Ventricular fibrillation

Introduction

• Cause: Can occur as electrophysiological complication of other "malignant" (ie prefibrillatory) ventricular arrhythmia such as very fast and sustained ventricular tachycardia (VT) in the presence of severe organic myocardial disease (eg sub-aortic stenosis or "Boxer dog cardiomyopathy").
  • In severe heart failure, when contractility is depressed and intraventricular diastolic filling pressure is high, VT can be a precursor to VF.
• Can also occur as a consequence of death: untreated ventricular fibrillation → to hemodynamic cardiac arrest and death ie often the result or final manifestation of death due to terminal cardiac or extra-cardiac disease.
• May be induced by lengthy or deep-planed general anesthesia (eg when lengthy and severe myocardial hypoxia present) or at onset of anesthesia induction, either in severely diseased patients or (rarely) in healthy animals.
• Respiratory arrest and ventilatory failure can lead to VF more commonly than does primary organic cardiac disease.
• Diagnosis: Electrocardiography. Chaotic/random, irregular and deformed, low-amplitude oscillations or undulations at a rate ranging between 150 and 300, or even >400 cycles/minute, with no distinct P waves (despite continuation of organized atrial electrical and mechanical activity), QRS complexes or T waves.
• Undulations either coarse (and relatively more amenable for cardioversion) or fine (less amenable for cardioversion).
• Many malignant ventricular arrhythmias that electrocardiographically and hemodynamically appear to be asystole (see Differential Diagnosis ) are, in fact, fine VF. Therefore, open-chest heart massage and direct observation of myocardial activity may be warranted early with this arrhythmia.
• Treatment: Direct current (DC) electrical countershock (defibrillation) only effective therapy.
• Prognosis: In the severely diseased, poor to grave, often even if promptly and successfully treated.
• More favorable if defibrillation performed immediately in otherwise normal animal with chemical etiology eg drug toxicity:
  • Macrolide antibiotics like tylosin when given to animals with pre-existing myocardial ischemia.
  • Anesthetic overdose such as halothane or barbiturates.
  • Drug-drug interaction.
    

Diagnosis

Clinical signs

• Cardiopulmonary arrest with acute unconsciousness, often either in patients that were previously asymptomatic despite severe myocardial disease, or following severe morbidity in patients with extracardiac disease or with terminal congestive heart failure.
• Pulseless peripheral arteries.
• Absence of auscultable heart sounds .
• Absence of palpable cardiac apex beat.
• Apnea.
  Apnea can occur with a normally beating heart, but normal respiration cannot last during cardiac arrest or VF.
• Cyanosis if ventricular fibrillation occurring for a few minutes or more.
• Centrally fixed, dilated pupils.
• Capillary refill time (CRT) not sensitive nor specific tool for diagnosing VF or cardiac arrest. Can remain within normal limits for as long as 30 minutes following cardiac arrest.
• Sustained pulseless VT.
• Sustained ventricular flutter.
• Ventricular asystole.
• Electro-mechanical dissociation (EMD).

Diagnosis

Differential diagnosis

• Other "malignant" ventricular dysrhythmic conditions that may lead to circulatory arrest.
  One of the relatively more treatable of these "arrest arrhythmias" is VF and immediate defibrillation is the treatment of choice. If the arrhythmia is not VF, one-time defibrillation will still do only minimal, if any, harm (see Treatment ).

Sequelae

Prognosis

• Prognosis for long-term survival in such cases is poor even if cardiorespiratory function is temporarily salvaged.

Sources

Publications

Refereed papers

• Crowe D T, Fox P R, Devey J J & Spreng D (1999) Cardiopulmonary and cerebral resuscitation. In: Textbook of Canine and Feline Cardiology. Principals and Clinical Practice. 2nd Edn. Eds: P R Fox, D Sisson D and N S Moise. WB Saunders Co, Philadelphia. pp 427-445.
• Kittleson M D (1998) Diagnosis and treatment of arrhythmias (dysrhythmias). In: Small Animal Cardiovascular Medicine. Eds: M D Kittleson and R D Kienle. Mosby Inc, St Louis. pp 449-494.
• Labato M A (1995) Cardiopulmonary arrest and resuscitation. In: Textbook of Veterinary Internal Medicine. Diseases of the Dog and Cat. 1 , Eds: S J Ettinger and E C Feldman. WB Saunders Co, Philadelphia. pp.71-79.
• Lunney J & Ettinger S J (1995) Cardiac arrhythmias. In: Textbook of Veterinary Internal Medicine. Diseases of the Dog and Cat. Eds: S J Ettinger and E C Feldman. WB Saunders Co, Philadelphia. pp 959-995.
• Detweiler D K (1988) The dog electrocardiogram: a critical review. In: Comprehensive Electrocardiology. Theory and Practice in Health and Disease. 2 , Eds: P W Macfarlane and T D Lawrie. Pergamon Press, NY. pp 1267-1329.

Other sources of information

• The Merck Veterinary Manual, Eighth Edition, Published by Merck & Co Inc, Whitehouse Station, NJ, USA. In cooperation with MERIAL LIMITED, a Merck and Aventis Companyhttp://www.merckvetmanual.com/(All rights reserved).