2. Role of the ECG Machine
The ECG machine is designed to recognise and record any electrical activity within the
heart. It prints out this information on ECG paper made up of small squares 1mm
squared
Each electrical stimulus takes the form of a wave and so patterns emerge made up of a
number of connected waves. A standard ECG is printed at 25mm per second or 25
small squares per second .In this way it is possible to calculate the duration of
individual waves.
10 small squares vertically is equal to 1 millivolt. So it is possible to calculate the
amount of voltage being released within the heart. If the line is flat at any time in the
duration of a series of waves, it indicates no electrical activity at that particular moment.
The direction in which the waves point indicates whether electricity is moving towards
or away from a particular lead.
The general direction in which electricity normally travels through the heart is a
downward diagonal line from the right shoulder to the left lower abdomen. This is
because the electrical stimulus originates in the SA node (upper right side of the heart),
travels through the AV node and bundle of His, and finishes mainly in the left ventricle.
(remember that there is more conduction in the left ventricle).
So different leads may have waves pointing in different directions. Eg. Lead AVR (right
shoulder/right arm/wrist) will always see the electrical stimulus travelling away from it,
therefore the waves expressed in AVR for sinus rhythm, part, will all point downwards.
3.
4. SITE OF ECG ELECTRODES
1. 6 chest leads
2. 4 limb or peripheral leads (one of these is "neutral")
5. Chest leads
The 6 leads are labelled as "V" leads and numbered V1 to V6. They are
positioned in specific positions on the rib cage. To position then accurately it is
important to be able to identify the "angle of Louis", or "sternal angle".
Right The space you are in is the 4th intercostal space. Where this space
meets the sternum is the position for V1.
Left 4th intercostal space. Where this space meets the sternum is the
position for V2.
The position for V4 is in the 5th intercostal space , in line with the middle
of the clavicle (mid-clavicular). V3 sits midway between V2 and V4.
Anterior line of 5th intercostal space to the left until your fingers are
immediately below the beginning of the axilla, or under-arm area. This is the
position for V5.
Midaxillary line of 5th intercostal space a little further until you are
immediately below the centre point of the axilla, (mid-axilla). This is the
position for V6
Limb Leads
Limb leads are made up of 4 leads placed on the extremities: left and right
wrist; left and right ankle.
The lead connected to the right ankle is a neutral lead, like you would find in an
electric plug. It is there to complete an electrical circuit and plays no role in the
ECG itself.
6.
7. That gives us nine wires and it is a 12-lead ECG. Where are
the other 3?
Well, so far we have nine wires. They all look directly at the
heart with tunnel vision. They only give information based
on what is immediately in front of them. These nine wires
are known as "unipolar leads".
The three active peripheral leads are AVr, AVL, and AVf.
Well, the 2 leads situated on the right and left wrist (or
shoulders), AVr and AVL respectively, and the lead situated
on the left ankle (or left lower abdomen) AVf, make up a
triangle, known as "Einthoven’s Triangle". Information
gathered between these leads is known as "bipolar". It is
represented on the ECG as 3 "bipolar" leads. So,
information between AVr and AVl is known as lead l.
Information between AVr and AVf is known as lead ll
Information between AVl and AVf is known as lead lll
8.
9. groups they make upRegions of the Heart
•AVL is on the left wrist or shoulder and looks at the upper left side of
the heart.
•Lead l travels towards AVL creating a second high lateral lead.
•AVf is on the left ankle or left lower abdomen and looks at the bottom,
or inferior wall, of the heart.
•Lead ll travels from AVr towards AVf to become a 2nd inferior lead
•Lead lll travels from AVL towards AVf to become a 3rd inferior lead.
•V2 V3 and V4 look at the front of the heart and are the anterior
leads.
•V1 is often ignored but if changes occur in V1and V2 only, these leads
are referred to as Septal leads.
•V5 and V6 look at the left side of the heart and are the lateral leads.
The ECG below shows where these leads are when printed.
10. • Intervals and segments
• PR Interval: From the start of the P wave to the
start of the QRS complex
• PR Segment: From the end of the P wave to the
start of the QRS complex
• QT Interval: From the start of the QRS complex
to the end of the T wave
• QRS Interval: From the start to the end of the
QRS complex
• ST Segment: From the end of the QRS complex
(J point) to the start of the T wave
11.
12. • Sinus Rhythm
• Sinus rhythm is the name given to the normal rhythm of
the heart where electrical stimuli are initiated in the SA
node, and are then conducted through the AV node and
bundle of His, bundle branches and Purkinje fibres.
• The P Wave
• The first wave (p wave) represents atrial
depolarisation. When the valves between the atria and
ventricles open, 70% of the blood in the atria falls through
with the aid of gravity, but mainly due to suction caused
by the ventricles as they expand.
• Atrial contraction is required only for the final 30% and
therefore a relatively small muscle mass is required and
only a relatively small amount of voltage is needed to
contract the atria.
• shape is generally smooth, not notched or peaked
• normal duration of less than or equal to 0.11 seconds
13. • The QRS Complex
• After the first wave there follows a short period where the line is flat. This is the point at
which the stimulus is delayed in the bundle of His to allow the atria enough time to pump all
the blood into the ventricles.
• As the ventricles fill, the growing pressure causes the valves between the atria and ventricles
to close. At this point the electrical stimulus passes from the bundle of His into the
bundle branches and Purkinje fibres. The amount of electrical energy generated is
recorded as a complex of 3 waves known collectively as the QRS complex. Measuring the
waves vertically shows voltage. More voltage is required to cause ventricular contraction and
therefore the wave is much bigger.
• Duration less than or equal to 0.12 seconds, amplitude greater than 0.5 mV in at
least one standard lead, and greater than 1.0 mV in at least one precordial lead. Upper limit
of normal amplitude is 2.5 - 3.0 mV.
• The Q Wave
• The picture below shows a small negative wave immediately before the large QRS complex.
This is known as a Q wave and represents depolarisation in the septum.
• Whilst the electrical stimulus passes through the bundle of His, and before it separates down
the two bundle branches, it starts to depolarise the septum from left to right. This is only a
small amount of conduction (hence the Q wave is less than 2 small squares), and it
travels in the opposite direction to the main conduction (right to left) so the Q wave points in
the opposite direction to the large QRS complex.
14. • The R Wave
• The QRS complex is made up of three waves. These waves
indicate the changing direction of the electrical stimulus as it
passes through the heart's conduction system.
• The largest wave in the QRS complex is the R wave.
• As you can see from the diagram, the R wave represents the
electrical stimulus as it passes through the main portion of the
ventricular walls. The wall of the ventricles are very thick due to
the amount of work they have to do and, consequently, more
voltage is required.
• This is why the R wave is by far the biggest wave generated
during normal conduction.
. R wave representing ventricular depolarisation
• More muscle means more cells. More cells means more
electricity. More electricity leads to a bigger wave.
15.
16. • The S Wave
• You will also have seen a small negative wave following the large R
wave. This is known an S wave and represents depolarisation in the
Purkinje fibres.
• The S wave travels in the opposite direction to the large R wave
because, as can be seen on the earlier picture, the Purkinje fibres
spread throughout the ventricles from top to bottom and then back up
through the walls of the ventricles
• So now it is possible to break down the QRS complex into 3 distinct
waves:
Q wave representing septal depolarisation
R wave representing ventricular depolarisation
S wave representing depolarisation of the Purkinje fibres
17. • The T Wave
• Both ventricles repolarise before the cycle
repeats itself and therefore a 3rd wave (t wave) is
visible representing ventricular repolarisation.
• T wave deflection should be in the same direction
as the QRS complex in at least 5 of the 6 limb leads
• normally rounded and asymmetrical, with a more
gradual ascent than descent
18. • The ST Segment
• There is a brief period between the end of the QRS complex and the
beginning of the T wave where there is no conduction and the line is flat.
This is known as the ST segment and it is a key indicator for both
myocardial ischaemia and necrosis if it goes up or down.
The U Wave
• Finally, it is possible to see another wave after the PQRST complex. This
is known as a U wave. It is not very common and is easy to overlook
• The U wave occurs when the ECG machine picks up repolarisation of
the Purkinje fibres. "U wave" can also occur with electrolyte
imbalances (potassium) but, again, this is not very common
19. • Normal Duration Times
P-R interval = 0.12 - 0.20 sec (3 - 5 small squares)
QRS width = 0.08 - 0.12 sec (2 - 3 small squares)
Q-T interval 0.35 - 0.43 sec
• Q wave < 0.04 s (1 mm) and < 1/3 of R wave
amplitude in the same lead.
20. • Approach to the ECG:
• Step 1: Determine the heart rate
• There are a number of strategies for determining the heart rate.
A simple, quick technique is to find a QRS complex that falls on
a major vertical grid-line (1), then count the number of large
squares to the next QRS complex (2). Dividing this number into
300 gives you the heart rate. In the ECG below, there are 2 large
squares between QRS complexes. 300/2 gives a heart rate of 150
beats per minute.
•
21. • Step 2: Measure important intervals
• The measurement of important
electrocardiographic intervals usually includes the
PR interval, the QRS interval and the QT interval.
At a standard paper speed of 25 mm/second, the
width of each small square (1mm) represents 0.04
seconds. One large square (5mm) represents 0.2
seconds.
22.
23. • Step 3: Evaluate the cardiac rhythm
• to check for regularity. This can be done by measuring the "p-p interval" or the "R-R interval".
To measure the P-P interval, place the edge of a piece of paper along the line of the rhythm and
mark the centre of 2 consecutive P waves. Compare this measurement with the next 2 P waves.
If the measurements are the same then the rhythm is regular.
• Similarly, to measure the R-R interval, measure the distance between the peaks of 2
consecutive QRS complexes (see above). Compare with the next 2. If they match then the
rhythm is regular.
• IS it Sinus Rhythm?
• To ascertain whether a rhythm is sinus or not you need to be able to identify key features.
There must always be a p wave.
The P wave should be a rounded shape
Each P wave should be the same shape
Each P wave should be followed by a QRS
The P-R interval should be 3-5 small squares and constant
The rhythm should be regular.
