Chapter 53: Arrhythmia

I. PROBLEM

An infant has an abnormal tracing on the heart rate monitor. Arrhythmias in neonates can be classified as benign or pathologic. They can be found in 1% to 5% of all neonates.

II. IMMEDIATE QUESTIONS

1. What is the heart rate? The heart rate in newborns varies from 70 to 190 beats/min (normally 120–140 beats/min; may decrease to 70–90 beats/min during sleep and increase to 170–190 beats/min with increased activity such as crying). See Table 53–1 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 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. If an infant shows clinical signs of heart failure that are believed to be have been caused by an arrhythmia, then this suggests the infant was in an abnormal rhythm for a significant period of time (hours to days). CHF resulting from rapid cardiac rhythms is unusual with heart rates <240 beats/min.

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 (SDs) above the mean for age (see Table 53–1).

      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 SDs below the mean for age (see Table 53–1). Transient bradycardia is fairly common in newborns (particularly premature infants); rates range from 60 to 70 beats/min.

      1. Benign causes. During defecation, vomiting or micturition, gavage feedings, suctioning, medications (eg, propranolol, digitalis, atropine, and infusion of calcium; maternal long-acting β-blockers to treat hypertension given within 24 hours of delivery).

      2. Pathologic causes

         1. Common. Hypoxia, apnea, convulsions, airway obstruction, air leak (eg, pneumothorax), CHF, intracranial bleeding, severe acidosis, and severe hypothermia.

         2. Uncommon. Hyperkalemia, cardiac arrhythmias, pulmonary hemorrhage, diaphragmatic hernia, hypothyroidism, and hydrocephalus.

2. Arrhythmias

   1. Benign arrhythmias. Includes any transient episode (<15 seconds) of sinus bradycardia and tachycardia and any of the benign causes of sinus tachycardia and bradycardia noted in Section III.A.1 and III.A.2. Sinus arrhythmia, a phasic variation in the heart rate often associated with respiration, is also benign.

      1. Premature atrial beats. These can occur in ...