Continuous Glucose Monitoring Diving and Diabetes: An Update of the Swedish Recommendations

Abstract

Divers travel to different countries to explore various diving sites worldwide. In 2005, the Divers Alert Network (DAN) published their guidelines for recreational diving and diabetes mellitus. However, although years have passed, there is still no consensus in the form of international guidelines on diabetes and diving. Large differences are noted with regard to the regulations in different countries. Furthermore, the diabetes technology has evolved rapidly and is not reflected in current international guidelines. This is potentially both a medical and an insurance problem for a diver with diabetes. We present a short summary of the recently updated Swedish recommendations for recreational divers with type 1 diabetes mellitus, focusing on the use of continuous glucose monitoring and continuous subcutaneous insulin infusion during such circumstances.

In Sweden, recreational scuba diving has been permitted since 1998 for individuals with diabetes mellitus in well-controlled and well-informed individuals with type 1 diabetes mellitus (DM1) on either multiple daily insulin injections (MDI) or insulin pump treatment, as well as in type 2 diabetes mellitus (DM2), on either insulin or combinations of oral hypoglycemic agents (OHAs).^1,2

In previous guidelines monitoring of blood glucose has been recommended in a specific time interval before and after diving: 90, 60, 30, and 10 minutes before and 10 minutes after diving.

An individual with diabetes able to show practical knowledge in handling their diabetes by achieving a lower HbA1c is better suited as a diver but there is a caveat: striving for too low HbA1c levels without reducing glucose variability results in increased risk of hypoglycemia. Therefore, we strongly believe that real-time continuous glucose monitoring (rtCGM) offers added value, given the information provided with this technology and the fact that rtCGM could be used either as a stand-alone system or as part of treatment, with continuous subcutaneous insulin infusion (CSII) combined with an autonomous insulin infusion stop or predicted low glucose management (PLGM) function to further reduce hypoglycemia.

Furthermore, studies have also shown that rtCGM provides the diver with information on the glucose levels during the dive, which is of importance when analyzing the outcome with regard to the undertaken measures.³ As hypoglycemia is the major obstacle for those on insulin treatment who want to perform recreational diving, we believe the guidelines need to be updated.

Recommendation

In the approval process, there is a need of a consensus between the responsible diabetologist, a physician authorized in diving medicine, and the diver.

The following criteria need to be met:

• Glycemic control with an HbA1c <7.9 % NGSP (<63 mmol/mol IFCC), without any long-term diabetic complications. Mild diabetic retinopathy is accepted.

• No episode of severe hypoglycemia during the last year and no report of hypoglycemic unawareness.

• The diver should have good knowledge of how to manage the diabetes disease, and have knowledge on how to adjust insulin doses and the amount of carbohydrates prior to physical activity.

• The use of continuous glucose monitoring (CGM) is recommended for risk evaluation prior to diving as well as at the evaluations made by the responsible diabetes physician.

• Yearly evaluation should be performed by a specialist in diabetology and a physician authorized in diving medicine.

The major hazard for an individual with diabetes during diving is linked to the risk of hypoglycemia. The symptoms of hypoglycemia, normally seen during other conditions, could mimic those that might occur during diving, that is, fatigue, shivering, and reduced cognitive function due to hypothermia. Moreover, hypoglycemia could be misjudged as decompression illness, which creates an even more complicated situation.⁴ It is easy to misjudge the symptoms, and this could lead to unnecessary acute air transportation and recompression treatment when the correct treatment would instead be to add carbohydrates or to administer glucagon. An untreated hypoglycemic episode during a dive might increase the risk for the person with diabetes as well as the diving buddy. Severe hypoglycemia during diving has been linked to fatalities.⁴ Pronounced and/or repeated hypoglycemic events have also been shown to impair the hormonal counter regulation.⁵

Therefore, glucose monitoring is of great importance before, during, and after diving. The most important advice is to prevent hypoglycemia. Bearing this in mind, it is also important to know how to treat low glucose levels during diving since a rapid ascent is not always possible.

Preventing Hypoglycemia

Glucose monitoring should be performed more frequently,⁶ either by glucometer or preferably by the use of rtCGM.

If self-monitoring of blood glucose (SMBG) is used, monitoring should be done frequently, 4-8 times per day during 1-2 weeks, prior to diving. Recommendation: Strive for a stable glucose level to minimize hypoglycemic events and to reduce the number of hyperglycemic events the week before diving.

Real-time CGM offers the possibility to adjust alerts to prevent such events both days and nights and offers the best way to control for possible hypoglycemia unawareness and to prevent hypo- and hyperglycemia. Besides the use of alerts and alarms, we recommend that glucose values, trends, and trend arrows be used as aggregated information to prevent hypoglycemia; thus rtCGM should be used proactively rather than reactively.

The beneficial effects of rtCGM on glucose control have been verified in injection therapy⁷ and MDI therapy,⁸ and in CSII treatment decreased HbA1c was shown along with decreased time spent in hypoglycemia.⁹ In the individuals having hypoglycemia unawareness lowered frequency of hypoglycemia has been noted when rtCGM was compared with SMBG, rtCGM was also associated with decreased frequency of nocturnal hypoglycemia and improved glycemic variability.¹⁰

During CSII treatment, the risk of hypoglycemia during diving can further be reduced by adjusting the basal rate (90-120 minutes before diving) and/or intake of carbohydrates prior to diving.¹¹ Under a regular dive (18-22 meters depth and with a 42- to 52-minute duration) the glucose level was reduced by 31 ± 68 mg/dl (1.7 ± 3.8 mmol/l) when the insulin dose was reduced prior to dive along with additional carbohydrate intake 10 minutes before dive.¹¹ This fall in glucose levels need to be accounted for and therefore the balance between insulin dose and carbohydrate is vital when diving. With the use of CSII including an autonomous insulin infusion stop/PLGM function, used before and after diving, we believe that safety could be further improved.

The timing of meals is important. Intake of a meal should take place at least 1.5 to 2 hours prior to all dives. With this approach, the insulin levels from the last bolus will be decreasing at the beginning of the dive and thus lower the risk of hypoglycemia during the dive. When aiming at a stable glucose level of 7-12 mmol/l prior to diving the insulin dose prior to a meal sometimes needs to be reduced. Furthermore, intake of additional carbohydrates, 15-30 g per 70 kg body weight (depending on glucose value) is recommended just before diving—this with no added insulin.

When repeated dives are planned, adjustment of the insulin dose is needed before the meal in between the dives in accordance to the increased insulin sensitivity seen during repeated physical exercise (as scuba diving).

When rtCGM is used, we then recommend a new calibration after the first dive or a blood glucose value to confirm that the sensor works well when using systems that do not require calibration.

Furthermore, it is recommended that a personal logbook is kept, where data on glucose levels, prior to and after dives, are registered as well as the intake of carbohydrates, insulin dose, description of the dive (depth, duration, and level of physical exertion). By analyzing these data, the diver can learn to dive safely. Today downloads could be used along with the specifics about the dive to analyze outcome of each dive.

Hypoglycemia: How to Inform the Buddy

It is very important for the diver to practice signaling and treatment of hypoglycemia.

The “L-signal,” with the index finger and thumb forming the letter L, is used to alert the diving buddy (Figure 1). If the signal is given, the diver and the instructor and/or diving buddy, should start ascending to the surface. Depending on the depth and length of the dive, the decision will be to go to either decompression level (usually 6-9 meters) or the surface. The individual with diabetes should at this point ingest carbohydrates (as carbohydrate gel or glucose/fructose solution) before even starting the ascent. These formulations should always be kept in pockets of the diving gear during all dives. We recommend that both the diver with diabetes and their diving buddy carry carbohydrate gel or glucose/fructose solution. Divers with diabetes should practice the procedure together with the diving buddy: first signaling the “L-signal” and then ingesting gel, first on ground and later underwater during controlled conditions.

Figure 1.

The L-signal, where the index finger and thumb form the letter L is used to call the attention of the diver’s buddy.

Following ingestion of carbohydrates, the dive should always be aborted following normal decompression routines.

Insulin Pumps: How to Manage During Diving

Individuals on subcutaneous insulin infusion pumps (CSII) should reduce the basal rate by approximately 50% 90-120 minutes before diving. This lowers the amount of insulin before and during the dive and reduces the risk of hypoglycemia.

The pump (conventional as well as patch pumps) should then be disconnected and removed shortly before the start of the dive. There is a risk of malfunction or permanent damage of the pump if used during diving. Most insulin pumps on the market are water-resistant, with an IP code IPX-8. The IP Code, International Protection Marking, is an international standard.¹² Thus, most CSIIs are according to the manufacturer waterproof up to a depth of 2.5-3.5 m for 30 minutes but are not approved for use underwater at increased pressures.

After the dive, the insulin pump should be reconnected as soon as possible.

Insulin pumps with an associated PLGM function further also reduces the risk of hypoglycemia.

MDI: How to Manage During Diving

Individuals on MDI treatment should, to reduce the risk of hypoglycemia during diving, reduce the basal insulin dose with 10-30% when planning a long or strenuous dive or when repetitive dives are to be performed. A decrease of a more pronounced long-acting insulin dose results in a postponed effect. In these cases, larger amounts of carbohydrates are recommended to prevent hypoglycemia. Carbohydrates might be added (10-15 g) just prior to the start of a dive if plasma glucose levels show a falling trend or being below 90 mg/dl (5 mmol/l).

International Aspects of Diving

Since divers often travel far to dive sites in various countries, it is of importance to conciliate the regulations on diving and diabetes worldwide. Having the aim at safer dives for individuals with diabetes also implies that the individual who wishes to dive also has to receive the necessary training in diabetes management in connection with physical activity. A JDRF initiative has resulted in important courses, training, and guidelines¹³ along with another recent published ISPAD guideline¹⁴ within this area.

Conclusions

Diving, even in the absence of diabetes, is a potentially dangerous activity with two to three fatalities annually in Sweden. Our recommendation is primarily aimed at individuals already diving who later develop diabetes, but the advice and recommendations are also valid for individuals with diabetes who would like to start diving.