What is relative bradycardia?

Relative Bradycardia: Definition and Clinical Significance

Relative bradycardia is a paradoxical clinical sign where a patient's heart rate is lower than expected for a given body temperature or clinical condition, despite the presence of factors that would normally increase heart rate. 1

Definition and Characteristics

• Relative bradycardia is not simply a heart rate below 60 beats per minute (which defines absolute bradycardia), but rather a heart rate that is inappropriately low for the clinical context, particularly during febrile illness 2, 1
• It represents a pulse-temperature dissociation or pulse-temperature deficit, where the expected increase in heart rate with fever (typically 8-10 beats per minute for each 1°C rise in temperature) does not occur 1
• The term should be used when there is a documented bradyarrhythmia that is inappropriate for the clinical condition, regardless of the specific heart rate 3

Clinical Significance

• Relative bradycardia serves as a sensitive but nonspecific clinical sign that may help narrow the differential diagnosis of both infectious and noninfectious diseases 1
• It is particularly associated with certain infections, most notably:
  • Typhoid fever (caused by Salmonella typhi) 2
  • Legionnaires' disease (caused by Legionella pneumophila) 2
  • Pneumonia caused by Chlamydia species 2
• Research suggests relative bradycardia is more commonly found in diseases caused by organisms that are both Gram-negative and intracellular 2
• Recognition of this sign may provide important bedside clinical clues when other diagnostic information is limited 1

Pathophysiological Mechanisms

• The exact mechanisms underlying relative bradycardia remain poorly understood 1
• Proposed mechanisms include:
  • Release of inflammatory cytokines affecting cardiac pacemaker cells 1
  • Increased vagal tone during certain infections 1
  • Direct pathogenic effects on the myocardium 1
  • Electrolyte abnormalities affecting cardiac conduction 1

Clinical Assessment

• When evaluating bradycardia, it's essential to assess its appropriateness for the clinical condition rather than focusing solely on an arbitrary heart rate cutoff 3, 4
• The 2018 ACC/AHA/HRS Bradycardia Guidelines emphasize that a slow heart rate may be physiologically normal for some patients (such as athletes), while a heart rate >50 beats per minute may be inadequate for others 3, 4
• Assessment should focus on whether the bradycardia is causing symptoms such as:
  • Syncope or near-syncope
  • Transient dizziness or lightheadedness
  • Confusion resulting from cerebral hypoperfusion
  • Heart failure symptoms
  • Exercise intolerance 3, 5

Clinical Implications

• The presence of relative bradycardia should prompt consideration of specific infectious etiologies, particularly those caused by intracellular pathogens 2, 1
• Treatment should rarely be prescribed solely based on heart rate being lower than an arbitrary cutoff 4
• Intervention is warranted only when bradycardia is symptomatic or likely to progress to a life-threatening condition (e.g., Mobitz type II second-degree AV block in the setting of acute myocardial infarction) 3
• Asymptomatic or minimally symptomatic patients with relative bradycardia do not necessarily require treatment 3

Pitfalls and Caveats

• The incidence of relative bradycardia varies widely in studies, partly due to inconsistent definitions and measurement protocols 1
• Do not confuse physiological sinus bradycardia (as occurs in highly trained athletes) with pathological bradyarrhythmias requiring intervention 3
• Avoid immediate intervention for all cases of bradycardia without first assessing the clinical context and symptoms 6
• Remember that relative bradycardia is a clinical sign, not a disease, and holds more value as a characteristic of specific diseases than as a predictive sign in individual patients 2