Page 999 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

974 CHAPTER 8

VetBooks.ir recent-onset AF. Intravenous administration may FIRST-DEGREE HEART BLOCK
be followed by oral administration if AF persists, as
long as toxicity is not observed.
Magnesium sulphate is the agent of choice for Overview/aetiology/pathophysiology
Normal finding in horses. The condition is associ-
quinidine associated torsades de pointes (Table 8.4). ated with high vagal tone, and it may be more preva-
In cases of quinidine toxicity, sodium bicarbonate lent in fit horses.
administration is beneficial in decreasing free quini-
dine levels by increasing protein binding. Clinical presentation/diagnosis
Quinidine is a negative inotrope (decreases Normal sinus rhythm. P waves and QRS complexes
force of contraction) and a positive chronotrope have normal conformation (Fig. 8.12). Prolongation
(increases heart rate), therefore it should be admin- of PR interval beyond 0.425 to 0.47 ms.
istered with caution in animals that are tachycardic
at rest or have poor cardiac function. The adminis- Management
tration of digoxin prior to, and during, therapy may No treatment is required.
counter some of these effects. Quinidine adminis-
tration is contraindicated in congestive heart fail- SECOND-DEGREE HEART BLOCK
ure. The prognosis for restoration and maintenance
of sinus rhythm is considerably reduced in older Overview/aetiology/pathophysiology
horses, larger horses, when underlying cardiac dis- Most often a normal finding in horses. The condi-
ease is present and when the duration of AF exceeds tion is associated with high vagal tone. This results
4 months. in decreased conduction through the AV node. It
Electrical cardioversion of AF has been described may be more prevalent in fit horses. Advanced block
in the horse. External cardioversion with bipha- (multiple cycles at a time) may be due to myocardial
sic defibrillator waveform was successful in one inflammation.
horse with AF duration of 3 weeks and concur-
rent antiarrhythmic medication administration. Clinical presentation/diagnosis
A technique for transvenous electrical cardiover- Normal or slightly slow ventricular rate. P waves
sion, where catheter-mounted electrodes are placed occur without associated QRS complexes. PP inter-
within the right atrium and left pulmonary artery val is consistent. QRS complexes are normal in
through a jugular vein approach, has been developed appearance (Fig. 8.12). Mobitz type I is more com-
and this technique appears to be highly effective. mon and is characterised by gradual lengthening of
Additionally, the technique has proven successful in PR intervals until a P wave is not followed by a QRS.
horses with a duration of AF of up to 7 years. The Mobitz type II is characterised by a constant PR
requirement for specialised equipment will limit this interval with intermittent block and is not as com-
procedure to referral centres. mon in the horse. Advanced block may be associated
with pronounced bradycardia and collapse.
Prognosis On auscultation the heart rhythm is irregular.
The prognosis for horses with lone AF that respond However, the blocked interval is a multiple (i.e. usu-
to conversion therapy is excellent and these horses ally twice) of the basal interbeat interval. The pre-
should return to their normal level of performance. dictable length of the blocked interval means that
Recurrence of AF is not uncommon, and horses this arrhythmia is usually identified as being regu-
previously diagnosed with AF that experience a sud- larly irregular. The arrhythmia should abate with
den decline in performance should be evaluated for increased heart rate or with decreased vagal tone,
recurrence. The prognosis for horses with AF sec- and it should therefore resolve with light exercise or
ondary to underlying heart disease is guarded. excitation.