Which device is commonly used to monitor a patient's heart rhythm?

The electrocardiogram (ECG or EKG) is the device specifically designed to monitor a patient's heart rhythm. It works by recording the electrical activity of the heart over a period of time. The ECG provides a visual representation of the heart's impulses, identifying irregularities in rhythm, rate, and overall heart function, which is crucial for diagnosing various cardiac conditions such as arrhythmias, heart attacks, and other heart diseases.

While other devices have their specific uses, a pulse oximeter measures blood oxygen levels rather than heart rhythm. A sphygmomanometer is primarily used to assess blood pressure, and a stethoscope allows healthcare providers to listen to heart sounds but does not provide a continuous or detailed recording of electrical activity like an ECG does. Thus, the ECG is the most appropriate and effective tool for direct monitoring of heart rhythm.