Cardioversion and defibrillation
Christopher R. Kennedy DVM, Dipl. ACVECC
Cardiac defibrillation and electrical cardioversion are similar but have key differences. Defibrillation is used to convert immediately life-threatening arrhythmias, specifically ventricular fibrillation and pulseless ventricular tachycardia, to sinus rhythm.1,2 Electrical cardioversion is used to convert other arrythmias and is typically a planned, rather than an emergency procedure, in veterinary medicine.2,3
Cardioversion describes converting an abnormal cardiac rhythm into a sinus rhythm.4 Electrical cardioversion achieves this by delivering an electrical shock synchronized to the peak of the R wave.2,4 It is often used for supraventricular tachyarrhythmias, such as atrial fibrillation.2,3,4 Electric cardioversion has not been reported in cats. Consultation with a cardiologist is recommended if considering this technique.
Ventricular fibrillation (VF) describes erratic electrical activities leading to abnormal impulse formation and conduction through the ventricle such that a pulse is not generated.2,5 It is recognized by chaotic waveforms on an electrocardiogram (ECG)5 (fig. 96.1). Pulseless ventricular tachycardia (PVT) describes an arrhythmia originating from the ventricles leading to rapid, ineffective ventricular contractions and hemodynamic failure.6 It can be monomorphic or polymorphic and is depicted on an ECG by more discernible (less chaotic) complexes.6 Together, VF and PVT constitute the “shockable rhythms”.1,7
Passage of a shock wave through the myocardium depolarizes the myocytes en masse, thus placing them into a refractory state, i.e., unexcitable. This allows re-establishment of a sinus rhythm originating from the sinus node.2,3
Defibrillation is most successful when performed immediately after the onset of a shockable rhythm.1 Within minutes of onset, myocardial energy substrates are depleted.1,7 In witnessed cardiac arrest with a shockable rhythm, manual chest compressions are recommended until the defibrillator is charged.1,7 In unwitnessed cardiac arrest with a shockable rhythm, a full cycle of compressions is performed prior to defibrillation.1,7 The ideal goal of defibrillation is to achieve sinus rhythm. However, this goal is not always realized; rather, defibrillation may cause rhythm conversion to asystole or pulseless electrical activity (PEA), the so-called “non-shockable rhythms”. In such instances, defibrillation is successful as the patient is no longer experiencing a “shockable rhythm” but continued basic life support (BLS) and advanced life support (ALS) are needed to achieve return of spontaneous circulation (ROSC) (fig. 96.2).
Modern defibrillators deliver biphasic shock waveforms.1 If using an older monophasic defibrillator, the energy delivered may be increased accordingly1,7 (table 96.1). Transformers within the device convert alternating current (AC) into direct current (DC), and a capacitor stores the generated charge until delivery into the patient. Most defibrillators have a battery for portability. Energy selection, charge, and delivery buttons are present on the front of the defibrillator along with an ECG monitor and a synchronization button (fig. 96.3). For defibrillation, the shock delivered is not synchronized.1,7
Pediatric paddles are perhaps better sized for cats (fig. 96.4). Larger paddles allow increased contact, decreasing impedance from the skin.2 Paddles in close proximity increases the risk for arcing.3 Electrode gels improve delivery; excessive gel overflows the paddles causing short circuits.2 Multifunctional electrode pads are available and allow both ECG monitoring and defibrillation. For open-chest scenarios, spoon-shaped paddles are designed to cradle and contact the heart (fig. 96.5).
Drug therapies and defibrillation
The role of pharmacotherapies concurrent with defibrillation is unclear. Some medications (e.g., amiodarone, lidocaine) may increase the defibrillation threshold, meaning higher energy is required for successful defibrillation.3 The American Heart Association (AHA) 2020 guidelines suggest amiodarone or lidocaine may be considered in shockable rhythms unresponsive to defibrillation.1 A recent report inferred conversion from VF to sinus rhythm in a kitten undergoing cardiac catheterization with esmolol, a beta-1-receptor antagonist, alone (i.e., pharmacological cardioversion).8
Potentially deleterious beta-agonist effects of adrenaline (epinephrine) have been described,1 and its role in ventricular fibrillation management has been questioned; however, the AHA recommends adrenaline for shockable rhythms, albeit after initial attempts at defibrillation.1
Hands-on defibrillation, double external defibrillation, and automated external defibrillators, described in human medicine, have not been evaluated in cats.
1. Panchal RA, Bartos JA, Cabañas JG, et al. Part 3: Adult Basic and Advanced Life Support: 2020 American Heart Association Guidelines for Cardiopulmonary Resuscitation and Emergency Cardiovascular Care. Circulation 142:S366-S468, 2020.
2. Pariaut R. Cardioversion and defibrillation. In Silverstein DC, Hopper K (editors). Small animal critical care medicine 2nd edition, St. Louis, 2015, Elsevier Saunders, pp 1054-1058.
3. Mellema MS, Cornell C, Kohen CJ. Defibrillation. In Burkitt Creedon JM, Davis H (editors). Advanced monitoring and procedures for small animal emergency and critical care. 2012, Wiley Blackwell, pp 235-244.
4. Willis R. Atrial fibrillation, In Willis R, Oliveira P, Mavropoulou A (editors). Guide to canine and feline electrocardiography. 2018, Wiley Blackwell, pp 127-146.
5. Mavropoulou A. Ventricular rhythms. In Willis R, Oliveira P, Mavropoulou A (editors). Guide to canine and feline electrocardiography. 2018, Wiley Blackwell, pp 169-187.
6. Foglesong A, Mathew D. Pulseless Ventricular Tachycardia [Updated 2020 Aug 8]. In StatPearls [Internet], StatPearls Publishing, 2020. [Available from https://www.ncbi.nlm.nih.gov/books/NBK554467, accessed November 16, 2020].
7. Rozanski EA, Rush JE, Buckley GJ, et al. RECOVER evidence and knowledge gap analysis on veterinary CPR. Part 4: Advanced life support. J Vet Emerg Crit Care (San Antonio) 22:S44-S64, 2020.
8. Meira C, Glaus T, Ringer SK. Reversal of ventricular fibrillation with esmolol in an anaesthetized cat. Vet Anaesth Analg 45:713-714, 2018.
Cardioversion and defibrillation