degree block or complete heart block occurs when atrial contractions are 'normal' but no electrical conduction is conveyed to the ventricles. The ventricles then generate their own signal through an 'escape mechanism' from a focus somewhere within the ventricle. The ventricular escape beats are usually 'slow'


Rhythm Regular

Rate Slow


P Wave unrelated

P-R Interval Variation.

Complete AV block. No atrial impulses pass through the atrioventricular node and the ventricles generate their own rhythm

Bundle Branch Block

Abnormal conduction through the bundle branches will cause a depolarization delay through the ventricular muscle, this delay shows as a widening of the QRS complex. Right Bundle Branch Block (RBBB) indicates problems in the right side of the heart. Whereas Left Bundle Branch Block (LBBB) is an indication of heart disease. If LBBB is present then further interpretation of the ECG cannot be carried out.

Rhythm Regular

Rate Normal


P Wave Ratio 1:1

P-R Interval Normal

Premature Ventricular Complexes


Due to a part of the heart depolarizing earlier than it should

Rhythm Regular

Rate Normal

QRS Duration Normal

P Wave Ratio 1:1

P-R Interval Normal

Ventricular Tachycardia (VT) Abnormal

Looking at the ECG you'll see that:

Rhythm Regular



P Wave Not seen

Rhythm - Regular

Ventricular Fibrillation (VF) Abnormal

Disorganised electrical signals cause the ventricles to quiver instead of contract in a rhythmic fashion. A patient will be unconscious as blood is not pumped to the brain. Immediate treatment by defibrillation is indicated. This condition may occur during or after a myocardial infarct.


Rhythm Irregular

Rate 300+, disorganized

QRS Duration Not recognizable

P Wave Not seen

Asystole - Abnormal

Rhythm Flat

Rate 0 beats per minute

QRS Duration None

P Wave None

Myocardial Infarct (MI)

Rhythm Regular

Rate 80 Beats per minute

QRS Duration Normal

P Wave Normal

STsegment Elevated

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The electrocardiogram (ECG) is a diagnostic tool that … · · 2014-05-19An ECG can not only tell you if an MI is present but can also show the approximate ... hypoglycaemia and