Cardio. SVT. Chap 47 Flashcards
Define SVT:
two types:
rapid cardiac rhythms originate in the atria or AV junction (above the bundle of His) or (2) involve the atria or AV junction as a critical component of a tachyarrhythmia circuit
atrial tachyarrhythmias or
AV node–dependent tachyarrhythmias
Tx why?
name:
abnormal rhythms result from structural heart disease & can cause of structural heart disease
tachycardia-induced cardiomyopathy
any young to middle-aged dog presenting with a clinical picture of DCM should have tachycardiomyopathy on ddx
C/S
poor CO
pulsing of the ears or bobbing of the head with HR
may be dx initially w. primary GI
Dx:
first ddx SVT vs VT:
suggested criteria:
SVTs w BBB that develops during SVT or antegrade conduction of a tachycardia over an accessory pathway
(rarely conduct antegrade that fast) is difficult to ddx:
1. p-wave SVT
2. fusion beat VT
3. vagal SVT
4. lidocaine trial VT
ddx SVT is atrial or AV node–dependent:
SVT is irregularly irregular = a-fib
ddx of regular SVTs involves several steps:
- initiation and termination of the SVT are important -
- if an SVT continues despite AV block, it is atrial in origin
- if VPC terminates the SVT, likely AV node–dependent
- if vagal maneuver term. SVT, likely AV node–dependent
Most common mechanisms of SVT:
- A-fib
- intraatrial reentrant tachycardia
- orthodromic AV reciprocating
(a macroreentrant circuit in which an impulse is carried
from the atria to the AV node–His-Purkinje system to the ventricles to a retrograde-conducting accessory pathway to the atria) - automatic atrial tachycardia
Treatment:
ID underlying cause if there is one:
Dual treatments:
- Acid-base, electrolyte, ischemia/anemia, hypoxemia
dual therapy:
one drug to slow AV nodal conduction
second drug to inhibit the atrial automatic focus or interrupt conduction
Tx:
Vagal:
may terminate the SVT if it is AV node dependent most effective vagal maneuver is carotid sinus massage
-sustained, gentle compression is applied for 5 to 10s carotid sinus, caudal to the dorsal aspect of the larynx
Tx pharm 4:
diltiazem, esmolol, adenosine, procainamide
direct current cardioversion and overdrive pacing
pre-cordial thump
Tx first line:
Diltiazem 0.125mg/kg
normal dogs demonstrated the superior efficacy of intravenous diltiazem in slowing AV nodal conduction while maintaining a favorable hemodynamic profile
Esmolol was a significantly less effective negative dromotrope and caused a severe drop in LV contractility
Adenosine, even at dosages of 2 mg/kg, was
ineffective in slowing canine AV nodal conduction
MoA B1 stimulation:
>HR because of stimulation of which channels:
Gs coupling of β1 receptors with adenyl cyclase
- > cyclic AMP production
= increased HR to stimulation
1. funny current (If)
2. L-type calcium current
3. enhanced myocardial contractility through L-type calcium current influx stimulating increased sarcoplasmic reticular calcium release
3. improved myocardial relaxation through phosphorylation of phospholamban
4. enhanced automaticity of subsidiary pacemakers
procainamide MoA:
sodium and potassium channel blocker
decreases abnormal automaticity
slows conduction
prolongs the effective refractory in atrial (and ventricular), accessory pathway, and retrograde fast AV nodal tissue.
DC - direct current cardioversion
failure to terminate
overdrive pacing can be performed without general anesthesia if the patient is depressed or moribund
jugular furrow can be locally anesthetized with lidocaine, a catheter introducer placed in the external jugular vein, and a multipolar catheter guided fluoroscopically
failure to terminate or rapid resumption of the tachyarrhythmia can indicate either an SVT caused by an automatic mechanism or successful termination but then rapid reinitiation
Long term tx:
Atrial tachyarrh. vs.
AV:
Atrial tachyarrhythmias typically are managed by dual antiarrhythmic therapy, one drug to slow AV nodal conduction and a second to terminate the atrial tachyarrhythmia itself
occasionally will respond to single- agent therapy aimed at slowing AV nodal conduction. In reality, however, these tachyarrhythmias most often require combination therapy as well.
Long term drugs:
Diltiezam or atenolol
Catheter abalation = only way to “cure” but first must map
radiofrequency energy is delivered to the tip electrode, causing thermal dessication of a small volume of tissue to permanently interrupt the tachycardia circuit.
