Friday, 5 April 2013

Pumping insulin

Squashes of all sizes and shapes
October 2012
In the early days of insulin-treated diabetes, people with Type 1 diabetes used to draw up insulin from a vial and inject using a hypodermic syringe. Nowadays, most people use 'pen' devices. These have a pre-filled insulin cartridge, a way of 'dialling' the number of units of insulin you want to inject, and a very tiny thin needle, which can be as little as 4 mm long. [Note: the linked website made me chuckle with its description of one of the insulin pens as "designed for people who don't like needles and children."]

However, today's blog is about one of the newer developments in insulin delivery, the insulin pump. This consists of an insulin reservoir containing rapid-acting insulin that provides continuous subcutaneous insulin infusion, 24 hours a day, via a cannula inserted into the skin. Most insulin pumps have a tube between the reservoir and the cannula, but there are tubeless types as well. The cannula can stay in place for two to three days. You can safely disconnect for about an hour, for showering, sports, trying on clothes etc.

For those of us who don't have Type 1 diabetes, our insulin level is adjusted minute by minute in response to all sorts of hormonal signals, such as levels of adrenaline, cortisol and growth hormones as well as the amount of blood-borne glucose. To imitate the insulin-secreting action of the normal pancreas, people on multiple daily injections (MDI) inject two types of insulin: long-acting once or twice a day, which is designed to release slowly into the circulation over a period of around 24 hours, and quick-acting insulin that lasts about 4 hours, injected at mealtimes to deal with the glucose entering the circulation from the carbohydrates that are eaten.

Like the human pancreas, an insulin pump only has one type of insulin, but two separate ways that it is delivered. There is a background or 'basal' rate that delivers the constant low-level drip of insulin day and night. At mealtimes, the pump user still has to decide how much 'bolus' insulin to give and at what rate - the pump doesn't remove the need to test blood glucose and calculate the carbohydrate content of food, and the injected insulin still doesn't act as quickly as insulin secreted by the pancreas directly into the blood circulation. But there are advantages to having more control over the amount of insulin going in.

For example, the basal rate can be adjusted on an hourly basis, and some pump manufacturers set up their pumps with a variable basal rate to match the general circadian pattern found in most people. A pump can also be of great benefit when exercising or drinking alcohol, because both of these activities tend to lower blood glucose, and it can be frustrating to have to take extra carbohydrate in order to avoid a hypo, especially if you're trying to lose weight. With a pump, a temporary lower basal rate can be set. [Note: exercise with diabetes is a fascinating physiological puzzle that I've been trying to figure out for the past few weeks, and I'm sure a blog post will appear on that subject very soon.] If you're on a once-daily injection of background insulin, then you can only adjust the background rate over a period of days.

There's lots of flexibility with the bolus insulin too (which could almost be done with MDI if you were prepared to give several extra injections). Many people find particular foods take ages to digest, especially if there is a lot of fat mixed with the carbohydrate (pizza is a typical example). If they inject just before or just after the meal, the rapid-acting insulin deals with the first surge of glucose but then its period of action is over before the food is fully digested, and their blood glucose level drifts upwards. With a pump, you can program the meal bolus to be delivered either in multiple bursts (just after the meal and then 30/60 minutes after or whatever) or at a constant rate over a period of time. This also works for really long meals, like a posh dinner with big gaps between courses, or for buffets and parties where you might be grazing over a long period of time.

Other advantages with the pump: it can help you with the sums, so you can be more accurate with your insulin dosage. For example, if you have calculated that your meal contains 48 grammes of carbohydrate and your ratio is 1.8 units of insulin per 10 grammes of carbs, the pump can do the calculation without blinking and deliver fractions of a unit (8.64 units in this case). On MDI, everything would be rounded off because we don't hold 1.8 times tables in our heads, and most pens can only deliver whole units of insulin (50 g carbs at 2 units/10g = 10 units of insulin). The ratio of insulin to carbs may change through a day (many people are more insulin resistant in the morning) and that can be programmed in to the pump software. And if you tell it your blood glucose reading, it can also suggest a correction dose, which can even take account of 'insulin on board', i.e. any insulin previously injected that may still be having an effect.

Of course there are disadvantages, not least the cost of the pump, which is available on the NHS only for those who meet the funding criteria. There's the inconvenience of wearing the pump at all times, which can place limits on clothing: at the beach, or on special occasions (e.g. with a party dress). The most worrying to me, based on what I've seen of patient care for diabetes in hospital, is that with a pump you have no long-acting insulin on board, and if your pump is disconnected for any reason your blood glucose level will start to rise after just an hour or two. If this happens at home, perhaps because of a blocked tube or kinked cannula, you should spot it and be able to deal with it. In hospital, the general awareness of diabetes is fairly minimal, and knowledge of insulin pumps is non-existent. There might be a risk of not being given insulin because it is assumed that you will have some level of background insulin on board, and that could have serious consequences.

Where I work, there are special clinics for pump patients, and I attended one of the monthly education sessions where pumpers can drop in to catch up on particular aspects of treatment. My colleague RSB is covering the pump clinic at the moment, so I don't yet have much contact with this group of patients, except for Mr M, who is a pump user, and who is always happy to enhance my knowledge of the issues.

[Update: Mr M informs me that contrary to my assumption, the first wearable insulin infusion pump was invented by Dean Kamen in 1970, while the first insulin injection pen device was introduced and marketed by Novo Nordisk in 1985. Thank you, Mr M!]

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