Fluid Management
Solutions and fluid balance in continuous kidney replacement therapy (CKRT)
CKRT solutions and their composition:
Dialysate and replacement solutions differ primarily in their concentrations of sodium, potassium and bicarbonate, as well as calcium, magnesium, phosphate and glucose. This permits precise, simultaneous and independent management of both fluid balance and plasma composition. For example, the plasma sodium concentration can be maintained at any target value while fluid balance is kept neutral, negative or positive.
The composition should be selected according to serial laboratory results, the treatment prescription and the local protocol, with a review of the solution's compatibility with the anticoagulant and the device [1,4].
Disorders of serum sodium
Using standard sodium solutions in a patient with severe hyponatremia may result in correction that is faster than desired. Described strategies include low-sodium solutions or the addition of 5% dextrose in water (D5W) to the circuit; the method and its rate depend on the prescription, the effluent rate, the target sodium concentration and serum sodium monitoring [1,2].
In IHD, the dialysate sodium concentration can be adjusted continuously within a limited range, usually 130–145 mmol/L, by changing the dialysis fluid containers and connecting them to the appropriate or desired reservoir.
However, achieving effective sodium concentrations outside this range is impractical because of the relatively high Qd used in IHD.
In CKRT, although the sodium concentrations of premixed solutions are fixed, the relatively lower Qd and Qr allow the effective sodium concentration to be adjusted more easily. This, in turn, permits slow, controlled correction of severe hyponatremia or hypernatremia.
Mixing equations can be used to estimate the final sodium concentration in the circuit, but the calculation alone does not constitute a clinical correction plan. Implementation requires frequent sodium monitoring. Any D5W must be included in the patient's fluid intake [1,2].
Correcting severe hyponatremia using CKRT
When correcting severe hyponatremia, the effective sodium concentration of the solution can be reduced by:
However, this method is generally impractical with ready-made commercial solutions, because large quantities of sterile water are not routinely available in hospitals owing to the risk of accidental systemic administration to the patient. Adding fluid to or withdrawing it from ready-made sterile bags also carries risks of inaccuracy or loss of sterility.
Institutions in which CKRT solutions are prepared in the pharmacy may be better able to customize low-sodium solutions, but this method also carries a risk of preparation errors and microbial contamination.
Using D5W to adjust sodium
Another method when using ready-made commercial solutions is to administer an additional infusion of Dextrose 5% in Water – D5W.
It can be administered into the CKRT circuit, usually post-filter, or as a separate systemic infusion.
In patients with hyponatremia, the volume of D5W to be administered in liters per hour to reach the target sodium concentration can be calculated using the equation:
VD5W = Qef × ([Na⁺]CKRT − [Na⁺]T) / [Na⁺]T
Where:
VD5W: the volume of D5W in liters/hour.
Qef: the total effluent flow rate in liters/hour.
[Na⁺]: the sodium concentration in the dialysate and/or replacement solution.
[Na⁺]T: the target sodium concentration.
Example
If we want to reach a sodium concentration of 125 mmol/L using a CKRT solution containing sodium at 140 mmol/L and a total effluent rate of 2.5 liters/hour, then:
VD5W = 2.5 × (140 − 125) / 125
= 0.3 L/h
That is:
300 mL/hour of D5W
The additional D5W volume is continuously removed by ultrafiltration according to the required net ultrafiltration rate.
Increasing the effective sodium concentration
When CKRT solutions with a high effective sodium concentration are needed, either to correct severe hypernatremia gradually or to achieve therapeutic hypernatremia, hypertonic saline can be used in one of two ways:
Example
For a patient weighing 60 kg, an appropriate effluent dose of 25 mL/kg/hour equals 1,500 mL/hour.
With sodium at a concentration of 140 mmol/L in the CKRT solution:
140 mmol/L infused at 1.5 liters/hour represents 210 mmol/hour of sodium, regardless of the CKRT modality used.
A 3% sodium chloride solution can then be infused at 50 mL/hour.
This can be administered either into the CKRT circuit, often as a post-filter replacement solution, or through a separate intravenous line.
3% sodium chloride contains 513 mmol/liter.
Thus, the additional amount of sodium infused at 50 mL/hour is:
513 mmol/liter multiplied by 0.05 liters/hour (50 mL/hour) equals 25.7 mmol/hour of additional sodium.
Combining the two sources gives: 210 mmol/hour (from the dialysis solution) + 25.7 mmol/hour (additional infusion) = 235.7 mmol/hour.
This sodium is contained in a total volume of 1.55 liters/hour (the replacement fluid volume plus the volume of 3% sodium solution over one hour).
The effective sodium concentration therefore becomes:
235.7 mmol/hour ÷ 1.55 liters/hour = 152 mmol/L.
The central clinical idea in this example is that standard CKRT with Na 140 would have created a large gradient: 154 in the patient at the start of treatment versus 140 in the dialysis fluid. This could cause an undesirable reduction in sodium and osmolality in a patient with cerebral edema and intracranial hypertension. Therefore, 3% NaCl was added to bring the effective CKRT sodium close to 152 mmol/liter, making the gradient very small and thus maintaining therapeutic hypernatremia more stably during CKRT.
Summary:
140 = Na concentration in the baseline CKRT solution.
152 = effective concentration of the prescription after adding 3% NaCl.
154 = the female patient's serum sodium at the beginning of the case.
Serum sodium after one hour requires a kinetic calculation and will not immediately equal 152.
Potassium and bicarbonate in CKRT solutions
Because the total daily solute clearance dose provided by CKRT is higher than that provided by IHD three times weekly or even daily, the potassium concentration required in CKRT solutions to control hyperkalemia generally does not need to be as low as that required in IHD.
Usually, 4 mmol/liter of potassium is sufficient unless hyperkalemia is severe.
Acetate or lactate were historically used as the principal buffers in KRT solutions, but modern CKRT solutions now rely almost exclusively on bicarbonate. The typical bicarbonate concentration ranges from 22–35 mEq/L.
CKRT solutions used with regional citrate anticoagulation (RCA)
CKRT solutions used with Regional Citrate Anticoagulation (RCA) are usually calcium-free, facilitating reduction of the calcium concentration within the filter.
These solutions also usually contain a lower bicarbonate concentration, approximately 25 mEq/L.
This compares with other standard CKRT solutions, in which the bicarbonate concentration is usually approximately 35 mEq/L.
The purpose is to account for the alkali load generated by citrate after its metabolism in the liver.
Unlike lactate, which produces an equimolar amount of bicarbonate when metabolized:
Each citrate molecule is metabolized into 3 bicarbonate molecules.
It is also important that some commercial phosphate-containing CKRT solutions are now available, whereas traditional CKRT solutions are phosphate-free.
Achieving Fluid Balance
As mentioned previously, one of the most important practical advantages of CKRT is its ability to remove fluid continuously and gradually.
CKRT machines use gravimetric or volumetric measurement techniques to measure fluid flow rates accurately.
This provides much greater precision in volume control than conventional pumps used to administer intravenous fluids.
Distinguishing UFnet from patient fluid balance
At the machine level, UFnet is the machine's net fluid removal after subtracting the replacement solution rate from the total ultrafiltration rate.
The patient's fluid balance, however, includes all fluid inputs and outputs, including medications, nutrition, intravenous fluids, urine and other outputs.
The UFnet value therefore does not equal the patient's overall fluid balance [4].
Machine UFnet ≠ Patient net fluid balance
Prescribing net ultrafiltration
There are several ways to prescribe net ultrafiltration.
The most common method is for nursing staff to make hourly adjustments to the machine's net ultrafiltration rate to achieve the prescribed patient net fluid balance targets, whether these targets are hourly or daily.
Although many trials are ongoing, rigorous data defining the best volume management strategy with CKRT remain insufficient.
For any critically ill patient requiring CKRT, the nephrology and critical care teams should use all available data relating to hemodynamic status and fluid status.
These data should preferably include dynamic measures of volume status to guide the net ultrafiltration prescription.
In addition, the constantly changing nature of critical illness requires frequent, serial reassessment of ultrafiltration targets and the patient's tolerance of ultrafiltration.
Phosphate in CKRT solutions
Many solutions used in CKRT do not contain phosphate; hypophosphatemia may therefore occur during continuous treatment. In a large observational study, phosphate-containing solutions were associated with a lower incidence of hypophosphatemia than phosphate-free solutions (21% versus 62%). This is an observational association and does not prove improved clinical outcomes [3].
Phosphate concentrations are reviewed periodically, and replacement or a phosphate-containing solution is selected according to the laboratory result, the available composition and the protocol.
Monitoring plan
References
[1] Yessayan L, et al. Management of Severe Hyponatremia with Continuous Renal Replacement Therapies. Semin Dial. 2018. https://pmc.ncbi.nlm.nih.gov/articles/PMC5969490/.
[2] Lippold C, Patel A. How To Prescribe And Troubleshoot Continuous Renal Replacement Therapy: A Case-Based Review. Kidney360. 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8741005/.
[3] Baeg SI, et al. Association of Phosphate Containing Solutions with Incident Hypophosphatemia in Critically Ill Patients Requiring Continuous Renal Replacement Therapy. Kidney360. 2021. PMID:33915554.
[4] KDIGO. Clinical Practice Guideline for Acute Kidney Injury. Kidney Int Suppl. 2012;2:1–138.