Troubleshooting the dialysis circuit

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The machine is alarming. A light has come on, and is blinking. You are alone in the room. You look around with increasing anxiety, but there is no nurse. The patient surfaces briefly from their sedation to regard you with one half-closed eye. Your confidence drains away beneath the cold scrutiny of their gaze; the alarm is very loud, and you can't make it stop. You don't even know where the silence button is. You begin to wish you were familiar with one of the local CRRT learning packages.

This nightmarish scenario has been played by the college in multiple past paper SAQs:

• Question 26 from the first paper of 2018
• Question 12 from the first paper of 2010
• Question 5.2 and Question 5.3 from the first paper of 2009

Low Access Pressure alarm: "Access pressure extremely negative"

This is the alarm for the venous side of the circuit.

The pressure gauge here measures the negative pressure generated by the access pump, which sucks blood out of the patient and pushes it into the filter.

The maximum tolerated pressure here is -250mmHg. Anything greater than this will cause a "low access pressure" alarm.

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.

Access disconnection alarm

Also known as "access pressure not nearly negative enough", this alarm is triggered if the access pressure is more positive than 10mmHg. Usually this means the patient has just pulled their vas cath out, and the machine is sucking air.

High Return Pressure: "Return pressure extremely positive"

This is the alarm for the arterial side of the circuit.

The pressure gauge here measures the positive pressure generated by the return pump, which sucks blood out of the filter and pushes it into the patient. The maximum tolerated pressure here is around 300mmHg. Anything greater than this will cause a "high return pressure" alarm.

Causes of high return pressure:

• The line is kinked
• The vas cath is kinked
• The vas cath return lumen is trapped against a vessel wall
• The vas cath has become occluded with clot
• The vas cath has dislodged, and the return lumen is emptying into a haematoma
• The vas cath was inserted into an artery, and nobody noticed.

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. You may need to withdraw it slightly.
  • Pause dialysis and aspirate the vas cath lumens, trying to suck out the clot (if there is one)
• Check the patient
  • Make sure the vas cath is actually in a vein, and that no haematoma is forming around it.

• Admit defeat
  • You may have to resite the vas cath. Furthermore, you may be unable to return the blood.
  • Swapping the lumens rarely helps in this situation, but its worth a try.

Low Return Pressure

This is the alarm for the arterial side of the circuit, generated by the return pump pressure gauge.

A "low return pressure" alarm would usually only be triggered by a negative pressure (the lower limit for the alarm is usually 0mmHg).

Causes of low return pressure:

• Low blood pump speed (i.e. low blood flow rate
• Line disconnection (venous access line is sucking air) - some machines will report this as a "disconnect" alarm
• Pre-sensor, post-pump line is clamped (the reading is thus 0 mmHg)

Troubleshooting:

• Check the circuit:
  • Start from the machine and work through the venous side of the circuit to ensure that there is no site of diconnection anywhere.
  • Make sure there is no embarrassing clamp anywhere.
• Check the patient
  • Make sure the vas cath did not get pulled out by the enraged delirious patient.

• Admit defeat
  • Increase the blood flow rate.

High filter pressure: "Filter pressure extremely positive"

This is the alarm for the filter. One usually hears it when the filter is dying. It occurs when the difference between filter pressure and return pressure is greater than 250mmHg. (In most machines, it will also go off when the transmembrane pressure is in excess of 450mmHg)

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.

High transmembrane pressure (TMP)

Some machines will report the TMP together with, or instead of, the filter pressure.

Again, one usually hears this alarm when the filter is dying.

Causes of high TMP

• TMP tends to rise gradually over the course of the dialysis session, as the filter becomes clogged with filth.
• If it rises suddenly:
  • The filtrate line (from the filter to the effluent bag) has become kinked or clamped
  • An embolus of clot or something has become lodged in the abovementioned line
  • The ratio between the blood and dialysate flows is too high

High TMPs with normal return pressures indicate a problem with the filter.

High TMPs with high return pressures indicate a problem with the return line and/or the filter.

Troubleshooting:

• Check the circuit:
  • Ensure that there
  • Make sure there is no embarrassing clamp anywhere.
  • Flush the filter through the rescue line - this may improve its function for a little while
• Admit defeat
  • Either increase the blood flow rate or decrease the dialysate flow rate, or both
  • Increase pre-dilution rate and decrease post-dilution rate

Air detected alarm

The bubble detector has found some bubbles.

Causes of the "air detected" alarm:

• There is air in your line
• The blood level in the bubble trap is too low
• The air detector is detached from the return line

Troubleshooting:

• Check the circuit:
  • Make sure the air detector is properly attached.
  • Ensure the bubble trap is working properly (i.e. fill it up from a syringe)
  • Look for actual bubbles
  • If possible, aspirate them
• Admit defeat
  • The circuit is full of froth, and you cannot give the blood back. You need to set up a new circuit.

Blood leak alarm

The blood detector in the effluent line has found something it thinks is blood.

If there is actual blood in the line, the filter has ruptured.

The situation may look like this:

Causes of a "blood leak" alarm

• There is actual blood in the line
• The blood detector is dirty (usually its the mirror)
• The patient has severe haemolysis, and the detector is picking up free haem.
• The patient has recently received a dose of hydroxycobalamin, which tends to discolour all body fluids, and which will cause the blood detector in the dialysis machine to alarm.

Troubleshooting:

• Check the circuit:
  • Does the effluent actually look like there is blood in there?
• Admit defeat
  • The filter has ruptured and needs to be replaced.

Set disconnect alarm

This one is triggered if the filter pressure is less than 10mmHg. Such a reading would suggest that either the filter line has come off the filter, or it is clamed above the pressure transducer (and thus nothing is being pumped). This can be readily identified by the presence or absence of a large amount of blood on and around the machine.

Lastly, there is a "Mute" button.