Cardiomegaly

Question 1

P wave amplitude

Answer 1

can vary according to autonomous stimulation and respiratory sinus arrhythmia
o ↑ amplitude and ↓PP with ∑ stim
 Shift in exit pathway in sinus node: more superior, closer to CrVC
o ↓ amplitude and ↑PP with p∑ stim

Question 2

RAE

Answer 2

• ↑ amplitude >40mV
• Tall, slender, peaked P waves → P pulmonale

Question 3

LAE

Answer 3

• ↑ duration >40ms (dogs), >35ms (cats) → P mitrale
• Bifid morphology, notched P wave
  o Intra-atrial conduction delays (firbrosis, anoexia, myocarditis) can cause bifid/bimodal P waves w/o LAE

Question 4

Ventricular enlargement

Answer 4

not possible to distinguish dilation and hypertrophy on ECG

Question 5

RVH

Answer 5

• Vectors of depolarization
  o Initial vector = IVS → unchanged
  o 2nd vector → ↑ amplitude
  o 3rd vector → inf to sup, L to R, post to ant
• Marked enlargement needed to cause changes on ECG (LV dominance)
• ECG characteristics
  o Normal QRS duration
  o Deep S waves in lead I, II, III, aVF
   >0.35mV in lead II
   R/S ratio
  o Q wave in aVR
  o MEA shift clockwise btw +100 to -80
• 80% of dogs will have R axis deviation + deep S wave
  o 40% will have deep S wave in lead II, aVF
  o Rarely deep Q waves in leads I, II, II, aVF

Question 6

Patterns of LVH

Answer 6

concentric, eccentric of mixed
o ECG voltage determined by muscle mass
o Some inaccuracy since also affected by distance of electrodes to heart
 Young, emaciated, narrow chested animals will have ↑ voltage
 Thoracic effusion, pneumothorax, obesity will have ↓ voltage

Question 7

LVH

Answer 7

• Vectors of depolarization
  o 1st and 2nd vectors are not affected
  o 3rd vector: ↑ magnitude, no change in direction
   Duration of depol can be increased, but cannot be distinguished from LBBB
• LBBB can have mid QRS notching/slurring
• Sudden ↑QRS duration and morphology change
• ECG characteristics
  o ↑R wave >3mV in lead II, aVF
   >0.5mV in lead I
  o Minimal ↑QRS duration >70ms
  o Depressed ST segment
   J point depression
   Downsloping of ST segment or upward bulge
  o T wave variation: >25% larger than R wave
   Inverted, asymmetric
  o Other changes:
   Notches
   ↓ slope of descending R wave
   Prolonged R-peak time
• 10% of dogs can display a moderate L axis deviation

Question 8

RBBB

Answer 8

• Latter part of QRS → represents RV depol
• Impulse travels from LBBB → R side of IVS below the block
  o Block can be in proximal (complete) or distal (incomplete) RBB
  o Delay in activation → wide and bizarre QRS
• Vectors of depolarization
  o 1st vector → normal (depol of IVS initiated by LBB)
  o RV activation vector directed post to ant, inf to sup, slightly to R
• ECG characteristics
  o ↑QRS duration >70ms
   Normal if incomplete block → S wave > R wave
  o R axis deviation: + in lead aVR, aVL, MEA can reach -110
  o Large/wide S waves in lead I, II, III, aVF
  o RR’ morphology in lead aVR

Question 9

Grades of RBBB

Answer 9

Grade I:
- Normal QRS duration
- S wave = 40-50% of QRS

Grade II
- Normal QRS duration
- S wave = >50% of QRS

Grade III
- ↑QRS duration btw 70-80ms
- S wave = >50% of QRS

Grade IV
- ↑QRS duration >80ms
- S wave = >50% of QRS

Question 10

LAFB

Answer 10

• Only slight prolongation of LV depol, main effect on direction
  o Shifted toward blocked fascicle and corresponding papillary muscle
• Vectors of depolarization
  o 1st vector → altered since LAF contributes to initial activation of IVS
   Oriented toward +80-90
  o 2nd vectors → normal
  o 3rd vector: 3a = depol of antero superior portion of LV
• Uncommon in dogs:
  o Vertical position of Ca heart → ↓ ECG changes
  o Changes consistent with LAFB must include lesions in anterior + septal branches → diffuse lesions
• ECG characteristics
  o Normal QRS duration  70ms
  o Marked L axis deviation: MEA -30 to -60
  o qR pattern in lead I and aVL
  o Deep S waves in leads II, III, aVF → rS pattern

Question 11

LBBB

Answer 11

• Block in LBB → RV activated first and late activation of LV
  o 1st vector: to the L with IVS activation from RBB
• Must be differentiated from LVE
• ECG characteristics
  o QRS duration >70ms
   + in leads I, II, III, aVF
   - in leads aVR, aVL
  o Small Q wave in lead I
   Early electrical activation from lateral wall of RV
  o Normal MEA: + in leads I, II, III, aVF and – in leads aVR, aVL

Question 12

RBBB + LAFB

Answer 12

o Anteroseptal infarct → RBB and LAF perfused by same coronary artery
o ECG characteristics
 ↑QRS duration >70ms
 Marked L axis deviation: oriented superiorly and to the L → -60 to -90
 Wide/deep S wave in lead I, II, III, aVF
 Rq pattern in leads I, aVL

Question 13

RBBB + LPFB

Answer 13

o ECG characteristics
 ↑QRS duration
 Prominent S waves in lead I and aVL
 R axis deviation