Page 993 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

968 CHAPTER 8

VetBooks.ir 8.3 alternatively, have a positive and a negative peak, or
may spontaneously change in character during the
ECG recording. Change in appearance of the P wave
in the horse is a variation of normal. This is termed a
wandering atrial pacemaker and is thought to repre-
RL sent variation in the pattern of depolarisation within
the atria.
RA Changes in QRS morphology are used to evalu-
ate the ventricular conduction system and ventricu-
LA
lar dimensions in humans and small animals, but not
in horses. The QRS complex is variable in appear-
ance between horses and is generally negative. The
QRS morphology should be consistent in the same
animal on successive recordings. T wave morphol-
ogy is labile. It may be positive, negative or biphasic
Fig. 8.3 Locations for electrode placement for Y-lead in nature and may vary within the same recording,
ECG recording in the horse. The right limb (RL) is particularly during exercise ECG recordings.
used as the ground lead and is here placed over the
left shoulder. The right arm (RA) is attached over the ECG interpretation
manubrium and the left arm (LA) is attached over The heart rate is readily evaluated by assessing the
the xyphoid. number of complexes within a given time. The heart
rhythm should then be assessed. The cardiac rhythm
8.4 can be evaluated by assessing intervals between suc-
cessive complexes (e.g. P to P and R to R intervals).
P T The cardiac rhythm should essentially be regular;
however, perfect regularity is as abnormal as an
arrhythmia and indicates loss of autonomic control
QRS of the heart. The complexes should then be evalu-
ated. The presence of P waves and their association
Fig. 8.4 Base–apex (Y-lead) ECG recording from a with QRS complexes should be evaluated, as should
horse in normal sinus rhythm. A bifid P wave is normal the conformation of recorded complexes.
in the horse. A typical QRS complex with only an R A wide range of arrhythmias has been docu-
(positive deflection) and an S (negative deflection) is mented in the horse. These arrhythmias are for
present. The T wave is highly volatile in the horse and the most part benign in nature, with some notable
can be a positive or negative (or both) wave form. The exceptions. Differentiation of arrhythmias based on
slight negative deflection following the P wave is due auscultation alone can lead to confusion. In particu-
to atrial repolarisation (atrial T wave).
lar, second-degree heart block and AF may be simi-
lar on auscultation in the resting horse. The clinical
importance when considering the possible effect on consequences are, however, drastically different.
future performance and when assessing the safety of Electrocardiography is indicated to determine the
rider or driver. clinical significance of any arrhythmia detected.
The waveforms generated on an ECG represent
different phases of the cardiac cycle (Fig. 8.4). The P Echocardiography
wave represents atrial depolarisation. In the horse the Ultrasonography provides a non-invasive procedure
P wave is often bifid (having two peaks). These two for the evaluation of cardiac structure, dimensions
positive peaks are often considered to represent the and function. In the horse, echocardiography pro-
two atria, although the correspondence is not as well vides the only means of assessing chamber enlarge-
correlated as it is in small animals. The P wave may, ment. Echocardiography allows the clinician to