It's the low body mass. When this was discovered, it was perhaps our first hint that body mass has survival implications. Joel Kopple, MD, showed that people who were taller and heavier do better on dialysis. At the same time, the biggest and heaviest patients were getting the lowest amount of dialysis. Kopple's conclusion was that heavier and taller patients had better survival rates, even though the URR was lower.
V is more of a proxy for body mass, and, therefore, nutritional status.
It could be that this has caused us to treat smaller patients who need the most dialysis with less than optimal therapy. Or, perhaps, no amount of increased dialysis will help them because they are dying of something else, something that is triggering a chronic inflammatory response. Maybe we should think more in terms of minimal clearance times time. The data is becoming more compelling that the time factor is also an independent outcome predictor. In 1970, we were dialyzing patients for six hours at a time, and I believe we were removing more volume.