Continuous glucose monitors (CGMs) monitor glycemia in diabetic dogs and cats without the need for obtaining blood samples, as these systems measure glucose in the interstitial space rather than in blood or plasma. Interstitial glucose is in equilibrium with blood glucose, but a lag of several minutes (the exact time period varies among species) elapses before a change in blood glucose is reflected in the interstitial space.1 CGMs collect glucose data over long periods (ie, weeks) and can provide on-demand, real-time information about glycemic status. CGMs record and display high- or low-glucose and glycemic trends, allowing intervention and application of preventive measures for hypoglycemia and hyperglycemia.
The CGM most commonly used in veterinary medicine uses a sensor with a subcutaneous filament that contains enzyme-mediated glucose-sensing technology to detect glucose in the interstitial fluid.2 Unlike earlier technologies, current sensors are factory calibrated and do not require users to perform calibration steps to ensure proper sensor function. The wireless sensor communicates data with a receiver (eg, dedicated electronic reader, smartphone).
Use of CGMs in diabetic dogs and cats has been reviewed and validated.1,3-5 The most common application is long-term glycemic monitoring in diabetic dogs and cats, but CGMs are also useful for in-hospital monitoring of patients with diabetic ketoacidosis.6,7 Anecdotal reports also include glycemic monitoring in nondiabetic disorders (eg, insulinoma, juvenile hypoglycemia, other conditions associated with dysglycemia).5,8 CGMs provide data that would be unavailable or impractical or difficult to obtain via conventional glucose-measuring methods. CGMs can provide real-time assessments of glycemia, as well as time-averaged glucose curves (ie, average glucose concentration over a period of time) constructed from hundreds of independent glucose measurements.
Functions & Features of Continuous Glucose Monitors
Following are some important functions and features of various CGMs.
The sensor automatically performs frequent measurements of interstitial glucose concentration for several weeks. For example, one CGM system measures glucose every minute and stores the values every 15 minutes over several weeks, which is the lifespan of the sensor.
CGM sensors are factory calibrated, so calibration steps are not needed before or after activation.
CGMs use a receiver (ie, reader, smartphone with a dedicated application) to acquire and analyze data.
User data can be stored on the receiver or uploaded to a web-based platform that stores data and has tools for analysis. These platforms allow data sharing between the user and the medical team.
CGMs may feature patient alarms to indicate glucose values that fall outside the desired range (high or low).
Will Continuous Access Cause Clients to Panic?
Efficient communication between the clinician, veterinary team, and client is imperative. For example, clients should understand that low glucose levels need to be interpreted in conjunction with clinical signs of hypoglycemia.
When outlining the plan for using a CGM, clients should be instructed to call the clinic immediately if they are concerned about an urgent or potentially hazardous situation. If a situation occurs outside the clinic’s normal hours, the client should contact an emergency hospital to ascertain whether actions should be taken at home or the pet should be taken to a medical facility.
Clients should be encouraged not to modify the insulin dose without first consulting the treating clinician. Clients with experience with human diabetes may be expecting to adjust insulin based on frequent readings, so it is important to be clear when discussing how the insulin dose is generally determined in cats and dogs. Fluctuations in glucose values are expected, and the continuous monitoring provided by this kind of system significantly mitigates the risk for situations that pose an immediate danger to the patient.
Step-by-Step: Application of a Continuous Glucose Monitor for Monitoring Glycemia in Diabetic Dogs & Cats
What You Will Need
Continuous glucose monitor sensor (new)
Receiver (ie, reader, smartphone with required CGM manufacturer application downloaded)
Clippers
Dilute chlorhexidine wipes, alcohol wipes, cotton balls, and dry gauze
Tissue adhesive
Procedure gloves (optional)
Step 1: Select the Application Area
Determine where the CGM will be applied, considering the sites most frequently used: dorsal aspect of the neck, withers between the scapulae, lateral trunk over the shoulder blade, hip, and skin over the lumbosacral spine. Choose a site where the CGM can be comfortably accommodated but not easily dislodged or removed by the patient.
Author Insight
Most sites on the trunk can be used, although some sites may be less suitable for certain body types. For example, the lateral thorax may be preferred over the withers in small dogs with a narrow thorax.
Step 2: Prepare the Site
Clip the hair as close to the skin as possible, with the clipped area only slightly larger than the size of the CGM. Take care not to injure the skin.
Once the hair is removed, gently cleanse the site with dilute chlorhexidine solution and alcohol wipes. Use clean dry gauze to remove excess fluid, and allow the skin to air-dry.
Author Insight
Protocols for preparing the skin for placement described in veterinary studies have been adapted and modified from manufacturer’s instructions for use on human skin. Regardless of the protocol used, the site must be clean, dry, and free from any material that might interfere with effective placement. Skin with obvious evidence of dermatitis or other lesions (eg, hyperemia, extensive scarring) should be avoided. Oily, excessively moist, and/or haired skin may interfere with attachment and result in failure or detachment.
Step 3: Prepare the Continuous Glucose Monitor
Prepare the CGM by precisely following the manufacturer’s instructions provided with each unit.
Author Insight
In the figures, the sensor pack (left) has been separated and the sensor applicator inverted to show the sensor loaded inside the applicator (right).
Step 4: Place the Continuous Glucose Monitor
Immediately prior to placing the CGM, invert the sensor applicator and inspect the loaded sensor unit. Apply several small drops of tissue adhesive to the underside of the sensor, taking care to avoid contact with the sensor needle and filament and any electronic components of the unit.
Place the sensor applicator over the application site and press firmly. Listen for a low clicking sound that will indicate the sensor is discharged. Hold the applicator in place for a few seconds, then gently pull the applicator away from the skin. Ensure the sensor remains adhered to the skin as the applicator is withdrawn.
Secure the sensor by gently pressing down on the sensor body, and move a finger along the adhesive material to ensure contact with the skin. If needed, use additional tissue adhesive to secure the adhesive material to the skin.
Discard the used sensor applicator and sensor pack materials according to local regulations.
Author Insight
It is important to be familiar with the manufacturer’s caution statements regarding the sensor apparatus and sensor placement.Sensor pack and sensor applicator codes must match so glucose readings are accurate. The sensor applicator contains a needle. The inside of the sensor applicator should not be touched, and the sensor should not be placed back in the sensor pack. Once the sensor is loaded, the sensor applicator should not be pushed down until it is in position over the prepared skin site. This will prevent unintended results or injury.
Step 5: Initialize the Continuous Glucose Monitor Sensor & Acquire Glucose Data
If using a dedicated reader, follow the screen prompts to prepare the reader to accept a sensor.
If using a smartphone, open the manufacturer’s application and follow screen prompts to prepare the phone to accept a sensor.
To initialize the sensor, follow the instructions on the device screen to scan and recognize the new sensor.
Author Insight
Following the manufacturer’s instructions is vital when linking a new CGM to a receiver to avoid a sensor recognition error, which requires application of a fresh CGM.
Whether a single CGM can be paired with >1 receiver (eg, dedicated reader and a smartphone) depends on the CGM being used. For CGMs that can send data to multiple receivers, there may be a specific order in which the devices must be linked to be recognized.
A 60-minute wetting period is required after placement so the sensor can equilibrate in the interstitial fluid and become functional. The monitoring screen will display a timer counting down from 60 minutes until the sensor begins to send data. At the end of the wetting period, the sensor should be checked to ensure it is providing readings, including current (real-time) glucose concentrations, a glucose trend arrow, and a glucose graph.
An error indicating an incorrect or aborted sensor initialization process or a sensor that does not respond after the 1-hour wetting period indicates a nonfunctioning sensor, which must be discarded. In this circumstance, a new sensor must be applied and the initialization procedure carried out again (as described in Steps 1-5).
Removing a Continuous Glucose Monitor
A CGM that has reached the end of its functional lifespan can be removed using an adhesive remover wipe or cotton ball soaked in baby oil applied liberally to the plastic sensor body and the apron of the adhesive material. If skin glue was used during the placement step, an adhesive remover used for bandage removal is preferred. Hair often regrows at the placement site, even under the sensor, so care should be taken during sensor removal to avoid epilating hair, which could damage the skin.
Complications of Using a Continuous Glucose Monitor
The most common complications of using a CGM are dermatologic, ranging from mild (eg, erythema, pruritus, mild crusting, discomfort) to severe (eg, erosions, ulcerations, infections, severe itching), and may be associated with the sensor adhesive or additional tissue adhesive.1
Premature sensor detachment is common. Most CGMs are designed to last 14 days, but studies have shown the maximum sensor life was reached in only 70% of dogs and 60% of cats with properly applied sensors.1,5,9 In cats, repeated application of sensors can help with becoming accustomed to wearing the device, resulting in a longer sensor lifespan.1,5,9