Page 996 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Cardiovascular system 971

VetBooks.ir agitated saline solution, allows for the evaluation of in humans, and skeletal muscular damage is com-
Contrast echocardiography, injecting a dye or
mon. The myocardial isoenzymes are also present
the direction of flow across shunts and across valves.
This is less commonly performed now, as colour-flow in skeletal muscle, and therefore, any skeletal muscle
damage may confound the detection of myocardial
imaging is available on many ultrasound machines. injury because of the skeletal origin of isoenzyme
In a fractious animal, sedation may be required to elevations. In the absence of skeletal muscle injury,
allow for safe examination. It is important to recog- CK and LDH isoenzyme elevation may be sugges-
nise, however, that functional assessment will not be tive of myocardial injury.
possible. Alpha-2 agonists have been shown to dra- More recently, troponin protein assays have been
matically reduce systolic function. developed to assess myocardial disease in humans.
Advanced imaging modalities including tissue Cardiac troponin T (cTnT) and I (cTnI) subunits are
Doppler imaging and speckle tracking, where myo- distinct from those of skeletal origin. Specific assays
cardial function may be closely interrogated may be have been developed and have been evaluated in the
of value in select cases (e.g. suspected toxic cardio- horse. cTnI has been considered more sensitive and
myopathy) This is limited to referral centres. specific to myocardial tissue than earlier assays for
troponin T because of the relatively higher concen-
Laboratory evaluation tration in cardiac muscle when compared with skel-
of cardiac disease etal muscle. New (3rd-generation) cTnT assays are
Routine serum biochemistry and haematology may considered to be of similar diagnostic sensitivity as
be beneficial. A murmur detected in a dehydrated or cTnI assays. Troponin assays should be performed
anaemic animal may relate to the underlying cause when myocardial disease or myocardial damage is
of the haematological abnormality rather than to suspected. High conservation across species means
cardiac disease. Electrolyte disturbances may be the that tests designed for human use should be capable
cause of arrhythmias. Haematological and biochem- of detecting troponin proteins in equine samples.
ical evidence of inflammation, such as increased There are many different cTnI tests, each targeting
white blood cell count, fibrinogen or globulins, may a separate portion of the cTnI protein. As much as a
raise suspicion of bacterial endocarditis as a cause for 100-fold variation in the reported value is possible.
a sudden-onset cardiac murmur. Laboratory assays It is essential, therefore, that the tests are compared
may be of value for assessing the systemic effects of only with themselves and normal values established
heart failure (e.g. azotaemia due to poor renal perfu- for each test.
sion). Arterial blood gas analysis may aid reaching a
diagnosis (e.g. low values [<40 mmHg] in congenital Cardiac catheterisation
shunting). Cardiac catheterisation may provide additional
Most antiarrhythmic medications have nar- information regarding the direction of flow across a
row therapeutic ranges, therefore serial analysis shunt and the cardiovascular effects of an abnormal
of serum levels of these medications is needed finding. This technique has declined in popularity
to optimise therapy. For example, blood levels with the increased availability of ultrasonography
of quinidine should be assessed repeatedly dur- and is limited to referral centres, where personnel
ing administration; this is to minimise the risks familiar with the procedure and specialised equip-
associated with quinidine toxicity in addition to ment are available. Interventional procedures (such
determining whether therapeutic levels have been as coil placement in patent ductus arteriosus [PDA]
attained. or transvenous pacemaker placement) involving car-
Elevations in serum creatine kinase (CK) and diac catheterisation are uncommon in the horse.
lactate dehydrogenase (LDH) levels are often asso- Angiography is infrequently used in horses. More
ciated with muscular damage. In humans, CK and often, it is used to evaluate limb perfusion, particu-
LDH isoenzymes specific to myocardial tissue have larly distal limb perfusion, rather than as part of a
been used to assess myocardial injury. In horses, cardiac examination. Echocardiography has replaced
myocardial disease is significantly less common than angiography in many cases.