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PR Intervals
Fast & Easy ECGs, 2nd E – A SelfPaced Learning Program
Fast & Easy ECGs, 2E
1
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
PR Intervals
• Another important step of assessing the ECG
is analyzing the PR intervals
Q
Fast & Easy ECGs, 2E
2
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
PR Interval
• Distance from the
beginning of the P
wave to the
beginning of the Q
wave
• Morphology is a P
wave and a flat line
I
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
PR Interval
• Denotes depolarization of the heart from the
SA node through the atria, AV node and HisPurkinje system
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
PR Intervals
• Their presence or absence as well as duration
can help us identify if and how well the
impulses are traveling through the heart’s
conductive pathway
– Important to know because delays or interruption
of conduction through AV node and/or junction
can lead to serious or even life-threatening
conditions
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Measuring PR Intervals
• If the PR intervals are
identifiable, measure
from where the P wave
begins to where the Q
wave begins
– To make it easier, find a
P wave that begins on a
vertical line and then
measure to where the Q
or R wave begins
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
PR Intervals
• Are considered abnormal if they are shorter,
longer, progressively longer, vary or absent
Fast & Easy ECGs, 2E
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Shorter P’R Intervals
• Shorter P’R
intervals occur
when the
impulse
originates in
the atria close
to the AV
junction or in
the AV junction
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Shorter P’R Intervals
• Can occur when an
impulse arises from a
supraventricular site
but travels through
abnormal accessory
pathways to the
ventricles
• Leads to premature
ventricular
depolarization called
preexcitation
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Longer P’R Intervals
• Can occur
when there is a
delay in
impulse
conduction
through the AV
node
Fast & Easy ECGs, 2E
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Varying P’R Intervals
• Can occur when
the pacemaker
site moves from
beat to beat
causing the P’
waves to appear
different and the
P’R intervals to
vary
Fast & Easy ECGs, 2E
11
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Varying P’R Intervals
• In one form of AV
heart block, the
PR intervals are
progressively
longer until a QRS
complex is
dropped and then
the cycle repeats
Fast & Easy ECGs, 2E
12
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Absent PR Intervals
• Occurs with:
– atrial rates that
exceed 250 BPM
and produce flutter
waves or a chaotic
baseline and
absence of P waves
preceding the QRS
complexes
– ventricular
dysrhythmias
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Absent PR Intervals
• Also seen in
the most
severe form of
AV heart block
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
More P Waves and Constant PR Intervals
• In on form of AV
heart block, some
sinus beats are
blocked in the AV
node and do not
reach the ventricles
• PR intervals
associated with P
waves that are
conducted through
to the ventricles are
constant
Fast & Easy ECGs, 2E
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© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• In this tracing, determine the type of PR
interval
I
Fast & Easy ECGs, 2E
16
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• In this tracing, determine the type of PR
interval
I
Fast & Easy ECGs, 2E
17
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• In this tracing, determine the type of PR
interval
I
Fast & Easy ECGs, 2E
18
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• In this tracing, determine the type of PR
interval
I
Fast & Easy ECGs, 2E
19
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Practice Makes Perfect
• Determine the type of PR interval
I
Fast & Easy ECGs, 2E
20
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• Another important step of analyzing an ECG
rhythm is examining the PR intervals
• PR interval is the distance from the beginning
of the P wave to the beginning of the Q wave
• Duration of the PR interval is normally 0.12 to
0.20 seconds
• PR intervals are considered abnormal if they
are shorter, longer, absent or vary
Fast & Easy ECGs, 2E
21
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• Shorter P’R intervals occur when the impulse
originates in the atria close to the AV junction
or in the AV junction itself
• AV heart block is the most common cause of
longer PR intervals
• When the pacemaker site moves from beat to
beat it causes the P’ waves to appear different
and the P’R intervals to vary
Fast & Easy ECGs, 2E
22
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• In one for of AV heart block, the PR intervals are
progressively longer until a QRS complex is dropped
and then the cycle repeats
• There is an absence of PR intervals in atrial rates that
exceed 250 BPM producing flutter waves or a chaotic
looking baseline and with ventricular dysrhythmias
• In the most severe form of AV heart block the PR
intervals are not measurable
Fast & Easy ECGs, 2E
23
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.
Summary
• In one form of AV heart block, some of the
sinus beats are blocked in the AV node and do
not reach the ventricles. The PR intervals
associated with the P waves that are
conducted through to the ventricles are
constant in duration
Fast & Easy ECGs, 2E
24
© 2013 The McGraw-Hill Companies, Inc. All rights reserved.