Typical energy level
Median successful energy for cardioversion of AF
Biphasic shocks have been shown to require the use of less energy than monophasic delivery and to have higher success rates of cardioversion. Currently biphasic shocks represent the present standard for cardioversion of atrial fibrillation . Other independent factors predicting successful cardioversion are thoracic impedance and duration of dysrhythmia . The American Heart Association provides guidelines on the energy requirements based on which arrhythmia is being treated, with 120–200 joules being the current initial requirement for biphasic cardioversion of atrial fibrillation . To avoid myocardial damage, some have suggested that the interval between consecutive shocks should be at least 1 min .
Cardioversion is contraindicated in dysrhythmias due to enhanced automaticity, such as digitalis toxicity and catecholamine-induced arrhythmias. Not only is cardioversion ineffective in these enhanced automaticity states, it is also associated with a higher incidence of post shock ventricular tachycardia and fibrillation. Additionally, cardioversion is contraindicated in multifocal atrial tachycardia as it does not address the underlying cause of dysrhythmia.
Focused History and Physical Findings in Patients Presenting for Elective Cardioversion
Patient assessment should begin with traditional pre-operative anesthetic evaluation, with particular attention paid to the cardiovascular system. The patient’s history should be carefully reviewed for the presence of syncope, near-syncope, dizziness, chest pain, palpitations, and history of known bradycardias or tachycardias. Evaluation should include assessment of the electrocardiogram (EKG) to characterize the patient’s current rhythm as well history of any known dysrhythmia triggers. It is important to be aware of prior pharmacologic and non-pharmacologic treatments of arrhythmias, as well as the details of any prior attempts at electrical conversion.
In the case of atrial fibrillation, the duration since onset may be unknown depending on the absence or presence of symptoms, and particular attention must be paid to the guidelines for evaluation of thrombus and appropriateness of anticoagulation. Further, the presence of implantable rhythm devices, pacemakers, ICDs, or dual devices, should be investigated. Improperly performed cardioversion in patients with these devices can damage the device or lead system and lead to malfunction or loss of capture. To ensure appropriate function, the device should be interrogated before and after cardioversion .
Is the Patient in an Optimum State to Proceed with Anesthesia
Preoperative management of known cardiac conditions, including arrhythmias, congestive heart failure, ischemia, and hypertension should be assessed. Review of the medication history should pay particular attention to use of beta blockers, antihypertensives, antiarrythmics, and statins. If the patient is on digoxin, the level should be checked and the patient should be assessed for signs of toxicity, however it is not necessary to routinely discontinue it prior to the cardioversion. Many patients undergoing cardioversion are on anticoagulation to prevent systemic embolization of thrombus, it is important to ensure that anticoagulation is adequate prior to proceeding with the procedure. An electrolyte panel is necessary for all patients undergoing cardioversion. Evaluation of labs and familiarity with patient’s history is essential as DC cardioversion is contraindicated in patients with digitalis toxicity or with hypokalemia .
Further testing, including echocardiogram for evaluation of the presence of thrombus, catheterization, and stress testing may be indicated based on the patient’s presentation in accordance with current guidelines. In all elective cases, NPO guidelines should be followed. In the event that cardioverion is being performed emergently, full stomach measures should be taken. These measures include preoxygenation and tracheal intubation. Succinylcholine is not contraindicated in this setting but bradyarrythmias may be precipitated and hyperkalemia may be worsened with its use. Occasionally, pregnant patients may present for cardioversion. Cardioversion has been found to be safe in this patient population, although it has been associated with fetal arrhythmias . In cases when the fetus is viable, it is prudent to monitor the fetal heart rate throughout the procedure.
Preparation and Commonly Used Anesthetic Technique
It is common for these procedures to be done in off-site, or non-OR locations. Off site anesthetic challenges include unfamiliar equipment, personnel unfamiliar with anesthetic aspects of care and remoteness from additional anesthetic support. Preparing for an off-site anesthetic it is essential to ensure immediate availability of emergency drugs and airway equipment. American Society of Anesthesiologists (ASA) guidelines for standard monitors apply, and the minimum for cardioversion should include continuous EKG, pulse oximetry, BP monitoring, and likely continuous capnography . An oxygen source, suction capability and a means to provide positive pressure ventilation should also be readily available.
A variety of anesthetics have been used successfully for cardioversion. Though external cardioversion is brief, the electrical stimulus can equal the intensity to that of surgical incision. An ideal anesthetic would provide analgesia, sedation, minimal hemodynamic compromise, and rapid recovery. The level of sedation required may depend on the setting and the patient, with adequate sedation allowing avoidance of recall of an unpleasant experience. Preventing or attenuating the effects of a catecholamine mediated stress response which could precipitate myocardial ischemia in this high risk population is paramount. The minimum appropriate anesthetic depth for elective cardioversion is likely deep sedation and often general anesthesia is required .
Volatile anesthetics are not often used for multiple reasons, including setting of the procedure often being offsite and very rarely requiring an endotracheal tube. There is no perfect IV drug however there are several small studies which have compared the efficacy of various IV agents including etomidate, propofol, fentanyl and benzodiazepines. Benzodiazepines have been shown to have a longer duration of effect and greater patient variability . Propofol is associated with hypotension and a higher incidence of apnea than other IV anesthetics, but in comparison recovery time is more rapid. Etomidate produces myoclonus and pain on injection, however it’s use is associated with less decrease in arterial blood pressure which may make it a better choice in patients with significant cardiac disease . Comparison has been made between the combinations of etomidate and fentanyl, and propofol and fentanyl, with the etomidate and fentanyl combination having a shorter induction time and greater degree of hemodynamic stability . A 2015 Cochrane database review looked at all the available literature and concluded that the evidence did not support superiority of any agent nor argue for the efficacy of additional analgesic medication to improve any metrics . No matter which agent is most appropriate for the individual patient, careful titration to avoid hemodynamic compromise in a high risk population is prudent.