Axis BASICs

A vector represents the magnitude and direction of the action potential generated by individual myocytes

The sum of all individual vectors generated through depolarization waves creates the electrical axis

Since each individual myocyte can produce an action potential, you can determine the axis for each wave and interval of the cardiac cycle

Knowing the axis of each and how they interact can give you insight into potential pathology

Ventricular axis determination is most commonly taught and discussed, but there is value in also determining the atrial axis in certain clinical situations

The left ventricle contains the majority of the cardiac muscle under normal circumstances, and generated the most electrical force that can be seen on an ECG

The normal ventricular axis is directed downward and slightly toward the left

The ventricular axis is determined using QRS complexes (ventricular depolarization), and is the sum of all individual vectors generated during ventricular depolarization

Each ECG electrode will record a signal as positive (vector points in the same direction as the lead), negative (opposite to lead), or isoelectric (perpendicular (90 degrees) to the lead)

There is some disagreement on the exact degrees that define each type of classifications, but there are some general cutoffs that can be used for QRS axis below:

The QRS axis moves leftward throughout childhood and adolescence and into adulthood

• • At birth, the normal axis lies between + 30 degrees and +190 degrees

  • 8-16 years, axis moves leftward between 0 and + 120 degrees

  • Normal adult QRS is between -30 and + 90 degrees (sometimes extended to + 100 degrees), which is downward and to the left