Final answer:
The lead that records electrical activity from a midpoint between the RA/LA to the LL is lead II. Lead II is most often graphed in an ECG as it is an indicator of heart-muscle function and is coordinated with arterial blood pressure.
Step-by-step explanation:
The lead that records electrical activity from a midpoint between the RA/LA to the LL is lead II.
In an electrocardiogram (ECG), leads I, II, and III measure components of the depolarization vector of the heart and are graphed to represent the electrical activity. Lead II is most often graphed as it is an indicator of heart-muscle function and is coordinated with arterial blood pressure.
Lead II shows the depolarization and contraction of the atria as they pump blood into the ventricles (P wave) and the depolarization of the ventricles as they pump blood to the lungs and body (QRS complex).