Preventricular Contractions (PVC's) are the most common
arrhythmia in the cardiac catheterization laboratory. The
ventricles normally contract out of synchronization with the
electrical stimulation of the SA node through the AV node. PVC's
are the result of ectopic foci electrically stimulating the
entire ventricular wall to contract. The PVC appears as a 
wide,
aberrant QRS complex. There is no P-wave because the
electrical
impulse for contraction did not originate from the SA node. The
ectopic foci can be located in either ventricle and can fire at
any time during the normal electrical conduction pattern.
PVC's can occur during any segment of the electrocardiogram.
There are specific names given to represent the number of PVC's
presented on the electrocardiogram or to define the PVC rhythm.
Bigeminy is the name
given to a PVC which occurs after every QRS
complex (i.e. every other beat is a PVC). 
Trigeminy defines those
PVC's which occur after every second beat (i.e. every two QRS
complexes are followed by a PVC). The term 
Quadgeminy means that
every fourth beat is a PVC.
PVC's can also be characterized by the similarity in the
shape of the QRS complex. Unifocal PVC's are those which
originate from the same ectopic foci in the ventricle. The shape
of each PVC wave is similar. 
Multifocal PVC's are those which
originate from two or more ectopic foci in the ventricle. These
ectopic foci can be located anywhere in either the right or left
ventricles. They can occur sporadically and each PVC has its own
shape.PVC's will be represented in all electrocardiogram leads.
In the cardiac catheterization laboratory, lidocaine is the drug of choice used for the treatment of PVC's because it suppresses ventricular activity. It is most often administered by the circulating technologist, when requested by the physician, and is introduced into the side port on the IV. Lidocaine toxicity can cause severe bradycardia, which may lower the heart rate to 30 or fewer beats per minute.
Since a PVC can occur at anytime during the electrical
conduction of the heart, the action potential of myocardial
contraction can also be affected. If a PVC occurs during the
normal T-wave it is called an 
"R on T" event. Since the T-wave
represents ventricular repolarization, it is during this stage of
the action potential of the ventricle that the muscle is
susceptible to any external stimulation which would cause
contraction. Ectopic foci can create a stimulus which can
overwhelm the ventricular musculature and result in an
arrhythmia. An "R on T" event can cause a resulting arrhythmia
which can 
progress
into ventricular tachycardia. PVC's are
considered dangerous and life threatening if there are more than
five occurring in a one minute period or if they are multifocal.