What happens if a person has bradycardia?

What Happens When Heart Rate Is Too Low

When a person's heart rate becomes too low (bradycardia, typically <50 bpm), the primary concern is whether the slow rate causes inadequate cardiac output leading to symptoms—if symptomatic, bradycardia can cause syncope with traumatic injury, acute heart failure, myocardial ischemia, shock, or even trigger life-threatening ventricular arrhythmias requiring immediate intervention. 1, 2

Clinical Consequences Depend on Symptoms

The critical distinction is between asymptomatic and symptomatic bradycardia:

Asymptomatic Bradycardia

• Heart rates of 40-45 bpm or even lower can be entirely physiologic and require no treatment in well-conditioned athletes, during sleep, or in young healthy individuals due to dominant parasympathetic tone 1, 2
• As long as stroke volume compensates for the decreased heart rate, patients can remain completely asymptomatic even with profound bradycardia 1
• There is no established minimum heart rate below which treatment is indicated—the key determinant is correlation between symptoms and bradycardia 1, 2

Symptomatic Bradycardia: Life-Threatening Manifestations

When bradycardia causes symptoms, serious complications can occur:

Neurological Consequences

• Syncope or presyncope represents one of the most debilitating symptoms, particularly dangerous due to sudden onset and risk of traumatic injury from falls 2, 1
• Altered mental status, confusion, or decreased responsiveness from inadequate cerebral perfusion 1, 2
• Dizziness occurs in approximately 22% of patients presenting with compromising bradycardia 3

Cardiovascular Consequences

• Hypotension or shock indicating hemodynamic compromise and end-organ hypoperfusion 1, 2
• Acute heart failure with dyspnea, pulmonary edema, and jugular venous distension 1, 2
• Ischemic chest discomfort or angina when bradycardia reduces coronary perfusion—seen in 17% of emergency presentations 1, 2, 3
• Bradycardia prolongs repolarization and predisposes to polymorphic ventricular tachycardia, especially torsades de pointes, which can be fatal 1, 4

Other Manifestations

• Fatigue and reduced exercise capacity are common with persistent bradycardia 1, 2
• Subtle symptoms include irritability, lassitude, inability to concentrate, apathy, and forgetfulness 1
• In cancer patients, bradycardia-related cardiomyopathy and adverse effects on brain function can occur 4

Severity and Presentation Patterns

Emergency Presentations

In a 10-year registry of 277 patients presenting to emergency departments with compromising bradycardia 3:

• Median heart rate was 33 bpm (range 30-40 bpm)
• Leading symptoms: syncope (33%), dizziness (22%), collapse (17%), angina (17%), dyspnea/heart failure (11%)
• 20% required temporary emergency pacing for initial stabilization
• 50% ultimately required permanent pacemaker implantation
• 30-day mortality was 5%

Underlying Mechanisms Matter

The clinical significance varies by mechanism 3:

• Primary cardiac conduction disturbances (49% of cases)
• Adverse drug effects (21%)—particularly beta-blockers, calcium channel blockers, digoxin, antiarrhythmics 1, 5
• Acute myocardial infarction (14%)
• Pacemaker failure (6%)
• Intoxication (6%)
• Electrolyte disorders (4%)

When Bradycardia Becomes Life-Threatening

Immediate intervention is required when bradycardia causes 1, 2:

• Acute altered mental status
• Ischemic chest discomfort
• Acute heart failure
• Hypotension (systolic BP <90 mmHg)
• Signs of shock
• Escape ventricular arrhythmias

High-Risk Bradycardias Requiring Urgent Treatment

Even if minimally symptomatic, certain rhythms warrant intervention due to risk of progression 1:

• Mobitz type II second-degree AV block, especially in acute myocardial infarction—the block is below the AV node and often progresses to complete heart block 1
• Third-degree (complete) AV block where no impulses pass between atria and ventricles 1
• Ventricular escape rhythms, which are unstable and require pacemaker implantation 1

Management Implications

Immediate Treatment for Symptomatic Bradycardia

Atropine 0.5 mg IV every 3-5 minutes (maximum 3 mg) is first-line therapy for acute symptomatic bradycardia 1, 6, 5:

• Most effective for sinus bradycardia and AV nodal blocks 6
• Doses <0.5 mg may paradoxically slow heart rate further 1, 5
• Less effective for infranodal blocks with wide-complex escape rhythms 6

When Atropine Fails

• Transcutaneous pacing serves as bridge to transvenous pacing 1, 5
• Dopamine or epinephrine infusions for refractory cases 5
• In severe cases (2 patients in one series), cardiopulmonary bypass was required for stabilization 3

Definitive Management

• Permanent pacemaker indicated for persistent symptomatic bradycardia after excluding reversible causes 2, 5
• Pacemaker placement follows ACC/AHA guidelines, with consideration of patient's functional status, life expectancy, and quality of life priorities 1, 2

Critical Pitfalls to Avoid

• Do not assume all bradycardia is benign—while athletes tolerate low rates, symptomatic patients require urgent assessment 7, 2
• Do not delay treatment in symptomatic patients—admission for monitoring alone is inadequate when hemodynamic compromise exists 6
• Do not use atropine for wide-complex bradycardia—infranodal blocks (Mobitz II, third-degree with wide QRS) are often atropine-resistant and may require immediate pacing 1, 6
• Consider ventricular tachyarrhythmias as cause of syncope in patients with conduction disease and left ventricular dysfunction, not just bradycardia 1
• In post-MI patients, use atropine cautiously as increased heart rate may worsen ischemia or extend infarct size 6