Page 248 - Clinical Small Animal Internal Medicine

216 Section 3 Cardiovascular Disease

250 bpm can induce severe myocardial dysfunction propagate within specialized muscular bundles of the
VetBooks.ir within 3–4 weeks in dogs. His–Purkinje conduction system, similar to normal
sinus beats. However, on occasion, SVTs can display
wide QRS complexes if a bundle branch block is present.
Epidemiology
In these challenging cases, the identification of P‐waves
Supraventricular tachycardias other than AF are rare in associated with QRS complexes confirms the origin of
both dogs and cats. the arrhythmia above the His bundle. P‐waves can occur
before, during or after the QRS complexes. They can be
positive or negative. Oftentimes, P‐waves are buried
Signalment
within the T‐waves. Although rarely effective, vagal
Large‐ and giant‐breed dogs, in addition to brachyce­ maneuvers can be done to slow AV conduction, reveal­
phalic breeds, are more commonly diagnosed with SVTs. ing hidden P‐waves associated to QRS complexes.
Tachyarrhythmias associated with an accessory pathway It is important to consider that ventricular tachycar­
have more frequently been reported in Labradors. dias are much more frequent than SVTs. Following care­
ful examination of the ECG, if uncertainty persists about
the origin of a wide QRS complex tachycardia, it should
History and Clinical Signs
be first treated as if it were ventricular tachycardia.
Signs of SVTs include weight loss, lethargy, exercise
intolerance, excessive panting, and dyspnea. However, it Ambulatory 24‐Hour Holter Recording
is not uncommon for dogs to be perceived as being non­ The information collected from Holter recordings
clinical. Owners of dogs with SVTs may report “seeing” includes the number of episodes and their duration, the
their pet’s heart pounding in the chest during episodes of modes of arrhythmia onset and termination, and infor­
tachycardia. Weakness and transient loss of conscious­ mation about the overall heart rate, especially when the
ness are less common with SVT. Additionally, SVTs may animal is in a familiar environment. Evaluation of heart
only be recognized at the time of physical examination in rate distribution over 24 hours in the light of an activity
dogs with signs of congestive heart failure. log completed by the owner during the period of record­
ing is a source of valuable information on the circadian
pattern of the arrhythmia and on the contribution of adr­
Diagnosis
energic tone to the initiation and rate of the arrhythmia.
Physical Examination
Cardiac auscultation is a useful diagnostic tool to calcu­ Echocardiography
late heart rate, detect occasional ectopic beats, and dis­ An echocardiogram is useful to assess for the presence of
tinguish between paroxysmal and sustained arrhythmias. structural cardiac disease and tachycardia‐induced
While paroxysmal SVTs are usually regular, SVTs may cardiomyopathy.
also be irregular. Some dogs experiencing episodes of
paroxysmal SVT will be in sinus rhythm at the time of Therapy
presentation. Femoral pulses may be weaker during
bouts of tachycardia. Decision to Treat
Treatment is justified when SVTs cause clinical signs,
Electrocardiography which are directly related to their rate and duration, as
It is critical to differentiate supraventricular from ven­ well as ventricular performance. In the absence of obvi­
tricular tachycardia. However, treatment can be initiated ous clinical signs, treatment should be considered if
without identifying the exact mechanism of an SVT. tachycardia‐induced cardiomyopathy is suspected.
The first step in the diagnosis of tachyarrhythmias is When SVTs are intermittent, relating the clinical signs to
recognizing that an uninterrupted and irregular (a varia­ the arrhythmia may be challenging. In that situation,
tion >100 ms between RR intervals) tachycardia is long‐term recording of the cardiac rhythm via ambula­
typically AF. Once AF is ruled out, SVTs must be differ­ tory 24‐hour Holter recording, or preferably with a wear­
entiated from ventricular tachycardia by looking at the able or an implantable loop event recorder, is indicated.
morphology of the QRS complexes. Ventricular tachy­ A rate‐control or rhythm‐control strategy, which con­
cardias have wide QRS complexes (>0.06 s in dogs; sists of restoring sinus rhythm, can be applied to the
>0.04 s in cats) followed by a large T‐wave directed treatment of SVTs. Drug selection is based on the type of
opposite to the QRS complex, whereas the main charac­ arrhythmia, the risk of adverse reactions, and the degree
teristic of SVTs is narrow QRS complexes, indicating of cardiac dysfunction. Other factors to take into account
that electrical impulses, once they reach the ventricles, include decreased oral medication absorption, drug