Here we see the EKG of a 15-year-old girl. The rhythm is irregular, with the heart rate fluctuating between approximately 60 and 90 beats per minute. All P waves are identical, and the PR interval is always constant. Therefore, a sinus arrhythmia is present. In this case, it is a respiratory sinus arrhythmia, which is commonly found in younger individuals. The heart rate increases reflexively during inspiration and decreases during exspiration. This physiological sinus arrhythmia is usually no longer present in older individuals.
Here is the ECG of a 3-year-old boy. Is there cause for concern? The ECG shows a sinus rhythm with significant sinus arrhythmia. The heart rate increases with inspiration and decreases with expiration, which is called respiratory sinus arrhythmia. The QRS-axis is between 60 and 90 degrees, which is physiological at this age. The negative T-waves in V1-V3 (V4) are also age-appropriately normal. Therefore, there is no reason for concern; the ECG is considered normal for the child's age.