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Basics ⬇

Basics

Definition

ECG rhythm characterized by absence of P waves; condition can be temporary (e.g., associated with hyperkalemia or drug-induced), terminal (e.g., associated with severe hyperkalemia or dying heart), or persistent.

ECG Features

Persistent Atrial Standstill

  • P waves absent.
  • Heart rate usually slow (<60 bpm).
  • Rhythm regular with supraventricular type QRS complexes.
  • Heart rate does not increase with atropine administration.

Hyperkalemic Atrial Standstill

  • Heart rate normal or slow.
  • Rhythm regular or irregular.
  • QRS complexes tend to be wide and become wider as the potassium level rises; with severe hyperkalemia (potassium >10 mEq/L), the QRS complexes are replaced by a smooth biphasic curve.
  • Heart rate may increase slightly with atropine.

Pathophysiology

Persistent Atrial Standstill

Caused by an atrial muscular dystrophy; skeletal muscle involvement common.

Hyperkalemic Atrial Standstill

Generally occurs with serum potassium levels >8.5 mEq/L; value influenced by serum sodium and calcium levels and acid-base status. Hyperkalemic patients with atrial standstill have sinus node function, but impulses do not activate atrial myocytes; thus, the associated rhythm is termed a sinoventricular rhythm. Since the sinus node is functional, an irregular rhythm may be due to sinus arrhythmia.

Systems Affected

Cardiovascular

Genetics

None

Incidence/Prevalence

Rare rhythm disturbance

Geographic Distribution

None

Signalment

Species

Dog and cat

Breed Predilections

Persistent atrial standstill-most common in English springer spaniels; other breeds occasionally affected.

Mean Age and Range

Most animals with persistent atrial standstill are young; animals with hypoadrenocorticism are usually young to middle-aged.

Predominant Sex

Hypoadrenocorticism more common in females (69%).

Signs

Historical Findings

  • Vary with underlying cause.
  • Lethargy common; syncope may occur.
  • Patients with persistent atrial standstill may show signs of congestive heart failure.

Physical Examination Findings

  • Vary with underlying cause.
  • Bradycardia common.
  • Patients with persistent atrial standstill may have skeletal muscle wasting of the antebrachium and scapula.

Causes

  • Hyperkalemia.
  • Atrial disease, often associated with atrial distension (e.g., cats with cardiomyopathy).
  • Atrial myopathy (persistent atrial standstill).

Risk Factors

  • Hyperkalemic atrial standstill
  • Hypoadrenocorticism
  • Conditions leading to obstruction or rupture of the urinary tract
  • Oliguric or anuric renal failure

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

  • Slow atrial fibrillation
  • Sinus bradycardia with small P waves lost in the baseline

CBC/Biochemistry/Urinalysis

Persistent Atrial Standstill

Normal

Hyperkalemic Atrial Standstill

  • Hyperkalemia.
  • Hyponatremia and sodium:potassium ratio <27 if atrial standstill secondary to hypoadrenocorticism.
  • Azotemia and hyperphosphatemia with hypoadrenocorticism, renal failure, and rupture or obstruction of the urinary tract.

Other Laboratory Tests

ACTH stimulation test if hypoadrenocorticism suspected

Imaging

Echocardiogram and electromyography if persistent atrial standstill suspected-cardiomegaly and depressed contractility may be seen.

Diagnostic Procedures

Skeletal muscle biopsy in animals with persistent atrial standstill.

Pathologic Findings

Persistent Atrial Standstill

  • Greatly enlarged and paper-thin atria; usually biatrial involvement, although one case of only left atrial involvement was reported.
  • Severe scapular and brachial muscle wasting in some dogs.
  • Marked fibrosis, fibroelastosis, chronic mononuclear cell inflammation, and steatosis throughout the atria and interatrial septum.

Treatment ⬆ ⬇

Treatment

Appropriate Health Care

Persistent Atrial Standstill

Not life-threatening condition; animal can be treated as an outpatient.

Hyperkalemic Atrial Standstill

Potentially life-threatening; often requires aggressive treatment.

Nursing Care

Aggressive fluid therapy with 0.9% saline often required to correct hypovolemia and lower serum potassium levels (see Hyperkalemia) in patients with hyperkalemic atrial standstill.

Activity

Restrict activity in patients with persistent atrial standstill and signs of CHF or syncope.

Diet

N/A

Client Education

Persistent Atrial Standstill

Clinical signs generally improve after pacemaker implantation; signs of CHF may develop, and weakness and lethargy may persist even after heart rate and rhythm are corrected with the pacemaker.

Surgical Considerations

Persistent Atrial Standstill

Implant permanent ventricular pacemaker to regulate rate and rhythm.

Hyperkalemic Atrial Standstill

Hyperkalemia secondary to urinary tract obstruction or rupture may require surgery.

Medications ⬆ ⬇

Medications

Drug(s) Of Choice

Persistent Atrial Standstill

Treat with diuretics and ACE inhibitor (e.g., enalapril or benazepril) if CHF develops.

Hyperkalemic Atrial Standstill

  • Treat the underlying cause (e.g., oliguric renal failure, hypoadrenocorticism).
  • Aggressive fluid therapy with 0.9% saline and possibly sodium bicarbonate or insulin with dextrose as discussed under Hyperkalemia.
  • Calcium gluconate-counters the cardiac effects of hyperkalemia; can be used in life-threatening situations to reestablish a sinus rhythm while instituting treatment to lower potassium concentration.

Contraindications

Avoid potassium-containing fluids or medications that increase potassium concentration in hyperkalemic patients.

Precautions

Diuretics lower preload and may worsen weakness in dogs with persistent atrial standstill and CHF unless a pacemaker has been implanted.

Possible Interactions

N/A

Alternative Drug(s)

N/A

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

  • Monitor ECG during treatment of hyperkalemia and periodically in animals with a permanent ventricular pacemaker.
  • Monitor electrolytes in patients with hyperkalemic atrial standstill.
  • Monitor patients with persistent atrial standstill for signs of CHF.

Prevention/Avoidance

N/A

Possible Complications

CHF in patients with persistent atrial standstill

Expected Course and Prognosis

Persistent Atrial Standstill

Clinical signs generally improve after pacemaker implantation. Signs of CHF may develop, and weakness and lethargy persist even after heart rate and rhythm are corrected with the pacemaker. There may be persistence of signs related to muscular dystrophy.

Hyperkalemic Atrial Standstill

Long-term prognosis is excellent if underlying cause can be corrected and hyperkalemia reversed.

Miscellaneous ⬆

Miscellaneous

Associated Conditions

Diseases causing hyperkalemia (e.g., hypoadrenocorticism, urethral obstruction or urinary tract tear, acidosis, and drugs).

Age-Related Factors

Persistent atrial standstill-usually diagnosed in young animals; hypoadrenocorticism-usually diagnosed in young to middle-aged animals.

Zoonotic Potential

None

Pregnancy/Fertility/Breeding

N/A

Synonyms

Silent atrial

Abbreviations

  • ACE = angiotensin converting enzyme
  • ACTH = adrenocorticotropic hormone
  • CHF = congestive heart failure
  • ECG = electrocardiogram

Suggested Reading

Kittleson MD. Electrocardiography. In: Kittleson MD, Kienle RD, eds., Small Animal Cardiovascular Medicine. St. Louis, MO: Mosby, 1998, pp. 72–94.

Kraus MS, Gelzer ARM, Moise S. Treatment of cardiac arrhythmias and conduction disturbances. In: Smith FWK, Tilley LP, Oyama MA, Sleeper MM, eds., Manual of Canine and Feline Cardiology, 5th ed. St. Louis, MO: Saunders Elsevier, 2015 (in press).

Tilley LP, Smith FWK, Jr. Electrocardiography. In: Smith FWK, Tilley LP, Oyama MA, Sleeper MM, eds., Manual of Canine and Feline Cardiology, 5th ed. St. Louis, MO: Saunders Elsevier, 2015 (in press).

Author Francis W.K. Smith, Jr.

Consulting Editors Larry P. Tilley and Francis W.K. Smith, Jr.

Client Education Handout Available Online