Back to Basics: Focusing on Hemodialysis Adequacy
The original Dialysis Outcomes Quality Initiative (DOQI) workgroup for hemodialysis adequacy released guidelines for hemodialysis adequacy in 1997. In 2000, the workgroup, re-named Kidney Disease Outcome Quality Initiative (K/DOQI) released updated adequacy guidelines. Goals set by CMS are based on K/DOQI recommendations. Currently, the MINIMUM recommended Kt/V is 1.2 (single-pool or spKt/V). Additionally, due to variances each treatment, it is recommended that prescribed spKt/V be 1.3 so that minimum dose is consistently achieved.
What is the difference between single-pool and double-pool (equilibrated or ekt/V) measurements? With a single-pool model, urea to be removed is considered to be in one-pool, that is, outside of cells, in the bloodstream, and ready for removal. With efficient dialysis, at least 65% of the urea in this pool is removed each treatment (URR of 65%). In actuality, urea is also contained inside cells and is must travel across cell walls into the bloodstream for removal. This second pool of urea is taken into account with a double-pool measurement method. In this case, urea levels rise after dialysis when urea (by diffusion) moves across cell walls. This rise, or rebound, typically occurs 30-60 minutes after treatment ends and requires post-dialysis BUN to be drawn only after this time has elapsed. For this reason, the vast majority of clinics use a single-pool model.
Having said that, exactly what does Kt/V mean?
- K represents clearance of urea — a function of the dialyzer. Dialyzers with a small surface area (amount of fibers) and small fiber pore size, clear less urea that do dialyzers with a larger surface area (more fibers) and larger permeability. Therefore, one avenue to increase Kt/V is to increase dialyzer size and/or dialysate flow rate —a commonly used intervention.
- t represents time — the length of time during which urea can be removed. Logically, longer dialysis treatments allow for more urea removal, which is why physicians may opt to increase dialysis time.
- V represents volume — the body volume of distribution for urea. Considered to be the same as total body water. Obese individuals have less total body water (fat tissue contains less water) while lean, muscular individuals have more total body water (muscle tissue contains more water).
- Based on the formula, a higher K or t, (the numerator) increases the product. Conversely, a higher V (the denominator) decreases the product. You may have noticed the change that volume makes in regard to young, muscular patients. Typically these patients (higher volume) must dialyze longer than patients with less muscle mass — if V is higher, either K or t must increase to yield a higher product. It is important to note, however, that volume “is what it is” — manipulating the volume for on-line clearance measurements to increase the Kt/V simply increases the number mathematically — not the actual clearance.
So, what can be done to increase Kt/V?
Before undertaking any intervention, make sure labs are drawn correctly! It may be well worth your time to review procedure with all staff — particularly if significant numbers of patients have poor adequacy outcomes. If labs are not drawn correctly and consistently correctly, adequacy numbers are worthless! Before conducting a full-blown adequacy investigation, make sure numbers are correct.
If labs are drawn correctly, the following tips may help improve adequacy
- Increase clearance (K)
- Access
- Is access functioning properly? Is prescribed blood flow rate obtainable? If not, ask MD for fistulagram order. Remember, if arterial pressure is more negative than –240, the pump speed displayed is NOT the pump speed delivered!
- Is recirculation occurring because needles are too close? Needle tips should ideally be two to three inches apart.
- Are bloodlines connected to access correctly (arterial bloodline to arterial limb)? It has been estimated that as many as 40% of patients have incorrectly connected bloodlines at any point in time!
- Can blood flow rate be increased? This may require larger needles. Remember, blood flow through access must be greater that prescribed flow. Average blood flow through an AVF is 500-800 mls/min and roughly 1,000 mls/min for synthetic graft. K/DOQI guidelines recommend fistulagram for access flows < 600 mls/min.
- Dialyzer
- Can dialyzer size be increased?
- Can dialysate flow be increased?
- Is dialyzer patent? If dialyzer is partially clotted, those fibers are not available for clearance—dialyzer is now, effectively, smaller than prescribed.
- Are dialysate hoses connected to dialyzer correctly? Only in special circumstances are hoses connected “backward” intentionally.
- Increase time (t)
- Is patient signing off early or starting treatment late?
- Is treatment time determined by machine clock or “wall clock”?
- If treatment is interrupted, is interrupt time added back to treatment time?
- Is treatment length adequate in and of itself?
- Volume (V)
- Again, volume “ is what it is”, however, if total body water is calculated based on body surface area, be sure that height and weight are correct in computer data base.
If above interventions are not helpful in improving adequacy, physician may consider measuring residual renal function to better assess overall clearance. Please feel free to contact Network 8 if you have any questions or need assistance in this important area of patient care.