ECG INTERPRETATIONS - LEARN BASICS OF ECG

Electrocardiograph (ECG or EKG [from the German Elektrokardiogramm]) is a transthoracic interpretation of the electrical activity of the heart over time captured and externally recorded by skin electrodes.^[1] It is a noninvasive recording produced by an electrocardiographic device. The etymology of the word is derived from the Greek electro, because it is related to electrical activity, cardio, Greek for heart, and graph, a Greek root meaning "to write". In English speaking countries, medical professionals often write EKG (the abbreviation for the German word elektrokardiogramm) in order to avoid confusion with EEG.

"The ECG learning modules are designed to help you learn how to look at ECG rhythm strips and interpret them. Is the strip the normal rhythm of the heart -- normal sinus rhythm? Or an abnormal and potential dangerous rhythm like ventricular fibrillation? You will also be introduced to the 12-lead ECG and learn how to look for signs of an acute myocardial infarction" -

ECG Basics
Normal ECG (Example)

An ECG is an electrical picture of the conduction of the heart. By interpreting changes from normal on an ECG, clinicians can identify a myriad of cardiac disease processes.

There are two ways to learn ECG interpretation; pattern recognition (the most common) or by understanding the exact electrical vectors recorded by an ECG. Most people learn a combination of the two and basing ECG interpretation on pattern recognition alone is often not sufficient.

Parts of an ECG

A normal ECG contains waves, intervals, segments, and one complex.

P wave: Indicates atrial depolarization
QRS complex: Indicates ventricular depolarization
T wave: Indicates ventricular repolarization

The P wave occurs when the SA node creates an action potential that depolarizes the atria. The P wave should be upright in lead II if the action potential is originating from the SA node. If this is so, the EKG is said to demonstrate a "normal sinus rhythm" abbreviated "NSR". As long as the atrial depolarization is able to spread through the AV node to the ventricles, each P wave should be followed by a QRS complex.

The QRS complex comes after the P wave when the SA nodal action potential travels through the AV node to the ventricles to cause ventricular depolarization. The first downward deflection is called the Q wave. The first upwarddeflection is called the R wave. The second downward deflection is called the S wave.

The T wave occurs after the QRS complex and is a result of ventricular repolarization. T waves should be upright in most leads (except aVR and V₁). T waves should by asymmetric in nature. The second portion of the T wave should have a steeper decline when compared to the incline of the first part of the T wave. If the T wave appears symmetric, cardiac pathology may be present.

An interval is the distance between two specific ECG events. For example:

PR interval: The time from the beginning of the P wave (atrial depolarization) to the beginning of the QRS complex (ventricular deploarization). A prolonged or shortened PR interval can indicate certain disease states.

QT interval: The time from the beginning of the QRS complex (ventricular depolarization) to the end of the T wave (ventricular repolarization). QT interval prolongation can be very serious.

Normal values:

PR interval: 0.12 - 0.20 seconds
QT interval: < 0.42 seconds
QRS complex duration: < 0.10 seconds

A segment is the length between two specific points on the EKG which are supposed to be at the baseline amplitude (not negative or positive). For example:

ST segment: The portion of the ECG from the end of the QRS complex to the beginning of the ECG. The ST segment should not be elevated or depressed. Any change from baseline may indicated cardiac disease.

TP segment: The portion of the ECG from the end of the T wave to the beginning of the P wave. This segment should always be at baseline and is used as a reference to see if the ST segment is elevated or depressed.

PR segment: The portion of the ECG from the end of the P wave to the beginning of the QRS complex. PR segment depression can indicate disease, however PR segment elevation usually does not.

EKG Leads

There are 12 leads on a standard ECG. Six of the leads of considered "limb leads" since they are placed on the arms and/or legs of the person. The other six leads are considered "precordial leads" since they are placed on the person's torso (precordium).

Reading an EKG

Introduction

When looking at a 12-lead ECG, there are a few logistics that must be known. First of all, the standard 12-lead ECG is a 10 second strip. The bottom one or two lines will be a full "rhythm strip" of a specific lead spanning the whole 10 seconds of the ECG. Other leads will only span about 2.5 seconds.

Each ECG is divided by large boxes and small boxes to help measure times and distances. Each large box is 0.20 seconds. There are five small boxes in each large box, thus each small box is 0.04 seconds.

If the entire ECG is 10 seconds, then there must be 50 large boxes (0.20 seconds X 50 large boxes). Each small box is also exactly 1 mm in length and so one large box is 5 mm. In genereal, when measuring amplitudes of waves/complexes, the units are expressed in millimeters (mm) and when measuring lengths for intervals, the units are expressed in time (seconds or milliseconds).

If each small box is equal to 0.04 seconds or 1 mm, then the standard ECG speed is 1 mm per 0.04 seconds or 25 mm per second.

The standard approach to reading an ECG includes in this order:

1) Examining the rate
2) Examining the rhythm
3) Examining the axis, intervals and segments
4) Examining everything else.

Rate

There are two different rates that can be determined on ECGs. The atrial rate is indicated by the frequency of the P waves and the ventricular rate is indicated by the frequency of the QRS complexes. Normally, the artial rate should be the same as the ventricular rate in the absence of disease, however certain conditions, such as third degree AV nodal block, can alter this normal relationship.

One quick and easy way to determine the ventricular rate is to examine the R to R interval and use a standard scale to find the rate. If two consecutive R waves are seperated by only one large box, then the rate is 300 beats per minute. If the R waves are separated by two large blocks, then the ventricular rate is 150 beats per minute. The scale continues down to show that if two consecutive R waves are separated by 8 large boxes, then the rate is 37 beats per minute. The pictoral explaination of this method is below:

Another quick way to calculate the rate is based on the fact that the entire ECG is 10 seconds. So by counting the number of QRS complexes and multiplying by 6, the number per minute can be calculated. This is a better method when the QRS complexes are irregular making the first method less accurate, since the R to R intervals may vary from beat to beat.

Rhythm

The rhythm is either sinus or not sinus. If there is a P wave before every QRS complex, the P wave is upright in lead II, and the P wave has a normal morphology, then normal sinus rhythm is said to be present. If there is sinus rhythm and the heart rate is greater than 100, then sinus tachycardia is present. If the there is sinus rhythm and the heart rate is less than 60, then sinus bradycardia is present.

If there are no P waves present or the P wave morphology is not normal, then the exact rhythm must be determined. See the reviews of different rhythms for more information.

Axis

The axis of the ECG is the major direction of the overall electical activity of the heart. It can be normal, leftward, rightward, or indeterminate. Usually, the QRS axis is determined, however the P wave or T wave axis can also be calculated.

To determine the axis, you must examine the limb leads (not the precordial leads). The depiction of the standard leads and their relationship to the cardiac axis is below.



Note that lead I is at zero degrees, lead II is at +60 degrees, and lead III is at +120 degrees. Lead aVL (L for left arm) is at -30 degrees, lead aVF (

F for "floor") is at +90 degrees, and the negative of lead aVR (R for right arm) is at +30 degrees. The positiveof lead aVR is actually at -150 degrees. Memorizing the above picture is crucial to accurately determining axis.


LAD = Left Axis Deviation
RAD = Right Axis Deviation
NW = northwest axis or indeterminate axis

One quick, non-specific method can be used to determine the QRS axis. If the QRS complex is upright (positive) in both lead I and lead aVF, then the axis is normal (see the above picture). If the QRS is upright in lead I and downward (negative) in lead aVF, then the axis is leftward. If the QRS is predominately downward in both leads I and aVF, then the axis is rightward. If the QRS is downward (negative) in lead I and upward (positive) in lead aVF, then the axis is northwest or indeterminate.

Another way to determine QRS axis, again using the first picture above, is to find which lead is the "isoelectric lead", that is the lead in which the upward deflection of the QRS is about the same as the downward deflection. The axis will then be exactly perpendicular to that isoelectric lead.

DOWNLOAD LINKS

• ECG Basics
• How to Analyze a Rhythm
• Normal Sinus Rhythm
• Heart Arrhythmias 1
• Heart Arrhythmias 2
• Heart Arrhythmias 3
• Acute Myocardial Infarction
• Advanced 12-Lead Interpretation
• Reading 12-Lead ECGs 1
• Reading 12-Lead ECGs 2

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• 50 ECG CASES -- LEARN ECG INTERPRETATIONS !!
• ECG QUIZ -- JUDGE YOURSELF ON ECG INTERPRETATIONS !!

ECG Archive

Check out the ALL NEW ECG quiz section with 100 ECG quizes!
Also 100+ USMLE questions now available and more on the way!

Below you will find the LearnTheHeart.com archive of ECGs. More will be added periodically. Currently over 200 available. If you would like to challange yourself to read the ECGs without the diagnosis given, check out the ECG quizes section (coming soon).

Normal EKG (Example)

Conduction abnormalities
Left Bundle Branch Block (Example 1)
Left Bundle Branch Block (Example 2)
Left Bundle Branch Block (Example 3)
Left Bundle Branch Block (Example 4)
Left Bundle Branch Block (Example 5)
Left Bundle Branch Block (Example 6)
Left Bundle Branch Block (Example 7)

Incomplete Left Bundle Branch Block (Example 1)

Right Bundle Branch Block (Example 1)    
Right Bundle Branch Block (Example 2)
Right Bundle Branch Block (Example 3)    
Right Bundle Branch Block (Example 4)
Right Bundle Branch Block (Example 5)
Right Bundle Branch Block (Example 6)
Right Bundle Branch Block (Example 7)
Right Bundle Branch Block (Example 8)

Incomplete Right Bundle Branch Block (Example 1)
Incomplete Right Bundle Branch Block (Example 2)
Incomplete Right Bundle Branch Block (Example 3)

Left Anterior Fascicular Block (Example 1)
Left Anterior Fascicular Block (Example 2)
Left Anterior Fascicular Block (Example 3)
Left Posterior Fascicular Block (Example)

Bifascicular Block - RBBB + LAFB (Example 1)
Bifascicular Block - RBBB + LAFB (Example 2)
Bifascicular Block - RBBB + LAFB (Example 3)
Bifascicular Block - RBBB + LAFB (Example 4)

Bifascicular Block - RBBB + LPFB (Example 1)

Trifascicular Block (Example 1)   
Trifascicular Block (Example 2)

First Degree AV Block (Example 1)
First Degree AV Block (Example 2)
First Degree AV Block (Example 3)
First Degree AV Block (Example 4)

Second Degree AV Block Type I - Wenkebach (Example 1)
Second Degree AV Block Type I - Wenkebach (Example 2)

2:1 AV Block (Example)

Third Degree AV Block (Example)

Ischemic Heart Disease
Anterior Wall ST elevation MI (Example 1)
Anterior Wall ST elevation MI (Example 2)
Anterior Wall ST elevation MI (Example 3)
Anterior Wall ST elevation MI (Example 4)
Anterior Wall ST elevation MI (Example 5)
Anterior Wall ST elevation MI (Example 6)
Anterior Wall ST elevation MI with RBBB (Example)
Old Anterior Wall MI (Example)

Inferior Wall MI (Example 1)
Inferior Wall MI (Example 2)
Inferior Wall MI (Example 3)
Inferior Wall MI (Example 4)
Inferior Wall MI (Example 5)
Inferior Wall MI (Example 6)
Inferior Wall MI with RBBB (Example 5)
Old Inferior Wall MI (Example 1)
Old Inferior Wall MI (Example 2)
Normal inferior Q waves - NOT old inferior MI (Exmaple)

Posterior Wall MI - Standard EKG (Example)
Posterior Wall MI - Posterior EKG (Example)
Inferior-Posterior Wall MI (Example 1)
Inferior-Posterior Wall MI Right-sided ECG (Example 1)
Inferior-Posterior Wall MI (Example 2)
Inferior-Posterior Wall MI (Example 3)
Inferior-Posterior Wall MI (Example 4)

Anterior Ischemia - (Example)
STEMI with a Paced Rhythm (Example)

Atrial Arrhythmias
Sinus Tachycardia (Example 1)
Sinus Tachycardia (Example 2)
Sinus Tachycardia (Example 3)
Sinus Tachycardia (Example 4)

Sinus Bradycardia (Example 1)
Sinus Bradycardia (Example 2)
Sinus Bradycardia (Example 3)
Sinus Bradycardia (Example 4)

Sinus Arrhythmia (Example 1)
Sinus Arrhythmia (Example 2)

Premature Atrial Contractions (PACs) (Example 1)
Premature Atrial Contractions (PACs) (Example 2)
Atrial Bigeminy (Example 1)

Atrial Fibrillation with Rapid Ventricular Rate (Example 1)
Atrial Fibrillation with Rapid Ventricular Rate (Example 2)
Atrial Fibrillation with Rapid Ventricular Rate (Example 3)
Atrial Fibrillation with Rapid Ventricular Rate (Example 4)
Atrial Fibrillation with Rapid Ventricular Rate (Example 5)
Atrial Fibrillation with normal Ventricular Rate (Example 1)
Atrial Fibrillation with normal Ventricular Rate (Example 2)
Atrial Fibrillation with normal Ventricular Rate (Example 3)

Atrial Fibrillation with bradycardia (Example 1)
Atrial Fibrillation with bradycardia (Example 2)

Atrial Flutter with Variable Conduction (Example 1)
Atrial Flutter with Variable Conduction (Example 2)
Atrial Flutter with Variable Conduction (Example 3)

Atrial Flutter with 2:1 Conduction (Example 1)
Atrial Flutter with 2:1 Conduction (Example 2)   
Atrial Flutter with 3:1 Conduction (Example 1)
Atrial Flutter with 4:1 Conduction (Example 1)     
Atrial Flutter with 4:1 Conduction (Example 2)
Atrial Flutter with 4:1 Conduction (Example 3)
Atrial Flutter with 5:1 Conduction (Example 1)
Atrial Flutter with 5:1 Conduction (Example 2)

Multifocal Atrial Tachycardia (Example)
Ectopic Atrial Tachycardia (Example 1)    
Ectopic Atrial Tachycardia (Example 2)
Ectopic Atrial Bradycardia (Example)
AV Nodal Reentrant Tachycardia (Example)
AV Rentrant Tachycardia (Example)

Ventricular Arrhythmias
Premature Ventricular Contractions (PVCs) (Example 1)
Premature Ventricular Contractions (PVCs) (Example 2)
Premature Ventricular Contractions (PVCs) (Example 3)
Ventricular Bigeminy (Example 1)
Ventricular Bigeminy (Example 2)
Ventricular Bigeminy (Example 3)
Ventricular Trigeminy (Example 1)
Ventricular Trigeminy (Example 2)
Ventricular Trigeminy (Example 3)

Accelerated Idioventricular Rhythm (Example 1)
Accelerated Idioventricular Rhythm (Example 2)
Accelerated Idioventricular Rhythm (Example 3)
Accelerated Idioventricular Rhythm (Example 4)

Monomorphic Non-Sustained Ventricular Tachycardia (Example)
Monomorphic Sustained Ventricular Tachycardia (Example 1)
Monomorphic Sustained Ventricular Tachycardia (Example 2)
Polymorphic Ventricular Tachycardia (Example)

Ventricular Fibrillation (Example)
Asystole (Example)

Junctional Bradycardia (Example)
Accelerated Junctional Rhythm (Example 1)
Accelerated Junctional Rhythm (Example 2)
Junctional Tachycardia (Example)

Chamber Enlargement and AxisLeft Atrial Enlargement (Example 1)
Left Atrial Enlargement (Example 2)
Left Atrial Enlargement with P-Mitrale (Example)

Right Atrial Enlargement (Example 1)     
Right Atrial Enlargement (Example 2)
Right Atrial Enlargement (Example 3)
Right Atrial Enlargement (Example 4)

Biatrial Enlargement (Example)

QRS Abnormalities
Left Ventricular Hypertrophy (Example 1)
Left Ventricular Hypertrophy (Example 2)Left Ventricular Hypertrophy (Example 3)Left Ventricular Hypertrophy (Example 4)Left Ventricular Hypertrophy (Example 5)Left Ventricular Hypertrophy (Example 6)
Left Ventricular Hypertrophy With Strain Pattern (Example 1)
Left Ventricular Hypertrophy With Strain Pattern (Example 2)Left Ventricular Hypertrophy With Strain Pattern (Example 3)Left Ventricular Hypertrophy With Strain Pattern (Example 4)
Right Ventricular Hypertrophy (Example 1)Right Ventricular Hypertrophy with Strain (Example 1)
Left Axis Deviation (Example)
Right Axis Deviation (Example)

MiscellaneousPericarditis (Example 1)Pericarditis (Example 2)
Early Repolarization (Example 1)Early Repolarization (Example 2)
LV Aneurysm (Example)Pulmonary Embolism (Example 1)
Pulmonary Embolism (Example 2)
Pulmonary Embolism (Example 3)
Pulmonary Embolism (Example 4)
WPW (Example 1)WPW (Example 2)
Low Voltage (Example 1)Low Voltage (Example 2)Low Voltage (Example 3)Low Voltage (Example 4)Pseudo-Low Voltage (Example)
Hyperkalemia (Example 1)
Hyperkalemia (Example 2)
Hyperkalemia (Example 3)
Hyperkalemia (Example 4)
Hyperkalemia before calcium bolus (Example 5)
Hyperkalemia just after calcium bolus (Example 5)
Hypokalemia (Example 1)
Hypocalcemia (Example)
Prolonged QT Interval (Example 1)
Prolonged QT Interval (Example 2)
Prolonged QT Interval (Example 3)
Prolonged QT Interval (Example 4)
Prolonged QT Interval (Example 5)
Wellen's Syndrome (Example 1)
Wellen's Syndrome (Example 2)
Wellen's Syndrome (Example 3)
Wellen's Syndrome (Example 4)
Wellen's Syndrome (Example 5)
Lown-Ganong-Levine (Example 1)
Lown-Ganong-Levine (Example 2)
Digoxin effect (Example)
Limb lead reversal (Example 1)
Limb lead reversal (Example 2)
Neurologic Insult (Example)
Brugada Syndrome (Example)
Arrhythmogenic right ventricular dysplasia (Example)

SOURCE: UCSF SCHOOL OF MEDICINE