In an ECG, what do horizontal lines measure?

In an ECG, horizontal lines measure time. Each small square on the ECG paper represents a specific amount of time, typically 0.04 seconds. The heart rate can be derived from these time measurements by counting the number of R-R intervals (the distance between successive R-wave peaks) over a set period. Therefore, the correct interpretation of horizontal lines relates directly to determining heart rate by observing the time intervals between heartbeats displayed on the ECG graph.

Other options focus on different physiological parameters that are not measured directly by the horizontal lines on an ECG. Blood pressure is assessed through different methods and is not graphically represented on an ECG. Cardiac output is a measure of the volume of blood the heart pumps and is calculated using other metrics, not through the time measurements on an ECG. Similarly, respiratory rate pertains to breathing patterns and is not represented in the time intervals on an ECG.