What type of medical equipment is primarily used for monitoring vital signs?

The electrocardiogram (ECG) machine is specifically designed to monitor vital signs, particularly focusing on the electrical activity of the heart. It provides crucial information regarding heart rate, rhythm, and overall cardiac function, which are key components of vital signs. The ECG records electrical impulses and displays them in a way that can be interpreted by healthcare professionals, allowing them to assess the heart's health and detect any abnormalities.

In contrast, an ultrasound machine uses sound waves to create images of internal organs and tissues, which is useful for diagnostic purposes but not primarily for monitoring vital signs. An infusion pump is used to deliver medications, nutrients, or fluids to patients in a controlled manner, rather than providing monitoring of vital signs directly. The magnetic resonance imaging (MRI) machine is utilized for detailed imaging of internal structures but again does not focus on monitoring vital signs. Thus, the ECG machine is the most relevant option for monitoring critical aspects of a patient's vital signs.