Page 231 - Feline Cardiology

236 Section F: Arrhythmias and Other Electrocardiographic Abnormalities

1999). Newer work supports the cardiovascular depres- derangements that predispose to cardiac arrhythmias.
sant effects of lidocaine in the cat: administration of Magnesium sulfate can be administered at a dosage of
lidocaine in anesthetized cats produces a significantly 30 mg/kg slow IV, with monitoring for mental depres-
lower heart rate, cardiac index, and stroke index com- sion or dullness, weakness, bradycardia, hypotension, or
pared to administration of equipotent amounts of hypocalcemia as signs of excess warranting termination
inhaled isoflurane (Pypendop and Ilkiw 2005; Pypendop of administration. Magnesium sulfate is a logical choice
et al. 2006). Therefore, cats with VT and concurrent in a cat with VT requiring antiarrhythmic treatment
systolic dysfunction (e.g., dilated cardiomyopathy, ter- when hypomagnesemia has been documented or is sus-
minal hypertrophic cardiomyopathy, possibly conges- pected, such as when electrolyte-wasting drugs [e.g.,
tive heart failure of most/all causes) should probably not furosemide] or disorders are present, or hypokalemia
be treated with lidocaine as a first choice antiarrhythmic. exists that is refractory to potassium supplementation.
For cats in whom systolic dysfunction is less of a concern Oral antiarrhythmics of choice for PVCs/VT include
(i.e., the majority of feline cases), lidocaine is dosed at sotalol and atenolol. Both are beta blockers (class II anti-
0.25–1 mg/kg IV as a bolus, which can be repeated twice arrhythmics) but sotalol (2 mg/kg PO q 12h; nonselec-
over a period of 15 minutes. Lethargy, mental depres- tive beta blocker) also has class III antiarrhythmic
sion, ataxia, collapse, or new-onset ECG abnormalities properties that atenolol (1–1.5 mg/kg PO q 12h; beta-1
Arrhythmias warrant immediate cessation of drug administration. tolerated by cats but should not be started during acute
selective blocker) does not. They are generally well-
(bundle branch block, AV block, sinus bradycardia)
fulminant congestive heart failure. No proof exists for
Seizures can respond to diazepam 0.5 mg/kg IV bolus
(Muir et al. 1999). If ongoing treatment is necessary
in cats, and broadly, atenolol might be chosen first, with
(and the drug is tolerated), an intravenous constant-rate the long-term benefit of these drugs as antiarrhythmics
infusion may be administered at 10–15 mcg/kg/min. a change to sotalol if a desired clinical response is not
An alternative to lidocaine for intravenous treatment achieved (Ferasin et al. 2002). Like most beta blockers,
of VT in cats is a beta blocker. The basis is both the they should be avoided in dilated cardiomyopathy and
narrow safety margin in cats for some first-line antiar- similar states of systolic dysfunction (e.g., end-stage
rhythmic drugs used commonly in other species (notably mitral regurgitation or VSD) since the mild decrease in
lidocaine) and the observation that beta-receptor stimu- systolic function conferred by these drugs may be poorly
lation via epinephrine can trigger ventricular arrhyth- tolerated by such patients, especially when medications
mias even in cats with normal hearts (Hikasa et al. 1996), are administered as fractions of tablets resulting in vari-
and likely more so in hypertrophied hearts (Rials et al. able doses (Margiocco et al. 2009).
1995). Esmolol and propranolol are routinely available. Amiodarone is a broad-spectrum antiarrhythmic,
Both bind beta-1 adrenergic receptors, and therefore with properties extending into all 4 Vaughn-Williams
their antiarrhythmic effect is not specific to the ventri- classes but mainly in class III (prolongation of repolar-
cles but is expected to be relevant when catecholamine ization as a means of reducing myocardial electrical
stimulation is contributing to, or triggering, VT. Beta heterogeneity/disorganization). It remains one of the
blockers must be administered in small, graded doses, most widely used antiarrhythmics in human cardiology.
with repeat dosing based on noting efficacy. A typical In cats, a dosage of 5 mg/kg has been shown in vitro to
protocol for propranolol in an average-sized cat with raise the fibrillation threshold, an important and encour-
sustained, rapid, overtly symptomatic VT would involve aging finding (Stoliarchuk and Storozhuk 1982).
administering 0.05–0.1 mg as an IV bolus and monitor- However, inducibility of ventricular arrhythmias in cats
ing for effect (decrease in sinus heart rate, decrease in with myocardial infarctions does not improve despite
VT rate and/or conversion to sinus rhythm) for approxi- amiodarone treatment (Marinchak et al. 1989). This
mately 1 minute. Failure of any effect would warrant finding, together with the thyroid (amiodarone is
repeat dosing with similar 1- to 5-minute monitoring approximately 1/3 iodine by weight) and hepatotoxic
intervals to a total dose of 0.4 mg; failure to respond at adverse effects noted in other species, make amiodarone
this cumulative dose suggests drug refractoriness or a poor choice for treatment of arrhythmias in cats.
logistical problems (e.g., IV catheter not in the vein). Given this assortment of possible therapeutic inter-
Esmolol would be administered as a constant-rate IV ventions, a reasonable starting point for intravenous
infusion at 25–200 mcg/kg/min. antiarrhythmic use in cats with rapid, persistent ven-
Magnesium sulfate can be considered for treating tricular tachycardia and/or any ventricular arrhythmia
ventricular arrhythmias in cats, particularly refractory causing syncope, after correcting reversible contributors
VT and ventricular fibrillation/cardiac arrest. Magnesium to ventricular arrhythmogenesis, is lidocaine 0.25–
deficiency may explain sodium- and potassium-related 1 mg/kg IV once, then repeated up to 2 more times over