Critical Care Trauma Centre




Line Blocking

Saline Flush



When using an ST 150 filter, the CRRT circuit is primed with 2 L of 0.9 NaCl. Usually the first bag contains 5,000 units/L of heparin, followed by a second unit of plain normal saline.  The heparin in the first bag will bind to the ST ("Surface Treated" to promote binding) filter. The second priming bag of plain normal saline rinses the tubing of any heparin so that the patient does not receive a bolus at the onset of treatment. Priming with heparin reduces clotting on the filter.  

Priming can be done without heparin using two bags of plain normal saline if heparin is ABSOLUTELY contraindicated (e.g., with Heparin Induced Thrombocytopenia or heparin allergy). If anticoagulation is being avoided due to bleeding risk, heparin can usually be added to the first bag as long as it is followed by a second bag of heparin FREE saline.

Priming ensures the removal of any air bubbles (air bubbles can precipitate clotting) and debris. It also removes any residues from gas sterilization.

Ethylene oxide (EtO) is a bactericidal gas used to sterilize most medical disposables, including dialysis circuits. Most of the EtO is eliminated by mixing it with carbon dioxide to produce a 10% solution. This produces a harmless waste at the end of the sterilization process, if a 1-2 week deaeration period is provided.

It has been shown that some EtO residual may remain at the end of the deaeration period, especially in the filter. If this gets into the blood stream, it can produce an allergic reaction. To ensure that all EtO residual is eliminated, the circuit must be primed to rinse out the EtO if it sits for a prolonger period of time. If the patient is not going to be connected immediately, prime up to the point when the Prime Test is required. A manual prime of 200 ml just prior to hookup is sufficient to clear any EtO that has reaccumulated. Manual priming cannot be done after the Prime Test is completed.



Administer a heparin bolus at the initiation of a treatment. The usual dose is 80 units/kg bolus (maximum dose 5,000 units). If the second bag of priming solution also had heparin, the volume of the circuit (approximately 250 ml) with 5 units per ml heparin should be considered (~1200 units). Usually, the second prime does not contain heparin so this does not need to be considered.

To adminiser the bolus of heparin at the start of the treatment, clamp the access limb, connect a syringe containing the heparin bolus, unclamp the access limb and inject the heparin into the catheter limb. The volume of heparin is less than the limb volume, therefore, the heparin bolus will not enter the blood directly. Reclamp the access limb and connect to the access tubing of the circuit. Start the treatment. As blood is pulled into the access limb of the catheter at the start of the treatment, the first blood to reach the filter will contain heparin.

PTT measurements for the purpose of titrating the heparin are only done q6h. Do not decrease heparin infusions before the 6 hour sample (unless active bleeding is the reason). Early reduction often leads to subtherapeutic levels. Systemic aPTTs should be drawn daily to monitor for screening of systemic levels.


When using citrate, measure a systemic ionized calcium before starting the treatment to ensure normal systemic levels. Administer bolus of calcium chloride if required (according to sliding scale) before starting the treatment. Start the systemic calcium chloride infusion 15 minutes before starting a treatment.

The citrate infusion is administered via the PreBlood Pump of the Prismaflex(TM) and is titrated to the post filter ionized calcium levels. The PBP infusion volumes are automatically removed and incorporated into the net fluid balance. A calcium chlorde infusion is administered via a central line that is not part of the dialysis circuit, and titrated to maintain a normal systemic ionized calcium. The calcium chloride infusion IS NOT accounted for by Prismaflex, therefore, it must be incorporated into the fluid balance calculation the same way that IV fluid is addressed.

PrismOcal dialysate is used with citrate because it is calcium free. Use of calcium containing dialysate increases the citrate requirements, which increases the calcium chloride requirements. PrismOcal has 0 mmol/L of potassium, therefore, add potassium according to the sliding scale.


Line blocking to maintain catheter patency between dialysis runs is done using 4% citrate (available in multidose vials by pharmacy), instilled into each limb for all dialysis catheters. Aspirate and flush each limb with saline prior to line blocking. Use a separate syringe for each limb to ensure line sterility is maintained. Instill undiluted citrate solution equal to the limb volume plus .1 ml. .


When priming and connecting the Prismflex circuit, add a saline flush infusion to the Y connector at the access port. This will make re-tranfusion quick and easy and allow prn flushing to assess filter patency. If you want to retransfuse or clear the filter of blood to inspect for clots:

  1. Close the stopcock to the access site and open it to the saline infusion
  2. Saline will now be drawn into the circuit at the blood flow rate
  3. Time the duration of time that the saline is open to calculate the volume administered (e.g., if the blood flow rate is 200 ml/min and the saline infuses for 30 seconds, the patient has received 100 ml).
  4. Incorporate the saline flush volume into the intake and output calculation; increase the fluid removal by an equivalent amount (if tolerated)

Remember that you will need to estimate the saline volume and record it on the intake record (saline flushes are not removed by the pump). If clotting is present during flushing, retransfuse.

Always try to retransfuse before the filter clots. The entire circuit with the Prismflex(TM) using ST150 filters and heater tubing contains about 250 ml of blood.


Prevent Excess Patient Removal/Treatment Shutdown

After changing a dialysate or replacement bag, remember to observe the fluid removal, dialysate and replacement volumes to ensure that the volumes are changing appropriately. Tubing impedance can occur on either the dialysate or replacement side. Kinked heater tubing, failure to fully crack the luer lock connection or a leak at the connection site can lead to over removal from the patient.

Any recurrent replacement, effluent or PBP alarm indicates that the bag volume is not changing according to the set flow rates. Before resetting the alarm, check for any clamps or flow restriction before resetting. Failure to do so will trigger a repeat alarm. For example, if the replacement clamp is closed and the replacement flow is set at 2 L per hour, the pump will automatically remove 2 L per hour of effluent, plus a volume equal to the net fluid removal + PBP volume. If the replacement flow is restricted, the same effluent volume will be removed. The missing replacement volume will be removed from the patient's plasma volume. To prevent excessive volume loss from the patient, safety alarms are in place. The newest software versin 7.11 is able to adjust pump speeds to accommodate some variation in flow, but persistent alarms may cause the sytsem to shut down.

Alarm Limits Automatically Adjusted

Remember that everytime a bag is changed or a flow rate is adjusted, the pressures may change. The machine automatically resets the pressure limits each time the pump resumes. Upper and lower pressures are automatically set as 50 mmHg above and below the operating pressures (once the flow rates have re-established). A slow rise in the filter, access or return pressures can eventually trigger the upper pressure limit. The next time the pump is reset, the upper limits (50 mmHg above the operating pressures) will be higher. This can allow the pressures to creep up and suddenly reach a level high enough to shut the pump down with a "clotted filter" alarm. Monitor the pressure trends carefully.

Access/Return Disconnection

The Prismaflex cannot tell if a catheter is truly disconnected. Instead, it measures the amount of "suck" required to pull blood from the access port (negative pressure) and the amount of "push pressure" that is required to return the blood to the return side. The Prismaflex expect that it will take at least - 10 mmHg to pull the blood and at least + 10 mmHb of pressure to return the blood. If the blood flow rate is too low, the access pressure may not become negative enough or the return pressure may not become positive enough. Thus, an access that is more positive than - 10 mmHg or a return that is less positive than + 10 mmHg will trigger an access or return disconnect.

If an access or return disconnect alarm occurs, check the connection for integrity. If the luer lock device is intact, increase the blood flow rate until sufficient access and return pressures are generated.

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Created: July 21, 2009

Updated: July 2, 2013, January 30, 2015