Chronic Hemodialysis Prescription and Adequacy of Dialysis
What is a haemodialysis?
- Hemodialysis is a type of dialysis— a procedure that aims at removing the waste products from the body of a person whose kidneys have failed. Haemodialysis prescription is
- While prescribing hemodialysis for a patient, the nephrologist has to take into account many factors like the patient’s pre-dialysis electrolyte levels, the amount of volume that needs to be cleared, the functional capacity of the diseased kidney, and the amount of waste products needed to be removed.
When is a hemodialysis patient considered to be adequately treated?
- The hemodialysis patient is considered to be adequately treated if he is physically fit, not malnourished, and has blood pressure within the normal range.
- The life expectancy of a patient on hemodialysis increases if he has been given adequate hemodialysis with carefully calculated parameters.
- It is considered approximately five to seven years for someone who is 60 years of age when the hemodialysis is initiated.
What is meant by hemodialysis dose?
- Hemodialysis dose means the amount of hemodialysis prescribed to the patient in terms of the duration and frequency of dialysis.
- The adequacy of dialysis or the hemodialysis dose is calculated by taking into account the extent of urea removal.
How is the hemodialysis dose measured?
- There are a number of models that can be followed to measure the hemodialysis dose, of which the Urea Kinetic Model is the most accepted one. It is a mathematical technique that, when used correctly, helps to calculate the amount of urea removed per haemodialysis session.
- Modern dialysis machines are very efficient. They can reduce the blood urea concentration by around 80% of the initial value in a single session.
How is the amount of urea cleared measured?
- The amount of urea cleared from the patient’s blood is measured in terms of two parameters— the Urea Reduction Ratio (URR) and the Treatment Index (KT/V).
What is Urea Reduction Ratio?
- Urea Reduction Ratio is the measure of the decrease in the blood urea nitrogen of the patient during the course of dialysis.
- It is calculated by comparing the post-hemodialysis blood urea nitrogen levels with the pre-hemodialysis blood urea nitrogen levels. It is expressed in terms of percentage.
- For example, suppose the urea level before dialysis was 60 mg/dl, and after dialysis, it is found to be 20 mg/dl. In that case, it means that the dialysis treatment has successfully removed 40 mg/dl of urea. Therefore, approximately 66% of urea was cleared.
- The suggested URR is more than 65%. Patients with more than 60% urea clearance tend to live longer and need less frequent hospitalisations.
- Ideally, URR is calculated once a month, after 12-15 treatments.
- One value can not be considered an indicator of dialysis adequacy; rather, an average of the values is taken.
What is the treatment index technique of urea clearance measurement?
- The treatment index is measured by KT/V.
- Here, K stands for the clearance of the dialysis machine, t indicates the time, and V denotes the volume of fluid from which urea has been removed per treatment.
- A KT/V of above 1.2 is considered to be ideal.
- KT/V is also supposed to be checked monthly.
- However, if the patient is non-compliant or there are frequent hurdles that prevent the delivery of the prescribed dose, more frequent KT/V values need to be obtained.
Both the URR and KT/V techniques were devised after experimenting with a large number of patients on dialysis. It has been found that patients with a low URR or KT/V have more mortality and morbidity rates. It is believed that patients who have a URR above 65% and KT/V above 1.2 are considered to be adequately dialysed.
What is a post-hemodialysis rebound?
- Immediately post-hemodialysis, the concentration of urea in the blood increases.
- This is because urea is shifted from the peripheral body compartments into the bloodstream, where it can be removed. It is over in approximately half an hour to an hour.
- Therefore to avoid getting false values, the post-dialysis blood sample for the calculation of urea should be taken at least after 30 mins-1 hour after dialysis.
- The amount of this rebound depends on the length of the dialysis session and the rate of dialysis.
What are the other parameters that affect hemodialysis adequacy?
Following are the parameters that affect hemodialysis adequacy:
Hemodialysis access recirculation:
- It is a phenomenon that occurs when the dialyzed blood that is coming back from the venous port re-enters the external dialysis circuit through the arterial port rather than going to the systemic circulation.
- This causes mixing of the dialyzed and undialyzed blood, leading to decreased efficacy of the dialysis procedure.
- Because of hemodialysis access recirculation, the amount of urea cleared decreases.
- It is one of the most important factors responsible for hemodialysis inadequacy in a patient.
Protein nitrogen appearance rate (nPNA):
- It is a parameter that indicates the total protein consumption in patients who are receiving maintenance dialysis.
- Several researches have established a connection between the protein nitrogen appearance rate and the treatment index(KT/V).
- A low nPNA is related to poor treatment outcomes, as it indicates poor dietary protein intake.
Residual renal function:
- In patients who have lost more than 70% of renal function, the residual renal function is estimated by calculating the average urea and creatinine clearances.
Are there any patient-specific factors that affect hemodialysis adequacy?
- Some studies have proved that males are more prone to dialysis inadequacy than females. This may be because they are taller, have more muscle mass, and have more body activity.
- Because men are taller, they have a higher urea distribution volume (V). As we know, dialysis adequacy depends on KT/V; increased V means lesser adequacy.
- These studies also suggest that patients whose hemoglobin levels are below 10 gm/dl are more susceptible to have inadequate dialysis than those whose hemoglobin levels are above 10 gm/dl.
- Therefore, there is no one factor that defines hemodialysis adequacy. It depends on many different parameters.
What are the NKF KDOQI Hemodialysis Adequacy Guidelines?
The National Kidney Foundation (NKF) released a Kidney Disease Outcome Quality Initiative (KDOQI) in 2006 that lays eight guidelines to ensure hemodialysis adequacy in clinical practice.
- It states that patients approaching kidney failure should be educated about kidney failure and its treatment options, including kidney transplantation.
- The glomerular filtration rate of the patient’s kidney and other related parameters like urea and creatinine clearance should be checked to get an assessment of the kidney function.
- Dialysis therapy should be started according to it. Renal replacement therapy should be commenced when the patient reaches Stage 5 of chronic kidney disease, after taking into consideration its risks, benefits, and disadvantages.
- It provides a road map for measuring hemodialysis dose.
- It suggests that the hemodialysis dose should be revised at regular intervals, at least once a month.
- The dose of the hemodialysis prescription is calculated by using the urea kinetic model discussed above.
- For the patients who undergo dialysis three times a week and have some residual renal function, the length of each dialysis session should be at least 3 hours.
- It discusses the proper way of taking pre-and post-dialysis blood urea samples. Both the samples need to be taken in the same treatment session to get accurate results.
- To avoid the inaccurate results due to access recirculation, the blood flow through the dialysis needs to be minimised before taking the sample.
- It helps to know the minimally adequate hemodialysis doses.
- The control of blood pressure of the patient undergoing hemodialysis improves the outcome of the procedure.
- Increased fluid volume is related to increased blood pressure, which is in turn related to increased blood pressure.
- If the patient is hypertensive, he should be advised to restrict fluid intake and cut down on sodium, and diuretics should be prescribed if necessary.
- Diuretics are medications that increase the filtration of water from the body. This brings the blood pressure to normotensive levels.
- It aims at preserving the residual kidney function. The amount of residual kidney function is directly related to the morbidity rate.
- The nephrologist has to aim to avoid kidney function loss as much as possible and preserve the residual renal function.
- It focuses on constant monitoring of the hemodialysis procedure and improving its quality to have better outcomes.
- All the members of the hemodialysis team should be adequately trained, and the dialysis centres should have all the necessary equipment.
- It provides rules for paediatric hemodialysis initiation.
What are the International Society of Nephrology (ISN) Hemodialysis Guidelines?
- ISN guidelines suggest that all the patients suffering from end-stage renal diseases must receive three dialysis sessions for four hours each week.
- If the patient prefers to receive two dialysis sessions instead of three, then the duration of each session should be increased to six hours.
- Dialysis is not considered to be adequate if there are less than two sessions per week.
- The recommended dialysis solution flow rate is around 500 ml/min, and the recommended blood flow rate is approximately 300 ml/min.
- The residual renal function should be monitored once every six months
- URR and KT/V values should be used to calculate the adequacy of the dialysis prescription
- For the patients who are stable, the hemodialysis prescription can be reviewed once per month. However, more frequent assessments need to be held for patients who are unstable on hemodialysis.
- If the URR or KT/V ratio is found to be low, the dialysis team is suggested to look for the cause.
How to make a hemodialysis prescription achieve the target clearance level?
- The haemodialysis prescription should consider the therapy goals, expected urea clearance, the volume that needs to be removed, and residual renal function. All these targets should be set, and the prescription should be formulated to achieve them.
- The URR and KT/V levels need to be checked monthly. The average KT/V ratio should be above 1.2, and the URR level should be more than 65%
- The NKF KDOQI and ISN guidelines should be considered before formulating the hemodialysis prescription
- If there is a failure to achieve the target clearance level, steps should be taken to troubleshoot the problem.
- The blood flow rate, length and frequency of sessions, and the dialysate flow rate should be reviewed.
- The patient’s electrolyte levels should be checked, and the concentration of the dialysis solution should be adjusted accordingly.
- If all the above parameters are found to be under control, a dialyzer with a larger surface area should be used.
Frequently asked questions
Q: What are the drawbacks of the URR technique?
- The URR technique has certain drawbacks. Though it can be roughly used to calculate hemodialysis adequacy, it can not be used to prescribe the hemodialysis dose.
- It does not take into account the efficacy of the dialyzer or the volume of body water distribution.
- Also, during gradual overnight dialysis sessions, URR does not accurately measure the amount of dialysis as more urea is produced during these sessions.
- Because of the drawbacks of the URR technique mentioned above, the Treatment Index (KT/V) is a more accurate parameter to measure dialysis adequacy as well as to prescribe dialysis doses.
- It is better because it takes into account the efficacy of the dialyzer used, the time taken to complete the dialysis, and the volume of body water distribution.
Q: Why is urea preferred to calculate the haemodialysis dose?
- It is because urea levels are usually high in renal failure patients.
- It is easy and cheap to measure
- It has a wide volume of distribution (almost the same as that of water)
- It can be removed by dialysis relatively easily.
- Also, the extent of urea clearance is closely linked to the mortality rates.
- Urea itself is not very toxic, but its high levels in the blood indicate the presence of other more toxic wastes, which are not easily measurable.
Q: What steps need to be taken if the treatment index is found to be consistently low?
- Ideally, the treatment index should be above 1.2
- If it is found to be consistently below 1.2, changes need to be made to the hemodialysis prescription to raise it and ensure adequate dialysis.
- The rate of blow flow through the dialysis machine will have to be increased as the KT/V value is directly proportional to it.
- Another measure that your nephrologist can take to increase the treatment index is to increase the dialysis time.