The insulin infusate

Where insulin is to be infused over prolonged periods, it is important to be sure that the concentration of the infusate remains stable. The dose is often estimated in milliunits of insulin per kilogram body weight, although using the body surface area is preferable, especially where subjects of different sizes are to be compared. Body surface area relates more precisely to volume of distribution. The specific requirements for individual studies are discussed below. Insulin sticks to plastic and so an insulin infusate will lose strength over time. To avoid this it is usual to make the insulin up in a 4 % solution of autologous blood, and it is critical that the insulin is mixed by slow rotation (to avoid bubbling). The solution is then run through the entire giving apparatus, to saturate the binding sites in the plastic from the beginning. For infusions lasting more than five hours, fresh solutions should be made. A small amount of all infusions should be kept for later estimation of the exact insulin concentration (in a diluted sample). This is particularly important when the investigator wishes to measure insulin sensitivity. In such studies, accuracy in the making of the insulin solution is critical and use of clinical grade syringes may not be adequate to achieve this reliably.

Line for infusing insulin and glucose

Hot box

Line for sampling with 3-way tap

Figure 5.4 Close up of arterialised venous sampling during induction of controlled experimental hypoglycaemia.

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