Page 998 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Cardiovascular system 973

VetBooks.ir in mechanism and maintenance between paroxys- function, left atrial dimension and left ventricular
performance indices should be evaluated.
mal and persistent AF have not been identified in
horses.
Management
Differential diagnosis In cases of lone AF, restoration of sinus rhythm
Second-degree AV block, atrial flutter, atrial tachy- should result in complete resolution of clinical signs.
cardia with variable AV response, sinus arrhythmia, If underlying cardiac disease is present, the progno-
third-degree AV block and AV dissociation should sis for restoration and maintenance of sinus rhythm
be considered. is poor and treatment may be contraindicated.
Traditional management has involved the admin-
Diagnosis istration of quinidine salts, either orally or intrave-
Physical examination should identify an arrhythmia nously. Oral therapy involves the administration of
that has no pattern of irregularity. The arrhythmia multiple doses of quinidine sulphate via nasogastric
is therefore described as being irregularly irregu- tube. A common protocol is the administration of
lar. Increasing the heart rate through exercise or 10 g quinidine sulphate orally every 2 hours until
excitement does not result in a return to regularity. restoration of sinus rhythm. Alternatively, a total
Signs of heart failure are not present unless AF is dose of 60 g can be administered. If AF persists, the
secondary. dosing interval may be increased to every 6 hours
AF is an electrocardiographic diagnosis (Fig. 8.11). for an additional 24–48 hours. Close monitoring
The criteria are absence of P waves, irregular ven- (i.e. hourly) of physical and ECG parameters is
tricular rhythm and irregular undulating baseline necessary. Administration should be under veteri-
waveforms (f waves) (irregular atrial electrical activ- nary guidance and not left to the owner. Quinidine
ity). In contrast, in atrial flutter the baseline undu- absorption rate and half-life are variable. Adverse
lations have a repeatable appearance. Holter monitor effects are common and may range from tolerable
testing or radio telemetry are indicated if paroxysmal (depression, mild tachycardia, mild hypotension)
AF is suspected (due to resting sinus rhythm). to severe and potentially life-threatening (tachyar-
Except in cases with advanced heart failure where rhythmias, neurological abnormalities, severe
increased cardiac dimensions or increased pulmo- hypotension, collapse). Therapy should be discon-
nary patterns may be encountered, no radiographic tinued if the heart rate exceeds 80 bpm or if the
abnormalities are present. Echocardiography is QRS duration exceeds 1.25 times the duration of
indicated in all cases where AF has been diagnosed. the resting QRS. Intravenous therapy with quini-
While echocardiographic changes are uncommon, if dine gluconate (0.5–2.2 mg/kg i/v q5–10 min to
present they have a significant impact on therapeutic a maximum of 12 mg/kg) and other formulations
management and prognosis. In particular, AV valve has also been described. This is most effective in

8.11

Fig. 8.11 Atrial fibrillation. Base–apex lead recorded from an 8-year-old Standardbred gelding with sudden
onset of poor performance during a race. Absence of P waves, presence of fibrillatory (f) waves, normal
ventricular complexes and irregular ventricular rhythm are seen. The ventricular rate of 40/minute is within
normal limits.