CARDIAC SESSION
QRS: it's NOT complex !
ECG Strips Analysis
by Dr. Dan Ohad
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CARDIAC SESSION QRS: it's NOT complex ! ECG Strips Analysis by Dr. Dan Ohad |
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The following 3 strips are recorded from the same 11-year-old, male castrated Cocker spaniel dog, in 3 different follow-up visits. Only during the second visit the dog was symptomatic. All 3 strips were recorded at 25mm/sec with a sensitivity of 1cm=1mV. What is your diagnosis for each strip? To make the most out of this case, analyze each strip using the provided hints (given with each strip individually), then, compare your own diagnoses to those offered below. Clues:
focus on the rate, rhythm, temporal
relationships between atrial and ventricular activity, and morphology
of abnormal QRS complexes. Please contact us for any questions or comments regarding this section
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About the
Author
Dr. Dan Ohad DVM - Koret School, Hebrew University. PhD - Hebrew University in applied cardiac electrophysiology Diplomate ACVIM Specialty of Cardiology
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Strip 1 : This tracing was recorded as a random screening "geriatric" test. It shows a ventricular rate of 55/min, and an atrial rate of 110/min.
hint for strip 1 Solution for strip 1
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Strip 2: Before the next visit the dog was lethargic and apathetic, and the recorded tracing showed a ventricular rate of slightly above 30/min, with an atrial rate of 210/min.
hint for strip 2 Solution for strip 2
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Strip 3: On a 3rd visit, the strip demonstrated a ventricular rate of 53/min and an atrial rate of 170/min.
hint for strip 3 Solution for strip 3
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Back to 1st strip Solution for strip 1
This strip demonstrates a 2:1 ratio between atrial and ventricular activity (only one out of every two P-waves is followed by a QRS-T complex).
Back to 2nd strip Solution for strip 2
Two QRS complexes on this
tracing are normal in morphology and are similar to those seen in the previous 2
visits. They can, therefore, serve as a "gold standard" to
compare other morphologies to. These "gold standard" complexes are
"sinus beats," conducted from the SA-node through fibers of a
specialized, fast conducting system to the ventricular myocardium. This is why
they are so narrow and short in duration.
There
are 2 wide QRS complexes that have slurred ST-segments and large, "discordant"
(opposite in polarity) T-waves (second and fourth complexes from the left).
Although they morphologically resemble VPCs, note that they are NOT premature!
In fact, they follow not only long and inconsistent PR-intervals, but also
follow relatively long PAUSES after their preceding QRS complexes. This makes
them, by definition, ventricular "ESCAPE
complexes" (VEC) and they represent a physiological,
compensatory mechanism that saves the dog from developing a bradycardia like the
one seen in the previous tracing. If we mistakenly diagnose these 2 beats as
VPCs and treat them as such, we will suppress this salvage mechanism and will
put the dog at risk. The only reason these escape complexes look like VPCs is
the fact that like VPCs, they originate from a ventricular focus and therefore
represent a depolarization front that propagates slowly, on a cell-to-cell basis
(rather than through the fast conducting, specialized conduction system fibers).
The
first QRS complex on the left happens to be a "FUSION" complex:
note that it shares morphological features with both the "gold
standard" sinus beats recorded elsewhere from this dog, as well as with the
ventricular escape complexes. The 2 morphologies "fuse" as a result of
two competing foci "firing" simultaneously from both the SA-node and
the ventricular escape focus, generating 2 "fusing" depolarization
fronts. The final morphology of a fusion complex depends on which of the 2
competing fronts captures most of the ventricular myocardial tissue, which, in
turn, depends on the temporal relationship between the 2 "firing"
foci. Therefore, different looking fusion complexes can result from the
interplay between the same two (supra-ventricular and ventricular) foci that
activate the ventricles at the same time.
Back to 3rd strip Solution for strip 3
Solution for Strip 1:
Note
that when P-waves are conducted, not only do they have a normal PR-interval, but
this interval is also very constant/consistent (130 ms) between different
conducted beats. This strip is an example of a "Type II", "low
grade", second degree Atrio-Ventricular (AV) Block. At this
stage the dog was not treated because it was hemodynamically normal and
asymptomatic.
Solution for Strip 2:
Although in this case the atrial and ventricular rates (210/min and 30/min, respectively) are not a precise multiplication of each other, this is still a second degree AV block, because PR-intervals are constant in duration (80ms), whenever P-waves are conducted. This strip is considered a "high grade" block since it has an A-to-V ratio of 6-7:1.
Note how the Sino-Atrial (SA) node is "trying" to compensate for the low cardiac output (due to the slow ventricular rate), by triggering rapid P-waves, most of which are not conducted through the AV-node.
At
this point, following a positive IV Atropine-test, the dog was treated with an
oral parasympatholytic drug (Propantheline bromide), and was scheduled for a
follow up visit later the same week.
Solution for Strip 3:
The
ECG diagnosis for this strip is, again, a high-grade 2nd degree
AV-block with an active physiological ventricular escape mechanism and
fusion complexes. At this point propantheline therapy was discontinued because
it was no longer needed, as the escape mechanism preserved a rapid enough
ventricular rate (and therefore a high enough cardiac output) to maintain normal
hemodynamics, at least at rest.