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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_2818_Библиотеки_им_академика_М_И_Перельмана
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https://t.me/med1917
Fig. 4.5 J point
4.6 S-T Segment
4 ECG Intervals andSegment
S-T segment starts at the end of S wave (J point) and ends at the beginning of T
wave (Fig.4.6). S-T segment is isoelectric. The average duration of the S-T segment
is less than 2 to 3 small squares (0.08–0.12s). It represents ventricular repolarization (phase 2 of repolarization, i.e. plateau phase). 1mmS-T segment elevation in
leads I, II and III and up to 2mm elevation is normal in some precordial leads (in
absence of clinical features of myocardial infarction). S-T segment elevation in only
one lead is not diagnostic of any disease.
Tips and Tricks
• The shape and deviation of S-T segment help us to diagnose life threatening
diseases like myocardial infarction, ischaemia and pericarditis.
• Often a dangerous looking S-T segment may be normal, and the reverse is
equally true.
• If S-T segment is elevated, think of myocardial infarction and pericarditis.
• If S-T segment is depressed, think of myocardial ischaemia, ventricular hyper-
trophy, abnormal intraventricular conduction and digitalis effect.
• Often minor degree of S-T segment deviation in absence of any symptoms is
seen in normal healthy persons.
4.7 Q-T Interval
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Fig. 4.6 S-T segment (normal)
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4.7 Q-T Interval
Q-T interval is calculated from the beginning of Q wave to end of T wave (Fig.4.7).
If Q wave is absent, the Q-T interval is measured from the beginning of R wave to
the end of T wave. It represents the duration of ventricular systole, both depolarization and repolarization. Normally, it comprises about 40% of the complete cardiac
cycle. Q-T interval varies with heart rate. It prolongs during bradycardia and shortens during tachycardia. Q-T interval corrected for heart rate is called Q-Tc.
Q-Tc=Q-T/√R-R interval (Bazett’s formula)
The normal range of Q-Tc is 0.35s to 0.44s (men) and 0.45s (women). It is
longer in females and in old age. Prolonged Q-T interval (more than about 0.48s)
may lead to a special type of ventricular tachycardia, called Torsades de Pointes.
Tips and Tricks
• At heart rate of 60/min, Q-T is the same as Q-Tc.
• Within heart rate of 60–100/min, Q-T interval should not be more than half of
R-R interval.
The various causes of abnormal Q-Tc are enumerated in Box 4.2, and the various
time intervals are enumerated in Box 4.3.
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Fig. 4.7 Q-T interval.
There are 10 small boxes
between the beginning of q
wave to end of T wave. In
this ECG the Q-T interval
is 10×0.4=0.40s
4 ECG Intervals andSegment
Q-TQ-T Q-T
Q-T = 0.40s
Box 4.2 Abnormal Q-Tc
Prolonged Q-Tc
Hypocalcaemia
Quinidine and procainamide effect
Acute myocardial infarction
Acute myocarditis
Congenital—Prolonged Q-T syndrome, Romano Ward syndrome
C VA
Torsades de Pointes
Tricyclic antidepressant drugs
Electrolyte imbalance
(a) Hypokalaemia
(b) Hypomagnesaemia
Hypothermia
Deep sleep
Idiopathic
Shortened Q-Tc
Hypercalcaemia
Digitalis effect
Hyperthermia
Phenytoin sodium therapy
Box 4.3 Various Time Intervals
QRS interval Less than 0.12s
P-R interval 0.12–0.20s
Q-Tc 0.35–0.44s
Heart rate 1500/R-R interval
QRS
QRS
4.9 Ventricular Activation Time
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P-R
Segment
COMPLEX
Q
R
S
S-T
Segment
J Point
Q-T
Interval
T
COMPLEX
R
P-R
Segment
P
S-T
Segment
T
P
T-Q
Q
S
Segment
Fig. 4.8 T-Q segment
4.8 T-Q Segment
T-Q segment represents phase 4 of action potential and corresponds to electrical
diastole. It starts from the end of T wave to the beginning of the next QRS complex
(Fig.4.8). This segment is used as isoelectric baseline for deviations of J point or
S-T segment. However, more precisely it is the T-P segment (end of T wave to
beginning of P wave) which is used as isoelectric baseline for measurement of deviations of J point or S-T segment. T-P segment is a part of T-Q segment.
4.9 Ventricular Activation Time
The time taken by an impulse to travel from endocardium to epicardium is known
as ventricular activation time (VAT). It is also known as time of onset of intrinsicoid
deection. It is calculated from the beginning of Q wave to the peak of the R wave
(Fig.4.9). If Q wave is absent, then it is calculated from the beginning of R wave to
the peak of R wave. Normally, it is 0.02s in lead V1 and 0.04s in lead V6. VAT
increases in ventricular hypertrophy.
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Lead V1
Lead V6
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Fig. 4.9 Ventricular
activation time
VAT 0.02 s
––––
+
+++
++++
–––
–
4 ECG Intervals andSegment
VAT 0.04 s
VAT
Self-Assessment Questions
1. J point is the junction of Q wave and R wave. True or false?
2. P-P interval helps in calculation of atrial rate. True or false?
3. Prolonged Q-T interval can lead to Torsades de pointes. True or false?
4. R-R interval helps in calculation of ventricular rate. True or false?
5. QRS interval indicates time taken for ventricular repolarization. True or false?
6. Which of the following is the wrong statement?
a. P-R interval is calculated from the beginning of P wave to beginning
of R wave.
b. S-T segment starts from the J point and ends at the beginning of T wave.
c. Q-T interval starts from the beginning of Q wave to the end of T wave.
d. Usually, Q-T interval is more than half of R-R interval.
7. P-R interval is mainly contributed by:
a. AV nodal delay b. Ventricular depolarization c. Atrial depolarization d.
Ventricular repolarization
4.9 Ventricular Activation Time
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8. S-T segment is used to diagnose:
a. First degree heart block b. Heart rate c. WPW syndrome d. Myocardial
infarction
9. Bazett’s formula is used to calculate:
a. Atrial rate b. Ventricular rate c. Corrected Q-T interval d. P-R interval
10. Ventricular activation time is increased in:
a. Atrial dilatation b. Complete heart block c. Electrolyte disturbance d.
Ventricular hypertrophy
Case Studies
1. Measure the duration of QRS complex and P-R interval in the ECG of Fig.4.10.
2. Measure the P-P and R-R intervals in the ECG of Fig.4.11.
3. Measure the duration of QRS complex and P-R interval in the ECG of Fig.4.12.
Mark the J point by placing arrow.
4. Measure the duration of QRS complex in the ECG of Fig.4.13. Mark all the
waves of the PQRST complex and the S-T segment.
Fig. 4.10 Measure the
duration of QRS complex
and P-R interval
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Lead II
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Lead V6
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4 ECG Intervals andSegment
5. Measure the Q-T interval in the ECG of Fig.4.14 and name one important condi-
tion where you will get this abnormality.
6. Measure the Q-T interval in the ECG of Fig.4.15 and name one important condi-
tion where you will get this abnormality.
Lead II
Fig. 4.11 Measure the P-P and R-R intervals
Fig. 4.12 Measure the
duration of QRS complex
and P-R interval and mark
the J point
4.9 Ventricular Activation Time
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Fig. 4.13 Measure the
duration of QRS complex
and mark all the waves
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Lead II
Fig. 4.14 Measure the Q-T interval
Lead II
Fig. 4.15 Measure the Q-T interval
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4 ECG Intervals andSegment
7. A 32-year-old gentleman had an electrocardiogram as part of a medical check- up.
Examine his 12-lead ECG given in Fig.4.16 and answer the following questions:
a. Calculate the P-R interval.
b. Calculate the QRS duration.
c. Is normal sinus rhythm present?
d. Calculate the Q-T interval.
8. A 28-year-old gentleman got his ECG done during renewal of medical insurance
policy. He has no chest pain, palpitation or dyspnoea. He does not smoke or drink
alcohol. He is not suffering from hypertension or diabetes. His lipid prole is normal. Examine his 12-lead ECG given in Fig.4.17 and answer the following questions:
a. Calculate the P-R interval.
b. Calculate the QRS duration.
c. Is normal sinus rhythm present?
d. What is the shape of the S-T segment?
e. Calculate the Q-T interval.
Fig. 4.16 Calculate the P-R, QRS and Q-T interval
Fig. 4.17 Calculate the
P-R, QRS and Q-T
interval. Identify the
rhythm
QRS
Lead II
4.9 Ventricular Activation Time
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Answers
1. False 2. True 3. True 4. True 5. False 6. d 7. a 8. d 9. c 10. d
Case Studies
1. The QRS duration (interval) is measured from the beginning of Q wave (R wave
in absence of Q wave) to end of S wave. Here, there are two small squares
between the beginning R wave (Q wave is absent) and end of s wave (Fig.4.18).
Hence, the QRS duration is 2×0.04s=0.08s.
The P-R interval is 9×0.04s=0.36s. This is prolonged P-R interval, also
known as rst degree heart block.
2. This is a rhythm strip. You must calculate all the P-P and R-R intervals (Fig.4.19).
The rst P-P interval is 17×0.04s=0.68s.
The second P-P interval is 17×0.04s=0.68s.
The third P-P interval is 22×0.04s=0.88s.
The fourth P-P interval is 22×0.04s=0.88s.
The fth P-P interval is 22×0.04s=0.88s.
The rst R-R interval is 17×0.04s=0.68s.
The second R-R interval is 17×0.04s=0.68s.
The third R-R interval is 22×0.04s=0.88s.
The fourth R-R interval is 22×0.04s=0.88s.
Fig. 4.18 Answer of Case
Study Question 1
(Fig.4.10)
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P-R
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