Kt/V

Kt / V is a parameter to determine the treatment effectiveness and an essential element for the assessment of dialysis efficiency. Another parameter to this evaluation is URR ( Urea Reduction Ratio).

The value can be obtained by the simplified formula

Be determined.

It is

• Clearance K is determined by the urea content of the blood before and after the dialysis.
• T Effective dialysis time in minutes
• V 60 % of body mass ( weight) in the blood can circulate (body water content)

The measured value was developed by Frank Gotch and John Sargent as a way to measure the dose of dialysis and to evaluate the effectiveness. The minimum value in a hemodialysis Kt / V of ≥ 1.3 is sought; in peritoneal dialysis is the goal of ≥ 1.7 per week.

Derivation of the Kt / V, as a parameter for measuring the dialysis efficiency

K ( renal clearance) multiplied by t ( time of dialysis ) results in a volume ( as ml / min × min = ml or l / h × h = l), and you can the product ( K × t) introduce or as milliliters liters liquid ( blood in this case ), which is purified over a treatment unit of urea. The V in the denominator is also a volume measured in milliliters or liters. Thus, the fraction K × t / V is dimensionless (ie without physical unit ). This fraction is the volume of purified blood in relation to the urea distribution volume. In the case of Kt / V = 1.0, a quantity of blood has been purified from urea, which is equal to the volume of distribution of urea.

The physical relationship between Kt / V and urea concentration C at the end of dialysis can be derived from an ordinary first order differential equation as follows. This models the clearance of any substance from the body, provided that the concentration of this substance in the body over time decreases exponentially ( exponential decay ):

It is

• C is the concentration of Subtanz, here urea [ mol/m3 ]
• T is the time [s]
• K clearance [m3 / s]
• V is the distribution volume [m3 ]

From the above equation it follows that the first derivative of the concentration with respect to time, that is, the change in the urea concentration over time. This differential equation can be separated, and can be integrated as follows:

By integration we obtain the equation

In which

• In (C), the natural logarithm of C
• Const, the constant of integration

Referred to. Turning to the natural exponential function to both sides of the equation (2b ), so we have:

In which

• E is the Euler number

Referred to. Basic calculation rules of algebra allow to rewrite this equation as:

In which

• C0 is the concentration at the start of dialysis (in [ mmol / l] or [ mol/m3 ] )

Referred to. This equation can be written alternatively as

By applying the rules for computing logarithms can see the formula (4 ) can also be written as

Being.

Urea Reduction Ratio relation to

The Urea Reduction Ratio ( URR ) is simply the proportionate reduction of urea during dialysis. By definition therefore applies URR = 1 -C/C0. From this 1- URR results = c/c0. By algebraic manipulation, namely by substituting in the above equation ( 4 ), thus obtained:

Calculated according to the formula of Daugirdas

The formula (4) neglects the following two facts: firstly, the body produces new urea during dialysis, and the other is removed by ultrafiltration urea ( which contributes to the clearance K, but the reduction is not affected ). Daugirdas has therefore proposed a modified formula for the calculation of Kt / V in order to take account of these factors:

It is

• T is the effective duration of dialysis in h;
• R postdialytische the urea content divided by the pre-dialysis urea content;
• KG the dry weight (in kg), i.e. the body weight of the dialysis treatment;
• UF the ultrafiltration volume of the same date ( in liters); the ultrafiltration volume (in liters ) as the difference between the body weight before dialysis, and the body weight computed by the dialysis treatment (both in kg).

According to the guidelines to ensure the quality of dialysis treatments, the Kt / V is calculated according to the formula (6).

Rebound after dialysis

The above-mentioned physical model assumes that urea diffuses evenly throughout the body, as if the whole volume, in the spread of urea, would consist only of liquid ( single- pool model ). A more accurate model uses the division into a plurality of compartments of the human body, in particular the extracellular space, in which, for example, the blood vessels are included, and the intracellular space. Observations have shown that still moves over about 30 to 60 minutes after completion of the hemodialysis, urea from the intracellular space to the blood circulation. Because the blood circulation in the short term more urea was removed as the intracellular space, the urea concentration is similar so again, so that the concentration in both compartments, finally again is the same - one talks of urea rebound. There are other formulas to calculate Kt / V based on the double- pool model which takes into account the various compartments.