Abstract
Direct current cardioversion involves the application of electrical energy synchronized to the QRS phase of the cardiac cycle, whereby electrical energy is applied across the thorax, stimulating the myocardium, simultaneously exciting cardiac myocytes, and inducing rhythmic ventricular contraction. The concurrent activation and contraction of ventricular myocytes subsequently causes the myocardium to enter an effective refractory period, abating reentrant circuitry propagation. Anesthesia for cardioversion is a common and safe procedure. A thorough preoperative assessment, with emphasis on resuscitation and stabilization in unstable patients, helps curtail potential complications.
Keywords
anesthesia, atrial fibrillation, cardioversion, sedation, tachyarrhythmias
Case Synopsis
An 88-year-old woman with a background of hypertension, angina, and mild asthma presented to the emergency department (ED) with a 10-day history of abdominal pain, nausea and vomiting, and reduced oral intake. She was initially being treated for acute dehydration secondary to gastroenteritis. On further questioning, she had a significant history of ischemic heart disease (IHD) limiting exercise tolerance, having suffered from a previous myocardial infarction in the right coronary territory, for which she underwent emergency (primary) percutaneous coronary intervention. On examination, the airway was patent, respiratory rate was labored at 30 breaths per minute, arterial saturations were 92% on 5 L of oxygen via facial mask, heart rate (HR) was irregularly irregular at 137 beats per minute, and blood pressure (BP) was 94/45 mm Hg. The laboratory results demonstrated a serum potassium of 3.1 mEq/L, blood urea nitrogen 33 mg/dL, creatinine 310 mmol/L, magnesium 0.57 mmol/L, and phosphate 0.38 mmol/L. The 12-lead electrocardiogram (ECG) showed new-onset atrial fibrillation (AF) with a rapid ventricular response, and Q waves and ST depression in leads 2, 3, and aVF. Following initial resuscitation, she was found to have a small bowel obstruction on abdominal computed tomography and was booked for urgent surgical intervention. Her HR was now 160 beats per minute and her BP was 60/35 mm Hg despite fluid therapy. To facilitate emergent direct current (DC) cardioversion within the ED, the anesthesiologist induced general anesthesia, and the patient was successfully cardioverted to sinus rhythm (SR) after the third shock at 150 J. Invasive monitoring was then instituted, and she was transferred to the operating room for urgent surgical intervention.
Acknowledgment
The authors wish to thank Dr. M. Franckowiak and Dr. N. D. Nader for their contribution to the previous edition of this chapter.
Problem Analysis
Definition
Cardioversion can be electrical or chemical therapy and is used to restore SR in a variety of contexts. DC cardioversion is exploited in three major situations: first, as the modality of choice for the management of tachyarrhythmias in unstable patients with hemodynamic instability; second, when pharmacologic therapy has failed, is contraindicated, or is not tolerated; and third, in an elective capacity in patients with dysrhythmias that have failed to be managed by pharmacologic therapy.
DC cardioversion involves the application of electrical energy synchronized to the QRS phase of the cardiac cycle. Electrical energy is applied across the thorax, stimulating the myocardium, simultaneously exciting cardiac myocytes, inducing rhythmic ventricular contraction. The concurrent activation and contraction of ventricular myocytes subsequently causes the myocardium to enter an effective refractory period, abating reentrant circuitry propagation. The rationale for failure of successful DC cardioversion by electrophysiologic evidence has been abstracted as two paradigms, the first being due to the critical mass theory and the second being the upper limit vulnerability theory. The former is based on the premise that a critical myocardial mass is required to generate and propagate an arrhythmia. Consequently, if all available myocytes are refractory and thus inaccessible to promulgate arrhythmogenic foci, this results in terminating the arrhythmia. The second theory is based on the premise that subtherapeutic defibrillations are unable to render a critical number of myocytes refractory, causing certain myocytes to become refractory while paradoxically stimulating other myocytes during a vulnerable phase in the cardiac cycle, culminating in arrhythmogenic reentrant circuits.
The usage and procedural conduct of DC cardioversion contrasts to defibrillation (DF). DF is not synchronized to a specific phase of the cardiac cycle, and it is used in emergent contexts such as pulseless ventricular tachycardia (VT) or ventricular fibrillation (VF). DC cardioversion can be used for a multitude of tachyarrhythmias, whether they arise from the atria, the atrioventricular (AV) node (together forming the supraventricular tachycardia [SVT]), or the ventricles. SVTs include circuit reentrant propagation; examples include atrial fibrillation and atrial flutter. Arrhythmias of ventricular origin (ventricular tachyarrhythmias) include VT and VF.
A defibrillator is a device that stores electrical charge and releases a predefined quanta of energy in a controlled and coordinated manner according to a predetermined and preset amount, and it is conventionally measured in joules, the SI unit for energy. The electrical current is discharged via defibrillation pads or paddles. Optimum pad positioning includes the anterolateral (anteroapical) position or the anteroposterior position. The anterolateral position is where one pad is placed under the right clavicle on the right sternal margin at the second intercostal level. The second pad is placed at the second intercostal level, over the cardiac apex. In the anteroposterior position, the anterior pad is placed over the left sternal margin overlying the cardiac apex, and the posterior pad is placed distally, between the scapulae. Modern defibrillators use conductive pads that improve electrical current delivery by reducing thoracic impedance. Some studies have suggested that the paddles may be associated with a higher incidence of successful cardioversion to SR.
Successful outcome in DC cardioversion can be broadly divided into patient-related factors and equipment-related factors.
Patient-related factors include the following:
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Thoracic impedance: energy level, electrode skin interference, distance, polarity, phase of ventilation, intrinsic myocardial properties, and overall ability to conduct electricity
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Type of arrhythmia
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Duration of arrhythmia
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Electrolyte imbalances (i.e., potassium, magnesium, and phosphate)
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Toxins and proarrythmogenic drugs (chronotropes, bathmotropes, and dromotropes)
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Concomitant chronic diseases
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Presence of implantable cardiac devices
Equipment-related factors include the following:
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Electrode: type, size, position
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Paddles versus adhesive pads
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Total energy used and duration
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Waveform: monophasic versus biphasic
An important distinction between defibrillation paddles and pads is that in the former, the operator must apply the paddles onto the patient’s thorax with a force of approximately 110 Newtons (25 pounds). Pads, on the other hand, do not require the operator to remain in contact with the chest during defibrillation, thereby improving safety. Pads also provide additional functionality such as the ability to undertake transcutaneous pacing.
Recognition
Acute issues presented within the case scenario include the following:
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Elderly
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Acute AF with a rapid ventricular response in context of IHD
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Severe dehydration
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Electrolyte abnormalities: hypokalemia, hypomagnesemia, hypophosphatemia
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Significant hemodynamic instability
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Shortness of breath (SOB) with supplementary oxygen
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Acute kidney injury
The case synopsis demonstrates that the patient has an unstable tachyarrhythmia compounded by dehydration and electrolyte imbalances. The juxtaposition of the acute physiologic derangement against the substantial history of cardiovascular complications (IHD, previous myocardial infarctions with the need for coronary stents, and diastolic dysfunction) leads to an oxygen demand versus supply imbalance. This imbalance culminates into AF with rapid ventricular response, reducing diastolic filling and further compounding the oxygen demand versus supply disparity.
The advanced cardiovascular life support (ACLS) and the advanced life support (ALS) guidelines both recommend DC cardioversion for the management of tachyarrhythmia for the treatment of unstable tachyarrhythmia in hemodynamically unstable patients.
Adverse features of an unstable rhythm include the following:
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Myocardial insufficiency, ischemia (chest pain, SOB, sympathetic stimulation)
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Cardiac failure (fatigue, SOB, orthopnea, peripheral and/or pulmonary edema)
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End-organ hypoperfusion (cardiac [ischemia, failure], renal [reduced urine output])
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Cerebral hypoperfusion (altered mental status, decreased consciousness, syncope)
DC cardioversion may also be recommended as the treatment modality of choice in preference to pharmacologic agents, even in stable patients, as they prevent untoward and proarrhythmogenic effects (see Chapter 161 ). This is particularly important in patients who may have structural heart disease or have a diseased myocardium, such as previous myocardial infarction, acute ischemia, and presence of heart failure (e.g., a reduced ejection fraction less than 35%). In these contexts, the side effects of drugs may indeed outweigh the benefits.
The main contraindications for cardioversion include the lack of operational experience and technical expertise, the presence of multiform atrial (ectopic) tachycardia, and dysrhythmias due to enhanced automaticity (i.e., digitalis toxicity). Pregnancy and the presence of an implantable device are not absolute contraindications; however, certain precautionary measures should be undertaken. These include using the lowest feasible energy, placing the DF pads at least 12 cm away from the implantable device, and if possible having the device switched off or reprogrammed to a nonsensing mode.
Risk Assessment
When faced with a patient with an arrhythmia, it is paramount to adopt a structured and protocolized approach to an unwell patient, using an airway (A), breathing (B), circulation (C), and disability (D; ABCD) paradigm. Furthermore, it is also important to undertake the simultaneous correction of any life-threatening conditions while undertaking the initial assessment. Using this approach may attenuate end-organ damage by restoring tissue and organ reperfusion. It is also important to emphasize that emergency cardioversion in unstable patients is time critical and requires prompt action.
In stable patients, arrhythmias do not require emergency correction, and recent evidence favors rate over rhythm control. If cardioversion is indicated, this should only be undertaken after anticoagulation, which can be organized in an elective capacity (discussed later).
Implications
The elective management of cardioversion in stable patients is both a safe and common procedure. In contrast, an emergency situation presents multiple challenges, in terms of patient-, equipment-, and location-related factors:
Patient-related factors:
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Emergency cases
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Deranged physiology
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Preexisting myocardial disease
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Concomitant chronic diseases
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Oxygen demand versus supply imbalance
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Equipment- and location-related factors:
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Remote site
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Unfamiliarity and limitation of available equipment
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Limited availability of anesthetic and resuscitation drugs
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Availability of the minimum standards of monitoring
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Presence of a trained assistant
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The advent of improved techniques, drugs, and training has sanctioned the safer delivery of cardioversion. Cardioversion that adopts biphasic waveform significantly reduces the amount of energy required for successful cardioversion. Biphasic cardioversion and DF delivers energy in two (opposing) vectorial planes, by alternating the polarity and thus electrical axis, contrasting to the unidirectional monopolar waveform. The former requires appreciably greater energy: 360 J versus 200 J for DF. Cardioversion involves the delivery of electrical current to stimulate the myocardium, triggering rigorous muscular contractions that can result in pain and discomfort. To mitigate these, anesthesia or sedation is required. Stable patients undergo elective cardioversion in a day surgery setting, providing they are hemodynamically stable and similarly fulfill the usual requirement for undergoing day surgery. Despite cardioversion having a long history of safe provision, especially in the elective environment, it is still fraught with several (potentially lethal) complications. These include the following:
Airway and breathing:
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Pulmonary aspiration of gastric contents, especially in patients who are nonfasted or have gastric stasis
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Failed intubation
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Difficulty in bag-valve-mask (BVM) ventilation
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Loss of airway
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Cardiovascular system:
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Failure to cardiovert to SR
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Hypotension and myocardial instability, compounded by anesthetic agents
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Bradycardias, particularly with concurrent AF
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Progression to a ventricular dysrhythmia
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Cardiac arrest: pulseless VT, VF, asystole, pulseless electrical activity
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Neurologic system:
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Venous thromboembolic events: cerebrovascular accident (CVA), transient ischemic attack (TIA)
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Altered consciousness, confound under general anesthesia
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Musculoskeletal system:
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Long bone and vertebral fractures, especially in patients with osteoarthritis, mineral deficiency, and metabolic bone disorders (1.5-fold increased risk)
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Myopathic pain
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