PREMATURE
CONTRACTION
Three common terms used to describe certain abnormal cardiac contractions are premature contractions (beats occurring early in time), ectopic beats (beats with sites of origin outside the sinus node), and extrasystoles (added beats). Only extrasystoles are truly added or additional beats, often interpolated or added between two normal beats without interfering with the basic rhythm.
Premature
atrial contractions are caused by an irritability of the atria, with early
contractions emanating from an impulse in the atria outside the SA node. Atrial
is differentiated from ventricular premature contractions by measurement of the
compensatory pause. With atrial contractions the compensatory pause is
incomplete, whereas with ventricular premature contractions the pause is complete
(see Plate 2-25). Measurement of the
compensatory pause for atrial premature beats follows:
•
Select the atrial premature beat (P wave) that appears different from the P
waves of the basic mechanism and is premature in time; this is the atrial
premature contraction.
•
Measure the interval from the premature P wave to the P wave immediately in
front.
•
Add this measured interval to the time between the premature P wave and the
P wave immediately following (<2×).
This total duration
is shorter than the time between two normal P-P intervals that do not include a
premature contraction (2×).
Nodal premature
contractions result from stimulation of the A-V node. Usually there is
retrograde conduction starting at the A-V node, with an accession wave moving
over the atria from the A-V node to the SA node, and P waves of an abnormal
form are written. At times there is no retrograde conduction, and P waves and
atrial contraction do not occur. The stimulus at the A-V node often is vagal or
is caused by disease (see Plate 2-25).
High Atrioventricular Nodal
Rhythm
High A-V nodal
rhythm prevails when the head of the A-V node becomes the pacemaker, and
atrial depolarization occurs in a retrograde fashion from the A-V node to the
SA node. With nodal premature contractions, inverted P waves are written
in leads II, III, and aVF because of retrograde auricular
depolarization. The P-R interval is short, P waves precede the QRS complexes,
and the QRS and T waves are of normal configuration.
When the junctional tissue is stimulated below the A-V
node near its center, atrial and ventricular depolarizations occur
simultaneously. Here the P waves fall within the QRS complexes, and the
summation complexes (QRS + P) are slightly different in appearance from the normal QRS complexes of the basic
mechanism.
If the
pacemaker is low in the junctional tissues, the ventricles are
depolarized before the atria, the QRS complexes are written first, and inverted
P waves in leads II, III, and aVF are written later.
RIGHT
VENTRICULAR IMPULSE
The P wave
rhythm is not disturbed. The ventricles contract early from a stimulus in the
region of the right ventricle. The accession wave travels from right to left
and, moving in this direction, produces upright QRS deflections in lead
I. The duration of this complex is long because of the abnormally long pathway
and is longer than 0.10 second, followed by S-T segments and T waves in the
opposite direction as the major deflections of the QRS complexes. The
compensatory pause is complete; that is, the interval between two normal
QRS complexes
that do not contain an ectopic beat is the same as the time from a QRS complex
before the ectopic QRS beat to the QRS complex that follows this beat (see Plate 2-25).
LEFT
VENTRICULAR IMPULSE
A pacemaker in
the LV wall produces an accession wave that travels from left to right, in lead
I resulting in negative (inverted) wide QRS complexes with positive (upward)
S-T segments and T waves and complete compensatory pauses.
