Page 108 - Small Animal Internal Medicine, 6th Edition

80 PART I Cardiovascular System Disorders

Supraventricular Tachyarrhythmias
BOX 4.3 Occasional premature beats do not require specific therapy.
VetBooks.ir Clinical Characterization of Common Heart Rate and Factors that predispose to these arrhythmias should be mini-
mized as possible (e.g., discontinue or reduce dosage of pre-
Rhythm Disturbances
disposing drugs, manage heart failure if present, and treat
Fast, Irregular Rhythms metabolic or endocrine abnormalities).
Atrial or supraventricular premature contractions Oral therapy for frequent supraventricular
Paroxysmal atrial or supraventricular tachycardia premature beats or paroxysmal tachycardia
Atrial flutter or fibrillation Initial oral therapy for frequent atrial premature complex-
Ventricular premature contractions es (APCs) or paroxysmal SVT can involve either diltiazem,
Paroxysmal ventricular tachycardia digoxin, a β-blocker, or a combination of these (Fig. 4.2). For
patients with CHF, if the arrhythmia is not sufficiently con-
Fast, Regular Rhythms trolled with diltiazem (or digoxin), along with other therapy
Sinus tachycardia indicated for heart failure, the combination of diltiazem with
Sustained supraventricular tachycardia digoxin (or a β-blocker with digoxin) may be effective. Cats
Sustained ventricular tachycardia with hypertrophic cardiomyopathy (HCM) or hyperthyroid-

Slow, Irregular Rhythms ism usually are treated with a β-blocker such as atenolol,
Sinus bradyarrhythmia although diltiazem could be an alternative. Refractory, fre-
Sinus arrest quent supraventricular tachyarrhythmias may respond to
Sick sinus syndrome amiodarone, sotalol, procainamide, or propafenone.
High-grade second-degree AV block Acute therapy for supraventricular tachycardia
More aggressive therapy is warranted for rapid, sustained
Slow, Regular Rhythms supraventricular tachyarrhythmias because of the hemody-
Sinus bradycardia namic impairment caused. A vagal maneuver can be tried ini-
Complete (third-degree) AV block with ventricular escape tially (see p. 82). IV access is secured, and fluid administered
rhythm to maintain blood pressure and enhance endogenous vagal
Atrial standstill with ventricular escape rhythm tone. However, caution is necessary. Patients with known
or suspected heart failure generally should not receive IV
AV, Atrioventricular.
fluid or should receive only a small volume administered
slowly. If a vagal maneuver does not terminate the arrhyth-
ingestion of stimulants or toxins (e.g., chocolate, caffeine), or mia, diltiazem IV (or oral loading) is a good first-choice
drugs (e.g., catecholamines, anticholinergics, theophylline, agent. However, it has a negative inotropic effect, so low
and related agents). The heart rate in dogs and cats with sinus doses and slow administration are important when used IV,
tachycardia usually is well under 300 beats/min, although especially in patents with suspected DCM. A slowly admin-
it could be higher in those with thyrotoxicosis or in those istered IV β-blocker (such as esmolol or propranolol) is an
(particularly cats) that have ingested exogenous stimulants alternative therapy but also has negative inotropic effects.
or drugs. Alleviation of the underlying cause and intrave- Occasional cases of reentrant SVT or automatic atrial tachy-
nous (IV) fluid administration to reverse hypotension (in cardia respond to IV lidocaine, which may be worth trying
animals without edema) should cause sympathetic tone and given this drug’s relative safety. Other strategies in refractory
sinus rate to decrease. cases include IV amiodarone, oral sotalol, or a class IA or
SVT of varying causes can be difficult to differentiate IC drug. Oral digoxin often is less effective than diltiazem
from sinus tachycardia. The heart rate with SVT may be but might be useful in some cases. Digoxin has a slower
greater than 300 beats/min, but it is uncommon for the sinus onset of action, and it increases vagal tone. IV digoxin is not
rate to be this rapid. Patients with SVTs usually have a recommended, as it also can increase central sympathetic
normal QRS configuration (narrow and upright in lead II). output. Adenosine has been ineffective for terminating SVTs
However, if an intraventricular conduction disturbance is in dogs. Further cardiac diagnostic tests are indicated once
present, SVT may resemble ventricular tachycardia. A vagal conversion is achieved or the ventricular rate has decreased
maneuver (see p. 82) can be useful in differentiating among to fewer than 200 beats/min. Options for longer-term oral
narrow QRS complex tachycardias. therapy to control recurrence include diltiazem, amioda-
Sustained, rapid arrhythmias lead to decreases in cardiac rone, a β-blocker, digoxin, or propafenone; combination
output, arterial blood pressure, and coronary perfusion. therapy may be necessary.
CHF eventually may result. Signs of poor cardiac output and Paroxysmal AV reciprocating tachycardia (AVRT) is a
hypotension include weakness, depression, pallor, prolonged reentrant tachycardia involving an accessory pathway and
capillary refill time, exercise intolerance, syncope, dyspnea, the AV node (see p. 44). It is interrupted by slowing conduc-
prerenal azotemia, worsening rhythm disturbances, and tion or prolonging the refractory period of either or both
sometimes altered mentation, seizure activity, and sudden tissues. A vagal maneuver may slow AV conduction enough
death. to terminate the rhythm. Diltiazem and β-blockers slow AV