PAROXYSMAL TACHYCARDIA
Paroxysmal tachycardia is the sudden attack of increased heart rate due to ectopic foci arising from atria, AV node or ventricle. It is also called Bouveret- Hoffmann syndrome.
Increase in heart rate due to ectopic foci arising from either atria or AV node is called supraventricular tachycardia (SVT). It differs from ventricular tachycardia, which does not depend upon atria or AV node. The attack lasts for a period of few seconds to few hours. It also stops suddenly. After the attack, heart functions normally. Symptoms include palpitations, chest pain, rapid breathing and dizziness.
Paroxysmal
tachycardia is of three types:
1. Atrial paroxysmal
tachycardia
2. AV nodal paroxysmal
tachycardia
3. Ventricular paroxysmal
tachycardia.
1. Atrial
Paroxysmal Tachycardia
Atrial paroxysmal tachycardia is the sudden increase in heart rate caused by ectopic impulses
discharged from atrial musculature. Heart rate is 150 to 220/minute. P wave in
ECG is inverted, with normal QRST.
2. AV Nodal
Paroxysmal Tachycardia – Bundle of Kent
AV nodal paroxysmal tachycardia
is the sudden increase in heart rate caused by ectopic foci arising from AV
node due to a temporary block in the conductive system. It also involves circus
movement. This type of tachycardia is very common in some healthy persons who
have got an additional conductive system. This system is formed by some
abnormal junctional tissues constituting a structure called bundle of Kent.
Bundle of Kent connects the atria and ventricles directly, so the conduction is
very rapid than through the regular conductive system.
Circus
movement – Re-entry and atrial echo beat
Circus
movement is defined as circuitous propagation of impulses around a
structural or functional obstruction, resulting
in re-entry of the impulse and re-excitation of heart. When there is a sudden and temporary block in normal conductive system, the impulses from SA node reach the ventricle through bundle of Kent. By this time, the blockage in normal conductive system disappears. Now, the impulse, which passes through bundle of Kent, after exciting the ventricular muscle, travels in the opposite direction through the normal conductive system and finally, it re-enters the AV node.
Re-entered impulse activates the AV node and depolarizes
the atria, resulting in atrial contraction. It is called atrial
echo beat.
Re-entered
nodal impulse simultaneously spreads to ventricle through normal conductive
system,
completing the circus
movement. This circus movement is repeated producing tachycardia called AV
nodal paroxysmal tachycardia. ECG shows normal QRST complex. But P wave is
mostly absent.
Wolff-Parkinson-White
syndrome
Wolff-Parkinson-White
syndrome is the condition characterized by repeated attacks of AV nodal
paroxysmal tachycardia in
persons with bundle of Kent. ECG shows short P-R interval with normal QRS
complex and T wave.
Lown-Ganong-Levin
syndrome
Lown-Ganong-Levin syndrome
is another condition characterized by AV nodal paroxysmal tachycardia.
This occurs in persons who
have got another type of abnormal conductive fibers like
bundle of Kent. These fibers also connect atria and distal
part of conductive system directly bypassing the AV node. So the impulse from SA
node reaches ventricle through the abnormal conductive
fibers. After exciting the ventricular muscle, the impulse travels in opposite
direction through normal conductive system and finally, it re-enters
the AV node. The re-entered impulse activates the AV node causing atrial
contraction. ECG shows short P-R interval with normal QRS complex and T wave.
3. Ventricular
Paroxysmal Tachycardia
Ventricular paroxysmal
tachycardia is the sudden increase in heart rate caused by ectopic foci arising
from ventricular musculature.
Sometimes, a part of ventricular muscle, particularly an ischemic area is
excited abnormally, followed
by a series of extrasystole. This condition is dangerous as the circus movement
is developed within ventricular muscle. This circus movement leads to ventricular fibrillation, which is fatal.
ATRIAL FLUTTER
Atrial
flutter is an arrhythmia characterized by rapid ineffective atrial
contractions, caused by ectopic foci
originating from atrial
musculature. It is often associated with atrial paroxysmal tachycardia. Both
the atria beat rapidly like the wings of a bird, hence the name atrial flutter. Atrial rate is about 250 to 350/minute. Maximum number of impulses
conducted by AV node is about 230 to 240 /minute. So,
during atrial flutter, the second degree of heart block occurs.
The ratio between atrial beats and ventricular beats is 2 : 1
or sometimes 3 : 1. Atrial flutter is common in patients suffering
from cardiovascular diseases such as hypertension and coronary artery
disease. Initially, it is marked by palpitations that are unnoticed. However,
prolonged atrial flutter may lead to atrial fibrillation or heart
failure.
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