Download THE ELECTROCARDIOGRAM (ECG)

Practical No : 10
THE ELECTROCARDIOGRAM (ECG)
Objectives:
At the end of the practical, the student should be able to,
1. Describe the spread of cardiac excitation.
2. Accurately place the ECG leads on a patient.
3. Explain the ECG complexes in relation to the leads.
4. Describe and interpret the significance of the amplitude, duration and direction of
ECG waves.
5. Identify the PR interval and QT interval and interpret their significance.
6. Calculate the heart rate from an ECG strip.
7. Calculate the cardiac axis from an ECG strip.
Usefulness:
Analysis of normal & abnormal rhythms
Detection & localization of changes in the myocardium due to pericardial
disease.
Detection of hypertrophy of walls of the atria and ventricles.
Detection of changes in electrical activity due to myocardial and
pericardial disease
Detection of changes in electrical activity of the heart consequent on
general metabolic changes.
ECG leads and their connections :
Bipolar Leads :
(Paste complex here)
-
+
Lead I
-
-



Lead II
Lead III

(Paste
complex
here)
(Paste
complex
here)

+
+
Unipolar Leads :
+
+
aVR
complex
aVL
complex





AVF complex
+
Chest Leads - Indifferent Electrode :
- ve + ve
V4





Augmented Unipolar Leads :
- ve + ve


LA
RA


aVF
Chest Leads :
MCL
AAL
MAL
V1 V2
V3
V6
V4 V5
V1
4th Intercostal Space - Right Sternal Edge
V2
4th Intercostal Space - Left Sternal Edge
V3
Halfway between V2 and V4
V4
5th Intercostal Space - Mid-Clavicular Line (MCL)
V5
Anterior Axillary Line (AAL), nearly horizontal to V4
V6
Mid -axillary line (MAL), horizontal to V4
Amplitude, Duration & Direction of Waves :
Sample of a normal ECG tracing :
R
T
P
S
- This tracing shows lead II, which is also used when obtaining a rhythm strip
- In lead II, the P, R & T waves show a positive deflection, while the S wave
shows a negative deflection
- The Amplitude of a wave is calculated by assessing its height.
- The Duration of a wave is calculated by assessing its length.
- The Rate of the galvanometer
 the paper runs at
R
ECG waves, segments and Intervals :
P
=
25 mm/sec
=
5 big squ/sec
=
25 small squ/sec
300 big sq/min or
1500 small sq/min
PR
Segment
ST
Segment
Q
S
QRS
duration
PR
interval
QT
interval
T
U
Calculation of heart rate :
Heart Rate is calculated by dividing 300 or 1500 by the distance between two Rs (R -R
interval)
Heart Rate =
300
1500
or
R-R interval in large squares
R-R interval in small squares
Heart rate of =
the above tracing
300
4
=
75 beats/min
Calculation of wave amplitudes, intervals and complexes :
P Wave :
Is a positive wave with a duration and an amplitude (calculate from strip
obtained).
Duration
=
no. of small squares x 0.04 sec
=
2 x 0.04
=
0.08 sec
Amplitude
=
=
=
height of wave in small squares x 1/10 mv
(10mm or 10 small squares or 2 big squares = 1 mv)
2 x 1/10 mv
0.2 mv
QRS Complex :
Consists of a negative Q wave, a positive R wave and a negative S wave.
Duration
=
length of entire complex in small squares x 0.04 sec
=
1.5 x 0.04 sec
=
0.06 sec
Voltage
Difference
=
=
=
height between tallest positive wave(R)
and deepest negative wave(S)
17 x 1/10 mv
1.7 mv
T wave :
Is positive in all leads except aVR
Duration
=
no. of small squares x 0.04 sec
=
3 x 0.04 sec
=
0.12 sec
Voltage
P - R Interval :
Duration
=
=
=
height of wave in small squares x 1/10 mv
2.5 x 1/10 mv
0.25 mv
=
=
no. of small squares x 0.04 sec
2 x 0.04 sec
=
0.08 sec
x
1/10 mv
Calculation of the mean electrical axis of the heart :
 Calculated using Einthoven's triangle.
 Perpendiculars are drawn from the midpoints of the sides of the equilateral
triangle, which then intersect at the center of electrical activity.
Lead I
RA
LA
Lead III
Lead II
LL
 Using the ECG strip, take each lead (I,II,III) and calculate the sum of the
height between R wave and the largest negative deflection (Q or S)
 Plot this distance on the triangle, starting from the midpoint of the lead, and
towards -ve or +ve direction, corresponding to the sum obtained.
 Drop perpendiculars from the end point thus obtained and find the point where
they intersect.
15
10
mm 5
0
-5
+5
-0
+5
I
-
+
+ 5 mm
-
-
+ 10 mm
+ 15 mm
15
10
mm 5
15
10
mm 5
+16
-1
+15
0
-5
III
+11
-1
+10
0
-5
+
+
II