Page 330 - Glucose Monitoring Devices

Exploring the equipment: components and characteristics of the 670G 337

insulin delivery, it does not bear FDA approval for nonadjunctive use of sensor
glucose values. Therefore, patients using this device are advised to treat based on
fingerstick glucose values. Studies assessing sensor accuracy by measuring the
mean absolute relative difference (MARD) have shown MARD of 8.7% [102]. In
this adult study, the sensor was placed on the arm and calibrations were performed
3e4 times a day [102]. When limited to only two calibrations per day, MARD rose
to 10.7% [102,103].

The Medtronic 670G insulin pump

The Medtronic 670G pump functions in two modes: manual mode or “auto mode.”
The manual mode includes pump therapy without the use of the Guardian 3 sensor
(or conventional pump therapy) as well as sensor-augmented pump therapy with
both a low glucose suspend (“suspend on low”) and a predictive low glucose suspend
(“suspend before low”) features. “Auto mode” is the term used to indicate the system
is working as a hybrid closed-loop insulin delivery method, increasing insulin deliv-
ery to curb hyperglycemia and decreasing insulin delivery for anticipated low blood
sugars.
The pump can hold up to 300 units of insulin and is approved for use with rapid-
acting insulin analogs. It has two bolus speed options: “standard” which delivers 1.5
units/min and “quick” providing 15 units/min. The pump is waterproof, protected to
a depth of up to 12 feet for up to 24 h. At altitude, it is approved for air pressures as
low as 70.33 kPa (10.2 psiA), or about 10,000 feet (3000 m) above sea level. It uses a
single AA battery. Basal rates can be given as low as 0 units per hour and up to 35
units per hour, or the maximum basal rate set in the pump, whichever is lower. A
maximum of 25 units may be delivered in aa single bolus.

Auto mode
The algorithm determining the basal rate modulation is housed in the insulin pump
itself. To use this feature, users must receive at least 8 units of insulin per day. Only
two settings may be tweaked during auto mode insulin deliverydthe insulin to
carbohydrate ratio and the active insulin time. Therefore, while there are preprog-
rammed basal rates in the pump, these do not impact the basal rates determined
by the PID controller with insulin feedback. The auto basal is administered via
microboluses every 5 min. The system setpoint is 120 mg/dL; however, if a correc-
tion dose is delivered, it targets glucose of 150 mg/dL. The system parameters are
adjusted each evening based on the total daily dose delivered over the preceding
days and fasting glucose levels. A temporary target can be set, which increases
the setpoint to 150 mg/dL for activities, like exercise. It can be set in half-hour
increments up to a maximum of 12 h.
To initiate auto mode, a minimum of 48 h of insulin delivery data is required.
The use of the system in the manual mode for a longer period of time gives the al-
gorithm more data upon which to derive initial algorithm parameters, leading some