Electrical Axis of the heart Flashcards
How does the direction of the electrical current affect the direction of an ECG waveform?
Current flow towards a lead = waveform deflects upwards (positive deflection)
Current flow away from a lead = waveform deflects downwards (negative deflection)
Current flow perpendicular (right angles) to a lead = waveform deflects in both directions (biphasic).
What is a vector?
The spread of the cardiac impulse gives rise to the main deflections of the ECG.
The total electrical activity at any one moment of time can be summated and represented by a single electrical force.
This force has magnitude and direction and is termed the instantaneous vector
Mean Cardiac Vector
All the vectors occurring in the ventricles during the production of the QRS can be averaged.
MEAN CARDIAC VECTOR – sum of all vectors at a given time
It is customary to measure this only in the frontal plane.
It is based on the orientation of the limb leads, I, II , III, aVR, aVL & aVF.
The limb lead with the tallest R wave will be the closest to the QRS axis.
What is cardiac axis?
The heart’s electrical axis refers to the general direction of the heart’s depolarization wavefront (or mean electrical vector) by using the polarity of the QRS complex in leads I, II, and III in the frontal plane.
What is the hexaxial reference system?
Used to determine the heart’s mean frontal electrical axis
Composed of the lead axes of the 6 frontal planes
They are rearranged so their centres overlay one another
Axes divide the plane into 12 segments (30 degrees)
Positive ends of the axes are labelled with the name of the lead
Hexaxial reference system
- aVF at top -90
+ aVL -30
+ I at 3’oclock 0
+II at 5oclock +60
+aVF +90 6oclock
+III +120 7oclock
-150 aVF 10oclock
How to derive the mean QRS axis..
- Find the limb lead on the ECG with the smallest equiphasic QRS complex.
-This will be at right angles to the direction of current flow in the heart. - Locate this lead on the hexaxial reference system.
- Find the lead that is located at 90º to the most equiphasic lead selected in step 1
-This lead will be in line with current flow/axis. - Is this lead (selected in step 3) mostly positive or negative?
- If this lead is positive stay with this lead, it is the mean QRS axis for this patient.
- If this lead is negative the lead diametrically opposite to this lead is the mean QRS axis for this patient.
(I.e. If the lead is positive take the axis value at the positive end of the lead, but if it is negative take the axis value from the negative end of the lead on the hexaxial diagram)
This will give the mean frontal plane QRS axis to within 30º
To correct the mean frontal plane QRS axis to the nearest 15º in addition to steps 1-6
- Look back at the original, most equiphasic, lead.
- If this is slightly more positive then correct the main cardiac vector 15º towards that original equiphasic lead (positive end).
- If this is slightly more negative then correct the main cardiac vector 15º away from that original equiphasic lead (towards the negative end).
This will give the mean frontal plane QRS axis to within 15º
Normal Range Mean Frontal QRS axis
In the normal adult the mean frontal plane QRS axis lies between -30º and +90º
Left axis deviation is present when the axis is less than -30º
Right axis deviation is present when the axis is greater than +90º
LAD
-90 to -30
Causes;
- Very mild LAD is sometimes seen in patients with normal hearts e.g. obesity, stocky build, pregnancy.
- Block of the anterior fascicle of the left bundle branch (anterior hemiblock), can be caused by MI or fibrosis.
- Pre-excitation syndromes
- Ventricular pacing
- Left Ventricular Hypertrophy.
- If this is also associated with RBBB and 1st Degree AV block (Trifasicular block) it can develop into complete heart block.
RAD
+90 to +/-180
Causes;
- Normal in paediatrics.
- Uncommon but tall slim adults can have RAD.
- Right Ventricular Hypertrophy
- Block of the posterior fascicle of the left bundle branch (posterior hemiblock).
- Pre-excitation syndromes
- Congenital heart disease
- Dextrocardia
