Parasystole is a kind of arrhythmia caused by the presence and function of a secondary pacemaker in the heart, which works in parallel with the SA node. Parasystolic pacemakers are protected from depolarization by the SA node by some kind of entrance block. This block can be complete or incomplete. Parasystolic pacemakers can exist in both the atrium or the ventricle. Atrial parasystolia are characterized by narrow QRS complexes
Two forms of ventricular parasystole have been described in the literature, fixed parasystole and modulated parasystole. Fixed ventricular parasystole occurs when an ectopic pacemaker is protected by entrance block, and thus its activity is completely independent from the sinus pacemaker activity. Hence, the ectopic pacemaker is expected to fire at a fixed rate. Therefore, on ECG, the coupling intervals of the manifest ectopic beats will wander through the basic cycle of the sinus rhythm. Accordingly, the traditional electrocardiographic criteria used to recognize the fixed form of parasystole are:

the presence of variable coupling intervals of the manifest ectopic beats;
inter-ectopic intervals that are simple multiples of a common denominator;
fusion beats.According to the modulated parasystole hypothesis, rigid constancy of a pacemaker might be expected if the entrance block were complete, but if there is an escape route available for the emergence of ectopic activity, then clearly there must be an effective ionic communication, not complete insulation, between the two tissues. If there is an electrical
communication between the two, then the depolarization of the surrounding ventricle may influence the ectopic pacemaker.
Locations
Colon
Adenomas of the colon, also called adenomatous polyps, are quite prevalent. They are found commonly at colonoscopy. They are removed because of their tendency to become malignant and to lead to colon cancer. Ashkenazi Jews have a 6% higher risk rate of getting adenomas, and then colon cancer, than do the general population, so it is important that they have regular actual colonoscopies, and specifically none of the less invasive diagnostic methods. Renal
This is a tumor that is most often small and asymptomatic, and is derived from renal tubules. It may be a precursor lesion to renal carcinoma. Adrenal
Adrenal adenomas are common, and are often found on the abdomen, usually not as the focus of investigation; they are usually incidental findings. About one in 10,000 is malignant. Thus, a biopsy is rarely called for, especially if the lesion is homogeneous and smaller than 3 centimeters. Follow-up images in three to six months can confirm the stability of the growth. While some adrenal adenomas do not secrete hormones at all, often some secrete cortisol, causing Cushings syndrome, aldosterone causing Conns syndrome, or androgens causing hyperandrogenism. Thyroid
About one in 10 people is found to have solitary thyroid nodules. Investigation is required because a small percentage of these is malignant. Biopsy usually confirms the growth to be an adenoma, but, sometimes, excision at surgery is required, especially when the cells found at biopsy are of the follicular type. Pituitary
Pituitary adenomas are seen in 10% of neurological patients. A lot of them remain undiagnosed.