Page 520 - ACCCN's Critical Care Nursing

Support of Renal Function 497

Normal clotting time, a laboratory test designed to production from tapwater at the bedside. This approach
measure the time taken for blood to clot under laboratory can be cheaper, requires no bag changing or reconstitut-
98
conditions, is used as a reference to determine a suitable ing by nurses, but involves installation of a complex and
therapeutic range of the anti-clotting drug during CRRT. expensive reverse osmosis machine, the cost of which
Different tests are applicable to different medications and would be offset if large volumes of fluid were then con-
their site of action in the clotting cascade. When using sumed from this online manufacture. This approach may
any anti-clotting agent, a balance is required between the alter fluid selection and use in the future, and is an essen-
benefits of increased coagulation suppression and the tial feature of chronic dialysis and use of ICU daily dialy-
higher risk of patient systemic bleeding. In each patient sis (EDDf) modes of therapy. 59-61
this risk may vary, depending on illness, accompanying Fluid balance maintenance is a key nursing responsibility
liver failure and administration of concurrent anti-clotting in managing CRRT. Most machines now available to
agents such as activated protein C.
provide CRRT incorporate a mode selection program,
Fluids and Fluid Balance including an automatic fluid balance system. This ensures

A key component of any CRRT is the administration of a delivery of a fluid prescription based on the input pro-
vided by the programming nurse. As many litres of fluid
replacement solution for the fluid removed during hae- may be exchanged in an hour (25–35 mL/kg), the default
mofiltration (see Table 18.4). This same physiologically setting is usually a net machine balance, where all fluids
balanced solution is also used as a dialysis fluid if a dialytic administered as either dialysate or replacement are recov-
mode is included. The solution may have a low potassium ered or balanced. The machine cannot, however, include
level, so this electrolyte can be removed rapidly if hyper- fluids administered or expelled directly from the patient,
kalaemia is part of the original indication for CRRT. Potas- so a fluid maintenance schedule must be established.
sium must therefore be added later to the solution to This schedule is also based on input and output being
ensure that hypokalaemia does not occur. In Australia and equal. For example, if more fluid is being infused into the
New Zealand these fluids are commercially prepared, with patient than being lost, as would be expected in ARF, then
the only major choice being between the type of acid additional fluid would need to be recovered via the CRRT
buffer: lactate or bicarbonate. Some small studies have circuit: that is, less replacement fluid administered or
compared the use of lactate and bicarbonate fluid buffer more waste created. Consider the calculation steps in the
because of a concern that the lactate solution reduced example in Table 18.5.
heart performance by causing cardiovascular depression
and made accurate determination of patient lactic acid Irrespective of the accuracy of fluid replacement assess-
accumulation difficult to interpret. 94-97 In most settings the ment prior to therapy, individual patient assessment for
use of lactate-buffered fluids in patients with cardiac and fluid status must occur at least twice a day. Subtle tem-
liver failure is avoided, as lactate accumulation (levels perature changes in the patient, fluid boluses, diarrhoea
above 5 mmol/L) in these patients indicates inadequate and variable absorption of feed may all contribute fluid
metabolism of lactate to bicarbonate by the liver, with losses not included in routine fluid maintenance. As treat-
13
increased acidaemia. While bicarbonate solutions may ment progresses over time, this may exacerbate a general
appear to have an advantage over lactate-based solutions
in critically ill patients, they are more expensive causing
some physicians to only prescribe them when lactate accu-
mulation is anticipated or after it occurs. TABLE 18.5 Example of CVVH fluid maintenance
schedule to calculate replacement solution dose/hour
The higher cost, problems with reconstituting bicarbon-
ate solution bags and manual handling of large (5 L) Fluids in (mL) Fluids out (mL) Fluid balance (mL)
fluid bags has increased interest in ‘online’ fluid
1. Drugs, IV, NG 2. drains = 10 mL/h + 85 mL (+ve
= 120 mL/h insensible losses = balance/h)
(heparin, 25 mL/h
inotropes,
TABLE 18.4 Typical replacement/dialysate fluid intraflows, NG
feeds)
constituents for CRRT
3. CVVH fluid removal
Bicarbonate-based Lactate-based dose = 2500 mL/h
Component solution (mmol/L) solution (mmol/L) (25–35 mL/kg/h)
4. Total in = 5. Total out = 35 mL/h
Buffer 25.00 45.00 120 mL/h (from 2) + 2500 mL
Potassium 0.00 1.00 ultrafiltrate =
2535 mL
Sodium 140.00 140.00
6. Patient overloaded 7. Replacement
Glucose 0.00 10.00 on clinical solution/h = 2535
examination; need (−120 mL fluid
Calcium 1.63 1.63 to remove an input; −25 mL to
Magnesium 0.75 0.75 additional 25 mL/h reduce fluid
overload) =
Chloride 100.75 100.75 2390 mL/h