Ecg basics lecture 3

Recording of the ECG
June 24, 2021 ECG intensive course for Medical Students 2

Times and speeds

The ECG Paper
• Horizontally:
One small box: 0.04 s
One large box: 0.20 s
• Vertically:
One large box: 0.5 mV

Components of an ECG Tracing

Assignment
• Are there any other sites for
Standard Limb Lead Electrode
Placement rather than these?

Methods for Calculating Heart Rate
• Heart rate is calculated as the number of times
the heart beats per minute.
• It usually measures ventricular rate (the number
of QRS complexes) but can refer to atrial rate
(the number of P waves).

• The method chosen to calculate HR varies
according to rate and regularity on the ECG
tracing.
Methods for Calculating Heart Rate

Method 1: Count Large Boxes
• Regular rhythms can be quickly determined by
counting the number of large graph boxes between
two R waves.
• That number is divided into 300 to calculate bpm.
• The rates for the first one to six large boxes can be
easily memorized.

• Remember: 60 sec/min divided by 0.20 sec/large
box 300 large boxes/min.

Calculate HR

Method 2: Count Small Boxes
• Sometimes it is necessary to count the number
of small boxes between two R waves for fast HR.
• That number is divided into 1500 to calculate
bpm.

• Remember: 60 sec/min divided by 0.04
sec/small box 1500 small boxes/min.

Examples:
a. If there are six small boxes between two R
waves: 1500/6 = 250 bpm.
b. If there are ten small boxes between two R
waves: 1500/10 =150 bpm.

♥ Clinical Tip
• If a rhythm is extremely irregular, it is best to count
the number of R-R intervals per 60 sec (1 min).

Components of the ECG complex
• PR interval: beginning of P wave to the beginning of
Q wave
• ST segment: end of S wave to the beginning of T
wave
• QT interval: beginning of the Q wave to the end of T
wave

Duration
• P wave: < 0.12 sec
• PR interval: 120-200ms, represented by 3-5 small
squares.
• QRS: ≤ 120ms, represented by 3 small squares
• QT interval: < 0.44 s

< 0.44 s > 0.44 s
Normal Long QT
QTc interval
o A prolonged QT can be very dangerous.
o Causes include: drugs, electrolyte abnormalities,
CNS disease, post-MI, and congenital heart disease.

Cardiac axis

• Axis refers to the mean QRS axis during ventricular
depolarization.
• We like to know the QRS axis because an abnormal
axis can suggest disease.

• Leads VR and II look at the heart from opposite
directions.
• The depolarization wave normally spreads
through the ventricles from 11 o’clock to 5 o’clock.

• A normal 11 o’clock-5 o’clock axis means that the
depolarizing wave is spreading towards leads I, II,
and III, and therefore associated with a predominantly
upward deflection in all these leads.
• The deflection will be greater in lead II than in lead
III.

• The normal QRS axis lies between -30o
and +90o
.
• A QRS axis that falls between -30o
and -90o
is
abnormal and called left axis deviation.
0o
30o
-30o
60o
-60o
-90o
-120o
90o
120o
150o
180o
-150o

• A QRS axis that falls between +90o
and +150o
is
abnormal and called right axis deviation.
• A QRS axis that falls between +150o
and -90o
is
abnormal and called superior right axis deviation.
0o
30o
-30o
60o
-60o
-90o
-120o
90o
120o
150o
180o
-150o

• Since lead I is orientated at 0o
a wave of
depolarization directed towards it will result in a
positive QRS axis.
0o
30o
-30o
60o
-60o
-90o
-120o
90o
120o
150o
180o
-150o
I

• Similarly, lead II is orientated at 60o
a wave of
depolarization directed towards it will result in a
positive QRS axis.
I
II
0
o
30o
-30o
60o
-60o
-90o
-
120o
90o
120
o
150
o
180o
-
150o

• Therefore, if the QRS complex is positive in both
leads I and II the QRS axis must be between -30o
and 90o
.
0o
30o
-30o
60o
-60o
-90o
-120o
90o
120o
150o
180o
-150o
I
II

• So the deflections in lead VR are normally mainly
downward (negative) and in lead II mainly
upward (positive).

• If the Rt ventricle becomes hypertrophied, the axis
will swing towards the Rt: the deflection in lead I
becomes negative and the deflection in lead III will
become more positive.
• This is called Rt axis deviation.

• When the Lt ventricle becomes hypertrophied, the axis
may swing to the Lt.
• So the QRS complex becomes predominantly negative
in lead III.
• This is called left axis deviation

• The QRS complex in the chest leads show a
progression from lead V1 to lead V6.
• The transition point, where the R and S waves are
equal, indicates the position of the interventricular
septum.

Assignments

Measure: cardiac axis and heart rate

Measure: cardiac axis

Ecg basics lecture 3

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Ecg basics lecture 3