The demand for anesthesia care outside of the operating room (OR) has been growing rapidly in recent years. Anesthesia providers may be used in a wide range of hospital settings including radiology (computed tomography [CT], magnetic resonance imaging, interventional radiology), the electrophysiology suite, cardiac catheterization laboratory, and gastroenterology and urology suites. Anesthesia services are also used in the office setting, with office-based surgery increasing 20-fold between 1984 and 2000.1
Many procedures are now being performed in out-of-OR settings because technology and medical advances have led to therapeutic options that are less invasive and do not require a traditional OR. Tumors may be destroyed with precisely targeted radiation therapy, cardiac arrhythmias managed with radiofrequency ablation (RFA), and coronary arteries imaged with CT.
Along with these advances in minimally invasive therapies, the role of the anesthesiologist in the out-of-OR setting is evolving. No longer does conscious sedation always provide the necessary depths of anesthesia and optimal working conditions for the interventionalist or electrophysiologist. General anesthesia has become standard practice in many locations. Although the basic principles of anesthetic management apply, certain techniques are particularly suited for the unique conditions that many of these minimally invasive procedures require.
The use of high frequency ventilation (HFV) is one technique that is finding a niche in out-of-OR anesthesia. Whereas it is unusual for the normal motion associated with respiration to adversely affect an open surgical case, this is not always true for arrhythmia ablation procedures, or those requiring precisely directed radiation therapy. In such cases, even slight motion artifact from spontaneous or positive pressure ventilation may significantly affect the procedure. It is through minimization of respiratory motion, and its ability to provide near static conditions of the chest and abdomen, that HFV is gaining traction as a viable technique for providing mechanical ventilation in sites such as the electrophysiology suite and diagnostic and interventional radiology. In the past 2 years, 64% of Acutronic Monsoon ventilators (used to provide high frequency jet ventilation [HFJV]) sold in the United States were for use in the electrophysiology suite (Travis Schaszberger, Susquehanna Micro, Inc., personal communication, September 17, 2010). Although the price of the Monsoon jet ventilator depends on the exact specifications, the cost is approximately US $35,000 per unit.
HIGH FREQUENCY VENTILATION
HFV encompasses a variety of ventilatory modes that use high respiratory rates (f) and low tidal volumes. In HFJV, the clinician will typically set f, inspiratory time, driving pressure, and expiratory pressure. Its benefits have been exploited in both the OR and intensive care unit (ICU) settings. In the OR, the technique is used for tracheal resection and reconstruction.2 In the ICU, it has been used in adults with acute respiratory distress syndrome, and in children with persistent pulmonary hypertension of the newborn.3,4 This article focuses on novel uses of HFJV by anesthesiologists in out-of-OR and ICU settings.
THE ELECTROPHYSIOLOGY SUITE AND ATRIAL FIBRILLATION ABLATION
Atrial fibrillation (AF) is responsible for 350,000 annual admissions in the United States and more than $6 billion in direct medical costs.5 The incidence of AF is projected to increase 2- to 4-fold by 2040. Although advances have been made in understanding the pathophysiology of this disease, few therapies have emerged that are curative. Pulmonary vein isolation and ablation is one technique that has been demonstrated to be effective in preventing recurrence of AF.6
Many factors enter into the choice of anesthetic technique for pulmonary vein isolation and ablation, and there are no randomized, controlled trials that demonstrate improvements in outcome with general anesthesia versus conscious sedation. In many centers, it is performed under conscious sedation, and an anesthesia provider may only be present for a cardioversion. A single anesthesia provider can support multiple procedure rooms under this model. When general anesthesia is needed, additional providers are required and a new model for anesthesia coverage must be adopted. Not only may general anesthesia improve patient safety and comfort, but HFJV may improve success rates, facility utilization and efficiency, and allow a broader range of patients to be treated.
Pulmonary vein isolation and ablation for AF requires sustained contact between the ablation catheter and the pulmonary veins for periods of up to 1 minute. The catheter is inserted via the femoral vein, traverses the vena cava, right atrium, and punctures the atrial septum to access the left atrium. Tidal ventilation may reduce catheter stability, both through changes in the length of the path and through changes in the dimension of the left atrium and pulmonary veins. An obstructed breathing pattern may further exacerbate this through enlargement of the left atrium during forced expiration. There is a strong association among obesity, obstructive sleep apnea, and AF.7 Patients with obstructive sleep apnea may be challenging to sedate for an ablation procedure. Undersedation may lead to discomfort from back pain, and patient movement may produce registration errors in the electroanatomic map. Oversedation may cause an obstructed breathing pattern. General anesthesia avoids these issues and may allow greater ablation catheter stability.
The study of HFJV in the electrophysiology suite is in its infancy, with only the University of Pennsylvania (Philadelphia, PA) and the University of Pittsburgh (Pittsburgh, PA) routinely using the technique. In a retrospective study of 36 patients undergoing atrial ablation procedures at the University of Pittsburgh, Goode et al.8 demonstrated a decrease in procedure duration and fewer ablation lesions required to obtain pulmonary vein isolation when HFJV was used, as compared with controlled mechanical ventilation.
The use of HFJV has expanded dramatically at the University of Pennsylvania. The first case was performed in 2007, and we have experienced significant growth in the number of cases using the technique each year (Fig. 1). We have 3 Monsoon jet ventilators (Acutronic Medical Systems, Fabrik im Schiffli, Switzerland), which are dedicated to the electrophysiology suite. Although improvement in ablation catheter technology has undoubtedly contributed to this, the use of HFJV is likely a major factor.
TECHNIQUE FOR HFJV FOR AF ABLATION
The Acutronic Monsoon ventilator and the Bunnell Life Pulse high frequency jet ventilator are available in the United States and internationally to provide HFJV. Sunshine Medical Equipment Company and TELI also manufacture jet ventilators in China. The initial settings we use are largely empiric and may require minor adjustments after initiating mechanical ventilation to optimize gas exchange. The variables that can be manipulated are respiratory rate, inspiratory time, and driving pressure.
The respiratory rate may be adjusted from 12 to 150 breaths per minute. The thorax and abdomen exhibit coupling of oscillatory modes in HFJV.9 The impact of this coupling on cardiac movement, as well as the optimal ventilatory frequency to maximize carbon dioxide (CO2) elimination and minimize cardiac movement, are both unknown. As the respiratory rate increases, the efficiency of CO2 elimination will decrease. We typically use respiratory rates of 120 breaths per minute based on our clinical observations that this provides satisfactory conditions for both ablation procedures and CO2 removal.
We initially set an inspiratory time of 40%. As this increases, lung volume will increase. Even in the morbidly obese patient, we have not had to alter the inspiratory time to maintain adequate oxygenation.
Driving pressure is the pressure the ventilator produces during inspiration, and can be varied from 0 to 45 pounds per square inch. We empirically start with a setting of 14 per square inch and increase as needed to maintain normocarbia. If an arterial line is not available to monitor arterial CO2, it can be assessed by converting from HFJV to controlled mechanical ventilation and measuring end-tidal CO2 (EtCO2). If arterial access is available, we typically obtain an arterial blood gas after approximately 30 minutes to assess the correlation between the EtCO2 and arterial CO2 values.
The Monsoon ventilator may be connected to either an endotracheal tube (ETT) or laryngeal mask airway. In an effort to minimize barotrauma, the ETT cuff may be left deflated, thereby creating an open system. This approach, however, results in greater entrainment of room air, leads to reduced lung volumes, and may increase atelectasis. Although the risk of barotrauma may be decreased in an open system, in an obese patient, the reduced lung volumes and increased atelectasis may worsen hypoxemia. Because our patients have not experienced barotrauma during HFJV, we inflate the ETT cuff, thereby creating a closed system. In an open system, placement of a jet ventilation catheter (Biro catheter, for example2) through an ETT and positioning it in closer proximity to the airway may help with CO2 elimination. However, we have not experienced any difficulty with hypercarbia when we connect the jet ventilation tubing directly to an elbow adaptor at the proximal end of the ETT or laryngeal mask airway (Fig. 2). By connecting the tubing at this position, it allows for easy transitioning between the jet and conventional ventilators. We have not conducted a randomized trial comparing the Biro catheter to our current technique.
There is no readily available method for delivering volatile anesthetics during HFJV, and total IV anesthesia is the mainstay for anesthetic maintenance. We have found that a combination of propofol and remifentanil infusions works well for anesthetic maintenance for AF ablation. The minimally stimulating nature of the procedure allows for lower than expected anesthetic doses, and we typically start propofol at 50 μg/kg/min and remifentanil at 0.1 μg/kg/min. We have not experienced any cases of anesthetic recall in patients undergoing HFJV for AF ablation. Initially, bispectral index (BIS) monitors were used to help assess intraoperative anesthetic depth. However, our observations that the BIS reading did not lead to alterations in anesthetic dosing and the lack of cases of intraoperative awareness have led us to abandon their use in these cases. There is no contraindication, however, to its use if the anesthesia provider prefers to titrate its anesthetic based on the BIS reading.
Muscle relaxants are used infrequently, and when they are used, it is frequently a depolarizing drug that facilitates intubation. Long-acting muscle relaxants are typically avoided because they interfere with the use of high output pacing to identify the phrenic nerves. Long-acting drugs are only used when muscle relaxation is needed for intubation and there is a contraindication to a depolarizing drug. The prolonged nature of AF ablation procedures makes remifentanil's metabolism via plasma esterases advantageous, because it will not accumulate. Although the effect of anesthetics on atrial triggers is incompletely understood, it is prudent to delay induction until cardiac mapping is complete, thereby giving the electrophysiologist the greatest chance of detecting any atrial trigger sites.
Standard anesthesia monitors are unable to provide continuous EtCO2 levels during HFJV. Although transcutaneous CO2 monitoring has been used in microlaryngosurgery,10 we rarely use this technique because of the duration of the cases. To obtain accurate transcutaneous CO2 readings in adults, the probe temperature must be maintained at 44°C, which results in a risk of burns unless the probe is moved every hour. We periodically switch to traditional ventilation (approximately every 30 minutes) to measure EtCO2, and directly measure arterial CO2 if an arterial catheter is available. There is evidence that thermal injury to the esophagus is more prevalent in patients undergoing pulmonary vein isolation and ablation under general anesthesia compared with conscious sedation.11 For this reason, intraluminal esophageal temperature is always monitored when the AF focus is in close proximity to the esophagus. The standard American Society of Anesthesiology monitors are used in every patient, including pulse oxymetry, arterial blood pressure monitoring, and electrocardiogram.
Every patient who presents for AF ablation is evaluated for the use of HFJV, and in our experience with the technique for this procedure, very few patients are deemed inappropriate candidates. In general, the greatest contraindication to HFJV is a patient who is too hemodynamically unstable to tolerate induction and general anesthesia with positive pressure ventilation. However, because AF ablation cases are almost always elective, this composes a very small portion of the patient population. In some patients, it may be challenging to maintain normocarbia, and it is imperative to monitor the arterial CO2 using either an arterial blood sample or an end-tidal measurement obtained via intermittent traditional mechanical ventilation. Although very unusual, hypercarbia refractory to adjustments in the driving pressure and inspiratory time may necessitate a permanent transition to conventional mechanical ventilation. We have not experienced any cases of hypoxemia leading us to abandon HFJV intraoperatively.
Thoracic and abdominal stability is critical to obtain high-quality CT images of chest and abdominal structures. In patients who are awake and breathing spontaneously, relative immobility may be attained by asking subjects to intermittently hold their breath. In patients requiring mechanical ventilation, transiently pausing ventilation will achieve a similar result. Although this may be sufficient for obtaining a series of CT images, such brief periods of respiratory stabilization are often too short to allow a CT-guided biopsy or RFA procedure. As in the case for AF ablations, the desire to minimize respiratory motion has prompted interest in the use of HFJV in these situations.
RFA is a less-invasive alternative to liver surgery for tumors with a diameter <4 cm.12 When performed under CT guidance, it is critical that the ablation probe be placed precisely at the correct position, avoiding the risk of nontarget injuries. A single probe activation during an ablation procedure for a liver mass may take longer than 10 minutes, and movement during this period when the procedure is performed under local anesthesia may be associated with parenchymal injury and hemorrhage. HFJV has been used in this situation to reduce respiratory and liver motion while imaging and treating hepatic lesions.12 In a series of 10 patients undergoing single-dose irradiation of liver tumors, Fritz et al.13 used HFJV along with a gold marker implant to enhance precision of the treatment radiation beam. Liver motion was limited to <3 mm in all directions, and the authors note that this technique may improve the accuracy of stereotactic body radiation therapy. HFJV has also been demonstrated as an acceptable technique for stereotactic body radiation therapy of stage I non–small cell lung cancer and lung metastases, allowing a reduction in the irradiation planning target volume.14 A larger planning target volume is associated with increased normal lung tissue damage.
HFJV may have a role in complex patients requiring diagnostic imaging procedures. Mandel et al.15 described the case of a 74-year-old man with ventilator-dependent respiratory failure who required coronary artery CT angiography before cardiac valve replacement surgery for endocarditis. HFJV was used via an uncuffed tracheostomy tube, allowing adequate images to be obtained without the interference of respiratory motion artifact.
FUTURE DIRECTION AND CHALLENGES
There is little doubt that in the future an increasing proportion of anesthesia providers' time will be spent in out-of-OR locations. Just as medical and surgical advances are allowing more cases to be performed minimally invasively, advances in anesthetic techniques and practices will be needed to optimize patient care in specialized settings such as the electrophysiology suite and interventional radiology. HFJV is one technique that may help improve outcomes in a variety of procedures including AF ablation, radiation therapy for lung and liver tumors, and certain diagnostic imaging studies. Further trials of this technique are clearly warranted to explore the use of HFJV in these settings.
Name: Jesse Raiten, MD.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Jesse Raiten approved the final manuscript.
Name: Nabil Elkassabany, MBBCh.
Contribution: This author helped conduct the study, analyze the data, and write the manuscript.
Attestation: Nabil Elkassabany approved the final manuscript.
Name: William Gao.
Contribution: This author helped analyze the data and write the manuscript.
Attestation: William Gao approved the final manuscript.
Name: Jeff E. Mandel, MD, MS.
Contribution: This author helped design the study, conduct the study, analyze the data, and write the manuscript.
Attestation: Jeff E. Mandel approved the final manuscript.
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