Page 514 - ACCCN's Critical Care Nursing

Support of Renal Function 491

outside the ICU. In North America, CRRT is not always ● a surgical joining of an artery and vein (usually in the
the preferred choice for supporting ARF in ICU, and inter- forearm), making a large vessel that is punctured with
54
mittent dialysis techniques prevail. As a result of this needles to both draw and return the blood (AV fistula)
mixed application of techniques, there have been a ● a catheter with two lumens to draw and return blood
63
number of publications highlighting the success, utility via a large central vein (veno-venous access
and methods for CRRT in the ICU setting, 47,55,56 with a catheter).
smaller number of publications either making compari- In the acute renal failure setting and where temporary
sons between approaches or continuing to support inter- treatment is anticipated, the two-lumen catheter is
mittent techniques using dialysis in the ICU. 57-60
recommended. 8,64
While this application of both CRRT and intermittent
haemodialysis (IHD) has prevailed for the management HAEMODIALYSIS, HAEMOFILTRATION AND
of ARF, new ‘hybrid’ approaches have evolved that HAEMODIAFILTRATION
combine some benefits of both CRRT and IHD, but also
do suffer from the compromise nature of the approach. There are numerous approaches to the delivery of RRT.
These are daily, limited time frame treatments, with one Haemodialysis, haemofiltration and haemodiafiltration
variant of the approach termed EDDf (extended daily are three common techniques used to achieve artificial
diafiltration), which provides a further treatment option kidney support in ARF. The basic blood path or circuit for
for ARF in the ICU. While daily treatments are emerging these therapies is indicated in Figure 18.10 and is useful
as the only method of RRT for ICU patients in a limited to review as a basis for understanding each of the three
number of hospitals 59,61,62 evidence as to their utility in circuits for each therapy and where the RRT fluids are
treating the most critically ill patients is lacking. This then applied to this circuit differentiating them as alter-
means that continuous forms of RRT prevail as the com- native techniques.
monest method for treating ARF in the ICU in Australia The extracorporeal component is a common factor in all
and Europe. these different circuit designs. The difference between
APPROACHES TO RENAL treatments is how the solutes (urea, creatinine and other
waste products) and solvent (blood plasma water) are
REPLACEMENT THERAPY removed from the blood as it passes through the filter
membrane (artificial kidney), and the intermittent versus
Both IHD and CRRT require a machine to pump blood continuous prescription of the therapy. This is deter-
and fluids; pressure and flow devices to monitor treat- mined by the way in which the dialysis fluids are mixed
ment; a tubing and filter membrane set that together with or exposed to the blood, the rate and direction of
create an extracorporeal circuit (EC) (outside the body blood and fluid flows, and how fluid loss or a negative
blood pathway); and a catheter connecting the patient’s fluid balance is achieved. The three physical mechanisms
circulation to the circuit (see Figure 18.10). This catheter of fluid and solute management are convection, diffu-
enables blood to be drawn from and returned to the sion, and ultrafiltration. Table 18.2 lists the commonly-
patient (known as ‘access’). Access can be achieved by used abbreviations to describe the timing of treatment,
three different techniques: blood access for the therapy and mode of solute removal.
● temporary catheters inserted via a skin puncture into
an artery (A) for drawing blood and a vein (V) to Convection
return the blood (AV access) Convection is the process whereby dissolved solutes are
removed with blood plasma water as it is filtered through
the dialysis membrane. The word is derived from the
Latin convehere, meaning ‘to remove or to carry along
65
EC blood path with’. This process is very similar to that occurring in the
native kidney glomerulus, as plasma water is filtered

Membrane
TABLE 18.2 Abbreviations describing modes of renal
Blood pump replacement therapy
and fluids monitor
Timing of Mechanism of solute
Return blood therapy Route of access removal
to patient
I = intermittent A = arterial H (or HF) = haemofiltration
C = continuous V = venous – convection
Blood from patient S = slow AV = arteriovenous D (or HD) = haemodialysis
VV = veno-venous – dialysis
HDF = haemodiafiltration
Access to patient – diffusion and
convection
UF = ultrafiltration –
FIGURE 18.10 Renal replacement therapy blood path circuit common to plasma water removal
all approaches.