Final answer:
An ECG records the heart's electrical activity, with the lead II potential used to measure heart rate. Heart rate is calculated by counting the QRS complexes on the ECG within a minute or by measuring the time interval between complexes on the printed ECG strip.
Step-by-step explanation:
An electrocardiogram (ECG) is able to provide a detailed picture of heart function by recording the electrical activity via surface electrodes placed on the body. One of the key uses of an ECG is to measure the heart rate. By analyzing the ECG, specifically the lead II potential, which measures the voltage between the right arm and the left leg, clinicians can determine the heart rate in beats per minute (bpm). To calculate the heart rate, one must identify the repeating patterns of the heart's electrical activity, commonly represented by the QRS complex, and measure the time interval between these complexes.
Assuming a constant time between beats, the number of QRS complexes that occur in a minute indicates the heart rate. For instance, if a single QRS complex occurs every second, this would translate to a heart rate of 60 bpm. If the ECG uses a standard recording speed, you can also measure the number of small squares between each QRS complex and use the formula of 1500 divided by the number of small squares to determine the bpm. In modern practice, 12-lead ECGs, offering a more comprehensive view with ten electrodes, and portable Holter monitors for continuous monitoring are often used.