This
is a regularly irregular rhythm with dropped beats. A
key clue that this is not Mobitz type I block is that
the characteristic prolongation of the PR intervals is
absent. On closer examination, it becomes apparent, especially
in lead II, that the T wave in each cycle is slightly
larger than the preceding T wave. This change in morphology
occurs because the premature P wave (P') has fallen on
top of the T wave. Because of the prematurity of the P',
it reaches the the AV node while it is still in its' refractory
period and conduction cannot occur. This results in a
non-conducted premature atrial beat that does not reach
the ventricles but does reset the SA node, resulting in
a brief pause in pacing. The P-to-P interval corresponds
to the P'-to-P interval due to this resetting of the SA
node. This pattern of two normal sinus beats with a dropped
beat secondary to a non-conducted premature atrial complex
results in a pattern of bigeminy on EKG.
Atrial
premature contractions (APC's or PAC's) are, in and of
themselves, benign. They occur in patients with and without
structural heart disease. APC's are more common with increasing
age and may be triggered by alcohol, stress, caffeine,
and tobacco. They can also be secondary to more serious
problems such as ischemia, acute myocardial infarctions,
electrolyte abnormalities, mitral valve disease, pericarditis,
and atrial hypertrophy or dilation. Patients with APC's
may be completely asymptomatic or may describe a feeling
of palpitations or noting a skipped beat. APC's may also
precede the development of a supraventricular tachycardia.
There is no clear evidence regarding the mechanism for
PAC's, but one theory is that they occur secondary to
a re-entrant type phenomena after depolarizations in atrial
muscle fibers near the SA node or other foci of automaticity.
The key finding on EKG is a P wave that has a different
morphology from the sinus P wave and occurs earlier than
expected. The premature beat may or may not be conducted
depending on its timing in the conduction cycle.
In the
asymptomatic patient, no treatment for PAC's is necessary.
If medical treatment is required, beta-blockers are first
line agents. Type IA, IC, and type III antiarrhythmic
agents can diminish the frequency of PACs and protect
against the development of a supraventricular tachycardia.
Digoxin, calcium channel blockers, and type IB antiarrhythmic
agents have not been clearly shown to be beneficial in
symptomatic patients.
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