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Recognizing Arrythmias

This is where you will be of most help to you patient and the

staff at your clinic. Think how much more interesting your job
will be if you watch a monitor during a surgery; you can be
responsible for alerting the doctor if abnormal beats occur.

To recognize arrythmias you need to know two things:

1. The site of origin of the abnormal beat.
2. Recognize deviations from the normal rate of automaticity
for that site.

Site of Origin

Three different sites can be identified on lead II by the

following features:

Atrial origin - These beats originate from
nowhere in the atria other than the SA mode.
They look just like a normally conducted
beat except that their timing is very early.
A big hint is that the P wave of the atrial
beat touches the T wave of the beat before it.

Junctional origin - These beats
originate near the AV node and
have a negative deflection P wave,
or no P wave, with a normally
conducted, short duration QRS
complex.

Ventricular origin - These beats
originate somewhere in the
ventricles. No P waves are
evident. QRS complexes are

wide and bizarre appearing

and may be positive or
negative polarity.

Intrinsic Rates of Automaticity

Atrial, junctional and ventricular sites each have a normal rate

of automaticity (the ability to initiate impulses), buy may
respond in the following abnormal ways:
Too fast (tachycardia)

Too irritable (premature)

Too slow (bradycardia)

Absent (block)

Normal pacemaker rates in the dog:

A great rule of thumb: Whichever site is fastest will drive the
heart.

A Case Example

Here’s an example to get you thinking along these lines. a 7-

year-old male boxer lethargically walks into the exam room
with a history of exercise intolerance. You run a lead II rhythm
strip that looks like the following:

How are we going to analyze it? Scanning from left to right

you see a P wave, PR interval, R wave (it’s OK for a QRS com-
plex not to have a Q an R and an S) and a negative T wave. In
other words, a normally conducted heat, which is followed by
two more normally conducted heats. The fourth beat has a
large, wide QRS complex with a big negative T wave. What
site do you think it originates from? It can’t be atrial because
there is no P wave. It can’t be junctional (near the AV node)
because it’s too wide and sloppy looking and junctional beats
should be of normal duration. But it definitely fits the criteria
for a ventricular origin beat! To further describe it we also use
the term “premature” because it comes before the next expected
normal beat. Next, are four more normally conducted beats fol-
lowed by another VPC (ventricular premature contraction).
Wouldn’t it be nice if they all came labeled! Can you spot the
next VPC on the strip?

This strip is recorded from the same dog one hour later. As

you can see, there are five normally conducted beats followed
by a long run of ventricular origin beats.

We have already decided the site of origin. the next step is to

decide if this an appropriate rate for that site. Here’s a chance to
practice the technique for calculating heart rate. Counting
between the markers (3 seconds at 25mm/second) we have 15
beats. If we multiply by 20 we will have the number of beats in
60 seconds. therefore, 20 x 15 = 300 beats per minute. That’s
way above the normal healthy ventricular conduction tissue rate
of 20-40 beats per minute indicated on the illustration of nor-
mal pacemaker rates in the dog. Now we want to apply the term
“tachycardia” to imply that the rate is too fast for that particu-
lar site. Ventricular premature contractions and ventricular
tachycardia are a significant finding in boxers. This breed is
predisposed to a form of dilated cardiomyopathy which has
life-threatening arrythmias.

A Second Case Example

A middle aged female miniature schnauzer faints while walk-

ing into the exam room, then gets up as if nothing happened.
The ECG you run looks like this:

Clearly, that long flat line is not normal. But how do we decide

what site in the heart is creating the problem? As was men-
tioned earlier, in the normal sequence of electrical activation in