The flat lines before the P wave, between the P and QRS and after the T wave are said to be at the baseline of that ECG tracing. The line connecting the QRS to the T wave is called the ST segment and is normally quite close to the baseline.
A QRS complex can have positive (upwards) or negative (downwards) deflections. If it starts with an initial negative deflection, that deflection is called a Q wave. The first upward deflection is called a R wave. A negative deflection following an R wave is called an S wave. If there is only one negative deflection without an R wave, that is called a QS complex. A second R wave following an S wave is called an R' ("R-primed") wave. The above tracing shows a large R wave and small S wave.
Time is represented on the horizontal, x-axis on ECGs. The distance between 2 vertical lines is 1 millimeter representing 0.04 seconds with a recorder sweep speed of 25 millimeters per second. The vertical, y-axis represents the amplitude or strength of the electrical signal in millivolts. Poorly reproduced in the above figure are horizontal lines spaced 1 millimeter apart. Each horizontal line represents 0.1 millivolts.
The heart rate is calculated by dividing 60,000 by the time (in milliseconds) between 2 consecutive R waves.
The time it takes for electricity to be conducted from the atria to the ventricles is represented by the PR interval. This is measured from the beginning of the P wave to the beginning of the QRS complex.
The time it takes for the ventricles to become electrically activated is represented by the QRS duration. This is measured from the beginning of the QRS to the end of the QRS.
The total amount of time the ventricles are electrically active (from onset of depolarization to completion of repolarization) is represented by the QT interval. This is measured from the onset of the QRS to the end of the T wave.
The voltage of the P wave and QRS complex is proportional to the total amount of muscle being depolarized. A higher than normal voltage implies overgrowth of the muscle of that chamber. Since the left ventricle has a lot more muscle than the right ventricle, the QRS complex primarily represents electrical events of the left ventricle.
An actual ECG is recorded by placing electrodes on each limb and 6 electrodes on the chest. The first chest lead is called V1 and is placed just to the right of the breastbone. Chest lead V2 is placed just to the left of the breastbone. Chest leads V3 through V6 are sequentially further to the left.
This allows the recording of 12 ECG leads:
- Lead I treats the right arm as negative and left arm as positive.
- Lead II treats the left arm as negative and the left leg as positive .
- Lead III treats the right arm as negative and the left leg as positive.
- Lead aVR treats the right arm as positive and the other limb electrodes as negative.
- Lead aVL treats the left arm as positive and the other limb electrodes as negative.
- Lead aVF treats the left foot as positive and the other limb electrodes as negative
Each of the 6 chest leads is a positive lead. The patient's back is considered the negative electrode for each.
A standard ECG machine records leads I, II and III simultaneously, then aVR, aVL, and aVF simultaneously, then V1, V2, and V3 simultaneously and finally V4, V5, and V6 simultaneously:
This 12 lead ECG recorded 3 heart beats from I, II, III; 3 beats from aVR, aVL, aVF; 3 beats from V1, V2, V3 and 4 beats from V4, V5, V6. You should be able to recognize a QRS complex for each beat in each lead and, in most leads, a preceding P wave and subsequent T wave.
The electrical signal that starts in the atria and travels down to the ventricle is of course moving through three dimensions. Each lead inscribes a positive deflection for that component of the net electrical vector that is travelling towards its positive electrode and a negative deflection for that component of the net electrical vector that is travelling towards its negative electrode. Thus, by knowing the position of each lead, you can determine the direction the electrical signal is travelling. Looking at the heart's electrical activity with 12 leads is like looking at a three dimensional object from 12 different angles.
Different lead sets "look" at different parts on the heart. Leads II, III and aVF all treat the foot electrodes as positive and thus reflect activity at the bottom or inferior wall of the heart.
Leads I, aVL, V5 and V6 have their positive electrodes on the left side of the body and thus reflect activity of the left-most (lateral) wall of the heart.
Leads V1, V2, V3 and V4 have their positive electrodes on the front of the body and thus reflect activity of the front (anterior wall) of the heart.
Here are some common things doctors look for on an ECG. There are examples of these throughout the
ECG gallery.
Depression of the ST segment below the baseline or inversion of the normal T wave deflection can indicate myocardial ischemia-a shortage of blood flow and oxygen that is not quite severe enough to permanently damage the muscle. Elevation of the ST segment above the baseline is the earliest ECG sign of a heart attack (myocardial infarction) where the blood flow and oxygen supply are so low or absent that the heart muscle is dying. Some hours later, Q waves will appear in leads that don't normally have them if the heart attack is not aborted in time.The ST segment elevation often resolves with time but the abnormal Q waves tend to persist.
The leads that show these ST segment abnormalities or Q waves will let you know which part of the heart is affected by these processes.
Abnormal heart rhythms are diagnosed by noting the rate of the P waves and QRS complexes, whether expected P waves or QRS complexes are absent and alterations in the normal 1:1 relationship between the P wave and QRS complexes. The ECG gallery also includes examples of abnormal heart rhythms
