During ECG interpretation, cardiac axis, or direction of electrical impulses, may be normal (physiologic) or abnormal (pathologic), suggesting abnormal cardiac conductivity. Although every deflection obtained on the ECG will have an axis, we will focus on the ventricular axis.
When we think of our cardiac monitoring lead placement, we have to understand cardiac Vectors, which is the sum of all electrical impulses. These are recorded on each individual lead, because impulses travel underneath the electrodes in multiple directions, therefore, incorrect lead placement will give a false axis.
Here, we will discuss Frontal Plane Leads (Limb Leads) which form the Ethovens Triangle:
- Lead I is obtained from the Right Arm (-) and Left Arm (+) electrodes, looking towards the lateral (left) aspect of the Left Ventricle (LV).
- Lead II is obtained from the Right Arm (-) and the Left Leg (+) electrodes, looking towards the inferior leftward portion of the LV., and follows the normal conductivity path (from the SA Node towards the ventricles), which explains why this leads is often used for rhythm monitoring.
- Lead III is obtained from the Left Arm (-) and the Left Leg (+) electrodes, looking towards the inferior wall of the LV and Right Ventricle (RV), having the most rightward view.
The Augmented leads are, as the name says, an augmented view obtained when we have the limb leads in place and are perpendicular to the angle obtained from the limb leads. These leads provide both positive and negative polarity views (unipolar leads).
- aVR (Augmented Vector Right), also known as Augmented Unipolar Right, is perpendicular to Lead III and vise versa, and looks towards the right upper portion of the heart.
- aVL (Augmented Vector Left) or Augmented Unipolar Left, is perpendicular to Lead II and vise versa, and looks towards the left upper portion of the heart.
- aVF (Augmented Vector Feet) or Augmented Unipolar Feet, is perpendicular to Lead I and vise versa, and looks towards the inferior portion of the heart.
***IMPULSES TRAVELING TOWARDS A LEAD WILL CAUSE A POSITIVE DEFLECTION IN THAT LEAD, WHILE IMPULSES TRAVELING AWAY FROM A LEAD WILL CAUSE A NEGATIVE DEFLECTION IN THAT LEAD. WHEN IMPULSES TRAVEL EQUALLY THROUGH A LEAD, THAT LEAD IS KNOW TO BE ISOELECTRIC***
After understanding this concept, we can understand easily how to identify vectors and axis.
The HEXAXIAL REFERENCE SYSTEM was developed to evaluate the degree of vector direction each lead looks at, which will help us determine the QRS axis. This graphic is often designed as a circle, which is divided into four quadrants.
Image from the amazing book, 12-Lead ECG: The Art Of Interpretation by Dr. Tom Garcia and Neil Holtz
- LEFT UPPER QUADRANT: between 0 and -29 degrees. Axis between these degrees is considered Physiologic Leftward Axis (Left Axis Deviation). Here, Lead I has a predominantly positive deflection with aVF having a predominantly negative deflection. Lead II will be predominantly positive.
A Leftward axis > -30 degrees, by most clinicians, but > -40 by some, is considered pathologic. This is differentiated from physiologic axis by Lead II being predominantly negative. This may be due to:
. Left Anterior Fascicular Block (LAFB)
– Pathologic Leftward axis from LAFB
- QRS axis > -30 degrees
- qR in Lead I and aVL
- rS in II, III, aVF
- Lead II predominantly negative
. Left Ventricular Hypertrophy
. Left Bundle Branch Block (LBBB)
. Ventricular beats/VT
. Right Ventricular Pacemaker
. Wolff Parkinson White (WPW) pattern
- LEFT LOWER QUADRANT: Axis between 1 to 89 degrees is considered normal range and all leads are for the most part positive. Lead III may or may not be positive.
- RIGHT LOWER QUADRANT: Axis between 90 to 180 degrees is considered pathologic. Here, Lead I is predominantly negative and aVF is predominantly positive.
Most common causes of Rightward Axis Deviation include:
- QRS axis >90 degrees
- rS in Lead I
- qR in II, III, aVF
- no hx of HTN, RVH or Pulmonary HTN
. Right Ventricular Hypertrophy (RVH)
- R >7 mm in Right Precordial Leads
- RV Strain pattern with asymmetric T wave inversion
. Hyperkalemia (RAD)
(click on the above ecg for Dr. Smith’s Hyperkalemia as STEMI mimic)
. TCA overdose
(click on the title for Dr. Smith’s TCA induced Brugada Pattern)
. Ventricular ectopi
- RIGHT UPPER QUADRANT: Axis between -180 and -90 degrees and is considered pathologic. This is also known as EXTREME RIGHT AXIS DEVIATION or NO MAN’S LAND. The most common cause are ventricular beats.
– Extreme or Severe RAD
- Lead I, II, III, aVF predominantly negative
An easy way to approximate the degree of ventricular axis:
1. Find the most isoelectric lead, which will be perpendicular to the main direction of the ventricular axis (QRS). This lead is often the smallest voltage lead.
2. Now, find that lead that is perpendicular to the isoelectric lead. For example, if Lead II is the isoelectric lead, then you know you have to look at aVL because aVL intercepts Lead II at a 90 degree angle, making them perpendicular to each other.
3. Now, determine the main vector direction of that perpendicular lead, is it predominantly positive or negative? For example, If aVL is predominantly positive, then you know impulses are traveling mainly towards that lead which looks at a -30 degree, and making the other opposite leads more negative, making it pathologic axis deviation.
By practicing this, you will be able to pretty much identify the degree of vector direction within a few seconds, pretty close (within <20 degrees) to the exact degree.
Click on the link below for further, detailed and excellent axis determination 6 part review, by our own, Tom Bouthillet.