Transcript The Cardiac Axis

THE CARDIAC AXIS & AXIS
DEVIATION
(Lecture 2 )
Associate Professor
Dr. Alexey Podcheko 1
Spring 2015
INTENDED LEARNING
OBJECTIVES:
To know :
CARDIAC AXIS
AXIS DEVIATION
CAUSES OF CARDIAC AXIS
DEVIATION
 AXIS DETERMINATION ON ECG
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Geometry of the vertical leads
(-30°)
(+210°)
(+120°)
(+90°)
“The Cardiac Axis”
• Six ECG leads (I, II, III, aVR, aVL, aVF)
examine the flow of depolarization
and repolarization in the heart in the
frontal plane. In this plane,
depolarizing forces of differing
magnitudes are moving through the
ventricles in many different
directions during the normal process
of ventricular depolarization.
If we sum together all of the
depolarization vectors occurring in
the frontal plane throughout the
duration of ventricular
depolarization, we can generate an
overall vector which has a magnitude
and direction.
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“The Cardiac Axis”
• The direction of this total frontal QRS
vector, defined by the angle (alpha) it
makes with lead I, is the cardiac axis
• In normal circumstances, the
direction of the frontal QRS vector is
dominated by the depolarization
forces generated in the large left
ventricular muscle mass.
• The pattern of depolarization of the
left ventricle is in turn dictated by the
precise anatomy of the
intraventricular conducting system.
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“The Cardiac Axis”
• Shortly after the anatomical
bifurcation of the bundle of
His into Right and Left
Bundle Branches the left
bundle branch itself quickly
divides into anterior fascicle
(the left anterior fascicle-LAF)
and a posterior fascicle (the
left posterior fascicle-LPF).
• Left anterior fascicle
depolarizes the anterior and
lateral walls of the left
ventricle,
• Left posterior fascicle
simultaneously depolarizes
the posterior and inferior
surfaces of the chamber.
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 In a normal heart these two depolarizing forces (LAF & LPF) are the
prime determinant of the cardiac axis.
 When added together, the total vector generated is travelling
downwards and to the left, more or less straight down lead II at
approximately 60 degrees relative to lead I.
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“The Cardiac Axis”
• There are extensive
anastomoses between the LAF
and LPF at their termination.
o If one of these major branches
is damaged and flow through it
blocked, the ventricle can be
depolarized by retrograde
flow of depolarization from the
intact branch via these
anastomoses.
o This will have a dramatic effect
on the direction of the frontal
QRS vector.
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o If disease blocks conduction in the
anterior fascicle (LAF) and the
lateral and anterior ventricular wall
are depolarized by current flowing
upwards from the LPF, the cardiac
axis is now travelling upwards and
to the left (due northeast). This is
termed “left axis deviation (or
LAD)”.
o With blocked conduction in the
posterior fascicle (LPF) the inferior
surface is depolarized by current
flowing from above and the sum of
these two major vectors is now
moving downwards and towards
the right.  This is termed “right
axis deviation (or RAD)”.
LAFB - LEFT
LPFB - RIGHT
Red Arrow shows direction
of the electrical axis of the
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heart
“Axis Deviation”
• Although the cardiac axis lies close to
lead II in many cases, its position
demonstrates a wide normal variation.
For example,
 short fat people tend to have an axis
towards the left
 while in tall thin individuals the axis
tends to be towards the right.
• The normal range lies between +90
degrees, or straight down aVF, to -30
degrees (…0 degree), straight along
aVL.
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• “Left axis deviation”, that is,
deviation of the cardiac axis beyond
– minus 30 degrees , is characterized
by a Strongly Negative QRS complex in
lead III combined with an Overall
Negative QRS in lead II and a Positive
lead I.
The commonest cause of this
abnormality is left anterior fascicular
block (LAFB).
• With “right axis deviation” –
deviation of cardiac axis beyond +90
degrees
The commonest cause of this
abnormality is leftposterior fascicular
block (LAFB).
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“Axis Deviation – Causes”
• While conduction defects in the left ventricle are
the primary cause of axis deviation on the ECG,
we should point out that in some situations other
vectors may come to the fore and shift the axis
independent of damage to conduction in this
chamber. Some of these are intuitively obvious. For
example,
a large Left lateral myocardial infarction, with loss
of left-directed depolarization vectors can shift
the cardiac axis to the Right.
Equally, a large Right sided infarction can shift the
axis to the Left by loss of rightward vectors.
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“LEFT AXIS DEVIATION: CAUSES”
• Left Anterior Fascicular Block
* Left anterior descending artery
(ischemia/infarction)
* LVH (Fibrosis / Ischemia)
* Cardiomyopathy
• Loss of vectors
* Large right sided infarct
• Gain of vectors
* LVH (unusual)
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“RIGHT AXIS DEVIATION: CAUSES”
• Left Posterior Fascicular Block
*Diffuse ischemia/infarction
*Cardiomyopathy
• Loss of vectors
*Left lateral infarction
• Gain of vectors
*RVH (unusual)
Note: There are many other causes of axis
deviation………………………….
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• Although ventricular hypertrophy in the
absence of conduction defects is a potential
cause of axis deviation, this is very unusual.
The great majority of patients with left
ventricular hypertrophy from whatever cause
do Not have LAD on their ECG.
Furthermore, and perhaps surprisingly, the
presence or otherwise of LAD in patients
with LVH bears No relation to the degree of
hypertrophy present.
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The same is true for right ventricular
hypertrophy acquired in adult life.
The majority of adult patients with RVH do Not
demonstrate RAD on their ECG.
Remember, axis deviation usually results
from conduction defects in the ventricles
axis deviation on an ECG is neither sensitive
nor specific in the diagnosis of ventricular
hypertrophy.
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“Axis Determination/Estimation”
• Many methods to determine cardiac axis
deviation:
General determination of electrical axis using
“Lead I & Lead aVF” method
More precise “Trigonometry/calculation”
method
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“Lead I & Lead aVF” method
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Trigonometry/calculation” method
• 1. Find limb lead (I, II, III, aVR, aVL,
aVF) where QRS is most isoelectric
(height and depth of R and S waves
are close to equal)
• 2. On the frontal axial chart draw
the line perpendicular to isoelectric
lead, put arrows on the both ends
of the line
• 3. Analyze morphology of QRS
complex in the leads near the line
you just have drawn (in this case leads II and aVR)
• The electrical axis is pointing in the
direction of the lead with positive
QRS complex (in this case – lead II)
• Measure angle between lead I and
your arrow – this is a value of an
electrical axis in this case (in this
case is +60degrees)
Exercise: Determine electrical axis on the
ECG films below
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