Arrhythmias

Question 1

What is a sinus arrhythmia

Answer 1

• Cyclic variation in HR synchronous w phases of respiration => vagal tone fluctuation
  o Non respiratory sinus arrhythmia possible when not associated to phases of respiration
  o Normal in dogs => strongly dominant because of influences of psymp system
   Rare in cats because of predominant symp system during examination (observed at home)
  o decr to disappear at faster HR

Question 2

Physiologic mechanism of sinus respi arrhythmia

Answer 2

• Sinus d/c incr w inspiration, decr w expiration
  o Activation of cardiovascular centers of medulla oblongata
   Inspiration decr vagal tone => incr HR
   Pronounced in brachycephalic breeds where airway obstruction cause incr force inspiration
  o Degree of lung inflation: Hering Breur Inflation reflex
   Stretch R in walls of bronchi/bronchioles → vagus nerve → dorsal respiratory center → ↓ inspiratory ramp → inhibition of vagal center
  o Cardiac acceleration reflex due to RA distension during inspiration
   Inspiration → thoracic cavity expansion + diaphragm descent → ↓ intrapleural pressure → lung, RA, RV and vena cava expansion → ↓ venous P and RAP
• Transmural pressure (RAP – intrapleural P) ↑ since RAP falls less than intrapleural P
• ↑ venous return and preload to R heart
   Expiration → ↓ LV filling → ↑ HR to maintain CO
• Pooling of blood into pulmonary circulation (↓PVR since pulmonary vessels in thoracic cavity)
• Shift of IVS to the L from ↑ RV filling
  o ChemoR and baroR reflexes
   incr BP after phase of cardiac acceleration
   Carotid massage incr vagal tone

Question 3

Distinguish sinus arrhythmia vs other bradyarrhythmia

Answer 3

• Responsive to atropine, which will suppress vagal tone

Question 4

What is wandering PM

Answer 4

Frequently observed w sinus arrhythmias
* Variation in P wave morphology/amplitude

Question 5

Physiology of wandering PM

Answer 5

changes in
o Site of impulse origin w/i SA node
o Exit pathways:
 incr psymp => inferior-posterior portion initially activated => impulse exit from inferior pathway -> along posterior and middle internodal tract => decr P amplitude
 incr symp => anterior-superior portion of SA node => impulse exit from superior pathway => anterior internodal tract => incr P amplitude

• Sinus node-AV junction wandering PM: more extensive migration from SA node to AV jct
  o P wave vary in polarity from + to – in lead II

Question 6

What is ventriculo phasic sinus arrhythmia

Answer 6

• Variation of P-P interval induced by ventricular systole
  o Ejection phase => stimulate aortic + carotid baroR => vagal stimulation
  o Start 600ms after QRS, last for 1s
• Occur w 2nd and 3rd ˚ AVB

Question 7

Methods to calculate HR variability

Answer 7

• Geometric:
  o 2D-3D tachograms of successive R-R intervals, HR tachogram, Pointcare plots of R-R intervals vs R-R+1 intervals, R-R interval histogram
  o Dogs tend to have double banding => R-R intervals w large reduction of # of R-R btwn bands
   Suggest exit block from SA node
• Time domain
  o Represent variability of R-R intervals
  o Information about psymp tone
• Frequency domain analysis: most complex to analyze

Question 8

Eq bradycardia

Answer 8

physiological => associated to incr psymp
o 2nd ˚ AVB is the most common bradyarrhythmia, most common vagally mediated arrhythmia
* Tachycardias are mostly pathological