Principles of hemodialysis
by
Marwa Saleh
Lecturer of nephrology
Before Dialysis
It is now difficult to imagine that less than 60 years ago, patients with
ESRD had only one prognosis—death.
Patients with renal failure were connected to patients with liver failure so
that each could be kept alive by the healthy organ of the other.
History of hemodialysis
1854:- Thomas Graham presented the principle of solute transport across
semipermeable membrane.
1913:- Abel, Rountree & Turner made the first artificial kidney.
1924:- first human dialysis by Hass.
1943-45:- Kolff was the first to made dialyzer.
1946:- Nils Alwall modified the Kolff kidney to remove excess water.
1946-60:- Nils Alwall invented A-V shunt made of glass & used to ttt
1500 patient in RF.
1960:- Scribner & Quinton modified the glass shunt & made them from
teflon(Quinton- Scribner).
1962:- Scribner started the first outpatient dialysis.
Drum dialyzer of Kolff Beginning of HD
It is of interest to reflect upon the differences between native &
artificial kidneys.
There are 1 to 2 million functioning nephrons in the 2 native
kidneys. The diameter of PT of nephrons is 40 mm & 14 mm in
length.
The artificial kidney, in its hollow-fiber format , contains
8000 to 10,000 fibers & provides a surface area for exchange
as high as 1.8 to 2 m2 .
The diameter of each hollow fiber is approximately 200 mm
and > 25 cm.
Kidney HD
Filtration ,secretion, reabsorption Filtration only
24 hours / week 12 hours / week
1000 ml / min 300 ml / min
molecules : 65000 D 30000 D
When to • Usually, dialysis started when
GFR is less than 15 ml/min,
start dialysis But we can start dialysis early
in the following conditions:-
?
Uncontrollable hyperkalemia, Severe metabolic acidosis
refractory to dietary restriction & resistant to bicarbonate
medical ttt ( > 6.5) treatment (<7.1).
Severe fluid overload &
Ureamic encephalopathy ,
Refractory
Intoxications pericarditis…
hypertension.
Hemodialysis
It’s one of RRT in which an exchange between patient
blood & dialysate across a semi-permeable membrane
in a countercurrent mechanism aiming to :-
Remove accumulated metabolic waste product
Remove excess fluid
Correct blood electrolytes composition
Solutes that accumulate in uremia
Free H2O soluble
LMW Protein bound Middle & high MW
• Na+ , k+ , H+ • Mg , Po4 • PTH
• Createnine • Indoxyl sulphate • B2
• urea • Hippuric acid macroglobulin...
• oxalate • Leptin….
• guanidine ….
Hemodialysis equipment has blood circuit and
dialysate circuit:-
Blood circuit
Vascular access
Principles of hemodialysis
Solutes removal
• Diffusion
• Convection
• Adsorption
Solvent removal
• Ultrafiltration
DIFFUSION
Water molecules &
solutes can pass
through membrane
pores via
CONVECTION (UF)
Hydrostatic
Osmotic
Diffusion
It is a passive process by which solutes moves from high
to low concentration gradient.
It
is particularly effective in the transport of small solutes
such as urea , K , Ca , & HCO3.
• Diffusive clearance of solutes by HD decreases
rapidly with increasing molecular size.
Movement of Diffusion or
molecules from
blood side clearance
Movement of
molecules from Back
dialysate side diffusion
Convection (UF)
When solution is moving
, the solutes dissolved
in it will move along.
(Given that membrane is
permeable for them)
Convective movement of solvent (water) from blood across the
semipermeable membrane by Ultrafiltration driven by the
transmembrane pressure ( hydrostatic or osmotic).
The convective transport of a solute depend on:
1- porosity of the membrane., known as the “sieving coefficient “
of the membrane.
2- TMP ( transmembrane pressure).
3-KUF ( ultrafiltration coefficient).
Blood
pressure in
hollow fiber is
+ve
Pressure
outside the
hollow fiber is
lower
The difference of the
pressure is the TMP which
regulate UF rate & amount
As solvent molecules move on a pressure
gradient , the dissolved solute molecules are
swept out
(solvent drag)
Adsorption
- Removing of PBUT by diffusion or convection is virtually impracticable .
- It can be removed by using the adsorptive properties of particular membranes as
PMMA which can adsorb LMW proteins than other.
- “Protein fouling” with time membrane pores will be closed by proteins.
Mg
Hippuric Indoxyl
Leptin Po4
acid sulphate
Po4
HD clearance of solutes
The volume of blood or plasma from which solutes
is completely removed in unit time (ml/min).
Most of solutes are removed from plasma water , So the
clearance of solutes decrease with increase Hct.
Clearance by diffusion
URR we use urea for measuring adequacy of dialysis as Urea
equilibrate faster from blood RBCs to plasma inside the dialyzer.
Clearance by convection
Kx = QUF * Sx ml/min
Convective clearance depend on
1- UF rate across membrane.
2- membrane sieving coefficient.
continuous
HD Modalities 1-Haemodialysis
Intermittent (CAVHD, CVVHD).
2- Hemofiltration
1- Hemodialysis (HD).
(CAVH, CVVH).
2- Hemofiltration (IHF.)
3- Hemodiafiltration
3- Ultrafiltration (UF).
(CAVHDF, CVVHDF).
4-Slow Low Efficiency
4- Ultrafiltration (SCUF).
Hemodialysis (SLED)
CAVH CVVH
CVVHDF
SLED
- The dialysate & the blood flow rate are decreased (100 and 200 ml/min,
respectively) & the session duration is increased (8–10 h).
- The lower clearance induces a lower solute removal rate & decrease of the
concentration gradient.
- Refilling from the interstitium to the vascular bed is enhanced, due to
prolonged duration of the treatment, the effective solute removal (i.e.,
efficiency) is increased.
- The haemodynamic tolerance reported to be equivalent to that of CRRT.
SCUF
- SCUF is used mainly to control fluid excess without a diffusive process,
thus, it is hemodynamically better tolerated.
- The usual blood flow ranges between 50 and 100 mL/min, adjusted to
achieve an UF volume of 2–5 mL/min, and replacement fluid is generally not
needed .
- This modality is useful for patients with heart failure.
