Correspondence (letter to the editor): Functional Impairment Does not Equal Injury

The glomerular filtration rate is proportional to cardiac output. Neither term was mentioned in the article (1), however. The glomerular filtration rate is the only objective measure for determining the severity of renal failure. Renal failure is only rarely caused by severe bilateral painful kidney injury or disease. In persons with healthy kidneys, the glomerular filtration rate is a measure not only for the severity of renal failure but also for the severity of heart failure. Heart failure is defined as too small a pumped volume of oxygenated blood. The stage of renal failure is never lower than that of heart failure. Too much emphasis is placed on the distinction between acute and chronic renal failure. If the extrarenal diseases are treated successfully, the renal failure will regularly recede.

Additionally, the terms mortality and case fatality rate (letality) were confused. The mortality rate is the death rate in the setting of this particular disease, whereas the case fatality rate is the death rate due to a particular disease. This is a fundamental difference.

Renal failure is only rarely caused by nephropathy or kidney disease. These are the reno-renal syndromes. “Acute kidney injury” should be replaced by “acute renal failure”, as long as no severe disease is found on histology. Functional impairment does not have to be due to a disease of the affected organ. Even the healthiest heart can pump only the blood that is available. Even the healthiest kidneys can filtrate only the plasma that is available.