Prescribed versus delivered dialysis: Importance of dialysis time
- Robert Hootkins, MD, PhD, FACP, FAHA, FASN
Robert Hootkins, MD, PhD, FACP, FAHA, FASN
- Section Chief, Nephrology and Hypertension
- Austin Diagnostic Clinic, Austin, TX
- William L Henrich, MD, MACP
William L Henrich, MD, MACP
- University of Texas Health Science Center at San Antonio
- Richard A Sherman, MD
Richard A Sherman, MD
- Professor of Medicine
- Rutgers - Robert Wood Johnson Medical School
Adequacy of hemodialysis is an important issue since it appears to be an important determinant of patient survival (see "Patient survival and maintenance dialysis"). One method of assessing the amount of dialysis prescribed is to assess urea removal normalized for a measure of body size, the Kt/V. This parameter can be estimated from the dialyzer urea clearance data supplied by the manufacturer (K), the duration of dialysis in minutes (t), and the patient's urea space (V) as derived from a standard height/weight nomogram.
A number of the major factors contributing to K are:
●The size of the dialysis membrane since larger surface area membranes can remove more urea per unit time.
●The blood flow rate to the dialyzer (Qb) since presenting new plasma with a high urea concentration maintains the favorable gradient for urea removal (figure 1).
●The dialysate flow rate (Qd) since delivering new fluid containing no urea also maintains the urea concentration gradient.
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- HISTORICAL BACKGROUND
- DETERMINANTS OF DIALYTIC SOLUTE CLEARANCE
- DIMINISHED UREA CLEARANCE DUE TO THE PARALLEL NATURE OF HEMODIALYSIS
- Vascular access recirculation
- DIMINISHED UREA CLEARANCE DUE TO DIMINISHED EQUILIBRATION WITH INTRACELLULAR STORES
- Regional blood flow model
- OTHER LIMITING FACTORS
- Inaccurate manufacturer's specifications
- Inaccurate and decreased blood flow rates
- Reduced fiber bundle volume from clotting
- Inefficient dialysis
- Errors in measurement of blood urea nitrogen
- Residual urea clearance
- TOTAL BODY-EFFECTIVE CLEARANCE AND TIME ON DIALYSIS
- INFORMATION FOR PATIENTS