What do the electrodes in the limb lead placements measure?

The electrodes in the limb lead placements measure the potential difference between the arms and legs. In an electrocardiogram (ECG), limb leads (I, II, and III) are positioned on the right arm, left arm, and left leg, allowing for the assessment of the heart's electrical activity from different angles. These electrodes detect the electric signals generated by the heart as it contracts and relaxes, converting these signals into a readable format. The potential difference between these electrodes helps to create a three-dimensional representation of the heart's electrical activity, which is crucial for diagnosing various cardiac conditions.

The other options, while related to heart health, do not pertain directly to the function of limb leads in an ECG. Heart rate is derived from the electrical activity but is not what the electrodes specifically measure. Blood oxygen levels are monitored through different means, such as pulse oximetry, and are not assessed via limb lead placements. Similarly, the electrical activity of the brain pertains to an entirely different field of study, typically evaluated using an electroencephalogram (EEG).