Page 72 - Basic Monitoring in Canine and Feline Emergency Patients

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Fig. 3.16. Lead II ECG (25 mm/s, 10 mm/mV) showing 3rd-degree AV block. The sinus rate (P wave rate) is
approximately 180 bpm, while the ventricular rate (actual heart rate) is 40 bpm. No P waves are conducted through
the AV node. The QRS complexes are ventricular escape complexes. There is no relationship between P waves and
QRS complexes.

Sinus node dysfunction / sick sinus syndrome Unlike high-grade AV block, SND/SSS is not an
emergency; sudden cardiac death has not been
Sinus node dysfunction (SND) and sick sinus syn-
drome (SSS) refer to a spectrum of abnormalities of reported with this condition. SND (asymptomatic)
SA node function leading to intermittent failure does not require any directed treatment, regardless
of the SA node to depolarize. These rhythms are of atropine response. Symptomatic SSS generally
characterized on ECG by an underlying sinus warrants treatment to decrease frequency of syncope.
rhythm (usually sinus bradycardia or sinus arrhyth- Most patients with SSS will respond to oral positive
mia) with periods of sinus arrest, meaning rhythm chronotropes (such as theophylline, propantheline,
pauses (with no SA node activity) lasting longer or hyoscyamine; see Table 3.6), and atropine
than two of the patient’s ‘normal’ R–R intervals response does predict long-term response to medi-
(see Fig. 3.17). These periods of sinus arrest may be cal management. Permanent pacemaker implanta-
terminated by another sinus-origin complex, or by tion is indicated for dogs with medically refractory
an escape complex originating from the AV node SSS. Bradycardia–tachycardia syndrome, while
uncommon, presents a particular treatment chal-
(junctional escape complex) or ventricles (ventricu- lenge. Medical management is problematic for such
lar escape complex). Sometimes, this arrhythmia
involves alternation between bradyarrhythmia/ patients because positive chronotropes (to increase
sinus rate and decrease periods of sinus arrest) may
sinus arrest and paroxysms of SVT (termed brady- exacerbate SVT, while drugs such as diltiazem (to
cardia–tachycardia syndrome). This suite of ECG
findings is referred to as SND in asymptomatic slow AV nodal conduction and resolve SVT) will
patients, whereas patients displaying symptoms of also suppress SA node automaticity and may exac-
this bradyarrhythmia (typically syncope) are diag- erbate periods of sinus arrest. Permanent pace-
nosed with SSS. maker placement is the treatment of choice in
Similar to AV block, there are two potential symptomatic patients with bradycardia–tachycardia
causes of SND/SSS: fibrosis or other structural dis- syndrome.
ease of the SA node itself, or increased vagal tone
causing physiologic decrease in rate of SA node Atrial standstill
depolarization. As with AV block, an atropine
response test can differentiate structural SA node Atrial standstill is an important bradyarrhythmia to
disease versus high vagal tone (see above and Table recognize in emergency practice because it can signal
3.6). ECG characteristics, including frequency or the presence of life-threatening electrolyte abnor-
duration of sinus arrest, do not predict atropine malities. Atrial standstill is characterized electrocar-
response. Miniature Schnauzers, West Highland diographically by the complete absence of P waves
White Terriers, Dachshunds, and Cocker Spaniels (see Fig. 3.18). In atrial standstill, the SA node is still
are overrepresented in this condition, particularly firing and conducting the impulse to the AV node
older female dogs within these breeds. through specialized atrial tracts, leading to ventricular

64 J.L. Ward