Confirmed over-infusion: Damaged syringe pump

1. Brief Description of the Incident

An infusion pump was set to deliver at 10ml/hr but was reported to deliver at 15ml/hr.
The over infusion was detected because of the routine practice, by nursing staff in the ward, of checking volume infused from the syringe barrel.

2. Device Details

A syringe pump whose rate is set in terms of a volumetric flow rate and which detects the syringe size in order to convert flow rate from the selected volume flow rate (ml/hr) to plunger movement in mm/hr. Most syringe pumps operate in this way.

3. Investigation

Workshop investigations confirmed the over-infusion; at 10ml/hr the average flow rate after an hour was 15.3ml/hr � an error of 53%.

The pump was tested with the BD Plastipak 50/60 ml syringes used in the ward.
Closer examination then revealed that the syringe size indicator (wrongly) recorded that a 30ml syringe was being used.
The pump does not require the user to verify the syringe size when the pump is used.
Internal examination showed that the syringe size indicator was damaged � the photo shows on the left a new part and, on the right, the damaged part.
The "syringe size sensor arm" rests on the syringe barrel rotating the "sensor arm flag" whose position is detected by opto-sensors to indicate the syringe size.
Because of the damage (hidden within the pump), the flag did not rotate in proportion to the syringe size and hence the 50/60ml syringe was detected as a 30ml syringe.
The ratio of the cross-sectional area of the syringes (50/60ml syringe is 5.54 cm²; 30ml syringe is 3.67 cm²), that is 1:1.51, accounts for the ~50% higher flow rate observed.

4. Possible causes

Failure to note that the syringe size indicated by the pump was not the same as the syringe being used.
Damaged syringe size indicator.
Cause of damage unknown. Damage could have been caused by storing pumps on top of each other, or if the pump had fallen or been knocked against another object with the force taken by the sensor.
A second pump was reported to the workshop from the same area soon after this with a completely broken syringe size indicator. A spot check revealed a further device with a sticking sensor.

5. Lessons to be learnt and remedial action to be taken

Report damage to medical devices. The cause of the damage is unknown. It is vital that staff report damage to medical devices. External appearance alone did not indicate any damage to the device. If a device is suspected of being damaged it must be thoroughly checked prior to its return to use.
Check the syringe size indicator prior to starting the pump. Know how your syringe pump displays the syringe size and check that it correctly detects the syringe size prior to setting up the pump.
Staff training to ensure that staff understand:
Why detection of the wrong syringe size caused the pump to over-infuse.
Why it is important to check the syringe size.
Why it is important to report damage to medical devices.
Why it is important to check the volume infused from the fluid container as well as totaliser.

6. Positive actions taken by staff

Checking volume infused from the barrel of the syringe showing what volume was actually delivered.
This ensured that staff detected that there was a fault with the pump and minimised the duration of the over-infusion.