Sinus tachycardia occurs with serious illness, fever, hypovolemia, anemia, anxiety or pain and sympathomimetic medications (e.g., dopamine, dobutamine, isoproterenol, epinephrine, caffeine, theophylline, and aminophylline), at heart rates up to 230 beats per minute in infants. With a sinus tachycardia, there is more rate variability than seen in many tachyarrhythmias and normal P waves precede the QRS complex, often “merged” into the preceding T wave at heart rates greater than 180 beats per minute (P waves positive in leads I, II, and aVF; negative in lead aVR). Pathologic supraventricular tachyarrhythmias can be differentiated from sinus tachycardia by usually faster rates, abnormal P wave axis or PR interval, and (when present) by an abrupt onset and termination or wide QRS complexes.

Premature depolarizations can originate from any conducting tissue. Atrial premature depolarizations occur in up to 30% of newborns (97). The diagnosis is reliably assigned when there is an identifiable, early, nonsinus P wave. However, the P wave may be hidden within the preceding T wave. Atrial premature depolarizations may be conducted to the ventricles normally, or with a bundle branch block pattern resulting in a wide QRS complex (if a bundle branch is refractory from the preceding beat), or may be “blocked” and not conducted to the ventricles (when very early and occurring when the AV node or proximal His bundle is refractory). When frequent, blocked atrial premature depolarizations result in ventricular bradycardia as a result of resetting of the sinus node with each premature atrial depolarization. In neonates with central venous catheters, frequent atrial premature depolarizations may be as a result of contact of the catheter with an atrial wall, and constitute an indication to withdraw the catheter from the atrium. Atrial premature depolarizations might also be secondary to electrolyte abnormalities or drugs (e.g., dopamine, dobutamine, isoproterenol, epinephrine, caffeine, theophylline, and aminophylline). Very rarely are atrial premature depolarizations secondary to myocarditis or cardiac tumors.