Download Diagnosis of Supraventricular Tachycardia

© SUPPLEMENT OF JAPI • april 2007 • VOL. 55
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Supplement
Diagnosis of Supraventricular Tachycardia
N Al-Rawahi, MS Green
T
his is a review of the recent knowledge regarding the
diagnosis of various forms of supra-ventricular tachycardia (SVT).
SVT are the most common tachycardia facing a physician with a wide
range of risk ranging from the most benign to the most malignant type
of tachyarrhythmia. A proper approach to the diagnosis of SVT will
affect mode of treatment, and ultimately the morbidity and even the
mortality of the patients.
Firstly, we must define 2 terms; SVT and narrow complex
tachycardia. All narrow complex tachycardia are SVT. SVT most of the
time are narrow complex but can be wide complex in two situations:
1) aberrant conduction due to functional or preexistent bundle branch
block or 2) preexcited tachycardia in the presence of anterograde
accessory pathway conduction.
Classifications
Many classifications have been used for SVT. One of the practical
classifications is to divide them according to the structures required to
initiate and maintain the tachycardia. By this classification there are two
types of SVT, those which require only atrial tissue for their initiation
and maintenance, and those that require the atrioventricular (AV)
junction (Table 1). This classification is more relevant and in keeping
with the electrophysiological assessment of the tachycardia.
SVT only requirIng Atrial tissue
for initiation and maintenance
Sinus tachycardia (ST): In this type of tachycardia the patient
will be having a heart rate more than 100 /min. The most common
causes of ST are in response to exercise and in conditions in which
catecholamine release is physiologically enhanced as in anger, stress,
flight, or fright. ST is not a paroxysmal condition but manifests a
gradual increase and decrease in rate.1 The ECG will show a P wave
as in normal SR. The PR interval will generally shorten as the rate
increases. Sometimes with very fast heart rates (more than 150/min)
the P wave may be difficult to see as it will be imposed in the T wave
of the previous beat. This may bring lead to confusion in diagnosis.
Manoeuvres such as carotid sinus massage or Valsalva may be helpful
in determining the P wave position.
Sinus nodal reentrant tachycardia (SNRT): This is an uncommon
arrhythmia. 2 It has an abrupt onset and termination which
distinguishes it clinically from ST.1 Episodes of SNRT may vary in
length. It can be sustained and even incessant.3 The ECG will show
a tachycardia (usually between 100 – 150 /min) with normal P
wave morphology. The key feature in differentiating this from sinus
tachycardia is the nonparoxysmal behaviour.
Focal Atrial tachycardia (FAT): This is a regular atrial rhythm at
a constant rate of >100 beats per min originating outside of the sinus
node.4 It may be sustained or terminate spontaneously.5 The ECG
will show P waves that have an abnormal morphology and/or axis.
The P wave morphology is not always reliable in localizing the atrial
focus, although a narrow “P” wave does suggest a septal origin. P
wave morphology depends on both on the arrhythmogenic site and
on intraatrial conduction6,7 (Fig. 1).
Multifocal atrial tachycardia (MAT): The average age of
presenting patients is approximately 70 years. These patients are
generally quite ill, with pulmonary, cardiac, and other serious
diseases. It has been estimated to occur in 0.05 to 0.32 percent of
electrocardiograms in general hospitals.8-12 MAT is an ECG diagnosis
and requires the following criteria:8,9
1.
Discrete P waves with at least three different morphologies.
2.
An atrial rate of over 100 beats/min.
3.
The P waves are separated by isoelectric intervals.
4.
The P-P intervals, the P-R duration, and the R-R intervals vary
(irregular).
MAT usually progress into other types of atrial tachycardia.
In one study 55% of patients progressed into atrial flutter or atrial
fibrillation.10
Atrial fibrillation (AF): AF occurs in the normal heart (lone
AF), and in the presence of organic heart disease of any cause. It is
classified into: paroxysmal, persistent and permanent according to
the duration of the tachycardia13 (Fig. 2). AF is associated with the
following ECG changes:
1.
Absent P waves.
2.
Fibrillatory or f waves are present at a rate that is usually between
350 and 600 beats/minute; the f waves vary in morphology,
amplitude, and intervals.
3.
R-R intervals are irregularly irregular.
4.
The heart rate may vary depending on the AV node.
Table 1 : Classification of SVT by structures required for
initiation and maintenance
Atrial tissue only
AV junction
Sinus tachycardia (ST)
Sinus nodal reentrant
tachycardia (SNRT)
Focal Atrial tachycardia (FAT)
Multifocal atrial tachycardia (MAT)
Atrial fibrillation (AF)
Atrial flutter (AFl)
AV nodal reentrant
tachycardia(AVNRT)
Atrioventricular reentrant
tachycardia (AVRT)
Division of Cardiology, University of Ottawa Heart Institute, 40
Ruskin, Ottawa, Ontario, K1Y4W7, Canada.
Fig.1: Atrial Tachycardia with 2:1 conduction. ECG of a 50 year old man
with palpitation. See abnormal P waves in lead II and V1 at a rate of
200/min with QRS rate of 100 /min.
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Fig. 2: Atrial Fibrillation. ECG of a 62 year woman. Clear irregular
irregular QRS complexes with no defined P wave.
Atrial flutter (AFl): AFl may be a stable rhythm or a bridge
arrhythmia between sinus rhythm and atrial fibrillation. It is an
arrhythmia most commonly seen in men, the elderly and individuals
with preexisting structural heart disease or chronic obstructive lung
disease.14 It is a macroreentrant type of tachycardia generally utilizing
right atrial tissue. The ventricular conduction rate is variable (2:1 or 4:1)
depending on the conduction in the AV node. There are two distinct
types of atrial flutter: Type one or typical also called counterclockwise,
and type two or atypical also called clockwise.4,15 Diagnostic ECG
features of both are listed in Table 2 (Figs. 3 and 4).
SVT requiring AV junction for
initiation and maintEnance
AV nodal reentrant tachycardia (AVNRT): This is a common
arrhythmia, accounting for approximately two thirds of cases of
paroxysmal SVT. It is paroxysmal and recurrent which can develop in
the normal heart or in the presence of organic heart disease. AVNRT
can present at any age. In a study of 253 patients, the mean age of
symptom onset in AVNRT was 32 years, with two thirds of cases
beginning after the age of 20 years .16 It is a microreentrant tachycardia
that utilizes the AV node and, usually, perinodal atrial tissue.17,18 It
Table 2 : ECG features type one and type two atrial flutter
Type one AFl
Type two AFl
Absent P waves
Biphasic “sawtooth” flutter
waves (F waves) range
between 240 to 340 beats/min
The F waves have an axis of
±90o and typically are prominently
negative in the inferior leads
(II, III, aVF) & V1
Absent P waves
Biphasic flutter waves (F
waves) ranging from 340
to 440 beats/min.
Positive F waves recorded
in the inferior leads
Fig. 3 : Atrial Flutter Type one. ECG of a 55 year old man. A clear
biphasic sawtooth F waves in leads II and aVF with 2:1 AV block.
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Fig. 4: Atrial Flutter Type two. ECG of 60 year old man. Note the positive
F waves in leads II, III and aVF. There is variable AV block.
requires dual conduction physiology in the AV node; a so-called fast
pathway and a slow pathway (Fig. 5). The ECG features of typical
AVNRT (slow-fast) are:
1.
Rate of 120 to 220 /minute.
2.
P wave buried in the QRS complex or occurring slightly before or
after (short RP) and usually in fusion with the QRS complex.
3.
Inverted P waves in leads I, II, III and aVF.
Atypical or fast-slow AVNRT has a long RP interval and may be
difficult to distinguish from orthodromic AVRT conducting down
the AV node and up a slowly conducting accessory pathway by the
surface ECG.19
Atrioventricular reentrant tachycardia (AVRT): It is less common
than AVNRT and the patients usually younger. The mean age of
symptom onset of 23 years and around 40% of patients had the initial
onset of symptoms after the age of 20.16 One main type of AVRT is
WPW syndrome. AVRT uses two distinct pathways, the normal AV
conduction system and an AV accessory pathway. Keeping this in
mind there will be two ways the conduction will travel. The first
most common variety occurs when the impulses travel down the
AV node to the ventricle and up the accessory pathway (AP). This is
called orthodromic AV reentry tachycardia. The second type occurs
when the AP is used in the anterograde direction to the ventricle and
retrograde conduction occurs up the AV node. This is called antidromic
AV reentry tachycardia. Both can be initiated by atrial or ventricular
Fig.5 : AVNRT. ECG of 27 year old woman with palpitations. Narrow
complex tachycardia at a rate of 200 / min. Note the P waves in the
terminal portion of the QRS complex (best seen in lead V1, magnified at
bottom).
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premature beats. The ECG features of both are:
1.
Orthodromic AVRT: The heart rate is regular and ranges between
150 to 220/min. The P wave is visible and inscribed within the
ST-T wave segment. The RP interval is shorter than PR interval
and constant. The QRS in narrow20 (Fig. 6).
2.
Antidromic AVRT: The heart rate is regular and ranges between
150 to 220/min. It is characterized by a pre-excited wide QRS
complex. The P wave may be more difficult to see because of
the wide QRS and ST and T wave. When the P wave is easily
visible the RP interval is constant and may be longer than the
PR interval, secondary to slow retrograde conduction over the
AV node.20 Rarely, in patients with more than one tachycardia the
circuit may use one AP in the anterograde direction and another
AP in the retrograde direction. This tachycardia may be virtually
impossible to distinguish from typical antidromic AVRT.
The most frightening tachycardia in patients with pre-excitation
is AF. Development of AF in combination with a short anterograde
refractory period of the AP may initiate ventricular tachycardia or
fibrillation (called atrioventricular fibrillation)21 (Fig. 7). The ECG
features include:
1.
2.
3.
Fig. 7 : Pre-excited Atrial fibrillation. ECG of a 25 year old man patient
known for WPW syndrome presented with palpitation and syncope.
Notice the irregularly irregular rhythm with bizarre nature of the QRS
complexes.
Irregularly irregular rhythm.
–
likely AV nodal independent
Rapid ventricular rate more than 180 to 200 /min, occasionally
with R-R intervals <200 msec.
–
rarely could be AVNRT;
–
absolutely rules out AVRT!
QRS complexes are wide and bizarre, and not resembling typical
bundle branch block aberrancy.
3.
Practical approach to SVT
In real life the physician will be faced by a patient with a
tachycardia. The above knowledge will be needed to figure out
what type the tachycardia the patient has. It is common that only
the response to treatment will give the physician the clue about the
diagnosis. With the above knowledge the following steps usually
help in diagnosing SVT:
1.
P’ wave position
–
 Usually
–
RP’ < P’R
 Usually AVRT

2. atrial tachycardia, but can be unusual variety
of AVRT or AVNRT
usually atrial fib or flutter with variable block
–
Sometimes multifocal atrial tachycardia
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Fig. 6 : Orthodromic AVRT: ECG of 23 year old man with WPW
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–
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