A muscle artifact during an ECG can be identified by which type of baseline?

A muscle artifact during an ECG is characterized by an erratic baseline. This type of baseline reflects the electrical activity generated by muscle contractions, which can interfere with the heart's electrical signals. The erratic appearance results from involuntary muscle movements or tension, causing the ECG tracing to display irregular peaks and valleys that do not correlate with the heart's activity.

In the context of the other choices, a wandering baseline typically indicates changes in the electrode position or skin impedance during measurement, while an interrupted baseline points to a disconnection or a problem in the lead connection. A flat, horizontal baseline suggests a lack of electrical activity, which may be associated with asystole or other conditions unrelated to muscle interference. Therefore, the identification of a muscle artifact specifically aligns with the erratic nature of a fluctuating baseline.