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.