CVS Practice Test 2025 - Free CVS Practice Questions and Study Guide

To determine whether the QRS axis is normal or deviated, examining leads I and II is crucial because these leads provide comprehensive information about the electrical activity of the heart along the frontal plane. Lead I measures the electrical activity from the right arm to the left arm, while Lead II measures the activity from the right arm to the left leg.

A normal QRS axis is typically between -30 degrees and +90 degrees in the frontal plane, and analyzing these two leads gives an effective snapshot of where the heart is oriented electrically. If the QRS complex is predominantly positive in both leads, this supports a normal axis. In contrast, variations in the electrical orientation noted in these leads will indicate deviations of the axis, whether towards left or right.

Leads V5 and V6 primarily evaluate the horizontal plane; lead aVR provides insight into right-sided heart activity but does not offer the same direct orientation regarding the frontal plane as leads I and II. Thus, they are less effective for determining axis deviation.