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 upward deflection 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:
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.
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