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Osmosis is the movemenr of fluid from an area of lesser solute concentration to an area of greater solute concentration. Ultrafiltration, the removal of water and fluid, is accomplished by creating pressure

gradients between the arterial blood and the dialyzer membrane or

compartment. IS

Another method to manage patients with ARF who are critically ill

is continuous renal replacement therapy, either through continuous

arteriovenous hemofiltrarion (CAVH) or continuous venovenous

hemofiltration (CYVH).lo.ls

Peritoneal Dialysis

Peritoneal dialysis (PO) involves using the peritoneal cavity as a

semipermeable membrane to exchange soluble substances and

water between the dialysate fluid and the blood vessels in the

abdominal cavity.8,lo.ls

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ACt.JTE CARE HANDBOOK FOR PHYSICAl THFRAPISTS

Dialysate fluid is instilled into the perironeal cavity through an

indwelling catheter that is either placed surgically or nonsurgically.

Surgical placement is preferable, as direct visualization during the

procedure facilitates berrer placement. After the dialysate is instilled

into the peritoneum, there is an equilibration period when water and

solutes pass through the semipermeable membrane. Once equilibration is finished, then the perironeal cavity is drained of the excess fluid and solutes that the failing kidneys cannot remove. Instillation, equilibration, and drainage constitute one exchange. 15

The process of perironeal dialysis can range from 45 minutes ro 9

hours, depending on the method of PO, and patients can have anywhere from four to 24 exchanges per day."

There are two types of PO, auromated PO and continuous ambulatory PD. Automated PD uses an automatic cycling device to control the instillation, equilibration, and drainage phase . ContiullOus

ambulatory PO involves manually exchanging dialysate fluids four

times a day and is schematically represented in Figure 9-3.'5

The following are indications for perironeal dialysis··s:

•

Need for less rapid management of fluid and electrolyte imbalances

• Staff and equipment for hemodialysis unavailable

•

Inadequate vascular access for hemodialysis

•

Shock

• Status post cardiac surgery (e.g., coronary arrery bypass grafting)

• Severe cardiovascular disease

•

Patient refusal of blood transfusions

The following are conrraindications for peritoneal dialysis:

• Peritonitis

• Abdominal adhesions

• Hernias

• Concurrent pulmonary complications, such as atelectasis or

pneumonia

•

Recent abdominal surgery

GENITOURINARY SYSTEM

589

A

B

Figure 9-3. Schematic illustratio" of coutinuous ambulatory peritoueal dialysis. A. l"stillatioN of dialysate fluid. B. Drainage of excess fluid and solutes.

(Artwork by Peter IV".)

Intermittent Hemodialysis

Kidney functions that are controlled by intermittent hemodialysis

include ( I) fluid volume, (2) electrolyte balance, (3) acid-base balance,

and (4) filtration of nitrogenous waStes. The patient's arterial blood is

mechanically circulated through semipermeable tubing that is surrounded by a dialysate solution in the dialyzer (artificial kidney). The dialysate fluid contains electrolytes similar to those in normal blood

plasma ro permit diffusion of electrolytes into Ot out of the patient'S

blood.6•s As the patient'S arterial blood is being filtered through the

dialyzer, "clean" blood is returned to the patient'S venous circulation.

Figure 9-4 illustrates this process.

590

ACUTE CARE HANDBOOK FOR I'HYSICAL THERAPI�TS

Infusion pump

Figure 9-4. Schematic represe"tation of hemodialysis. (With permission from

1M Thompson, GK McFarland, IE Hirsch, et al. (eds(. Mosby's Clinical Nllrsing (]rd ed(. St. LOllis: Mosby, 199];9]8.)

Vascular access is attained either through cannula insertion (usually for temporary dialysis) or through an internal arteriovenous fistula (for chronic dialysis use), which is surgically created in the forearm. The arteriovenous fistula is created by performing a side-toside, side-to-end, or end-to-end anastomosis between the radial or ulnar artery and the cephalic vein. If a native fistula cannot be created, then a synthetic graft is surgically anastomosed between the arterial and venous circulation.6.8.ls Figure 9-5 illu trates these various types of vascular access.

Patients who require chronic intermittent hemodialysis usually

have it administered three to four times per week, with each exchange

lasting approximately 3-4 hours. The overall intent of this process is

to extract up to 2 days' worth of excess fluid and solutes from the

patient'S blood.IO