Download PR Interval

SKILLS – ECG Reading & Interpretation
Background
An ECG is printed on paper covered with a grid of squares.
Notice that five small squares on the paper form a larger
square. The width of a single small square on ECG paper
represents 0.04 seconds. To successfully interpret ECGs,
you must have this value committed to memory. Do this now. If each small
square represents 0.04 seconds, then a second will be 25 small squares
across. If you print out a minute's worth of your heart's electrical activity,
the paper would be 1500 small squares wide. If something on an ECG is,
let's say, 12 small squares in width, that means that it lasted 12 x 0.04, or
almost half a second. A common length of an ECG printout is 6 seconds;
this is known as a "six second strip."
The first little hump is known as the P wave. It occurs when the atria
depolarize (i.e. trigger).The next three waves constitute the QRS
complex. They represent the ventricles depolarizing. These three are
lumped together because a normal rhythm may not have all three.
Many times, you'll only see a R and an S. This is not abnormal. If there
are less than three, how do we know which one is which? Well, the R
wave is the first wave ABOVE the isoelectric line. You then name the
waves in relation to the R wave. If it falls before the R wave, it is called
the Q wave; after the R wave is the S wave.
Normal Values
Feature
Description
Duration
RR
The interval between an R wave and the next R wave is the inverse of the
0.6 to 1.2 sec
interval heart rate. Normal resting heart rate is between 50 and 100 bpm
The PR interval is measured from the beginning of the P wave to the
beginning of the QRS complex. The PR interval reflects the time the electrical
PR
impulse takes to travel from the sinus node through the AV node and
0.12 to 0.20 s
interval
entering the ventricles. The PR interval is therefore a good estimate of AV
node function.
The QRS complex reflects the rapid depolarization of the right and left
QRS
ventricles. They have a large muscle mass compared to the atria and so the 0.08 to 0.12 s
complex
QRS complex usually has a much larger amplitude than the P-wave.
ST
The ST interval is measured from the J point to the end of the T wave.
0.32 s
interval
The QT interval is measured from the beginning of the QRS complex to the
QT
end of the T wave. A prolonged QT interval is a risk factor for ventricular
0.30 to 0.43 s
interval tachyarrhythmias and sudden death. It varies with heart rate and for clinical
relevance requires a correction for this, giving the QTc.
EXAMPLE
Calculating the time for some of the
intervals helps medical professionals
interpret possible cardiac abnormalities. In
the diagram to the left calculate the
following:
PR Interval
4 ¼ boxes = 4.25 x 0.04 sec =0.17 sec
QRS Interval
2 ¼ boxes = 2.25 x 0.04 sec = 0.09 sec
QT Interval
10 ¼ boxes = 10.25 x 0.04 sec = 0.41 sec
ST Interval
8 boxes = 8 x 0.04 sec = 0.32 sec
Activity
Calculate the R-R, PR, QRS, QT, and ST intervals for the following ECG strips. Show your work!
A
R-R = __________________________
PR = __________________________
QRS = _________________________
QT = __________________________
ST = ___________________________
B
R-R = __________________________
PR = __________________________
QRS = _________________________
QT = __________________________
ST = ___________________________
C
R-R = __________________________
PR = __________________________
QRS = _________________________
QT = __________________________
ST = ___________________________
D
R-R = __________________________
PR = __________________________
QRS = _________________________
QT = __________________________
ST = ___________________________
E
R-R = __________________________
PR = __________________________
QRS = _________________________
QT = __________________________
ST = ___________________________
F
R-R = __________________________
PR = __________________________
QRS = _________________________
QT = __________________________
ST = ___________________________
G
R-R = __________________________
PR = __________________________
QRS = _________________________
QT = __________________________
ST = ___________________________