Gary Scheiner - Practical CGM: Improving Patient Outcomes through Continuous Glucose Monitoring

Essentials of Continuous Glucose Monitoring

Theres a reason so many major companies have invested heavily in fingerstick glucose monitoring. Point-in-time glucose readings are valuable, and not just for those who are pricking their fingers. They rake in billions for medical device manufacturers because they allow us to titrate (fine-tune) insulin and medication doses, see cause-and-effect relationships within our daily lives, conduct cutting-edge research on new treatments, and fix elements of diabetes management programs that just arent working. Like a photograph or a painting, fingerstick blood glucose (BG) readings depict what is happening at a particular moment in time, but they also leave a lot to the imagination. Take any 10 people and show them a portrait of da Vincis Mona Lisa, and youll hear 10 different interpretations about who she was, what she was like, and what she was thinking about.

As we will discuss in Chapter 2, adding a continuous glucose monitor (CGM) to ones diabetes management is like taking all those images and turning them into a movie, complete with story notes. CGM lets us see and understand the full picturewhere we came from, where we are, and to an extent, where were headed. CGMs alert features provide the user with an early warning system to guard against severe hypoglycemia and prolonged hyperglycemia. And the ability to analyze CGM data retrospectively gives health-care providers and their patients an opportunity to make decisions based on facts rather than assumptions.

These are among the reasons that CGM is growing in popularity. There is no question that the technology still has considerable room for improvement, and our ability to interpret CGM data is still evolving. Nevertheless, CGM has already been shown to be an effective tool for improving glucose control and quality of life for users.15

GET TO KNOW THE EQUIPMENT

Currently, Dexcom and Medtronic offer CGM systems in the United States. Although systems by other manufacturers are available in other countries, our discussions will focus on devices that are approved by the U.S. Food and Drug Administration (FDA) and available for sale in the U.S. In addition to real-time systems that allow the user to see, learn from, and react to their own data, both Medtronic and Dexcom offer professional use systems. Professional systems are borrowed for a fixed period of time (typically 67 days). The only significant difference between systems is whether the user is able to see their data while they wear the sensor. The Dexcom G4 Professional system has an option that allows the user to see their data in real time or to blind it (not showing it on the display) until the receiver is downloaded. The Medtronic Professional system, called iPro, is always blinded to the user. Health-care providers download the professional system after removing the sensor from the patient and analyze the information to make therapy adjustments.

Regardless of the manufacturer and whether the device is for real-time or professional use, three components are common to all CGM systems: a sensor, a transmitter, and a receiver or display.

Sensors

The sensor is a thin, flexible, metallic filament that is about a finger-width long (Fig. 1.1). It is placed in the layer of fat below the skin using a push-button insertion device (Fig. 1.2). The insertion device pops a small needle (with the sensor attached) into the skin and then retracts the needle, leaving only the sensor below the skin. The insertion process is virtually painless when done properly; training by a device manufacturer representative is highly recommended. Sensors are indicated for ~1 week of use (6 days for the Medtronic Enlite; 7 days for the Dexcom G4). Anecdotally, users have reported the ability to use sensors for longer than their FDA-approved indication, but at present no controlled studies have evaluated the safety and efficacy of prolonged sensor use.

Figure 1.1Sensors.

Figure 1.2Insertion devices.

Transmitters

Once below the skin, the sensor reacts with glucose molecules by generating a miniscule electric current. This current travels up to the base of the sensor on the skin surface where it connects to a transmitter (Fig. 1.3). The transmitter is about the size of a thumbnail. It contains its own power source and a radio transmitter. Transmitters require periodic charging or replacement to keep the power fresh.

Figure 1.3Transmitters. The image on the left also shows the transmitter charger.

Receivers and Displays

The radio frequency generated by the transmitter varies based on the magnitude of the electrical impulse coming from the sensor. Each transmitter has a unique ID and is linked to a receivera device that interprets the radio signals and displays the corresponding glucose data (Fig. 1.4). The Dexcom CGM utilizes a handheld receiver about the size of a small cellular phone. It also transmits to the Animas Vibe insulin pump and soon will be able to transmit to other insulin pumps. The Medtronic CGM display is integrated into Medtronic insulin pumps (model x22 and higher). Medtronic also offers a stand-alone receiver (not an insulin pump) called Guardian. The Guardian receiver has similar functionality to the CGM component of Medtronics insulin pumps, although it is only approved for use with Medtronics previous-generation Sof-sensor and uses an older algorithm to interpret the signals.

Figure 1.4Receivers and displays.

CGM receivers display a variety of important real-time information, including trend graphs covering recent time intervals; trend arrows indicating the current direction the glucose is headed; and, of course, the latest glucose value generated by the sensor. Both Medtronic and Dexcom receivers provide updated information every 5 min. The receivers also emit audible and vibratory alerts. Users and their clinicians can customize the alert settings to suit individual preferences and achieve specific goals. Alert options include the following:

Basic high and low alertsas soon as the sensor detects a glucose concentration that has crossed a user-set threshold, an alert is triggered