How to… use sampling catheters to avoid repeated venipuncture
Sampling catheters are a great alternative to repeated needle sticks in patients who need frequent blood samples!
They can be really useful in diabetic/ketoacidotic patients, patients with electrolyte disturbances which require regular assessment, or those on supplemented fluid therapy (e.g. high-rate potassium supplementation). There are lots of different types available - my practice typically uses long-line catheters which are inserted through a larger, normal IV catheter into a saphenous vein. This way, if the sampling catheter fails, still have a standard IV catheter.
Sampling catheters can be used as a normal IV for administration of medications and fluid therapy, but can also be used to collect samples. They are handled aseptically, with gloves worn, ports cleaned with alcohol prior to use, and flushed regularly with 0.9% saline. Traditionally, the three syringe technique was used to collect blood from sampling lines - firstly, a small volume of flush was instilled into the catheter. This was then aspirated back with 0.5-1ml of blood, before a fresh syringe was used to collect a blood sample (not contaminated with any IV flush). The catheter was then flushed.
This technique can quickly lead to anaemia, especially in small patients or those requiring repeated sampling. More recently, the push-pull technique has been reported. This allows a sample not contaminated with IV flush to be collected, but avoids discarding any blood. The push-pull technique involves inserting a volume of flush into the catheter, then, using the same syringe, drawing a blood volume 3x the dead-space volume of the catheter into the syringe, and re-infusing this into the patient. This is repeated three times, before drawing your blood sample into a separate syringe.
This method has been studied in human paediatric medicine, and at Penn vet school.
