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Module title = Tutorial: Origins of Activity
Lesson title = P wave rules
This is lesson 7 of 8 in this module
Rule: More P waves than QRS complexes = conduction block
If you have a P wave without a QRS after it, then the electrical signal that generated the P wave did not travel into the ventricles to generate a QRS complex. Therefore, the electrical signal was
blocked
.
The only way to have
more P waves
than QRS complexes is to have one or more P waves without a QRS after it. There are 2 situations in which you will see this:
The
AV node is diseased
and is not conducting the electrical signals from the SA node down to the ventricles.
The AV node is normal, but there are too many P waves (more than 200 per minute) that can be conducted through the AV node, so there is a conduction block for
normal physiological reasons.
Listing these situations above as ECG diagnoses, they are:
Second or third degree heart blocks
Atrial flutter (with a physiological block)
Note that atrial fibrillation is not listed because there are no P waves with atrial fibrillation and first degree heart block is not listed because with
first
degree block, there are an
equal
number of P waves with QRS complexes.
If these terms are not familiar to you yet, don't stress. We will cover them in the
Rhythm Diagnostic Criteria tutorial
.
Rule:
P waves all the same size and shape = 1 origin for the P waves
This should make sense. If the P waves are all starting from the
exact same location
within the heart, then they will travel through the heart in the same way and create the
same deflection
every time on the ECG.
Rule:
P waves
NOT
all the same size and shape = 2 or more origins for the P waves
If you depolarize the atria from a different location, then the signal will travel through the atria in a
different direction
and have a different vector and therefore create a different waveform on the ECG. Below is atrial depolarization by a sinus, atrial, and junctional pacemaker. Each will create a
unique wave of electricity
through the atria and therefore create a uniquely shaped P wave.
SA node beat Atrial beat Junctional beat
Sometimes we can see a mixture of pacemakers. For example, the SA node might be producing most of the heartbeats but then occasionally an extra beat from the atria or from the AV node might sneak in for a cameo.
Summary:
more P waves than QRS complexes = conduction block
think 2nd or 3rd degree AV block
think atrial flutter (or other fast atrial rhythms)
P waves
not
all same size and shape = more than one origin for the P waves
think about a combination of sinus, atrial or junctional beats
Lesson 7 of 8
That was the last lesson!