One issue that arises is the suitability of SVG patency, and SVG narrowing as a<unk><unk>  for<unk>  events.  A<unk>  marker (or<unk> <unk>  is term used in medical research for a change to the human body that is believe to be necessary to an eventual outcome or<unk> .
In<unk><unk> , a<unk><unk>  is a measure of the effect (often what is known as a biomarker like  cholesterol) of a certain treatment that is substituted in the  evaluation of a new drug or device in place of a "hard<unk> " like  death or heart attack. The National Institutes of Health (USA) defines<unk><unk> s as: "A biomarker intended to substitute for a<unk> <unk> ".  The use of a<unk><unk>  can lead to more rapid and efficient completion of<unk><unk> , but the use of a<unk><unk>  has  been criticized as reductions of a substitute maker are not always predictive of improvements in<unk>  outcomes.  The classic example of a  failed<unk>  marker is the use of PVC suppression as a substitute  marker of anti-arrhythmia effectiveness.  Class III anti-arrhythmias reduced PVCs or extra heart beats, but were associated with a higher risk of death in the Cardiac Arrhythmia Suppression<unk> (CAST).<unk>  markers may be used when it is unethical to look for the<unk>  (e.g., death) in the experiment, or when the number of<unk>  events is very small, thus making it impractical to conduct an experiment to look for the<unk> .  The measurement of<unk>   markers provides a way to test the effectiveness of a treatment for a fatal disease without having to wait for a statistically significant number of deaths to occur. The FDA will often accept evidence from<unk><unk>  that show a benefit with respect to<unk>  markers instead of hard<unk> <unk> s.
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