Arrhythmia refers to irregular heartbeat or disturbance in the rhythm of heart. In arrhythmia, heartbeat may be fast or slow or there may be an extra beat or a missed beat. It occurs in physiological and pathological conditions.
CLASSIFICATION
In arrhythmia, SA node may
or may not be the pacemaker. If SA node is not the pacemaker, any other
part of the heart such as
atrial muscle, AV node and ventricular muscle becomes the pacemaker.
Accordingly,
arrhythmia is classified into two types:
A. Normotopic arrhythmia
B. Ectopic arrhythmia.
NORMOTOPIC ARRHYTHMIA
Normotopic arrhythmia is
the irregular heartbeat, in which SA node is the pacemaker.
Normotopic
arrhythmia is of three types:
1. Sinus arrhythmia
2. Sinus tachycardia
3. Sinus bradycardia.
SINUS ARRHYTHMIA
Sinus arrhythmia is a
normal rhythmical increase and decrease in heart rate, in relation to
respiration.
It is also called respiratory
sinus arrhythmia (RSA). Normal sinus rhythm means the normal heartbeat with SA node
as the pacemaker. Normal heart rate is 72 per minute. However, under physiological
conditions, in a normal healthy person, heart rate varies according to the
phases of respiratory
cycle. Heart rate increases
during inspiration and decreases during expiration.
ECG Changes
ECG is normal during sinus
arrhythmia. Only the duration of R-R interval varies rhythmically according to
phases of
respiration . It is shortened during inspiration and
prolonged during expiration.
Cause
Sinus
arrhythmia is due to fluctuation in the discharge of impulses
from SA node . During inspiration,
the lungs
are inflated and the intrathoracic pressure
decreases.
This increases the venous return. Inflation
of lungs stimulates the
stretch receptors of lungs, which
send impulses to
vasodilator area (cardioinhibitory
center)
through afferent fibers of vagus. It leads to
reflex
inhibition of vasodilator area and reduction in
vagal tone. Because of
these two factors, heart rate
increases. Simultaneously,
increased venous return
initiates Bainbridge reflex that causes increase in heart
rate (Chapter 101).
During
expiration, the lungs are deflated and
intrathoracic pressure
increases. This decreases the
venous
return. During deflation of lungs, the stretch
receptors are not
stimulated and vasodilator area is not
inhibited. So, vagal tone
increases, resulting in decreased
heart rate.
Simultaneously, decreased venous return
abolishes
Bainbridge reflex. It also decreases the heart
rate.
SINUS TACHYCARDIA
Sinus tachycardia is the
increase in discharge of
impulses from SA node,
resulting in increase in heart
rate. Discharge of
impulses from SA node is very rapid
and the heart rate
increases up to 100/minute and
sometimes up to
150/minute.
ECG
Changes
ECG is normal in sinus
tachycardia, except for short R-R
intervals because of increased heart rate
Conditions
when Sinus Tachycardia Occurs
Sinus tachycardia occurs
in physiological as well as
pathological conditions.
Physiological
conditions when tachycardia occurs
1. Exercise
2. Emotion
3. High altitude
4. Pregnancy.
Pathological
conditions when tachycardia occurs
1. Fever
2. Anemia
3. Hyperthyroidism
4. Hypersecretion of
catecholamines
5. Cardiomyopathy
6. Valvular heart disease
7. Hemorrhagic shock.
Features
of Sinus Tachycardia
1. Palpitations (sensation
of feeling the heartbeat)
2. Dizziness
3. Fainting
4. Shortness of breath
5. Chest discomfort
(angina).
SINUS BRADYCARDIA
Sinus bradycardia is the
reduction in discharge of
impulses from SA node
resulting in decrease in heart
rate. Heart rate is less
than 60/minute.
ECG
Changes
ECG shows prolonged waves
and prolonged R-R
interval (Fig. 96.4).
Conditions
when Sinus Bradycardia Occurs
Sinus bradycardia occurs
in both physiological and
pathological conditions.
It occurs during sleep. It is
common in
athletes due to the cardiovascular reflexes,
in response to increased
force of contraction of heart.
Physiological
conditions when sinus
bradycardia
occurs
1. Sleep
2. Athletic heart.
Pathological
conditions when sinus
bradycardia
occurs
1. Disease of SA node
2. Hypothermia
3. Hypothyroidism
4. Heart attack
5. Congenital heart
disease
6. Degenerative process of
aging
7. Obstructive jaundice
8. Increased intracranial
pressure
9. Use of certain drugs
like beta blockers, channel
blockers, digitalis and
other antiarrhythmic drugs
10. Atherosclerosis.
Bradycardia due to atherosclerosis
of carotid artery, at the
region of carotid sinus is
called carotid
sinus syndrome.
Features
of Sinus Bradycardia
1. Sick sinus syndrome
2. Fatigue
3. Weakness
4. Shortness of breath
5. Lack of concentration
6.
Difficulty in exercising.
Sick
sinus syndrome
Sick sinus syndrome is the
common feature of sinus
bradycardia. It is the
condition characterized by dizziness
and unconsciousness.
ECTOPIC ARRHYTHMIA
Ectopic arrhythmia is the
abnormal heartbeat, in which
one of the structures of
heart other than SA node
becomes the pacemaker.
Impulses produced by these
structures are called ectopic foci.
Subtypes
of Ectopic Arrhythmia
Ectopic
arrhythmia is further divided into two subtypes:
1. Homotopic arrhythmia,
in which the impulses for
heartbeat arise from any
part of conductive system
2. Heterotopic arrhythmia,
in which the impulses arise
from the musculature of
heart other than conductive
system.
Different
Ectopic Arrhythmia
1. Heart block
2. Extrasystole
3. Paroxysmal tachycardia
4. Atrial
flutter
5. Atrial
fibrillation
6. Ventricular fibrillation.
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