a) Draw and label a diagram to show the key components of a continuous veno-venous haemofiltration circuit. (40% marks)

b) The following pressures are displayed on your Continuous Renal Replacement Therapy (CRRT) machine that is providing continuous veno-venous haemofiltration (normal values are provided in brackets).

                       Access pressure:        -240 mmHg* (-50 to -150 mmHg)

                       Pre-Filter pressure    46 mmHg*      (100 to 250 mmHg)

                       Return pressure:        38 mmHg*      (50 to 150 mmHg)

          Describe your approach to dealing with the problem  (40% marks)

c) The problem resolves but the following day you are presented with a new issue

Access pressure: -110 mmHg    (-50 to -150 mmHg)

Pre-Filter pressure. 450 mmHg*   (100 to 250 mmHg)

Return pressure: 40 mmHg*     (50 to 150 mmHg)

What is the likely cause?(20% marks)

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College answer

a)

college CVVHF diagram

Either pre-or post-filter replacement fluid acceptable

Effluent pressure monitor not required

b)

Statement that this is a venous access problem

  • Check patient: hypovolaemic? Potentially give volume
  • Check settings: is blood flow excessive?
  • Check proximal limb of circuit: kinking or clots?
  • Access catheter: check for kinking due to insertion angle (replace) or clotting (try aspirating and flushing). Try withdrawing catheter slightly. Consider replacing catheter (different site, larger bore, longer catheter in femoral site, end-hole catheter instead of side hole)
  • If all else fails try reversing lines

c)

Imminent clotting of the filter 

 Examiners Comments:

 This was answered well by most candidates.

Discussion

a)

It is pleasing to see the college offer some positive remarks to the candidates, a downtrodden species generally accused of having serious knowledge gaps. This time, they did well. It is equally pleasing to see the college offer a reasonable-looking diagram in their model answer. This one is not as easy to track down as some of the other Google images they used (it only appears in this presentation, although it very likely has some other source). As the official college diagram, it is superior to the locally available non-peer-reviewed options, and should be viewed as the Definitive CVVH Diagram for the purposes of exam revision.

b)

The pressure are:

                       Access pressure:        -240 mmHg* (-50 to -150 mmHg)

                       Pre-Filter pressure    46 mmHg*      (100 to 250 mmHg)

                       Return pressure:        38 mmHg*      (50 to 150 mmHg)

This is just short of the values which in the Prismaflex machines give a "Access Pressure Extremely Negative" alarm (-250 mmHg). The following troubleshooting section is lazily cut-and-pasted from the chapter on the troubleshooting of the dialysis circuit:

Causes of low access pressure

  • The vas cath is kinked
  • The line is kinked
  • The vas cath is sucking against a vessel wall
  • The vas cath has become occluded with clot
  • The vas cath is of poor design (i.e. you should have inserted a short widebore vas cath, with a circular lumen crossection)
  • The blood flow to the vas cath is poor:
    • The patient is hypovolemic
    • There is increased intrathoracic or intraabdominal pressure, decreasing venous flow past the catheter tip
    • The patient is breathing without positive pressure ventilation, and is hyperventilating (deep panicked breaths create a strongly negative intrathoracic pressure during inspiration, which pulls blood in the opposite direction, out of the vas cath).

Troubleshooting:

  • Check the circuit:
    • Start from the machine and work along the circuit, checking for kinks
    • Check the vascath for kinks; ensure it has not become dislodged
    • Obviously, unkink the kinked bits.
    • Try to gently rotate the vas cath, if possible. Some positions may be better than others.
    • Pause dialysis and aspirate the vas cath lumens, trying to suck out the clot (if there is one)
  • Check the patient
    • An agitated patient who is constantly repositioning themselves will play havoc with the access pressure. Encourage their cooperation.
    • Ensure there is enough venous blood (i.e. consider the possibility that there is hypovolemia)
    • Ensure the respiratory pattern is not responsible (for intrathoracic catheter tips and spontaneously breathing patients)
  • Admit defeat
    • First, just press "continue"*. Whatever the phenomenon was, it may have passed.
      *This rarely works.
    • You may have to resort to decreasing the blood flow rate. This will decrease the solute clearance somewhat, but at least the machine will stop alarming.
    • If nothing is working, one may find that swapping the access and return lumens can help. However, even if this does work, the recirculation will be massive (much of the returned blood will get sucked into the circuit again). Conventional wisdom holds that this is a useless strategy. You may as well not be dialysing at all, they say. However, a study by Carson et al (2005) suggests that reversing the lumens does not appear to hamper the clearance of urea. Still, in most circumstances, it is still better to resite the vas cath.

c)

The pressure we are  presented with:

Access pressure: -110 mmHg    (-50 to -150 mmHg)

Pre-Filter pressure. 450 mmHg*   (100 to 250 mmHg)

Return pressure: 40 mmHg*     (50 to 150 mmHg)

The extremely high pre-filter pressure and the unusually low return pressure suggests that the filter is clotting. This would generate a "Filter Pressure Extremely Positive" alarm.

Causes of high filter pressure

  • Filter pressure tends to rise gradually over the course of the dialysis session, as the filter becomes clogged with filth. This is part of the natural filter degradation.
  • If it rises suddenly, there are few options as to why:
    • The line from the pressure gauge to the filter has become kinked or clamped
    • An embolus of clot or something has become lodged in the abovementioned line
    • You set the pre-dilution replacement solution flow rate too high

Troubleshooting:

  • Check the circuit:
    • Ensure the line going into the filter is free from kinks.
    • Make sure there is no embarrassing clamp anywhere.
    • Assure yourself that the pre-dilution replacement fluid flow rate is appropriate
  • Admit defeat
    • The filter has clotted. Get another one.

References

References

The Gambro PRISMA Systems Operator's Manual is a wealth of information. However, it is very long.

This excellent nursing resource from Nepean ICU by Keren Mowbray is both succinct and complete.

So is this one (also from Nepean, by Alison Bradshaw - but it appears to be in Comic Sans)

Ricci, Zaccaria, Ian Baldwin, and Claudio Ronco. "Alarms and troubleshooting."Continuous Renal Replacement Therapy (2009): 15.

Carson, Rachel C., Mercedeh Kiaii, and Jennifer M. MacRae. "Urea clearance in dysfunctional catheters is improved by reversing the line position despite increased access recirculation." American journal of kidney diseases 45.5 (2005): 883-890.

Sutter, Mark, et al. "Hemodialysis complications of hydroxocobalamin: a case report." Journal of Medical Toxicology 6.2 (2010): 165-167.