How do hypo‑ and hyperglycemia affect heart rate and cause tachycardia in adult patients, particularly those with diabetes?

How Blood Sugar Impacts Heart Rate and Tachycardia

Hypoglycemia Causes Bradycardia, Not Tachycardia—Especially at Night

Hypoglycemia primarily causes bradycardia (slow heart rate) and dangerous arrhythmias rather than sustained tachycardia, particularly during nocturnal episodes, while the initial sympathetic response may briefly increase heart rate before vagal predominance takes over. 1, 2

The Biphasic Heart Rate Response to Hypoglycemia

The heart rate response to hypoglycemia follows a predictable pattern:

• Initial tachycardia phase: When blood glucose drops below 70 mg/dL, the body releases counterregulatory hormones (epinephrine, glucagon, cortisol) that initially cause tachycardia, sweating, and shakiness as sympathetic activation occurs 3, 4

• Secondary bradycardia phase: As hypoglycemia persists or worsens (especially <54 mg/dL), excessive compensatory vagal activation after the counterregulatory phase causes bradycardia and associated arrhythmias 2, 5

• Nocturnal episodes are particularly dangerous: During sleep, the counterregulatory response is blunted, resulting in prolonged hypoglycemia with multiple nadirs. The initial sympathetic activity at glucose nadir is replaced by increased vagal activity, causing significant bradycardia 6, 2

Cardiac Arrhythmias During Hypoglycemia

Bradycardia and atrial/ventricular ectopic beats occur significantly more frequently during nocturnal hypoglycemia compared to euglycemia, providing a mechanism for the "dead-in-bed syndrome" and increased cardiovascular mortality. 1, 2

Specific arrhythmias documented during hypoglycemia include:

• Marked sinus bradycardia 1
• Ventricular tachycardia (32.8 ± 60 episodes in patients with hypoglycemic events versus 0.9 ± 4.2 in those without) 7
• Second- and third-degree AV block 6
• QTc prolongation (>500 ms in some patients), which increases risk for torsades de pointes 6, 2
• Atrial and ventricular ectopic beats 2

Cardiac Autonomic Neuropathy and Permanent Tachycardia

In contrast to acute hypoglycemia, diabetic cardiac autonomic neuropathy (CAN) causes permanent resting tachycardia due to parasympathetic denervation, not episodic changes. 3

• Severe CAN presents with permanent tachycardia and orthostatic hypotension as clinical symptoms 3
• The prevalence of CAN increases with diabetes duration and poor glycemic control 3
• CAN is present in 20% of diabetic patients when defined by two or three abnormal heart rate variability tests 3
• CAN increases risk of sudden death secondary to serious cardiac rhythm disorders 3

Hyperglycemia Effects

While the evidence focuses predominantly on hypoglycemia, hyperglycemia contributes to:

• Development of cardiac autonomic neuropathy over time, leading to baseline tachycardia 3
• QTc prolongation (though less acutely dangerous than hypoglycemia-induced changes) 3
• Long-term cardiovascular complications including heart failure (2-3 fold higher risk in diabetic patients) 3

High-Risk Populations for Hypoglycemia-Induced Arrhythmias

Patients at highest risk include those with nephropathy, autonomic neuropathy, prolonged QTc interval, those on negative chronotropic medications (beta-blockers, calcium channel blockers, digoxin), and elderly patients with underlying heart disease. 1

Additional risk factors include:

• Recent severe hypoglycemia (within 3-6 months) 3
• Impaired hypoglycemia awareness 3
• End-stage kidney disease 3
• Cognitive impairment or dementia 3
• Intensive insulin therapy 3

Critical Management Principles

After any episode of bradycardia associated with hypoglycemia, immediately treat the hypoglycemia with glucose (not cardiac pacing), then reevaluate the diabetes treatment plan to prevent recurrence. 1

• Glucose is the preferred treatment for conscious patients with glucose <70 mg/dL 3, 1
• Cardiac pacing is NOT indicated for hypoglycemia-induced bradycardia—treat the underlying hypoglycemia 1
• Review and potentially reduce negative chronotropic medications 1
• Screen for impaired hypoglycemia awareness at every clinical encounter 3
• Consider continuous glucose monitoring for high-risk patients 3

Common Pitfall to Avoid

Beta-blockers mask early hypoglycemic symptoms including tachycardia, making recognition of hypoglycemia more difficult and potentially allowing progression to dangerous bradycardia. 1 This creates a particularly hazardous situation where patients may not recognize hypoglycemia until severe neuroglycopenic symptoms or arrhythmias occur.