A trial of the Pen Pump infuser Anne Henry SRN Staff Nurse

Janet Cheesman SRN Dip N (Lond) Ward Sister

Joanna Sheldon MD FRCP Consultant Physician

Diabetic Unit, Royal Sussex County Hospital, Brighton.

Correspondence Dr Joanna Sheldon M D FRCP, Consul tant Physician Royal Sussex County Hospital, Brighton, East Sussex, BN2 5BE.

Introduction The Pen P ~ m p ‘ ’ ~ ) ( R a n d Rocket Ltd,

Hitchin, Herts) has been devised to allow 100 units/ml strength insulin (UIOO) to be given intermittently through an indwelling subcutaneous catheter and needle. It is primarily of use, therefore, for patients in whom short-acting insulin before meals and snacks may provide better control than one or two injections daily. It does not provide continuous insulin administration.

By virtue of its design, however, the ‘Pen Pump’ has the added advantage of extremely accurate delivery of a given dose, even in the partially-sighted. I t should only be used with UlOO insulin, and not with any other strength. We have supplied the Pen Pump to 12 patients and present the results of this trial.

Materials and patients The Pen Pump consists of (i) A 3ml plastic syringe barrel with

Figure 1. f e n Pump barrel and infusion catheter

plastic finger grips and a sterile inner sleeve. At the distal end of the barrel is a small black rubber plunger. A plastic subcutaneous infusion catheter with attached needle can be attached to the syringe nozzle. The catheter has a volume of 0.16m1, ie contains 16 units of UlOO insulin (Figure I).

(ig A non-disposable, screw-threaded metal piston, which can be pressed into and thus fixed to the black rubber plunger; the piston is equipped with a plastic yoke which can be spun u p or down the piston, and which clips onto the finger grips of the syringe barrel. The piston is designed so that the dosage knob when turned clockwise through 360° deliveries 4 units of UlOO insulin. There is an audible and palpable click at every quarter turn, so that single units of insulin can, in fact, be accurately delivered (Figure 2).

Figure 2. Screw-threaded metal piston

(iii) A fountain-pen-like metal cap with

To assemble the Pen Pump, the yoke is spun u p the metal piston until it reaches the dosage knob a t the proximal end (Figure 4). The pointed end of the piston is then pressed into the black rubber plunger of the syringe (Figure 5). The piston is then withdrawn to its limit, and the protective plastic piston sheath removed and discarded (Figure 6). The inner wall of the syringe thus remains sterile. The syringe is then filled with insulin in the ordinary way, except that a larger volume is withdrawn, as the syringe also acts as a storage reservoir for the insulin (Figure 7).

pocket clip (Figure 3).

When filled, the yoke assembly is spun down the piston and locked in place around the finger grips of the plastic syringe barrel (Figure 8). The needle is then discarded and replaced by the infusion catheter (Figure 9). Air is

Figure 3. Metal cap for Pen Pump

42 Practical DIABETES May/June 1985 Vol 2 No 3

-- A trial of the Pen Pump infuser

Figures 4-12. Assembling the Pen Pump

expelled from the syringe by turning the dosage knob to screw the piston down, rather than by pressing the plunger down in the usual way. The metal cap is then fitted in place over the piston, and the Pen Pump attached to clothing, or t o a chain around the neck as desired (Figure 10). The needle is then inserted subcut- eously into the abdominal wall, in either a horizontal or upward direction (Figure //), and anchored with adhesive tape so as to cover the needle (Figure 12). The catheter and needle are usually changed every one to three days, using a fresh injection site.

patients (9 female; 3 male), whose ages ranged from 19-76 years and who had previously been on insulin for periods ranging from 6 months t o 27 years. In 10 cases, the decision to try a Pen Pump was made on the basis of poor control, on twice-daily mixtures of shor t a n d medium-acting insulins, in patients who nevertheless were genuinely striving for good control and who were keen to try the Pen Pump. In one case, control was not bad on the patient’s twice-daily insulin injections, but nevertheless she wished to try to improve it further.

The remaining patient was insulin- resistant, requiring 196 units daily before startine the Pen PumD. which was used in Patients and methods

Y I ,

We have supplied Pen Pumps to 12 order t o provide frequent smaller doses.

Practical DIABETES May /June 1985 Vol 2 No 3

However, he was changed from bovine to human insulin at the same time as starting use of the Pen Pump, and there was a rapid fall in his insulin require- ment, probably due to the change in the species of insulin. He is now using only 28 units daily but, being partially-sighted, prefers t o continue the Pen Pump for more accurate delivery of the required dose.

All patients were admitted to the Diabetic Unit. During the first week, we tried to achieve optimal control on their twice-daily insulin regime. On days 7 and 8, o r later if better control appeared likely, blood glucose was measured: (a) before each of the three main meals; (b) one and a half hours after each of the

43

-- three main meals; (c) before the bedtime snack. Patients were then started on the Pen Pump and one week later the same blood glucose measurements were made. Haemoglobin A, (Ref) was estimated during the first week of admission, before starting the Pen Pump, and three months later in those patients who continued its use for this period (reference range: 5.4- 9.2%).

Results Of the 12 patients, only 9 were still

using the Pen Pump after three months. One 20-year old boy stopped using the

Pen Pump after seven weeks. We had not been able to achieve better control with it than he was able to obtain with four conventional injections daily. On five daily doses of short-acting insulin given via the Pen Pump, the last dose being given at bedtime, he still had persistently high fasting blood glucose levels (12.5- 19.1 mmol/l). When Initard was used instead of Neusulin, it sedimented in the catheter. Thus, it was necessary to give an additional injection - given convent- ionally - of a medium-acting insulin a t bedtime and the patient preferred to revert to conventional insulin injections. He also found the Pen Pump an uncomfortable encumbrance.

One 70 year old woman stopped using the Pen Pump after one week, because

she was apprehensive a b o u t the equipment and had been frightened by an episode of ketosis occurring while using it, probably due to kinking of the catheter and failure of delivery of insulin.

One 33-year old woman stopped using the Pen Pump after only two days. She was a patient who suffered from a chronic anxiety state, for which she had received psychiatric treatment; she was particularly anxious about her poor diabetic control but, far from providing reassurance, the Pen Pump frightened her even more and she was unable to persist with it. In addition, her fasting blood glucose was high, even when the last dose of short-acting insulin was given a t bedtime via the Pen Pump.

The remaining 9 patients have been using Pen Pumps for periods ranging from 3 to 20 months, and have been extremely pleased with them, preferring them to conventional insulin injections.

Difficulties encountered by these 9 patients using the Pen Pump were minor. Two patients reported breakage of the plastic yoke once, requiring replacement of the piston; one patient’s catheter broke off a t the junction with the hub on one occasion, and another noticed a leak at the junction of the syringe and catheter. The finger grips on one batch of syringes

Figure 13. Blood glucose profiles in 8 patients, before and on Pen Pump

A trial of the Pen Pump infuser

were not gripped adequately by the yoke, though the equipment was still usable. One patient found for a time that the needle was frequently obstructed by blood, but this problem resolved itself spontaneously.

For assessment of diabetic control, we have excluded the 3 patients who stopped using the Pen Pump after short periods, and the insulin-resistant patient, whose use of it coincided with the change from bovine to human insulin. His improved control (Haemoglobin A, dropping from 18.8%, before changing the species of insulin and starting the Pen Pump, to 10.4% three months later) could not therefore be attributed only to the use of the Pen Pump.

Mean blood glucose levels with optimal control before using the Pen Pump and one week after starting the Pen Pump are shown in Figure 13. Each point represents the mean of all blood glucose estimations at each time, including two estimations from each of the 8 patients. Mean fasting blood glucose was significantly higher when patients were using the Pen Pump. Six of the 8 patients had been using mixtures of short and medium-acting insulins twice daily, o r medium-acting insulins alone twice daily, before using the Pen Pump, which could

15 14 13 12 11 10

BLOOD

GLUCOSE a (mmolll) 7

6 5 4 3 2 1

NS NS * NS NS * T

%- BEFORE PEN PUMP

0- AFTER 1 WEEK ON

PEN PUMP

* SIGNIFICANTLY DIFFERENT

(p < 0.05)

$ SIGNIFICANTLY DIFFERENT

(p < 0.02)

NS NOT SIGNIFICANTLY DIFFERENT

44 Practical DIABETES May/June 1985 Vol 2 No 3

A trial of the Pen Pump infuser

account for their lower fasting blood glucose levels then.

The increase in blood glucose after breakfast on the Pen Pump was less than before, reflecting the effect of what was usually an increased amount of short- acting insulin before breakfast when using the Pen Pump. From before lunch onwards mean blood glucose levels were lower on the Pen Pump than before using it, and significantly lower one and a half hours after lunch and before dinner.

Mean haemoglobin A, levels measured just before and three months after starting the Pen Pump in these 8 patients were 12.94% (+ 3.49) and 10.83% (+ 2.48). This improvement after three months on the Pen Pump did not achieve statistical significance, perhaps due to the small numbers involved.

Discussion Even though the improvement in

diabetic control obtained in patients satisfied with the Pen Pump, who continued to use it for periods ranging from 3-20 months, was not statistically significant when assessed by the change in haemoglobin A, levels, the individual

patients preferred the Pen Pump for various reasons. It allowed less frequent insertions and, although the needle was re-sited in most patients at 2-3 day intervals, one patient usually used the same site for 5 days without difficulty. This patient even went swimming with the Pen Pump tucked into her bikini bottom!

Our oldest patient using the Pen Pump, aged 76, whose insulin dose was reduced, probably as a result of a change in the species of insulin, continued to use the Pen Pump because, being partially sighted, it gave him greater indepen- dence. He could see sufficiently well to assemble the equipment, but did not have to see graduation marks on the syringe, and could thus deliver the required dose more accurately. For the intelligent user, the Pen Pump also allowed greater flexibility, both in the timing and in the content of meals, though not the same degree of flexibility as can be expected using a continuous infusion of insulin with pre-meal boosters.

However, certain patients who seemed suitable candidates for the Pen Pump turned out not to be so. This in 2 cases was for psychological reasons, both patients being anxious individuals who

found the handling of the equipment worrying. The main limitation of its usefulness, however, would seem to be the failure of the Pen Pump in somecases adequately to control fasting blood glucose, requiring the addition of a medium-acting insulin by conventional injection at bedtime. Designed for the frequent injection of short-acting insulin, we found that the needle blocked on the only occasion we tried to overcome this problem by using a mixture of short and medium-acting insulins (Initard insulin, Nordisk Limited).

The price of the Pen Pump has now been reduced to 290.00, which is cheaper than the infusion pumps, but it is not available on the NHS. Supplies of disposable syringes, catheters and needles cost 24.52 for 10 packets.

In summary, we feel that for selected patients the Pen Pump offers significant advantages in management, and may improve diabetic control.

Reference Hall P M, Cook J C H, Could B J. An inexpensive. rapid and precise affinity chromuto~raphy method for the measure- ment of glycosylated haemoglobins. Ann Clin Biochem, 1983; 20: 129-35.

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