Download Where are the P waves?

Diagnostic Tip:
Where are the P waves?
What do you do when you perform a lead II rhythm strip on a dog or cat and you don't see
any P waves? The answer depends on the clinical history, the heart rate and the regularity
of the R-R intervals.
Sometimes when evaluating a lead II rhythm strip ECG you don't see P waves. The
differential diagnosis for this situation:
•atrial standstill
•atrial fibrillation
•supraventricular tachycardia with P waves fused with the T waves
•sinus rhythm with small P waves lost in the baseline
•baseline artifact (eg, 60 cycle or muscle trembling) obscuring the P waves
Atrial fibrillation is characterized by a rapid irregularly irregular rhythm, generally with
normal appearing but possibly large QRS complexes. No p waves are visible, but the
baseline is usually not flat, because fibrillation or flutter waves can be seen in the baseline.
This dog's ECG shows a heart rate of 200 and atrial fibrillation with one ventricular
premature complex in the middle of the strip. Note the high heart rate and irregularity of
the rhythm.
Atrial standstill is characterized by a slow or normal heart rate, absence of P waves with
a flat baseline. The appearance of the QRS and T waves will vary with the cause of atrial
standstill. If atrial standstill is due to hyperkalemia, the T waves are often tall, spiked and
have a narrow base. As the potassium levels rise the QRS complexes become wider and
bizarre in appearance.
This ECG shows a rate of 120, irregular rhythm and no P waves. This is termed a
sinoventricular rhythm as it is still controlled by the sinus node and hence has the
irregularity of a sinus arrhythmia. P waves are absent because the atria cannot depolarize
due to hyperkalemia in this dog.
Supraventricular tachycardia with fusion of P and T waves is characterized by a rapid
rate, regular R-R interval and generally normal appearing QRS complexes. Performing a
vagal maneuver will sometimes slow the rate or break the tachycardia, permitting
visualization of the P waves.
This dog's ECG shows a heart rate of 300, regular R-R interval and no P waves. The
very high heart rate causes the P and T wave to fuse, so a distinct P wave is not
evident.
P waves are sometimes difficult to see in patients with a rapid ventricular tachycardia.
However, as the sinus node and atria are activated at a different rate than the
ventricles in animals with ventricular tachycardia, P waves can usually be identified
moving in and out of the QRS complexes.
In patients with a normal heart rate, normal appearing QRS complexes, and absence of
P waves on the rhythm strip, you have to consider the possibility of a sinus rhythm
with P waves being small and lost in the baseline. If you suspect that P waves may
be present, hat can you do to try to make this diagnosis? The following techniques will
often allow you to visualize P waves when they don't appear on the lead II rhythm
strip:
•Repeat an ECG obtaining all 6 limb leads (I, II, III, aVR, aVL, aVF). Sometimes
the P waves are isoelectric in lead II, but will be visible in one of the other limb
leads.
•If P waves aren't visible on any of the limb leads and the QRS complexes are
small, increase the sensitivity setting on the ECG machine to make everything
larger.
•If P waves are still not visible, run a chest lead. The chest lead typically makes
the complexes larger. To run a chest lead place the exploring lead (5th lead) on
the chest over the left apex of the heart. If your ECG machine does not have a
5th lead wire, move the lead from the left hind leg to the left chest wall and run
the ECG on lead II.
•If P waves are still not visible and the rate is slow, you can administer atropine
at 0.04 mg/kg (0.02 mg/lb) IM and repeat the ECG 20-30 minutes later.
Depending on the cause of the bradycardia the atropine will speed up the heart
rate and often makes the P waves larger.
Summary
When you can't see P waves, the first step is to eliminate baseline artifact and then
evaluate multiple leads and sensitivities to make sure that small P waves aren't lurking
in the baseline. Once you are convinced that P waves don't exist, you are probably
dealing with either atrial fibrillation (rapid irregularly irregular rhythm),
supraventricular tachycardia (rapid regular rhythm), or atrial standstill (slow regular or
irregular rhythm).