The Flushing Of Cannulas

Goose. Geese. Moose. Meese. Cannulas. Cannulae. Whatever!

Blood Extraction

Why throw away the first 10mL of blood when collecting it from an existing cannula? It’s because that portion is theoretically contaminated, such as by previous infusions.

Saline Flush

The purpose is, also theoretically, to keep the cannula open and working. Patent, as they say. That’s why this is recommended after giving IV medications or just periodically. Basically, after each use.

Not Water

Normal saline is injected, not water. This is because a pure water injection would lead to the water rushing into red blood cells, due to that trusty process of osmosis. This sudden water influx would make them explode.

Which, as you can imagine, would run counter to the usual hospital purpose of trying to heal people, not atomically bomb their bloodstream.


  1. Blood collection from intravenous cannula. (2019). Retrieved from
  2. Keogh, S., Flynn, J., Marsh, N., Mihala, G., Davies, K., & Rickard, C. (2016). Varied flushing frequency and volume to prevent peripheral intravenous catheter failure: a pilot, factorial randomised controlled trial in adult medical-surgical hospital patients. Trials, 17(1), 348. doi:10.1186/s13063-016-1470-6
  3. Labos, C. (2019). Why can I taste saline and medications when they’re inserted into my IV?. Retrieved from

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