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Chapter 12
Detailed Answers to Assess Your Understanding
With atrial dysrhythmias the atrial waveforms differ in appearance from normal sinus P waves.
In wandering atrial pacemaker, the pacemaker site shifts between the SA node, atria, and/or AV
junction. This produces its key characteristic; P’ waves that change in appearance. It is also called
multifocal atrial rhythm.
Wandering atrial pacemaker produces an irregular rhythm with normal QRS complexes and P’
waves that differ as often as from beat to beat. There must be at least three different looking P’ waves
to classify it as wandering atrial pacemaker.
Depending on from where they originate, premature atrial complexes may have normal P’R
intervals. However, PACs that originate in the atria near the AV node may have a shortened PR interval.
a: (True) A PAC arises earlier in the cardiac cycle and will interrupt the regularity of the underlying
rhythm. Wherever there is irregularity, the P-P and R-R intervals are shorter than the P-P and R-R
intervals of the underlying rhythm. Another key feature of PACs is they are not followed by a
compensatory pause.
c: The heart rate characteristic of atrial tachycardia is 150 to 250 beats per minute.
a: The QRS complexes seen with atrial tachycardia are normally 0.06 to 0.10 seconds in duration. Other
common characteristics include: although there is one P’ wave (unless there is a block) preceding each
QRS complex, it deviates in appearance from the normal P wave and is typically buried in the T wave of
the preceding beat. If present, the P' waves may be flattened or notched. The P’R intervals are typically
indeterminable because the P’ waves tend to be buried. If visible, the P’R interval is often shortened, but
it may also be normal. The dysrhythmia arises above the ventricles, so the QRS complexes are normal
unless there is aberrant conduction.
b: With multifocal atrial tachycardia the heart rate is 100 to 250 beats per minute. MAT is often
misdiagnosed as atrial fibrillation with rapid ventricular response but can be distinguished by looking
closely for the clearly visible but changing P’ waves. The P’ waves change in morphology as often as
from beat to beat. It results in three or more different-looking P waves. The P’ waves may be upright,
rounded, notched, inverted, biphasic, or buried in the QRS complex. It also results in varying P’R
intervals and narrow QRS complexes. You may notice that the only difference between wandering
atrial pacemaker and MAT is the heart rate is faster in MAT.
c: Atrial flutter has a characteristic saw-tooth pattern causing some to analogize that “you can cut a tree
down with that rhythm.”
c: Atrial flutter has an atrial rate of between 250 and 350 beats per minute. The atrial rhythm is regular,
and depending on conduction ratio, the ventricular rhythm may be regular or irregular. The ventricular
rate depends on ventricular response; it may be normal, slow, or fast. A 1:1 atrial-ventricular conduction
is rare; it is usually 2:1, 3:1, 4:1, or variable. The PR interval is not measurable. QRS complexes are
usually normal.
c: Atrial fibrillation has normal QRS complexes (unless there is aberrant conduction or a ventricular
conduction defect). The ventricular rate depends on how many impulses bombarding the AV node are
conducted through to the ventricles. It may be normal, slow, or fast.
b: The atrial waveforms associated with atrial fibrillation are indiscernible and the PR intervals are
nonexistent. Instead of P waves there is a chaotic baseline of fibrillatory waves, or f waves,
representing atrial activity. This is a key characteristic of atrial fibrillation. There are no PR intervals
present because of the absence of P waves.
d: Atrial fibrillation has a totally irregular rhythm. This is a standout feature of atrial fibrillation. An
irregularly irregular supraventricular rhythm is atrial fibrillation until proven otherwise.
d: Atrial fibrillation has an atrial rate of greater than 350 beats per minute.
c: Regarding the patient in the scenario at the beginning of the chapter, the saw-toothed P waves are
known as flutter waves which are characteristic of atrial flutter. The QRS complex will be narrow as
long as there is no conduction defect in the bundle of His and will respond in a patterned manner as a
generally consistent ratio of flutter waves to QRS complexes.
b: As the heart rate increases there is less time for the ventricles to fill between beats. The amount of
blood ejected (stroke volume) diminishes and the cardiac output decreases despite the rise in heart
rate. This is particularly true in the elderly who have less efficient contraction of the ventricles secondary
to aging.
c: Cardiac output is equal to heart rate multiplied by stroke volume.