Page 234 - Cote clinical veterinary advisor dogs and cats 4th

100 Atrial Standstill

Atrial Standstill
VetBooks.ir Etiology and Pathophysiology

BASIC INFORMATION
• Atrial myopathy–associated atrial standstill ○ ECG must display multiple leads (e.g.,
not just lead II) to make a diagnosis of
Definition ○ Atrial stretch and replacement of atrial atrial standstill. Otherwise, P waves may
Cardiac arrhythmia with total absence of myocytes with fibrous tissue, disrupting be isoelectric in one lead, which may give
depolarization of the atria, despite normal myocyte-to-myocyte conduction of electri- the impression of atrial standstill on single-
sinoatrial (SA node) impulse formation. The cal impulses through the atria lead ECGs of patients in normal sinus
two main causes are hyperkalemia (temporary) ○ Typically, the ventricular depolarization rhythm.
and atrial myopathy (permanent). is initiated at the atrioventricular (AV) • Serum electrolyte panel
junction (junctional escape rhythm). ○ Serum potassium concentration is an
Synonyms ○ Most commonly occurs in cats with essential test in every patient with atrial
Atrial paralysis, silent atrium marked atrial enlargement due to cardio- standstill. Hyperkalemia severe enough
myopathy; sporadic cases occur in dogs to produce atrial standstill is a medical
Epidemiology and cats without atrial enlargement emergency.
SPECIES, AGE, SEX (normokalemic). ○ Absence of hyperkalemia in a patient with
Dogs and cats: any age, either sex ○ Atrial standstill is typically permanent. atrial standstill provides the diagnosis of
• Hyperkalemia causing atrial standstill atrial myopathy (by exclusion).
RISK FACTORS ○ Rising serum potassium concentrations • Digoxin level (if relevant, given the patient’s
Diseases that can cause hyperkalemia (pp. 495 decrease the transmembrane concentration medication/exposure history)
and 1235) and therefore atrial standstill include gradient of cardiomyocytes, slowing repo-
• Hypoadrenocorticism larization (phases 1-4 of depolarization). Advanced or Confirmatory Testing
• Urethral obstruction ○ High serum potassium concentrations • Thoracic radiographs and echocardiography:
• Urinary bladder rupture slow or may totally inhibit cardiac atrial structure should be evaluated if atrial
• Acute renal failure sodium channels and attendant phase 0 standstill coexists with normokalemia. Atrial
• Iatrogenic (improper addition of KCl to depolarization. contractility may be inherently reduced with
intravenous [IV] fluids; direct IV admin- ○ Atrial myocardium is exquisitely sensitive atrial myopathy.
istration of undiluted KCl or potassium to the paralytic effects of hyperkalemia, • Serum ionized calcium level: should be
penicillin) much more so than ventricular myocar- evaluated in cats with atrial standstill second-
• Muscle necrosis (reperfusion injury, massive dium. SA nodal tissue is most resistant. ary to urethral obstruction–related hyper-
trauma) ○ The heartbeat originates from the SA node, kalemia. Hypocalcemia is commonly present
travels normally through the atria along and may be arrhythmogenic.
Clinical Presentation the internodal pathways, and reaches the
DISEASE FORMS/SUBTYPES AV node normally but fails to depolarize TREATMENT
+
• Hyperkalemia-induced (generally, K ≥ the surrounding atrial tissue along the way
7.0 mEq/L) (sinoventricular rhythm). No P waves are Treatment Overview
• Atrial myopathy-induced seen on the ECG. Identify cause, and reverse it if possible.
• Quinidine or digitalis toxicosis–induced ○ Atrial standstill caused by hyperkalemia
• Hypoxemia/hypothermia/myocardial is reversible with normalization of serum Acute General Treatment
infarction–induced potassium. Depends on cause:
• Treatment for hyperkalemia if present
HISTORY, CHIEF COMPLAINT DIAGNOSIS (p. 495)
• Hyperkalemia induced: generally reflective of • Treatment for atrial myopathy–associated
underlying disorder (e.g., lethargy, inappetence, Diagnostic Overview atrial standstill involves treatment of the
vomiting in dogs with hypoadrenocorticism) The diagnosis is suspected when a patient is underlying cardiac disorder as indicated,
• Atrial myopathy induced: exercise intoler- bradycardic, hyperkalemic, or both. The ECG regardless of atrial standstill. If overt clinical
ance, lethargy, weakness, possibly syncope (multiple leads) is confirmatory. signs (e.g., exercise intolerance, syncope, con-
or congestive heart failure gestive heart failure) occur despite treatment
• Quinidine or digitalis toxicosis induced: drug Differential Diagnosis for the underlying heart problem, pacemaker
history, systemic signs (e.g., vomiting, inap- Bradycardia on physical examination: implantation may be necessary.
petence common) • Second-degree AV block • Treatment for digitalis toxicosis involves
• May be an incidental finding on electrocar- • Third-degree AV block supportive care with adjusted antiarrhythmics
diogram (ECG) • Sinus bradycardia/sinus arrhythmia and temporary pacing if necessary.
• Sick sinus syndrome/sinus node dysfunction
PHYSICAL EXAM FINDINGS PROGNOSIS & OUTCOME
• Bradycardia is a suggestive finding. Initial Database
○ Typical ventricular rate = 40-100 beats/ • ECG is the gold standard of clinical • Hyperkalemia-associated atrial standstill:
min in dogs with atrial standstill. diagnosis. immediate prognosis is guarded. Successful
○ In cats, bradycardia may be present; ○ P waves absent in all ECG leads management of hyperkalemia and its inciting
however, cats with severe, life-threatening ○ Regular rhythm (constant R-R interval) cause often results in a good long-term
hyperkalemia causing atrial standstill may ○ Heart rate: bradycardia (dogs: typically prognosis.
still have heart rate > 200 beats/min. < 100 beats/min) or any heart rate (cats: • Atrial myopathy–associated atrial standstill:
• Other signs depend on the inciting cause 120-260 beats/min in most cases, but guarded prognosis, specifically depends on
(e.g., markedly enlarged urinary bladder with possibly < 80-100 beats/min if terminally degree of atrial enlargement and severity of
urethral obstruction). ill) clinical signs

www.ExpertConsult.com