Page 67 - Small Animal Internal Medicine, 6th Edition

CHAPTER 2 Diagnostic Tests for the Cardiovascular System 39

block). In some cases, the premature impulse is conducted
BOX 2.3 slowly (prolonged P′Q interval) or with a bundle branch
VetBooks.ir Causes of Sinus Bradycardia and Sinus Tachycardia block pattern. Although P′ waves usually do not precede
junctional complexes, retrograde conduction into the atria
sometimes causes a negative P′ wave to follow, be super-
Sinus Bradycardia
Hypothermia imposed on, or even precede the associated QRS complex.
Hypothyroidism If the specific origin of the ectopic complex(es) is unclear,
Drugs (e.g., some tranquilizers, anesthetics, β-blockers, the more general term supraventricular premature complex
calcium entry blockers, digoxin) (or supraventricular tachycardia) is used. Clinically it often
Increased intracranial pressure is more important to determine whether an arrhythmia
Brainstem lesions originates from above the AV node (supraventricular) or
Ocular pressure below it (ventricular) rather than the more specific local-
Carotid sinus pressure ization. Supraventricular premature complexes that also
Other causes of high vagal tone (e.g., airway
obstruction) depolarize the sinus node reset the sinus rhythm and
Sinus node disease create a noncompensatory pause (i.e., the interval between
Severe metabolic disease (e.g., hyperkalemia, uremia) the sinus complexes that precede and follow the prema-
Normal variation (athletic dog) ture complex is less than that of three consecutive sinus
Cardiac arrest (before or after) complexes).
Sinus Tachycardia Supraventricular Tachycardias
Hyperthermia/fever Tachycardias of supraventricular origin often involve a reen-
Hyperthyroidism trant pathway using the AV node (either within the AV node
Anemia/hypoxia or using an accessory pathway); a premature supraventricu-
Heart failure lar or ventricular impulse can initiate reentrant supraven-
Hypotension
Shock tricular tachycardia (SVT). During episodes of reentrant
Sepsis SVT in animals with ventricular preexcitation (see p. 44), the
Anxiety/fear PR interval usually normalizes (or becomes prolonged) and
Excitement retrograde P′ waves may be evident. The QRS complexes are
Exercise of normal configuration unless a simultaneous intraventric-
Pain ular conduction disturbance is present.
Drugs (e.g., anticholinergics, sympathomimetics) Atrial tachycardia is caused by rapid discharge of an
Toxicities (e.g., chocolate, amphetamines, theophylline) abnormal atrial focus or by atrial reentry (repetitive activa-
Electric shock tion caused by conduction of the electrical impulse around
Other causes of high sympathetic tone an abnormal circuit within the atria). In dogs, the atrial
activation rate usually is between 260 and 380/minute. The
P′ waves are often hidden in the QRS-T complexes. Atrial
tachycardia can be paroxysmal or sustained. It generally
When a premature complex follows each normal sinus QRS, is a regular rhythm unless the rate is too fast for the AV
this creates a bigeminal pattern; the origin of the prema- node to conduct every impulse, in which case physiologic
ture complexes determines whether the rhythm is described AV block causes irregular ventricular activation. A consis-
as atrial or ventricular bigeminy. Fig. 2.29 contains exam- tent ratio of atrial impulses to ventricular activation (e.g.,
ples of ectopic supraventricular and ventricular complexes 2 : 1 or 3 : 1 AV conduction) preserves the regularity of this
and rhythms. arrhythmia. Sometimes the impulses traverse the AV node
but are delayed within the ventricular conduction system,
Supraventricular Premature Complexes causing a bundle branch block pattern on the ECG. Differ-
Supraventricular premature complexes are impulses that entiation from ventricular tachycardia can be challenging in
originate above the atrioventricular (AV) node, either in these cases.
the atria or the AV junctional area. Because they are con-
ducted into and through the ventricles via the normal con- Atrial Flutter
duction pathway, their QRS configuration is normal (unless Atrial flutter is produced by rapid (usually > 400 impulses/
an intraventricular conduction disturbance also is present). min) waves of electrical activation regularly cycling through
Premature ectopic complexes arising within the atria (APCs, the atria. The ventricular response may be irregular or
or PACs) usually are preceded by an abnormal P wave (of regular, depending on the pattern of AV conduction. The
either positive, negative, or biphasic configuration) called a ECG baseline shows “saw tooth” flutter waves that represent
P′ wave. If an ectopic P′ wave occurs before the AV node the fast, recurrent atrial activation. Atrial flutter is not a
has completely repolarized, the impulse may not be con- stable rhythm; it often degenerates into atrial fibrillation
ducted into the ventricles (an example of physiologic AV (AF), or may convert back to sinus rhythm.