Cardiac Axis
Learn cardiac axis determination — normal ranges, axis deviations, and how to assess the electrical axis on a 12-lead ECG.
Normal Cardiac Axis
The normal cardiac axis represents the mean direction of ventricular depolarization, falling between -30° and +90° in adult humans. It reflects balanced left and right ventricular forces and intact normal conduction through the His-Purkinje system.
-30° to +90° (human); +40° to +100° (canine); +0° to +160° (feline)Left Axis Deviation
Left axis deviation (LAD) is a cardiac axis more negative than -30°, indicating that the dominant vector of ventricular depolarization is directed superiorly and to the left. It is one of the most common ECG abnormalities and has numerous cardiac and non-cardiac causes.
-30° to +90° (human normal); LAD is defined as more negative than -30°Right Axis Deviation
Right axis deviation (RAD) is a cardiac axis more positive than +90° in adults, indicating that the dominant ventricular depolarization vector is directed rightward and inferiorly. It reflects increased right ventricular forces, decreased left ventricular forces, or altered septal depolarization.
-30° to +90° (human normal); RAD is defined as more positive than +90°Extreme Axis Deviation
Extreme axis deviation (also called northwest axis or no man's land axis) occurs when the cardiac axis falls between -90° and ±180°. In this quadrant, both lead I and lead aVF are negative, indicating that ventricular depolarization is directed superiorly and to the right — opposite to the normal inferior-leftward direction.
-30° to +90° (human normal); extreme axis: -90° to ±180°Left Ventricular Hypertrophy
Left ventricular hypertrophy (LVH) is an increase in left ventricular myocardial mass, most commonly from chronic pressure overload. On ECG, LVH produces increased QRS voltage, repolarization abnormalities, and often a leftward shift in cardiac axis. ECG detection of LVH has moderate sensitivity but high specificity.
Cardiac axis in LVH: typically -30° to 0° (mild leftward shift; frank LAD less common)Right Ventricular Hypertrophy
Right ventricular hypertrophy (RVH) occurs when the right ventricle enlarges due to chronic pressure or volume overload, shifting the cardiac axis rightward and increasing right-sided ECG forces. The ECG findings of RVH reflect the reversal of the normal left ventricular dominance over the electrical axis.
Cardiac axis in RVH: typically +90° to +180° (right axis deviation is a hallmark finding)How to Determine Cardiac Axis
Determining the cardiac axis is a fundamental ECG interpretation skill that identifies the mean direction of ventricular depolarization in the frontal plane. Multiple methods exist, ranging from rapid bedside assessment using leads I and aVF to precise calculation using the hexaxial reference system.
Normal human axis: -30° to +90°; target of axis determination methodsCardiac Axis in Veterinary ECG
Cardiac axis interpretation in veterinary ECG requires species-specific normal ranges that differ substantially from human values. Dogs have a normal axis of +40° to +100°, cats have a wide normal range of 0° to +160°, and the clinical significance of deviations reflects veterinary-specific pathologies such as heartworm disease and hypertrophic cardiomyopathy.
Canine: +40° to +100°; Feline: 0° to +160°; human reference: -30° to +90°