Glucose sensors and insulin pumps in hypoglycemia prevention

Yes. A sensor with alarms is one of the best measures for preventing hypoglycemia and is now the standard for glucose monitoring in adults with type 1 diabetes [1]. The sensor brings the greatest benefit to patients at higher risk of hypoglycemia [2] [3]. Examples are people who no longer feel their hypoglycemic episodes or who have had severe hypoglycemia in the past year [2] [3]. In adults at this level of risk treated with insulin injections, the sensor reduces hypoglycemic episodes lasting at least 20 minutes below 54 mg/dL (3.0 mmol/L) by 72% [2]. This reduction is calculated in comparison with using a glucometer alone [2]. In patients who no longer feel their hypoglycemic episodes, severe episodes fall to less than half while they use the sensor, compared with periods with a glucometer alone [3].

The sensor also reduces hypoglycemia in adults treated with insulin injections who have poorer metabolic control, with a glycated hemoglobin (HbA1c) of at least 7.5% [4] [5]. In this case, the number of episodes lasting at least 20 minutes with blood glucose below 54 mg/dL (3.0 mmol/L) falls by about 30% [4]. Time spent below 70 mg/dL (3.9 mmol/L) falls by about 40% during the day and by almost half at night, compared with periods with only a glucometer [5].

The threshold is the value from which the sensor's low glucose alarm sounds. You choose it together with your care team, based on your risk of hypoglycemia. To start with, a reasonable threshold is 65–70 mg/dL (3.6–3.9 mmol/L) [6]. Your care team teaches you how to set the alarms when you start using the sensor [7].

Remember that the value shown by the sensor lags 5–15 minutes behind blood glucose [1]. That is why an alarm with a threshold that is too low may sound when hypoglycemia has already set in, as the page on confirming hypoglycemia also explains. If you have frequent or severe hypoglycemia or no longer feel your hypoglycemic episodes, your doctor may set glycemic targets that are less strict than usual [1]. The threshold of the low glucose alarm is set individually and can be adjusted over time [8].

It is the predictive alarm, which not all sensors have. A sensor with this alarm assesses how quickly blood glucose is falling and warns you before the displayed value drops below a certain threshold [1]. For example, one sensor with this alarm warns you when it estimates that the value will reach 55 mg/dL (3.1 mmol/L) or less within the next 20 minutes [9]. This gives you time to take carbohydrates before blood glucose reaches that value.

If you switch to a sensor with a predictive alarm, you may notice that the time spent below 70 and below 54 mg/dL (3.9 and 3.0 mmol/L) decreases [9]. This time decreases whether the low glucose alarm threshold is set at 70 or 80 mg/dL (3.9 or 4.4 mmol/L) [9]. The benefit of the predictive alarm does not depend on how often you open the sensor app, because the alarm sounds on its own anyway [10]. A sensor that gives fewer false low glucose alarms helps you treat nocturnal hypoglycemia faster, because you are less likely to ignore its alarms [11].

It is the function by which the pump stops the basal rates on its own. The basal rates are the insulin that the pump delivers continuously, in small amounts. In the version called low glucose suspend, the pump stops the basal rates when the sensor value reaches a set threshold. Some pumps have predictive low glucose suspend and stop the basal rates earlier, when they estimate that the sensor value will reach the set threshold within the next 30 minutes. The pump then resumes insulin delivery when the sensor value rises into a range considered safe. Low glucose suspend alone reduces nighttime hypoglycemia by about a third, without raising HbA1c [12].

In children and adolescents, predictive low glucose suspend significantly reduces the time spent in hypoglycemia, especially at night [13] [14]. This benefit comes without an increase in time spent in hyperglycemia or in the number of ketoacidosis episodes [14]. An automated insulin delivery (AID) system is a sensor-connected pump driven by an algorithm [1]. The AID pump continuously adapts its insulin delivery in both directions, based on the values received from the sensor and their trend [1]. In adults at high risk of hypoglycemia, the AID pump reduces the time spent below 70 mg/dL (3.9 mmol/L) by about 60% [15]. The figure below compares the three technologies.

No. An AID pump continuously adapts its insulin delivery in both directions, and some models also deliver small correction doses of insulin [7]. However, no AID pump can remove insulin that has already been delivered from the body [7]. Hypoglycemia can still occur after a meal bolus that is too large or after physical exercise, especially if you did not tell the pump in time [6]. The physiological lag of the values estimated by the sensor behind blood glucose can also contribute [6]. After switching from injections or from a pump without a sensor to an AID pump, severe hypoglycemia becomes less frequent in routine practice [16] [17]. Compared with a sensor-connected pump with predictive low glucose suspend, AID pumps generally did not bring a further reduction in severe hypoglycemia in randomized trials [1]. In routine practice, this decrease is sometimes also seen after switching from such a pump [16].

About one in five adults with type 1 diabetes who use an AID pump report at least one episode of severe hypoglycemia in a year [18]. Patients who have had severe hypoglycemia most often name an insulin dose that was too large or an overestimated amount of carbohydrates as the cause [18]. That is why it is important to enter carbohydrates correctly into the pump, to turn on exercise mode in good time and to keep the alarms on [1]. If you use an AID pump, hypoglycemia is usually treated with 5–10 g of carbohydrates, because the pump has already been reducing or stopping insulin delivery for some time [1] [14] [19]. The exceptions are hypoglycemia during physical exercise and hypoglycemia after a meal insulin dose calculated for far more carbohydrates than were eaten [6]. In these situations, you treat hypoglycemia with the usual amount of about 15 g of carbohydrates, as the page on treating hypoglycemia explains [1].

For most adults with diabetes, the target for time spent below 70 mg/dL (3.9 mmol/L) is under 4% of the day (under an hour) [20]. Within this time, the target for time spent below 54 mg/dL (3.0 mmol/L) is under 1% of the day (under 15 minutes) [20]. In older patients or those at high risk of hypoglycemia, the target for time spent below 70 mg/dL (3.9 mmol/L) is under 1% of the day (under 15 minutes) [20]. In pregnancy, in women with type 1 diabetes, the target of under 4% of the day applies to time spent below 63 mg/dL (3.5 mmol/L) [20].

The percentages in the sensor report are reliable if the sensor recorded values for at least 70% of the time, over at least 14 days [20]. These percentages show the risk of hypoglycemia better than the episodes you remember, because 25–30% of patients with type 1 diabetes no longer feel their hypoglycemic episodes [1]. A single episode of level 2 or 3 hypoglycemia is enough for your care team to review your treatment, so report it without waiting for your next appointment [21].

Start with the standard 14-day report, called the ambulatory glucose profile (AGP report) [7]. First look at the time spent below 70 mg/dL (3.9 mmol/L) and below 54 mg/dL (3.0 mmol/L), because these values show the risk of hypoglycemia [20]. The main graph overlays the blood glucose values from the 14 days on the same 24-hour axis. For each hour of the day, the lower edge of the colored band shows the lowest values recorded over the 14 days.

Look for patterns, not isolated episodes of hypoglycemia. A pattern means values below 70 mg/dL (3.9 mmol/L) at the same hour on several days. Such a pattern helps you identify an insulin dose that may be too high. You then discuss it with your care team, as the page on adjusting insulin also explains. Compare nights with days, days with exercise with days without it, and weekend days with working days. The report also shows the hypoglycemic episodes you did not feel [22]. Your care team adjusts your treatment based on these data [22].

Yes, it is possible. This is called alarm fatigue, and it occurs when you receive so many alarms that you no longer respond to them correctly [23]. The risk is that you turn off all alarms, including the low glucose alarm [23]. That is why alarm settings are decided together with your care team [7].

Keep only the alarms that require immediate action from you [6]. Give priority to the low glucose alarm [8]. You can add the high glucose alarm, for example, with an initial threshold of 250–300 mg/dL (13.9–16.7 mmol/L), then adjusted to your needs [6]. Review your alarm settings with your care team at every appointment. Never turn off the low glucose alarm overnight.

An insulin pump that is not connected to a sensor can reduce the number of severe hypoglycemic episodes compared with multiple daily insulin injections [14] [24]. The greatest benefit occurs in patients who had many severe hypoglycemic episodes while on injections [24]. After an education program in which they learn to adjust their insulin doses, patients treated with injections have severe hypoglycemia less often [25]. Their risk becomes close to that of patients who use a pump [25].

The greatest benefit comes when the pump is connected to a sensor and stops the basal rates or adapts its insulin delivery on its own [1] [12] [15]. In children, switching to a pump with predictive low glucose suspend or to an AID pump reduces time spent below 70 mg/dL (3.9 mmol/L) by more than a third [26]. AID pumps are now the preferred method of insulin delivery in type 1 diabetes, in children even from diagnosis [7] [14]. If you do not have access to an AID pump, the next choice is a pump with predictive low glucose suspend [14]. If you use a pump that is not connected to a sensor, talk to your doctor about switching to an AID pump.

Measure your blood glucose with a glucometer until you have access to a new sensor. Every patient who uses a sensor must always also keep at hand a glucometer with test strips that are within their expiry date [7]. Blood glucose should be checked before meals and snacks, at bedtime, before driving and whenever you have signs of hypoglycemia [7]. As long as you do not have a working sensor, you do not receive low glucose alarms. That is why it is wise to measure your blood glucose once during the night as well on nights with a higher risk of hypoglycemia [8]. Examples are nights after intense physical exercise or after drinking alcohol.

If it does not receive values from the sensor, the AID pump can no longer adapt its insulin delivery. Most AID pumps then switch to manual mode, delivering the programmed basal rates without automatic adjustment [7]. You also need a written backup plan for the case in which the pump stops working altogether. You need to know which insulin doses you can take with an insulin pen [1]. After a new sensor is inserted, there is a warm-up period during which the sensor shows no values and gives no alarms [7]. During this time you can measure your blood glucose with a glucometer, as the page on the warm-up period also explains. Keep a spare sensor at work and another one in your luggage when you travel.