Final answer:
With therapeutic levels of digoxin, we are most likely to see a prolonged PR interval due to increased vagal tone on the atrioventricular node.
Step-by-step explanation:
If a patient's digoxin levels are in the therapeutic range, the most likely electrocardiogram (ECG) change to see would be a prolonged PR interval. Digoxin is known to increase vagal (parasympathetic) tone, which can lead to an increased PR interval. However, digoxin does not typically cause long QRS complexes, long QT intervals, or ST-elevation unless in cases of digoxin toxicity or interaction with other factors affecting cardiac depolarization and repolarization.
In summary, digoxin's effect on the heart's electrical conduction primarily manifests as a prolonged PR interval, which is a sign of increased vagal tone on the atrioventricular (AV) node. It is important for clinicians to monitor for signs of digoxin toxicity, which can include other ECG changes.