Bent cannulas
The soft tube folds on insertion, the site looks perfect, and the pump reports no problem. Why a correction that does nothing is the real clue.
A bent cannula is the small soft tube that goes under your skin folding on the way in, so insulin either goes in badly or does not go in at all. It is invisible from the outside, the pump reports everything as normal, and it is one of the commonest reasons a pump user gets unexplained highs.
- It happens on insertion, and you usually cannot tell by looking. The site can appear perfect.
- The pump does not know. It reports delivery as normal because it delivered; the insulin just went nowhere useful.
- The sign is unexplained highs that corrections do not fix, sometimes starting soon after a set change.
- Because there is no long-acting insulin behind a pump, this leads to ketones within hours rather than days.
- The response is to change the set and follow the plan your team gave you, rather than repeating doses through the same one.
- Watch afterwards for the other direction: corrections that were stacking up while nothing arrived can all land once a working set is in, so a hypo (a low below 4 mmol/L where you feel shaky, sweaty or confused and need fast sugar) can follow at the time or shortly after, and again hours later including overnight.
What is actually going wrong under the skin
Most infusion sets place a soft flexible cannula using a needle that is then withdrawn, leaving the tube behind. If the insertion meets resistance, or the angle is off, or the site is over firm or scarred tissue, the soft tube can kink as the needle comes out. What is left is a folded tube that either restricts insulin or blocks it completely.
The reason it is so hard to catch is that everything visible looks right. The adhesive is flat, there is no leak, no blood, and the pump is delivering on schedule and reporting no problem. A pump can only detect a blockage when pressure builds enough to trigger an occlusion alarm, and a partly bent cannula often never reaches that point.
Firm patches under the skin make it considerably more likely, which is where this connects to site rotation: a set going into tissue that has been used repeatedly meets more resistance and has more to fold against.
The pattern, since looking will not tell you
The usual shape is a rise that starts within hours of a set change and does not respond to corrections given through the same set. That last part is the useful bit: if you correct and nothing moves, the most likely explanation is that the correction did not arrive rather than that it was too small.
Other clues are worth knowing. Insulin smell around the site, or damp skin under the adhesive, suggests it is leaking back out rather than going in. Soreness at the site, or a site that hurts when the pump delivers, suggests the tube is not sitting properly. And a set that has been in longer than it should be is likelier to have developed a problem.
The safest habit is to treat a set change as a thing to watch for a couple of hours rather than a job that is finished. Most people find it useful to check a while after changing, particularly overnight changes, because a set going in badly at bedtime has all night to cause trouble.
Change the set, do not chase it
The instinct on an unexplained high is to correct and wait, and with a suspected set problem that is the wrong move: more insulin down a route that is not delivering achieves nothing except spending the time in which ketones build. Changing the set puts insulin back into the equation, which is the thing that actually needs to happen.
Your team will have given you a plan for a high that will not come down, and it will usually include changing the set, checking ketones, and how to get insulin in another way while a new set settles. That plan is worth knowing before you need it, because working it out while high and worried is harder than reading it in advance.
Then keep the spares. A set failure with no replacement is an emergency rather than an inconvenience, and it is the reason a pump user carries a backup way of getting insulin in. That is not pessimism, it is the standing advice, and this is the specific scenario it exists for.
What makes it more likely
Overused sites are the biggest factor, and they are also the one you control. Very lean areas offer less room for a cannula to sit properly. Moving vigorously soon after inserting can dislodge a cannula that went in fine. And sets vary: some people find a different angle or a steel cannula solves a recurring problem entirely, which is a conversation worth having rather than persevering.
How it presents
Examples, not instructions or doses.
High after lunch, you correct, and two hours later you are no lower. The likely explanation is that it never arrived, not that it was too small.
Set changed at bedtime, and you wake high with ketones. A bad insertion at night has the longest possible run before anyone notices, which is why a later check is worth it.
Adhesive flat, nothing to see, no alarm from the pump. That is exactly what a bent cannula looks like from the outside, which is why the pattern matters more than the inspection.
When it's urgent
A pump has no long-acting insulin behind it, so a set that is not delivering leads to ketones and DKA within hours. If your glucose is high and a correction has not worked, change the set rather than repeating the dose, check for ketones, and follow your team’s plan. If you are being sick and cannot keep fluids down, have stomach pain, are breathing fast or deeply, or feel very unwell, contact your diabetes team or call NHS 111. If you are drowsy, confused or breathing very fast, call 999.