Which diagnostic tool is primarily used to monitor heart rhythm continuously?

The electrocardiogram (ECG) is the primary diagnostic tool used for continuous heart rhythm monitoring. It provides a real-time graphic representation of the electrical activity of the heart, allowing healthcare professionals to observe and assess the rhythm, rate, and overall electrical conduction. This capability is crucial for detecting arrhythmias or any irregularities in heart function, making it an essential tool in both emergency settings and ongoing clinical evaluations.

In contrast, a chest X-ray is primarily used for imaging the structure of the heart and lungs, which does not provide information about the heart's rhythm. Similarly, an echocardiogram utilizes ultrasound to evaluate heart structure and function but is not designed for continuous rhythm monitoring. A stress test assesses the heart’s performance under physical strain and can provide insight into coronary artery disease, but it does not continuously monitor the heart's rhythm during everyday activities or across a prolonged period. Thus, the ECG is uniquely suited for the continuous assessment of heart rhythm, making it the correct choice in this context.