Start here: www.drjohnm.org/2013/06/benign-pvcs-a-heart-rhythm-doctors-approach/www.doctorslounge.com/cardiology/forums/backup/topic-24286.html
Though ectopic beats (PVCs, PACs, and a few others) can be a sign of ischemic heart disease, the vast majority of the people that get them have no pathology, in terms of heart structure or blood supply to the heart. This may seem difficult to believe, but most people can't feel them. My wife worked for a cardiologist, she said a patient came in one day that had so many PVCs she couldn't take his pulse. An EKG was done, the doctor examined the patient, and while talking to the patient, took the patient's pulse while asking him if his heart ever missbeats or skips a beat. The patient said no. The doctor didn't inform him of his benign rhythm issue, lest he worry about
Adrenaline can aggravate ectopics, and therein lies the rub, for when you react to them, the more adrenaline you get, and the more likely you are to get more. Too many PVCs for too long can reduce the strength of the heart, but the first link I provided, an Electrophysiologist (a heart rhythm specialist) said that 20,000 PVCs per day would need to occur before weakening of the heart muscle might start to occur, and my own E.P. said the same. Online, an article said that 20% of the total number of beats would need to ectopics before it's a concern for weakening the heart. For the average heart with 100,000 beats per day, this is where they get the 20,000 i.e. 20%. If someone had more than 100,000 beats per day, no doubt the total of acceptable PVCs per day would be slightly higher.
Pregnancy is known to cause ectopics, as is overeating. Seemingly anything competing for space in your chest and abdomen. I'm sure hormone and changes of certain substances in the blood may also contribute. Hyperthyroidism is something else known to cause them.
They learned years ago that, with the exception of Beta Blockers, the use of more potent Antiarrhythmics to treat benign ectopics has the potential to cause dangerous rhythm issues, and the treatment of benign ectopics is now reserved for patients that are symptomatic, having dizziness and lightheadedness etc., and extreme anxiety and disruption of day to day life. Treatment with more than most Beta Blockers or Calcium Channel Blockers usually requires close supervision during the initial dose, dose increases, and periodic screening for heart and other health issues.
Hope this is of some help. My last Holter showed 6000 PVCs, I'm in the midst of an ectopic outbreak, and have been having similar outbreaks yearly for nearly 40 years.