Sinus rhythms

Question 1

Wandering pacemaker

Answer 1

large sinus node in dogs (40mm) → exit pathway depend on autonomic tone

Question 2

Atrial depol

Answer 2

P wave
* Originates from SA node PM 
* From R → L, superior → inferior, anterior → posterior
* P wave: + in leads II, III, aVF
o Initial part: RA
o Mid portion: both atria
o Final portion: LA

Question 3

Atrial repol

Answer 3

Ta wave
* Ta wave: opposite polarity to P wave
o Smaller amplitude, 2.5x longer

Question 4

PQ seg

Answer 4

• Depol of nodal and intraventricular conduction system → not working myocardium

Question 5

Ventricular depol

Answer 5

• Start from L side of IVS: postero-inferior and antero-superior divisions of LBB
  o 1st vector: IVS depolarization
  o Q wave: positive in lead aVR (R wave)
  o Inferior → superior, L → R
• Then activates R side of IVS: posterior → anterior and inferior → superior (apex to base)
  o 2nd vector: RV depolarization
• 3rd vector: LV depolarization
  o Superior → inferior, anterior → posterior, R → L
  o R wave
• Endocardium → epicardium

Question 6

Ventricular repol

Answer 6

• Repol 2-3x longer vs depol: T wave longer than QRS
  o Epicardium → endocardium
  o Transmural dispersion 2nd to differences in AP duration (longer M cells en endocardium vs epi)
• T wave: +, -, diphasic
  o Ascending portion: sub-epicardial repol
  o Descending portion: steeper
• U wave: repol of M cells or Purkinje
  o Endocardial to epicardial gradient during IVRT

Question 7

Start of atrial depol

Answer 7

• SA node → propagate to entire atrium, // to the surface
  o R to L
  o Superior to inferior
  o Anterior to posterior

Question 8

Vectors of atrial depol

Answer 8

o Vector I: RA depol → sup to inf, post to ant, slightly to L
o Vector II: LA depol → ant to post, R to L, slightly sup to inf
o Mean vector: sup to inf, ant to post, R to L
 + in inferior leads (I, II, III aVF)
 - in superior leads (aVR, aVL)
* P wave

Question 9

Atriogram

Answer 9

 SA node activation not visible on surface ECG
 Initial portion = RA activation
 Final portion = LA activation

Question 10

Normal values P waves

Answer 10

 Amplitude dogs <0.4mV, cats <0.2mV
 Duration dogs/cats <40ms (<50ms for giant breeds), cats ms, Eq < 160ms
* Reprensent time for impulse to transmit from SA node → AV node
 Polarity should be constant: axis 0-75 degrees

Question 11

P wave variation healthy dogs

Answer 11

• Wandering PM
• Bifid P waves
• Higher amplitude with high ∑ tone

Question 12

P waves horses

Answer 12

o Can change w HR: bifid at ↓HR
o Successive P waves not always identical in normal Eq

Question 13

Vector of atrial repol

Answer 13

sup to inf, ant to post, R to L

Question 14

Appearance Ta wave

Answer 14

• Opposite polarity from P wave
  o Smaller amplitude
  o 2.5x duration

o Dome shape deflection following P wave w/o interposition of isoelectric line
 Usually hidden by QRS
 Can extend to J point

Question 15

Factors playing a role in visualization of Ta wave

Answer 15

o PQ interval duration
 2 or 3AVB can facilitate since not all P waves are followed by QRS
o Vagal tone
o Conduction properties of atrial myocardium

Question 16

Normal Ta waves value

Answer 16

 Ta/P amplitude ratio  0.35
 Ta/P duration ratio  2.99

Question 17

Wandering PM

Answer 17

• Depend on the exit pathway from sinus node
  o Superior exit pathway: inspiration, high ∑ tone
  o Inferior exit pathway: expiration, high vagal tone