Night Hypoglycemia Linked to Slow Heart Rate

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Harvard Medical School
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Night Hypoglycemia Linked to Slow Heart Rate

News Review From Harvard Medical School

April 23, 2014

News Review From Harvard Medical School -- Night Hypoglycemia Linked to Slow Heart Rate

People with diabetes may have potentially dangerous heart rhythm changes if their blood sugar drops very low at night, a new study finds. The findings may help to explain other research that has shown links between very low blood sugar (hypoglycemia) and sudden death, especially at night. The new study included 25 people with type 2 diabetes. All of them had heart disease or a high risk of developing it. All had been taking insulin for their diabetes for at least 4 years. Everyone wore devices for 5 days to continuously monitor their blood sugar and heart activity. Overall, people in the group had hypoglycemia for 134 hours during the 5 days. When this occurred at night, people were 8 times as likely to have a slow heart beat called bradycardia as they were when blood sugar was normal. Abnormal heart rhythms also were more likely to occur at night if blood sugar was low. Slow heart rates didn't occur during the day. The journal Diabetes published the study. HealthDay News wrote about it April 22.


By Howard LeWine, M.D.
Harvard Medical School


What Is the Doctor's Reaction?

Controlling high blood sugar is a primary goal of diabetes treatment -- and for good reason. Keeping the blood sugar close to normal can prevent further health problems linked with diabetes. These include nerve, eye and kidney damage.

But efforts to lower blood sugar can also cause trouble. Medicines prescribed to lower blood sugar are potent. They can easily lower blood sugar too much. This is called hypoglycemia. Both diabetes pills and injected insulin can cause hypoglycemia. But people who use insulin are at greatest risk.

People with type 1 diabetes must use insulin. Those with the much more common type 2 diabetes usually are first treated with diet and pills. But over time insulin is often added to control blood sugar levels.

The American Diabetes Association (ADA) recommends that most people with diabetes strive for a hemoglobin A1C blood level of less than 7%. Doctors call this tight control. A1C reflects a person's average blood sugar during the last 2 to 3 months.

A normal A1C is less than 5.7%. A1C levels between 5.7% and 6.4% are used to identify people with pre-diabetes. A level of 6.5% or higher confirms a diagnosis of diabetes.

For many people, an A1C of less than 7% has meant more frequent hypoglycemia.

The ADA has recently softened its stand on tight control. There was good reason for change. Recent studies have found that we have been underestimating the dangers of hypoglycemia. Some estimates suggest that it may cause up to 10% of deaths in people with diabetes.

Animal and human studies show a link between very low blood sugar and sudden death, especially during sleep. The link is strongest for people who have known heart disease and a high risk of blocked heart arteries.

This study provides insight into how hypoglycemia and sudden death may be linked. The researchers recruited 25 people with type 2 diabetes and likely coronary artery disease. They all took insulin, with a goal of tight blood sugar control.

The researcher continuously monitored electrical heart activity and sugar levels for five days. No one died. Hypoglycemia occurred often.  The episodes at night during sleep were more frequent, more severe and longer than those during the day.

During hypoglycemia, people often had abnormal heart electrical activity. This also was seen more during sleep.

The link between the abnormal heart activity and hypoglycemia is strong. So it provides a possible explanation for the risk of death from hypoglycemia. But it is NOT proof.


What Changes Can I Make Now?

If you have type 2 diabetes, your treatment plans and goals should be tailored to your own circumstances. Things to consider include:

  • Your age
  • Your overall health
  • Whether you have heart disease, a seizure disorder or problems with memory or thinking
  • Whether you take pills and insulin for your diabetes, which increases your risk of hypoglycemia
  • How often you have low blood sugars
  • Your ability to pay for your medicines

In general, the blood sugar goal for a newly diagnosed person with type 2 diabetes should be near normal. But often this is not the best goal for someone who has had diabetes for many years. This is especially true for people with multiple health problems.

For older people with diabetes and those with other medical problems, 8% is a reasonable goal for A1C. And if hypoglycemia still occurs, it is probably safer to accept "looser" blood sugar control.

Controlling blood sugar is only one part of diabetes treatment. It's just as important to:

  • Control weight with a healthy diet and plenty of physical activity
  • Reduce blood pressure to normal levels
  • Achieve normal LDL cholesterol levels

But these goals also may need to be set at less aggressive targets. For example, taking multiple medicines to lower blood pressure to an ideal level may cause fainting, falls and fractures. Like blood sugar goals, blood pressure and cholesterol goals need to be based on each person's needs.


What Can I Expect Looking to the Future?

It's not likely that a study will give us a definite answer regarding the risk of death from abnormal heart activity directly caused by hypoglycemia. Doctors already may aim for "looser" blood sugar control in the elderly to avoid hypoglycemia. This strategy also makes sense to help prevent it in people with heart disease and longstanding type 2 diabetes.

Last updated April 23, 2014

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