Direct-current cardioversion is a common cardiological procedure, consisting of delivering an electric shock to the patient’s heart with the goal of restoring normal sinus rhythm. The common indication for the procedure is atrial fibrillation or flutter and less commonly other supraventricular arrhythmias or ventricular tachycardia. The shock is usually delivered by a pair of conductive pads applied to the patient’s skin on the anterior and posterior chest or, if the patient already has an implanted cardioverter-defibrillator (ICD), by discharge from the internal device. In either case, the electric shock is painful and provokes anxiety in conscious patients. An anesthetic is usually administered in conjunction with the cardioversion. The main challenge to the anesthesiologist involved with a cardioversion is that although the procedure is simple and short, the patient often has comorbid cardiac and pulmonary conditions that require careful titration of anesthetic depth and duration against circulatory and respiratory embarrassment. Transesophageal echocardiogram (TEE) is often obtained just before direct-current cardioversion to discern the presence of a left atrial (LA) thrombus, which may be dislodged by the shock or subsequent resumption of synchronized atrial contractions. Both procedures are performed under the same anesthetic at many institutions, significantly extending its length and the likelihood of hypotension, hypoventilation, or oxygen desaturation episodes.
Cardioversion became a method of restoring normal sinus rhythm in the early 1960s after Lown and colleagues published a report of successful treatment of patients with atrial fibrillation with a synchronized direct-current shock by a device they termed a cardioverter. Their innovation featured the synchronization of the discharge to the R-wave of the electrocardiogram (ECG) to avoid inducing ventricular fibrillation during the vulnerable portion of the cardiac cycle. In the years since, electric cardioversion has been accepted for use as a treatment of all cardiac arrhythmias except ventricular fibrillation, which requires asynchronous defibrillation. More recently, devices using biphasic current waveform have supplanted the older monophasic cardioverters, allowing greater success rates at lower energies. TEE has an excellent ability to visualize the left atrium and atrial appendage in detail to exclude the presence of atrial thrombus. Such information is valuable in patients with symptomatic arrhythmia or hemodynamic compromise but uncertain duration of atrial fibrillation and inadequate state of anticoagulation. For this reason, TEE extended the therapeutic reach of cardioversion to patients who would otherwise face a delay in treatment of several weeks to establish proper anticoagulation. The proportion of TEE-guided cardioversions is growing and now exceeds 30% in some large centers.
Over the last half century, the majority of intravenous sedative-hypnotic drugs, alone and in combination, have been used for procedural sedation and anesthesia in conjunction with direct-current cardioversion with or without TEE. Overall, anesthetic goals have remained the same—elimination of pain from the shock and blunting of recall of this unpleasant event. Early observations led to speculation whether electric shock might be tolerated by patients without anesthesia, but subsequent experience convinced physicians this was not the case. Considering the need in many patients for repeat cardioversions over their lifetime, the anxiety engendered by recalling a prior painful shock likely would act as a deterrent to obtaining the necessary treatment. Many patients with atrial dysrhythmias currently receive cardioversion on an outpatient basis, on a prearranged date after a 3- to 4-week period of anticoagulant therapy, and with emphasis on the briefest duration of preprocedure stay and prompt discharge thereafter. In this setting, sedative premedication is usually impractical, the use of all but the shortest-acting opiates is inadvisable, and neuromuscular blockade is contraindicated. Therefore the intravenous anesthetics suitable for the procedure are the short-acting barbiturate methohexital, etomidate, and, most commonly today, propofol. However, prospective studies supporting a decisive advantage for any of these agents during cardioversion are lacking.
Preoperative Assessment
In addition to the usual preanesthetic history, full review of systems, and a physical examination, several issues typically need to be addressed. First, the anesthesiologist should inquire about prior cardioversion attempts and the patient’s satisfaction with the associated anesthetic. Old anesthetic records are a good guide to selecting anesthetic medications and dosages and can help anticipate postprocedure cardiorespiratory problems. Physicians’ stated concern for suppressing recall is reassuring to most patients, but a possibility of recall should be frankly discussed with patients with complex cardiac or pulmonary problems.
Second, patients should be questioned about their compliance with an anticoagulation regimen, if any. For patients on warfarin, the absence of weekly International Normalized Ratio (INR) measurements in the therapeutic range (usually 2-3 or over >2.5 in the presence of mechanical heart valves ) over 3 to 4 weeks is an indication for pre-cardioversion TEE to exclude LA thrombus or sludge. Many patients on newer anticoagulants will not have a record of INR measurements and do not need TEE if they are compliant. These medications include the oral direct thrombin inhibitor dabigatran (Pradaxa) and direct factor Xa inhibitors rivaroxaban (Xarelto) and apixaban.
Third, a careful review of medications may point to potential issues arising during the procedure. Many patients will be on antiarrhythmic and heart rate control medications. Among them, those taking calcium-channel and beta-adrenergic blockers will exhibit an early tendency toward hypotension and bradycardia with the restoration of sinus rhythm. Patients in atrial fibrillation on the newer antiarrhythmic drug dronedarone (Multaq) also may become bradycardic after cardioversion. Additionally, concern exists over QT prolongation if these patients are given some of the common antiemetic drugs, such as ondansetron (Zofran).
Fourth, a history of cardiomyopathy, congestive heart failure, ischemic heart disease, or dyspnea with mild exertion should alert the anesthesiologist to the need for a cautious induction of anesthesia with ample time for circulating the drug between incremental bolus doses. Patient history of obstructive sleep apnea correlates with the need to provide positive-pressure mask ventilation during the anesthesia. Head and neck and airway examinations will predict the difficulty of mask ventilation and prepare the physician for a potentially challenging intubation.
Necessary laboratory data include a recent potassium level; significant hypokalemia will need correction before cardioversion. If the patient is on warfarin, the INR over the preceding 3 to 4 weeks should be reviewed. In patients on digoxin (now uncommon), its levels must not exceed therapeutic range because of concern for cardioversion triggering ventricular tachyarrhythmias. The patient’s electrocardiographic activity on the monitor should be reviewed just before starting the anesthetic. Not infrequently, patients can undergo spontaneous reversion to sinus rhythm during preparations for cardioversion.
Intraoperative Management
Cardioversions take place in established outpatient procedure areas, emergency wards, and at the bedside of hospitalized patients. Anesthesia machines typically are not available in these locations, and the anesthesiologist is usually expected to bring in the supplies and medications—both those likely to be used during the procedure and those reserved for emergency treatment. Regardless of location, the patient should be monitored according to the American Society of Anesthesiologists guidelines. Cuff blood pressure measurements must be reliably obtained. Insertion of an arterial line is rarely necessary. Audible variable-tone pulse oximetry is most helpful, because a monitor may not be directly facing the anesthesiologist. A primary and a backup supply of oxygen and a working suction line must be present. A single reliable intravenous catheter is sufficient for the procedure. If multiple clinical services are involved—for example, one for TEE and another performing the cardioversion—all practitioners must be on hand before induction of anesthesia to minimize the duration of anesthetic.
Echocardiographers skilled in TEE often have a routine for topical administration of local anesthetics in the oropharynx and conscious sedation before insertion of the TEE probe. For TEE expected to be followed immediately by a cardioversion, the anesthesiologist has two alternatives. The first is the topical and conscious sedation routine of the echocardiographer during the TEE portion, followed by the induction of a brief general anesthetic for the cardioversion portion. The advantages of this approach are potentially avoiding general anesthesia altogether, if TEE finding of a thrombus precludes immediate cardioversion, and minimizing the duration and depth of general anesthesia in high-risk patients. Patients with significantly compromised myocardial function, pulmonary diseases, morbid obesity, a history of obstructive sleep apnea, and especially the combination of several of these conditions, will benefit from the first approach. The second approach is the induction of a general anesthetic of somewhat longer duration and sufficient depth to cover both portions of the combined procedure. Patients with intact cardiac and pulmonary reserve and reassuring airway features are best served by receiving general anesthesia from the beginning, because this option is more comfortable to the patient, it does not impair protective airway reflexes during recovery secondary to topical anesthesia of the tongue and hypopharynx, and the procedure is briefer and less labor-intensive.
For direct-current cardioversion, as for all surgical procedures, a time-out protocol is a useful tool for avoiding preventable errors in treatment. At time-out, the patient and procedure are identified, all staff make their introductions, and the presence of adequate rescue supplies is confirmed. For both external and internal (by ICD) cardioversion, positioning is the same: conductive pads are placed anteriorly and posteriorly (or laterally) on the patient’s chest, such that the vector of the current passes maximally through the atria (for atrial fibrillation or flutter); the patient is positioned supine, with all pressure points padded; and the arms are lightly tucked next to the body to avoid being thrown in the air by the energy of the shock.
General anesthesia is induced, after several minutes of thorough preoxygenation by tight-fitting mask at fraction of inspired oxygen (Fi o 2 ) 1.0, by gradual titration of an intravenous hypnotic agent, typically propofol (initial bolus dose 1 mg/kg). Further 10- to 20-mg boluses of propofol may be given until the eyelash reflex is abolished. These are timed at longer intervals for patients with impaired myocardial performance. Patient readiness for the shock (or TEE probe introduction, if done under the same general anesthetic) can be ascertained by an absence of response to a firm mandibular thrust. If precardioversion TEE is performed under general anesthesia, the examination is typically brief and focused on excluding LA thrombus. The midesophageal two-chamber view (near 90 degrees) usually affords the best visualization of the left atrial appendage, but additional views should be sought by changing the probe angle from 0 to 120 degrees and by withdrawing and advancing the probe to scan the full height of the left atrium. The finding of spontaneous echo contrast (“smoke”) in the left atrium and measuring pulsed-wave Doppler velocities less than 40 cm/second in the LA appendage further increase the suspicion for thrombus or sludge.
Once the patient is sufficiently deeply anesthetized and the decision is made to proceed with the cardioversion, the device’s capacitor is charged and everyone at the bedside is reminded to stay clear and not touch the patient. The only exception is the person who will (optionally) apply firm pressure to the anterior chest pad through a dry towel, which serves to decrease the chest wall impedance and enable higher energy delivery to the myocardium for a given cardioverter setting. If the anesthesiologist is providing positive-pressure ventilation to the patient, the device discharge should take place after allowing a full exhalation to minimize impedance due to pulmonary air. The presence of a groan or a grimace in the patient receiving a shock, while often disconcerting to the physicians and nurses present at cardioversion, does not correlate with the recall of the event. After the direct-current shock, the patient’s ECG is closely monitored for the return of sinus rhythm. Failure to convert may necessitate another attempt at a higher energy setting or with a different pad position. In this case, care must be taken to provide adequate respiratory support between the shocks, assess hemodynamic stability, and reconfirm the depth of anesthesia before proceeding.
If cardioversion is successful, the uncomfortable step of removing the conductive pads is best performed while the patient is still under general anesthesia. Whereas some patients will maintain a patent airway and spontaneous respirations under adequate general anesthesia, a large proportion will need maneuvers to open the airway and require intermittent positive-pressure ventilation. This is quite problematic during the TEE portion of the procedure, when bag-mask ventilation of the patient is not an option. The three-pronged approach to this problem is to (1) thoroughly preoxygenate the patient; (2) communicate to the echocardiographer the need for a speedy, highly focused examination; and (3) use an oral airway device compatible with TEE. Especially promising are the multifunctional devices now becoming available that combine the features of a bite block, an oral airway, and a conduit for oxygen insufflation. The use of a nasopharyngeal airway is relatively contraindicated because of the possibility of epistaxis resulting from the anticoagulated state in most patients.
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