Tip:
Highlight text to annotate it
X
Although, the flow of depolarisation and repolarisation through the myocardium is a three dimensional
process, it is very important to realise that each of the leads of the ECG / EKG machine
examines the movement of these forces through the heart in one plane only. In fact, based
on the plane in which they analyse electrical events in the heart, the 12 leads of the ECG
/ EKG divide into two groups of six. The six chest leads (also referred to as the precordial
leads) V1 to V6 examine the flow of depolarisation and repolarisation through the heart in the
horizontal or transverse plane while a second group of six leads, the frontal leads examine
these electrical events in the vertical or frontal plane. We will first deal with the
transverse group-the precordial or chest leads. The 6 chest leads, V1 to V6 are placed on
the surface of the chest wall in an arc, from V1 in the 4th right intercostal space to the
right of the sternum to lead V2 in the fourth left intercostal space to the left of the
sternum and then at roughly equal intervals to lead V6 in the fifth left intercostal space
in the mid-axillary line. These leads examine the flow of depolarisation and repolarisation
in the transverse plane from these perspectives. If we take a representative transverse section
through the thorax, you will appreciate that with the heart in anatomical position, the
atria lie posterior to the ventricles and the right ventricle lies somewhat anterior
to the left in this plane. Leads V1 and V2, therefore, face the surface of the right ventricle.
However, note that they also face the much larger muscle mass of the interventricular
septum. These leads are, strongly influenced by electrical events in this structure and
V1 and V2 are, therefore, often referred to as the septal leads. V3 and V4 face the anterior
wall of the left ventricle while V5 and V6 face the lateral wall of the left ventricle.
Modern machines present the printed ECG / EKG readout landscaped on an A4 piece of paper
and the signal from each of the chest leads is recorded on the left hand side of this
A4 ECG readout in numerical order as shown here.
We can consider the frontal leads in two groups of three, the standard limb leads, leads I,
II and III, and the augmented vector leads, aVR, aVL and aVF. The frontal leads examine
the flow of depolarisation and repolarisation through the heart in the frontal plane. If
we take a representative frontal section through the thorax it will help you understand the
perspective of each of these leads on cardiac electrical events. To remember the position
of all 6 of the frontal leads relative to the heart use lead I as your reference point.
Lead I looks directly at the heart from the patients left hand side and defines zero degrees
in all further discussions of the frontal leads, lead II looks at the heart at an angle
600 further clockwise while lead III is positioned a further 600 clockwise from lead II, these
angles will become very important in the next section of this course.The readout from the
standard leads, Leads I, II and III are recorded down the right hand side of the ECG / EKG
paper as shown here. We are now left with three further leads to
remember, the augmented vector leads aVR, aVL and aVF. aVL looks at the heart from the
left (L is for left) but at 30o anticlockwise from lead I, aVR looks at the right side of
the heart (R is for right), and, just like aVL, it is 300 above the horizontal relative
to lead I, as aVL and aVR are set at 30o off the horizontal plane, you can think of them
as the left and right Wings or 'vings' of the ECG / EKG. aVF looks straight up at the
inferior surface of the heart and is therefore at 90o clockwise from lead I, think of aVF
as looking straight up at the heart from the feet (F is for feet). On the ECG readout,
recordings from the augmented vector leads are positioned between the standard leads
and the chest leads from aVL to aVr and down to aVF at the foot of
the page. Now that we understand the perspective of
all 12 ECG leads on cardiac depolarisation and repolarisation, you may ask, can we use
the simple principles outlined in video 2 to build a predicted normal 12 lead readout?
Well, the answer is yes certainly for the chest leads. But for reasons which hopefully
will become clear to you this approach is slightly less satisfactory when dealing with
the frontal leads. Understanding the readout in the frontal leads, particularly the morphology
of the qrs complexes, requires a slightly more sophisticated but still comprehensible
approach. The electrical forces of depolarisation and repolarisation are vectors and before
you go on we would urge you to watch the next two videos outlining some principles of vector
mathematics. We will then use these principles and what we have learned so far to construct
a predicted normal 12 lead ECG / EKG. Before leaving the topic of lead perspectives
there is one more important point to consider. For reasons which will become obvious to you
in subsequent sections, in addition to the division into frontal and chest leads based
on the plane in which they analyse cardiac electrical events, the leads are also considered
in groups which face adjacent anatomical areas of the heart. Three of the frontal leads,
II, III and aVF face the inferior surface of the heart and are termed the inferior leads.
Leads V1 and V2 face the interventricular septum, while leads V3 and V4 face the anterior
surface of the left ventricle. These four chest leads are referred to as the anterior
or anteroseptal leads while the left lateral leads face the left lateral surface of the
left ventricle. The designations shown here will be very useful to us when we consider
disease states but do not make the mistake of thinking that these groups of leads analyse
cardiac electrical events only in the anatomical areas indicated. As you will see shortly,
the lead groups analyse electrical events in all areas of the heart in the relevant
plane from the perspective indicated by their designations.