Determination of the electrical
axis of the heart
MICHAEL R. STAFFORD, DO
DONALD G. STAFFORD, PhD
The rapid, accurate determin-ation
of the electrical axis of a patient's
heart is very useful for diagnosis. This de-termination
can be made readily by using
six electrocardiographic leads. The proce-dure
for interpreting the data from these
leads and determining the electrical axis
of the heart is charted. This method uses
key leads I and aVF to determine the quad-rant
where the axis is located and secon-dary
leads II, III, aVR, and aVL to find
the axial position within the quadrant.
When either I or aVF is isoelectric, only
one key lead is used.
(Key words: Electrical axis, electrocar-diographic
leads, axis deviation, QRS
vector)
The rapid determination of the heart's elec-trical
axis can be a very valuable ability for
the physician. Sometimes this axis is the only
significant change on the electrocardiogram
(ECG) to alert a physician that something is
abnormal.
A change in the electrical axis can also give
a clue to the diagnosis of a problem. For ex-ample,
left axis deviation, axis beyond – 30
degrees, may be caused by either left ventricu-lar
hypertrophy, dilation, or left anterior
hemiblock (axis > – 45 degrees). Hyperten-sion,
aortic stenosis, and right ventricular in-farctions
will also produce left axis deviation.
When the axis is beyond + 120 degrees (right
axis deviation) problems such as chronic ob-structive
pulmonary disease or pulmonary
valve stenosis are possibilities. There are, of
course, several other possibilities for deviation
from the normal axis of the heart. The proce-dure
outlined in this article allows an easy,
rapid detection of the axis abnormality to help
Reprint requests to Michael R. Stafford, DO, 1414 Ar-lington,
Ada, OK 74820.
the physician arrive at the diagnosis.
The electrical axis of the heart can be de-termined
from an ECG by use of a simple, but
very accurate, method that is summarized in
the electrical axis chart (Figure). If the method
is learned and applied, the electrical axis can
be determined correctly within a few degrees.
Method
This method requires simple interpretation of
six leads from the ECG. The leads used are
I, II, III, aVR, aVL, and aVF. Lead I and aVF
are used as key leads to determine the quad-rant
in which the electrical axis is located.
Leads II, III, aVR, and aVL are used as secon-dary
leads to establish the position of the axis
within the quadrant.
The first step in interpreting the ECG to
find the electrical axis of the heart is to deter-mine
if the QRS vector of a lead is positive,
negative, or isoelectric. A vector is positive ( + )
if the presentation is more elevated above the
baseline than depressed. The vector is nega-tive
( – ) if the presentation is more depressed
below the baseline than elevated. The vector
is isoelectric (0) if the vector is equally ele-vated
above and depressed below the baseline.
If the vector is only slightly positive or slightly
negative, assign the value as ( + ) or ( – ): The
magnitude of the positive or negative value
is not important for this determination.
Next, use the two key leads I and aVF to
establish the quadrant in which the electrical
axis is found (Figure). For example, if both
leads I and aVF are positive, the electrical axis
is in quadrant 1. If lead I is negative and aVF
is positive, the electrical axis is in quadrant 2.
After leads I and aVF have been used to de-termine
the quadrant in which the electrical
axis is located, secondary leads can be used
to determine the position of the axis within
that quadrant.
Clinical practice • Stafford and Stafford JAOA • Vol 93 • No 1 • January 1993 • 103
Axis
quadrant
Key
leads
Secondary
leads
Vector
value
Heart
axis
( — ) +15°
— 90°
I(+) III (0) +30°
( + ) See aVL . . .
180° 0°
( + ) +45°
aVF (+) aVL (0) + 60°
+ 90°
Quadrant 1 ( — ) + 75°
I(+),aVF(+)
( + ) —15°
— 90°
I(+) II (0) — 30°
( — ) See aVR .. .
180° 0°
( — ) —45°
aVF ( — ) aVR (0) — 60°
+ 90°
Quadrant 4 ( + ) 75°
I(+),aVF(—)
( — ) +105°
— 90°
I ( — ) aVR (0) + 120°
(+) See II . . .
180° 0°
( + ) +135°
aVF (+) II (0) + 150°
+ 90°
Quadrant 2 ( — ) + 165°
I(—),aVF(+)
( + ) —105°
— 90°
I ( — ) aVL (0) —120°
180° "' 0 °
( — ) See III . . .
( + ) —135°
aVF ( — ) III (0) — 150°
+ 90°
Quadrant 3 ( — ) — 165°
I ( — ), aVF ( — )
I (0) +90°
Special cases
aVF (0) 0°
Figure. Electrical axis chart.
104 • JAOA • Vol 93 • No 1 • January 1993
Clinical practice • Stafford and Stafford
Examples
• If both leads I and aVF are positive, the axis
is in quadrant 1. Next, use the secondary
leads in this way. If lead III is negative ( – ),
the axis is + 15 degrees. If lead III is isoelec-tric
(0), the axis is + 30 degrees. If lead III
is positive ( + ), then go to the aVL lead. If
the avL lead is positive ( + ), the axis is + 45
degrees. If the aVL lead is isoelectric (0), the
axis is + 60. If lead aVL is negative ( – ), the
axis is + 75 degrees. Because the normal po-sition
of the heart is in quadrant 1, the elec-trical
axis will be found there most of the
time.
• If lead I is positive ( + ) and lead aVF is nega-tive
( – ), the axis is in quadrant 4, the sec-ond
most common case. Again, use the sec-ondary
leads in the chart to determine the
position of the axis in that quadrant. For ex-ample,
if lead II is positive ( + ), the axis is
– 15 degrees. If lead II is isoelectric (0), the
axis is – 30 degrees. If lead II is negative
( – ), then go to the aVR lead. If the aVR lead
is negative ( – ), the axis is – 45.
There are two special cases shown in the
Figure in which only one key lead is needed
to determine the axis. These are the cases
when either lead I or aVF is isoelectric. If lead
I is isoelectric, the electrical axis is + 90 de-grees.
If the aVF lead is isoelectric, the elec-trical
axis is 0 degrees.
The Figure is arranged in order of decreas-ing
frequency in which the indicated electri-cal
axis is found. Begin each time at the top
of the chart and scan downward. The proce-dure
is the same in each case. Use the key
leads first to determine the quadrant and then
go to the secondary leads to establish the value
within the quadrant.
Comment
As soon as the interpretation procedure is
learned, the electrical axis chart can be used
alone. After a clinician has used this chart a
few times, the method will become almost auto-matic.
Clinical practice • Stafford and Stafford JAOA • Vol 93 • No 1 • January 1993 • 105