Dialysis Adequacy – Theory of Kt/V

1. Dialysis Adequacy – Theory of Kt/V Personal and confidential communication. Only for employees of the B. Braun Group. Copies, including excerpts, prohibited.

2. Introduction Since the earliest days of haemodialysis there has been a continual search for a method of identifying the correct dialysis dose for individual patients, and for assessing the effectiveness of the delivered dialysis treatment. Before the introduction of urea kinetic modelling (UKM) the methods used were inconsistent and relied upon generalised standard procedures. These were in turn influenced by the experience and expertise of the renal team. The general use of predialysis blood values depended upon experienced nephrology staff detecting when a patient is under-dialysed, which in turn was dependent upon their ability to interpret clinical signs and symptoms which can often be disguised by co-morbid conditions.

3. Because of the difficulty in assessing complex patients to determine an adequate dialysis dose interest turned away from purely clinical assessment towards physical parameters that could describe the efficiency of solute removal by an artificial dialyser. In the 1970’s this included measurement of permeability coefficients, mass transfer coefficients and clearance. Of these parameters, clearance remains the principle parameter in any mathematical method of assessing dialysis adequacy. The publication of the National Co-operative Dialysis Study in the US in the early 1980’s set the scene for the use of urea as then main measurement parameter for dialysis adequacy. It is now widely accepted that UKM is the current measurement of choice. However, there is still some controversy over it’s use, and it is important to remember Kt/V should be used alongside clinical assessment.

4. Determinants of Adequacy There are many aspects to dialysis adequacy, and for the benefit of patient care all of these separate components should be considered when assessing the patient. However for the purpose of this presentation we will concentrate on the issue of Kt/V only. Fluid & electrolyte homeostasis Biocompatibility Acidosis correction Anaemia correction Adequate solute removal DIALYSIS ADEQUACY HD schedule & duration Blood Pressure Control Good nutrition Patient Outcome Quality of life Rehabilitation Morbidity Mortality

5. What is an Adequate Dialysis Dose or Kt/V? The National Co-operative Dialysis Study (NCDS) identified that in patients dialysed 3 times a week with a Kt/V of 0.9, it was sufficient to maintain the morbidity risks at an acceptable level. A Kt/V of less than 0.9 was associated with an increased mortality and morbidity risk. Subsequent studies and international standards and guidelines have since shown that the minimum Kt/V target should be 1.2. But it is important to also note that evidence is also showing that patients should also dialyse for a minimum of 12 hours a week. Held et al (1991) reported a tendency to improve survival by 8% for each incremental increase in Kt/V of 0.1 up to a Kt/V of 1.4. But what about patients who dialyse only twice a week? Studies have indicated that a Kt/V of between 1.8 – 2.0 should be targeted for twice a week dialysis.

6. Theory behind KtV Urea kinetic modelling uses urea, a waste product of protein catabolism, as a marker molecule for clinically important uraemic waste products. It is this measurement that is used to assess dialysis efficiency, but it can also be used to calculate individual dialysis prescriptions and measure protein intake. Urea has a number of advantages as a marker of dialyser clearance and dialysis adequacy. It is present in high concentrations easy to measure it is a stable low molecular-weight compound soluble in water & easily removed in dialysis equally distributed throughout the body These characteristics make it the best marker so far identified.

7. Measuring KtV There are a number of different methods of measuring and assessing Kt/V, but they all rely upon the fact that urea is equally distributed throughout the body in the total body water. We will now consider the two most common methods of assessing the Kt/V 1: Single Pool Kt/V 2: eKt/V Avitum uses the eKt/V as it’s preferred method of choice. In order to understand how dialysis prescriptions can affect the patients Kt/V we need to explore how the Kt/V calculations are made.

8. Calculating V Because urea is equally distributed throughout the total body water, single-pool Kt/V regards water as being in one ‘pool’ called the urea distribution volume or ‘V’. This volume can be estimated, taking total body water as 58% of lean body mass. But because patients are all different sizes and shapes, V can be more accurately calculated by taking into account the patient’s height, weight and gender (the Watson Formula). These are displayed below. Don’t worry, you are not expected to remember these equations, but just understand how different body shapes and genders could affect the Kt/V. Male V = 2.477-(0.0952xAge(years))+(0.017xHeight(cm)))+0.336xWeight(kg)) Female V = 2.097-(0.0952xAge(years))+(0.017xHeight(cm)))+0.247xWeight(kg))

9. Single Pool KtV The generation of urea into this ‘single pool’ is determined by the rate of protein catabolism. And removal of urea form the pool is equal to the sum of dialysis clearance and residual renal function (if any). Different dialysers have different clearance rates, and so will affect the amount of urea cleared during a dialysis treatment. It is this clearance factor that gives us ‘K’ from the term Kt/V. The amount of time that a patient spends on dialysis will also determine the amount of urea (and other solutes) cleared during treatment. This gives us ‘t’. Therefore K = clearance (determined by the size and type of dialyser in mls/min) t = time (actual dialysis treatment time in minutes) V = volume (the calculated urea distribution volume in litres)

10. eKtV eKt/V considers urea to be distributed in two pools in the body the intracellular fluid (ICF) the extracellular fluid (ECF). and the formula used to calculate the Kt/V is altered accordingly. During dialysis, changes in the urea concentration of the ICF lag behind changes in the ECF, and following the end of dialysis a ‘rebound’ in the serum urea will occur. This lag is due to the diffusion of urea from the ICF into the ECF, which continues until equilibrium is reached. During the high efficiency solute exchange of today’s haemodialysis the dual-pool nature of urea is significant and the dual-pool formula used to calculate Kt/V takes this into account. The target is to achieve a minimum Kt/V of 1.2 for all patients on 3 times a week dialysis, unless there is significant residual renal function.

11. KtV Example Any change to each of the elements of KtV will affect the result. Changing dialysis time (t) or the dialyser (K) will affect the dialysis adequacy. The only variable that you are unable to influence will be ‘V’, as this is patient dependent.

12. The following is a very simplified example to illustrate the influence of dialysis time and changes in a dialyser can make to the Kt/V. A 48yr old man, weighing 100 Kg dialyses for 3 hours using a Hips 15 dialyser. Using the Watson formula his V = 34.6 3 hours dialysis: t = 180 mins K = 0.173 So if we put this into the Kt/V formula Kt/V = 0.173 x 240 / 34.6 = 0.9 This is less than the minimum required of a KtV >1.2, and indicates that he may be underdialysed.

13. So how can we improve this? So what happens if we increase his dialysis time to 4 hours and recalculate? Using the Watson formula his V = 34.6: 4 hours dialysis: t = 240 mins: K = 0.173 So if we put this into the Kt/V formula Kt/V = 0.173 x 240 / 34.6 = 1.2 This results in a KtV of 1.2, so that he is now achieving the minimum standard of dialysis adequacy. This just illustrates the relationships between different aspects of dialysis that you can influence to improve the quality of treatment your patient receives.

14. Blood Sampling Two blood samples are required for calculation of the patients KtV. A predialysis urea level and a post dialysis urea sample. Timing of the sample collection is important for both samples. The pre-dialysis sample should be taken via the arterial needle of central venous catheter (CVC) arm before dialysis commences. The timing of the post dialysis sample is important to ensure consistent sampling and therefore results. If different staff use a different technique then it is impossible to make comparison of treatments for each individual patient over a period of time.

15. Blood Sampling – Timing is all The following post-dialysis sampling technique has been adopted throughout the Avitum Dialysis centres. At the end if dialysis slow down the blood pump speed to 100mls/min then count to 15 seconds and take the blood sample from the arterial needle or CVC arm.

16. Common errors in post dialysis blood sampling The formula Avitum uses to calculate the patients KtV makes a number of assumptions and is designed to estimate the KtV from the sampling technique just described. 1:Taking the blood too soon This results in an inaccurate high KtV because your are taking a sample of mixed venous return blood and arterial blood. 2: Not reducing the blood pump speed Again this will result in an inaccurate high KtV due to risk of venous return blood mixing with the arterial blood in the fistula – recirculation 3: Waiting too long to take the sample If you wait for too long after reducing the blood pump speed then there will be a rebound of the urea from the interstitial compartments into the blood resulting in a lower than expected KtV.

17. Limitations of KtV Kt/V results for specific patients should be carefully applied and it’s limitations appreciated. It should be remembered that Kt/V deals only with urea clearance and cannot give a full picture regarding the clearance of all uraemic toxins, though it does provide an estimation of dialysis efficiency as a whole. There remains a poor understanding of the relationship between uraemia and specific solute concentrations, so the extension of this mathematical model to include other components of the dialysis prescription continue to be explored. Therefore it remains important to undertake a full clinical assessment of the patient as a whole, and not rely just on the Kt/V results for your patients. The clinical considerations behind the Kt/V result will be explored further in the next module “Dialysis Adequacy 2 – Influences on Effective Treatment”.

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