2-11. VENTRICULAR ARRHYTHMIAS.
a. General. Ventricular arrhythmias are the most serious arrhythmias. The
definition of arrhythmia is an abnormality in the rate, rhythm, or conduction of the
heartbeat. A heartbeat irregularity may be an indication of some disease or a normal
response to the body's activity. Arrhythmias are classified on the basis of the site of
origin of the arrhythmia: SA node, atria, AV node, or ventricle. As with some heart
blocks, the QRS complex measurement is equal to or greater than 0.12 seconds. A
QRS of less than 0.12 of a second indicates a supraventricular pacemaker. If the QRS
is 0.12 of a second or greater, the problem could be ventricular or supraventricular with
ventricular conduction defect. All ventricular arrhythmias will have a QRS complex of
0.12 of a second or greater. The reason ventricular rhythms are the most serious
arrhythmias is that the heart is less effective than usual and is functioning on its last
level of backup support. Four types of ventricular arrhythmias will be covered here:
premature ventricular contractions, ventricular tachycardia, ventricular fibrillation, and
idioventricular rhythm.
b. Premature Ventricular Contractions.
(1) Analysis. A premature ventricular contractions (PVCs) is a single
ectopic (heartbeat arising from a place other than the SA node) caused by irritable focus
in the ventricles. The heartbeat comes earlier than expected and interrupts the
regularity of the underlying rhythm of the heart. The heartbeat rate is determined by the
underlying heartbeat rhythm, PVCs not being included in the rate. P waves will not
come before an ectopic heartbeat; however, P waves may be seen near a premature
ventricular contraction. Since the heartbeat comes from a lower focus, there will be no
P-R interval. The QRS complex will be wide and bizarre (at least 0.12 or more of a
second). The T wave is usually in the opposite direction from the R wave. Premature
ventricular contractions may be quite serious, particularly in certain combinations and in
the wake of acute myocardial infarction. Dangerous signs include frequent PVCs, runs
of PVCs, multifocal PVCs, and the R-on-T phenomenon. In a perfectly healthy person,
PVCs may not always be considered an abnormality; however, if the casualty is
experiencing acute myocardial infarction, PVCs must be treated instantly.
Characteristics. The following are characteristic of premature ventricular
(2)
contractions:
(a) A compensatory pause usually follows a PVC. The distance
between the R wave of complex preceding the PVC and the R wave of complex
following the PVC is exactly twice the R-R interval of underlying rhythm.
(b) The PVC does not have to have a compensatory pause. The beat
can be "interpolated" between two sinus beats without interrupting the underlying
rhythm.
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