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232 Chapter 6: Genitourinary system

Renal transplantation is a special form of RRT which remove and return blood, the patient must have vascu-
most closely restores a physiological state, including the lar access such as an arteriovenous fistula or a double-
endocrinefunctions.Patientsmayswitchmodalitymany lumen central venous line. The blood needs to be anti-
times whilst on RRT. coagulated (usually with heparin) to stop it clotting in
the dialysis machine.
Haemodialysis Although many patients cope very well with dialy-
Blood has to be pumped from the patient, and passed sis, common symptoms include headache, joint pains
through a ‘dialyser’, sometimes called an artificial kid- and fatigue during and after a dialysis session. Com-
ney. The dialyser consists of an array of semi-permeable plications include hypotension, line infections, dialysis
membranes. The blood flows past the membrane on one amyloid and increased cardiovascular mortality.
side, whilst on the other side, a solution of purified wa-
ter, sodium, potassium, calcium, magnesium, chloride, Haemofiltration
dextrose and bicarbonate or acetate is kept flowing in the Continuous renal replacement therapy, as takes place
other direction (counter-current – see Fig. 6.3). on intensive care units, uses ultrafiltration as the main
Solutes diffuse across the semi-permeable membrane method rather than dialysis. Convection carries water
downaconcentrationgradient.Smallsoluteswithalarge and solutes across a highly permeable membrane and
concentration gradient diffuse rapidly, e.g. urea and cre- this is removed. Before the blood is returned to the body,
atinine,whereasdiffusionisslowerwithlargermolecules fluid is replaced using a lactate or bicarbonate-based so-
or if the concentration gradient is low. lution containing solutes in the desired concentrations
The diffusion gradient is kept high by keeping blood (see Fig. 6.4).
and dialysate flows high, and also by using counter- The advantage of continuous therapy is that removal
current flow. Proteins are too large to cross the mem- of fluid and changes in electrolyte concentration take
brane. fluid is removed by generating a transmembrane place more gradually, reducing this risk of hypotension.
pressure, so that fluid is pushed out from the blood com- Itisexpensiveinbothmaterialsandstaffingandtherefore
partment into the dialysate compartment. Water drags only takes place on intensive care units.
small and medium molecular weight solutes with it. This
is called ultrafiltration. Several hours of dialysis, usu- Peritoneal dialysis
ally three times a week, are needed to achieve satisfac- In peritoneal dialysis (PD), fluid and solutes are ex-
tory urea and creatinine clearance. Underdialysis (lack changedacrosstheperitonealmembranebyputtingdial-
of adequate dialysis) is associated with an increase in ysis solution into the abdominal cavity. Peritoneal access
cardiovascular and other morbidity and mortality. To is obtained by putting a tube through the anterior ab-
dominalwall.Dialysateisrunundergravityintotheperi-
toneal cavity and the fluid is left there for several hours.
Blood from Blood to Small solutes diffuse down their concentration gradients
patient patient
between capillary blood vessels in the peritoneal lining
and the dialysate. Fluid removal takes place by osmo-
sis. The osmotically active agent in PD fluid is usually
Semipermeable dextrose.
membrane
Dialysate exchanges are usually performed by the pa-
tient,fourorfivetimesaday(CAPD–continuousambu-
latoryperitonealdialysis),orsomeorallofthe exchanges
can by performed by an automated cycling device to
which the patient is attached at night (APD – automated
peritoneal dialysis). Patients often develop some consti-
Dialysate out Dialysate in pation which can limit the flow of dialysate, they are
treated with laxatives. Hernias should be repaired before
Figure 6.3 Principles of haemodialysis. starting PD.