What is a normal cardiac rhythm for an infant?

Normal Cardiac Rhythm in Infants

The normal heart rate for infants ranges from 90-166 beats per minute (bpm) in the first week of life and 107-182 bpm in the first month, with sinus rhythm being the predominant cardiac rhythm. 1

Heart Rate Parameters by Age

Heart rate varies significantly based on age, with specific normal ranges established by the European Society of Cardiology:

• 0-1 days: 93-154 bpm (mean 123 bpm) 1
• 1-3 days: 91-159 bpm (mean 123 bpm) 1
• 3-7 days: 90-166 bpm (mean 129 bpm) 1
• 7-30 days: 107-182 bpm (mean 149 bpm) 1
• 1-3 months: 121-179 bpm (mean 150 bpm) 1

Physiological Variations in Heart Rate

Several factors influence infant heart rate:

• Sleep state: Heart rates are typically lower during sleep (minimum 72-120 bpm) compared to waking states (minimum 78-140 bpm) 2
• Activity level: Maximum heart rate while awake can range from 150-222 bpm 2
• Position: Heart rate is approximately 4.9 bpm higher when on mother's chest compared to in a cot 3
• Sex: Girls typically have heart rates about 1.6 bpm higher than boys 3
• Temperature: Heart rate increases by approximately 0.5 bpm per 0.1°C increase in rectal temperature 3

Normal ECG Parameters

Beyond heart rate, other important ECG parameters in infants include:

• QRS axis: 55-200° in full-term neonates, decreasing to ≤160° by 1 month 1
• QRS duration: Narrow (<80 ms) in newborns and infants 1
• QT interval: Mean QTc on the 4th day of life is 400±20 ms, with upper normal limit of 440 ms 1
• T waves: Normally variable in the first week; after 1 week, negative in lead V1 and positive in V5 1

Defining Bradycardia and Tachycardia

• Bradycardia: Heart rate below 91 bpm in the first week and below 107 bpm in the first month 4
• Tachycardia: Heart rate above 166 bpm in the first week and above 179 bpm in the first month 4

Common Rhythm Variations

• Sinus arrhythmia: Mild irregularity in rhythm is common in infants, with 24 out of 29 infants showing mild irregularity in one study 2
• Isolated premature atrial contractions (PACs): Can occur in healthy infants and are generally benign 2
• AV junctional escape rhythm: May occur after sinus pauses 2

Clinical Implications

• The traditional definition of bradycardia as <80 bpm appears inappropriate for newborns given the age-related increase in heart rate 5
• Heart rate <60 bpm with signs of poor perfusion requires immediate intervention 4
• In the first minutes after birth, median heart rate is <100 bpm at 1 minute, rising to 139 bpm by 2 minutes and 163 bpm by 5 minutes 6
• Preterm infants typically show a slower rise in heart rate after birth compared to term infants 6

Common Pitfalls in Assessment

• Using adult heart rate parameters to evaluate infant cardiac rhythm
• Failing to account for normal physiological variations based on sleep state and activity
• Not recognizing that mild sinus arrhythmia is normal in infants
• Using a universal threshold (like 80 bpm) for bradycardia without considering age-specific references 5
• Misinterpreting normal QRS axis values (right axis deviation in adults is normal in infants)

When assessing infant cardiac rhythm, it's essential to use age-appropriate reference ranges and consider physiological factors that influence heart rate variability.