When a lead deflects the current at right angles to its direction, the trace will be what?

When a lead deflects the current at right angles to its direction, the trace will be what?

• A. Isoelectric
• B. Upright
• C. Inverted
• D. Mixed

Answer: (A)

The key idea is that a lead’s trace is a projection of the heart’s electrical activity onto the lead’s axis. The amplitude of the deflection depends on the angle between the depolarization vector and the lead. When the current deflects at right angles to the lead, that angle is 90 degrees, and the projection becomes zero (cosine of 90 degrees is zero). So there is no net deflection and the trace appears flat—an isoelectric line. Upright or inverted deflections occur when the wavefront aligns with or moves away from the lead axis, respectively; a trace that’s partially deflected at other angles would be biphasic or mixed. In this case, perpendicular means isoelectric.