Which cardiac procedure can a medical assistant perform without assistance?

The holter monitor application is a procedure that a medical assistant can perform independently, as it involves placing electrodes on a patient's chest and attaching a portable device that records the heart's activity for an extended period. This task requires basic technical skills and an understanding of how to instruct the patient on wearing the device, but it does not need the direct supervision or assistance from a healthcare provider.

In contrast, the other procedures listed often require more specialized skills or the presence of a trained technician or physician. For instance, a cardiac stress test involves monitoring a patient's heart while they engage in physical exercise, which necessitates close observation and immediate access to medical personnel in case of emergencies. An echocardiogram is a diagnostic test that uses ultrasound to examine the heart’s structure and function, typically performed by a trained sonographer or cardiologist. An electrocardiogram (ECG) can also be done by a medical assistant, but it often requires a healthcare provider to interpret the results immediately. Thus, the holter monitor application stands out as a task suitable for a medical assistant to conduct independently.