Diagnostic Tip: Where are the P waves?

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• Diagnostic Tip Where are the P waves?
• What do you do when you perform a lead II rhythm
  strip 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.
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• 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.
  
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• This 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 , 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).