Case 4: Fever and Bradycardia in an 8-month-old Girl

An 8-month-old previously healthy girl presents to the emergency department with fever (as high as 102.7°F [39.2°C]) for 2 days. She has no cough, vomiting, diarrhea, rash, sick contacts, or recent travel. She is making 4 to 5 wet diapers per day. She normally takes 30 oz of formula daily but has taken 20 oz over the past day. She was seen at her pediatrician’s office the initial day of fever and was diagnosed as having a viral illness. The mother brought her to the emergency department owing to concern about the persistent fever.She was born at term via cesarean delivery. She was noted to have a murmur in the nursery that resolved. Her medical, surgical, and family histories are normal. She was seen 1 time previously in the emergency department at 7 months of age for minor head trauma, and she was discharged after brief observation.On physical examination, her temperature is 102.4°F (39.1°C), heart rate is 90 beats/min, respiratory rate is 45 breaths/min, blood pressure is 109/51 mm Hg, and oxygen saturation is 99%. Her physical examination findings are within normal limits. A test performed based on the girl’s vital signs leads to an unexpected finding.A review of the vital signs was concerning for relative bradycardia. It was noted that the girl’s heart rate was inappropriately low for being in a febrile state. A 12-lead electrocardiogram demonstrated her to be in complete heart block (Fig). She was subsequently admitted to the pediatric intensive care unit. An echocardiogram revealed no evidence of congenital heart disease. Laboratory investigations revealed a normal complete blood cell count and C-reactive protein level, as well as negative blood and urine cultures, respiratory panel findings, Lyme titers, and parvovirus B19 antibody titer. The patient also had normal creatine kinase and troponin levels. On further questioning, the mother reported no history of systemic lupus erythematous. The girl remained in complete heart block but hemodynamically stable during her hospital course. Her heart rate ranged from 65 to 100 beats/min. Her fever defervesced after 36 hours and was suspected to be viral in etiology. She remained in the pediatric intensive care unit for 2 days and was discharged with a Holter monitor and instructions to follow up closely with cardiology. No cause for the complete heart block was identified.Relative bradycardia is when the heart rate is inappropriately slow for the current medical condition. The heart rate does not necessarily need to be in the bradycardic range to be called relative bradycardia. Heart rate varies due to a variety of conditions in children. For example, body temperature, hydration status, anxiety, and medications can all affect the heart rate. Relative bradycardia is most notably identified when there is a pulse-temperature discrepancy. During a febrile state, the heart rate should normally elevate with body temperature by increasing approximately 10 beats/min per 1°C. In this patient, the heart rate was lower than expected for the height of the fever. A normal heart rate for a 6- to 12-month-old is 110 to 170 beats/min. Because the girl had a temperature of 102.4oF (39.1°C), it would be expected that her heart rate should be approximately 20 beats/min higher than normal, placing her in the range of 130 to 190 beats/min.Relative bradycardia can be a diagnostic finding in a variety of conditions. Most notably, relative bradycardia is known to be associated with infectious conditions, but it can also be a diagnostic finding in noninfectious conditions, as seen in our case. Infectious diseases that have been shown to be associated with relative bradycardia include Chagas disease, babesiosis, legionellosis, psittacosis, Q fever, typhoid fever, typhus, malaria, leptospirosis, yellow fever, dengue fever, viral hemorrhagic fevers, trichinosis, and Rocky Mountain spotted fever. Relative bradycardia can be used to help differentiate between infectious diseases that have similar presentations. The underlying mechanism of how relative bradycardia occurs in infectious conditions is unclear. Noninfectious conditions that can be associated with relative bradycardia include the use of β-blockers, cardiac bradyarrhythmias, central nervous system lesions, lymphomas, factitious fever, drug fever, and hypothyroidism.In complete heart block, the main diagnostic finding is a slow heart rate. Complete heart block is confirmed by an electrocardiogram. Patients may present in utero, in the neonatal period, or in childhood. As many as 40% of cases present in childhood (mean age, 5–6 years). Complete heart block can be intermittent at first but then usually becomes persistent over time. Neonatal lupus is responsible for 60% to 90% of congenital complete heart block.Treatment of relative bradycardia depends on the underlying cause. In the case of congenital complete heart block, most patients will ultimately have a pacemaker inserted, irrespective of the time of presentation. Management of bradycardia diagnosed in utero and in the neonatal period includes corticosteroid therapy if associated with lupus antibodies (anti-Ro/SSA and anti-LA/SSB), isoproterenol, or pacemaker placement after birth.