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.

Complex ECGs like this one have to be approached systematically. Firstly, we can see a normal sinus rhythm. A is the first beat of a wide complex tachycardia. This must be a ventricular tachycardia. Although there is a P-wave before the first beat of the tachycardia, it is not premature. Therefore, there is no SVT with aberrant conduction. The first beat of the tachycardia looks different from the subsequent beats because there is a fusion beat present. The VT conducts 1:1 back to the atria (a small negative P-wave can be seen at the end of the QRS complex in V1).

76-year-old man, with a history of inferior wall myocardial infarction. He experiences recurrent episodes of brief palpitations, often lasting only 5-15 seconds. In this ECG, at the beginning, the last part of a wide-complex tachycardia is visible. After 2 sinus beats, another wide-complex tachycardia begins (with the same QRS configuration).

ECG 1 is from a 57-year-old male with no prior cardiac disease. He reports acute shortness of breath for 2 days. We see a sinus rhythm with left anterior fascicular block (LAFB) and conspicuous T-wave inversions in the inferior leads and in V1-V6. These are typical ECG changes that may indicate a pulmonary embolism. ECG 2 was taken from the same patient 1 year earlier. The patient has an acute pulmonary embolism. Sinus tachycardia may be present in acute pulmonary embolism. However, as in this example, the heart rate can also be completely normal.