PVC

Premature Ventricular Contraction

PVCs are characterized by the premature occurrence of strangely shaped QRS complexes, with the electrocardiographic wave (QRS) width usually greater than 120 msec. These complexes are not preceded by a P-wave, and the T-wave is usually large and opposite in direction to the major deflection of the QRS.

In other words, the left and right ventricals of the heart contract before they have time to fill with blood again. This has the potential to disrupt the rhythm of the heart.

In the United States, PVCs are one of the most common arrhythmias. They occur in patients with and without heart disease. In healthy, middle-aged males, more than 60% will show PVCs on routine Holter monitoring. In patients with prior myocardial infarction (MI), more than 80% will show evidence of PVCs on Holter.

In networking context PVC stands for Permanent Virtual Circuit, a software-defined logical connection in a frame relay, X.25 or ATM network. The network provider (or sometimes the client) defines endpoints and the required bandwidth (often called CIR, Committed Information Rate) to the network devices which handle the use of physical networks and manage the traffic to achieve the defined connections. Usually there are multiple PVCs sharing a physical network path at the same time, but otherwise the connection is much like a leased or dedicated real circuit.

While PVC (poly-vinyl chloride) is used everywhere from toys and medical devices to construction materials, it must be acknowledged that there are serious problems associated with it.

In order to be pliable vinyl plastic containers require the addition of a plasticizer. One commonly used to soften PVC is "DEHP, di(2-ethylhexyl)phthalate." This soft PVC presents some hazards when used in medical devices and/or infant feeding bottles. "DEHP is not chemically bound, but is dissolved physically in the plastic film." DEHP is known to move into the contained liquid and then into the human body as said liquid is ingested or infused. This is more of a problem in vulnerable individuals with frequent exposure to the product(s) such as bottle fed infants and kidney dialysis patients. In the case of the bottle feeding infant; feeding bottles should be constructed of hard plastic and/or glass or of a material that does not contain DEHP. In the case of chronically ill populations in need of soft plastic medical devices such as intravenous tubing; efforts are being made to find safe alternatives by groups such as the United States Department of Health and Human Services.

PVC, which is a heavily chlorinated plastic, gives off dioxin when burned and this dioxin in turn gets into the air and eventually onto plants and into the food chain. Because PVC is used in many disposable applications, a lot of it gets burned. Bad, bad idea.

Occupational exposure to vinyl chloride was implicated in a famous cancer cluster case involving liver cancer among workers in Kentucky and reported by the Center for Disease Control.

These are not good things, in fact the environmental protection organization, Greenpeace, recommends "an overall dioxin prevention strategy that ultimately leads to a sunset on the production and use of PVC plastic" and on 4/25/01 launched a "unique international database, which finally offers a real choice for the construction industry between PVC products and more environmentally friendly alternatives". This can be found on their web page, referenced below.

It looks like this:

Cl       H
  \     /
   C = C
  /     \
 H       H

     Cl   H  Cl   H  Cl   H
      |   |   |   |   |   |
- - - C - C - C - C - C - C - - -(with thousands of repeats)
      |   |   |   |   |   |
      H   H   H   H   H   H