ECG
of the Month
Hide and Seek
Keith J. C. Finnie, MB, ChB, FRCPC
An 80-year-old woman complains of recurrent self-limiting episodes of palpitations. An ECG
(Figure 1) is obtained during a typical episode.
Figure 1. ECG obtained during a typical episode of palpitations.
1. What is the differential diagnosis for the arrhythmia shown?
©
t
n
h
o
i
t
g
i
u
r
b
i
y
str
i
D
Cop
l
a
i
rc
ad,
ownlo
d
n
e
a
c
ers
mm
o
ised us personal use
C
r
o
r
h
t
u
o
A
or correct diagnosis?
2. What measures
might
d. establishing
fthe
le sbee phelpful
ein
a
S
e copy
rohibit
l
r
g
n
i
o
s
f
u
Not uthorised and print a
Una
, view
display
Perspectives in Cardiology / April 2008 1
ECG
of the Month
This Month’s ECG Diagnosis
ECG shows a regular narrow QRS com1. The
plex tachycardia at 115 bpm. In lead V , each
1
QRS complex can be seen to be preceded by
P waves. Examination of lead II reveals that the
P wave morphology is not consistent with origin from the sinus node; the P waves are of very
low amplitude and have a slightly negative
polarity. This is not sinus tachycardia with a
long PR interval, but some other form of regular supraventricular tachycardia (SVT).
When dealing with SVT where the P waves
are visible, the relationship of the P waves to
the preceding and following QRS complexes
can be helpful. Short RP intervals (P wave closer to the preceding QRS than the following
one) are seen in the common form (slow antegrade pathway, fast retrograde pathway) of
atrioventricular node reentrant tachycardia
(AVNRT) and in atrioventricular reentrant
tachycardia (AVRT) utilizing retrograde conduction over an accessory pathway. In AVNRT
the retrograde P wave is usually closer to the
QRS complex than in AVRT and may be
obscured within the terminal portion of the
QRS complex. Long RP intervals (P wave closer to the following QRS than to the preceding
one) are suggestive of:
• an ectopic atrial tachycardia,
• the uncommon form of AVNRT (antegrade
conduction via a fast pathway and
retrograde conduction via a slow pathway)
and
• AVRT with slow retrograde conduction
through an accessory pathway.
In this patient, the P waves are approximately
mid-way between the QRS complexes, making
characterization by RP interval difficult.
hen dealing with SVT where the P waves are
visible, the relationship of the P waves to the
preceding and following QRS complexes can be
helpful.
W
Angiotensin II Receptor Blocker
Angiotensin II Receptor Blocker/Diuretic
Please consult product monographs for warnings and precautions.
Product monographs available upon request at 1-866-INFO BMS
(1-866-463-6267), Bristol-Myers Squibb Canada, 2365 Côte-de-Liesse,
Saint-Laurent, Quebec H4N 2M7.
An agreement between Bristol-Myers Squibb and sanofi-aventis for the codevelopment and
marketing of irbesartan and clopidogrel, two compounds from sanofi-aventis research.
CDN.IRB.06.02.11E
ECG
of the Month
2.
The diagnosis and treatment of regular SVT
can often be assisted by the response of the
arrhythmia to measures which slow AV node
conduction. Carotid sinus massage and
Valsalva strain can be performed at the bedside
and a response is likely to be diagnostic; the
absence of a response is not helpful.
Alternatively, IV adenosine will profoundly
and transiently slow AV node conduction and
may be given safely with ECG and BP monitoring. This patient received 6 mg of IV adenosine and the response is shown in Figure 2. AV
node conduction is blocked for a few seconds,
revealing twice as many P waves as had been
suspected initially. The rhythm is atrial tachycardia with 2:1 AV conduction, possibly due to
enhanced atrial automaticity or intra-atrial
reentry. Because it does not require the AV
node to sustain it, AV node blockade will not
terminate atrial tachycardia. And, because the
initial ECG only revealed every other P wave,
attempts to characterize this patient’s arrhythmia by RP relationship are rendered invalid and
likely to prove misleading. PCard
PCard
Figure 2. ECG showing patient’s response after IV adenosine.
Dr. Finnie is a Professor, Department of
Medicine, Schulich School of Medicine and
Dentistry, University of Western Ontario and a
Cardiologist, LHSC University Hospital, London,
Ontario.
Publication Mail Agreement No.: 40063348
Return undeliverable Canadian addresses to:
STA Communications Inc.
955 St. Jean Blvd.,
Suite 306
Pointe-Claire, QC,
H9R 5K3
Perspectives in Cardiology / April 2008 3