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48: Arrhythmia

I. PROBLEM

An infant has an abnormal tracing on the heart rate monitor.

II. IMMEDIATE QUESTIONS

  1. What is the heart rate? The heart rate in newborns varies from 70–190 beats/min. It is normally 120–140 beats/min but may decrease to 70–90 beats/min during sleep and increase to 170–190 beats/min with increased activity such as crying. See Table 48–1 for normal heart rate values.

  2. Is the abnormality continuous or transient? Transient episodes of sinus bradycardia, tachycardia, or arrhythmias (usually lasting <15 seconds) are benign and do not require further workup. Episodes lasting >15 seconds usually require full electrocardiogram (ECG) assessment.

  3. Is the infant symptomatic? A symptomatic infant may need immediate treatment. Signs and symptoms of some pathologic arrhythmias include tachypnea, poor skin perfusion, lethargy, hepatomegaly, and rales on pulmonary examination. All of these signs and symptoms may signify congestive heart failure (CHF), which may accompany arrhythmias. CHF resulting from rapid cardiac rhythms is unusual with heart rates <240 beats/min.

Table 48–1.HEART RATES IN NEWBORNS
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Table 48–1.HEART RATES IN NEWBORNS
Term Infants Preterm Infants
Age Mean HR Range HR Mean HR Range HR
1 minute 80 20–140 100 50–145
2 minutes 140 80–200 120 80–160
3 minutes 150 100–200 140 105–175
4 minutes 160 120–195 155 120–183
5 minutes 160 120–190 160 120–180
10 minutes 160 110–185 160 129–195

Normal Newborns
Up to 24 hours 119 84–145
1–7 days 133 100–175
8–30 days 163 115–190

HR, heart rate.

Data from Hastreiter AR, Abella JE. The electrocardiogram in the newborn period: I. The normal infant. J Pediatr. 1971;78:146. Dawson JA, Kamlin CO, Wong C, et al. Changes in heart rate in the first minutes after birth. Arch Dis Child Fetal Neonat Ed. 2010;95:F177–F181, 2010. Brady JP, James LS. Heart rate changes in the fetus and newborn infant during labor, delivery and immediate neonatal period. Am J Obstet Gynecol. 1962;84:1–12. Davidson S, Reina N, Shefi O, Hai-Tov U, Akselrod S. Spectral analysis of heart rate fluctuations and optimal thermal management for low birth weight infants. Med Biol Eng Comput. 1997;35:619–625.

III. DIFFERENTIAL DIAGNOSIS

  1. Heart rate abnormalities. Heart rates in the normal newborn vary dramatically. Some evidence, using computer programs to assess heart rate variability in the neonatal period, suggests that the lower heart rates early on are due to the inability of the infant's sympathetic system to inhibit the parasympathetic (or vagal) response.

    1. Tachycardia. Heart rate >2 standard deviations (SD) above the mean for age (see Table 48–1).

      1. Benign causes. Postdelivery, heat or cold stress, painful stimuli, medications (eg, atropine, caffeine, epinephrine, intravenous glucagon, pancuronium bromide, tolazoline, and isoproterenol).

      2. Pathologic causes

        1. Common. Fever, shock, hypoxia, anemia, sepsis, patent ductus arteriosus, and CHF.

        2. Uncommon. Hyperthyroidism, metabolic disorders, cardiac arrhythmias, and hyperammonemia.

    2. Bradycardia. Bradycardia is a heart rate >2 SD below the mean for age ...

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