Page 1002 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Cardiovascular system 977

VetBooks.ir Management feedback. Respiratory sinus arrhythmia is not com-
monly found in the resting horse.
The underlying disease or electrolyte imbalance
should be treated.
Clinical presentation/diagnosis
ATRIAL TACHYCARDIA Respiratory sinus arrhythmia is characterised by
variations in RR interval associated with respiration
Overview/aetiology/pathophysiology (Fig. 8.16). With exercise-associated arrhythmia,
This is an uncommon condition that is considered the heart rate slows suddenly then rises gradually
clinically relevant. The underlying pathology is such that the cardiac deceleration has a step-like
unknown. It may be due to myocardial inflamma- appearance.
tion or disease.
Management
Clinical presentation/diagnosis No treatment is required.
Atrial tachycardia is characterised by multiple APCs
(four or more in a row) and supraventricular tachy- SINUS ARREST/BLOCK
cardia with a rate often between 100 and 200 bpm.
QRS complexes are normal in appearance and are Overview/aetiology/pathophysiology
preceded by a P wave. The P wave may have a dif- Sinus block is considered a normal variation in rest-
ferent conformation than normal. Superimposition ing horses. It is often associated with variations in
on the T wave may lead to difficulty identifying the vagal tone. Persistence at elevated heart rates may
P wave. Second-degree heart block may occur and in indicate a pathological basis. Prolonged (4 seconds
those cases the heart rhythm is irregular. or greater) blocked intervals or those associated with
syncope are also clinically relevant.
Management
Identification and removal or treatment of the Clinical presentation/diagnosis
underlying cause, such as electrolyte imbalance Sinus block is characterised by a normal cardiac
or inflammation, are the most important aspects. rhythm, with periods when neither P waves nor QRS
Antiarrhythmic medication may be indicated. complexes occur. During these ‘blocked beats’ the
Quinidine has been used but it should be used RR interval is usually twice the normal RR interval
cautiously, especially if a second-degree block is (Fig. 8.17). In contrast, with sinus arrest the periods
present. Quinidine increases conduction through of electrical silence are variable.
the AV node and conduction of previously blocked
P waves will increase ventricular rate. Digoxin can Management
be used to slow the ventricular rate and limit AV No treatment is required for the benign form.
nodal conduction (Table 8.4). Pacemaker implantation would be the only option
for advanced block.
SINUS ARRHYTHMIA
PRE-EXCITATION SYNDROME
Overview/aetiology/pathophysiology
Sinus arrhythmia is characterised by minor fluc- Previous name: Wolff–Parkinson–White syndrome
tuations in cardiac rhythm associated with respi-
ration-induced changes in vagal tone (respiratory Overview/aetiology/pathophysiology
sinus arrhythmia). The term is also used to describe Pre-excitation syndrome is caused by an accessory
fluctuations in cardiac rhythm associated with exer- conduction pathway between the atria and the ven-
cise (exercise-associated arrhythmia). This arrhyth- tricles or bundle of Hiss and surrounding tissue and
mia is commonly found in fit horses during cardiac is rare in the horse. It is associated with poor per-
deceleration following submaximal exercise and is formance and may result in periodic collapse. It may
likely to be associated with a variation in autonomic also predispose to paroxysmal AF.