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Anesthetic Management of Patients Undergoing Deep Brain Stimulator Insertion

Venkatraghavan, Lashmi, MD, FRCA, FRCPC; Luciano, Michelle, MD; Manninen, Pirjo, MD, FRCPC

doi: 10.1213/ANE.0b013e3181d2a782
Neuroscience in Anesthesiology and Perioperative Medicine: Review Article
Free
SDC
CME

Deep brain stimulation is used for the treatment of patients with neurologic disorders who have an alteration of function, such as movement disorders and other chronic illnesses. The insertion of the deep brain stimulator (DBS) is a minimally invasive procedure that includes the placement of electrodes into deep brain structures for microelectrode recordings and intraoperative clinical testing and connection of the DBS to an implanted pacemaker. The anesthetic technique varies depending on the traditions and requirements of each institution performing these procedures and has included monitored anesthesia with local anesthesia, conscious sedation, and general anesthesia. The challenges and demands for the anesthesiologist in the care of these patients relate to the specific concerns of the patients with functional neurologic disorders, the effects of anesthetic drugs on microelectrode recordings, and the requirements of the surgical procedure, which often include an awake and cooperative patient. The purpose of this review is to familiarize anesthesiologists with deep brain stimulation by discussing the mechanism, the effects of anesthetic drugs, and the surgical procedure of DBS insertion, and the perioperative assessment, preparation, intraoperative anesthetic management, and complications in patients with functional neurologic disorders.

Published ahead of print February 8, 2010

From the Department of Anesthesia, Toronto Western Hospital, University Health Network, Toronto, Ontario, Canada.

Address correspondence and reprint requests to Lashmi Venkatraghavan, MD, FRCA, FRCPC, Department of Anesthesia, Toronto Western Hospital, University Health Network, 399 Bathurst St., Toronto, ON, Canada M5T 2S8. Address e-mail to Lashmi.venkatraghavan@uhn.on.ca.

Accepted December 16, 2009

Published ahead of print February 8, 2010

Deep brain stimulation is used for the treatment of patients with neurologic disorders who have an alteration of function that is not usually accompanied by gross structural or anatomical changes, such as Parkinson disease, essential tremors, dystonia, and certain psychiatric conditions.1,2 The aim of this procedure is to improve the quality of life of the patient. The insertion of the deep brain stimulator (DBS) is a minimally invasive procedure that includes the placement of electrodes into deep brain structures for microelectrode recordings (MERs) and macrostimulation for clinically testing the patient and connection of the DBS to an implanted pacemaker. The anesthesiologist plays a key role in the management of patients for the insertion of a DBS. The anesthetic technique varies depending on the traditions and requirements of each institution performing these procedures and has included monitored care under local anesthesia, conscious sedation, and general anesthesia.37 There are many challenges and demands for the anesthesiologist in the care of these patients, whichever anesthetic technique is used. The purpose of this article is to familiarize anesthesiologists with the surgery of DBS insertion, the influences of anesthesia on the procedure, and to describe the anesthetic management of patients undergoing DBS insertion.

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DEEP BRAIN STIMULATION

History and Mechanism

Historically, surgical procedures for functional neurologic disorders, such as Parkinson disease, consisted of making a lesion in deep brain structures with procedures such as thalamotomy, pallidotomy, and cingulotomy. Lesioning procedures were irreversible and were associated with several permanent side effects.8 After the discovery of the beneficial effects of intraoperative electrical stimulation, deep brain stimulation was first described in 1987 as an alternative treatment to ablative procedures to reduce the symptoms of Parkinson disease.9,10 Reversibility and the option of bilateral stimulation along with the ability to titrate the stimulation have revolutionized the use of the DBS for functional neurosurgery. The safety and long-term benefits of the DBS are well documented.11 After its initial success in patients with Parkinson disease, the indications and applications have now expanded to include many other disorders such as dystonia, other tremors, obsessive-compulsive disorder, epilepsy, chronic pain, Alzheimer disease, and multiple sclerosis.1,2,12 The exact mechanism of deep brain stimulation is incompletely understood and may differ depending on the site of stimulation. The primary target sites vary with the patient's symptoms and include the subthalamic nuclei (STN), globus pallidus pars internal (GPi), and the ventralis intermedius nucleus of the thalamus (Vim)13 (Fig. 1). The effects of stimulation on the various nuclei differ. For example, stimulation of the STN causes hyperpolarization or “neuronal jamming,” and this consequentially results in the inhibition of its activity.14,15 In addition, STN inhibition decreases the production of glutamate, and this has been suggested as a mechanism of neuroprotection in patients with Parkinson disease after insertion of a DBS.16 Stimulation of the GPi nuclei may result in activation of γ-aminobutyric acid (GABA)ergic axons, which in turn inhibits GPi neurons.15 In contrast, stimulation of the Vim nucleus of the thalamus activates output to the neurons in the reticular nucleus, which then sends inhibitory efferents back to the thalamic nuclei.14 The inhibition of these targeted nuclei will then result in improvements of the patient's symptoms. The effects of a DBS are also shown to be frequency dependent, with the greatest relief of symptoms at >100 Hz and no therapeutic effect at <50 Hz.13

Figure 1

Figure 1

The DBS system consists of 3 components: the intracranial electrode, an extension cable, and an implanted pulse generator. The electrode is a coiled wire insulated in polyurethane with 4 platinum iridium electrodes for implantation in the target neural tissue. The lead is connected by an extension cable to the implanted pulse generator that is a battery-powered neurostimulator encased in titanium housing, which sends electrical pulses to stimulate the target site. It is placed subcutaneously below the clavicle or in the abdomen and can be programmed to optimize symptom suppression and control side effects.

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Surgical Technique

The overall surgical procedure involves 2 stages: insertion of the electrode(s) into the target area of the brain, and the internalization of the lead(s) and implantation of the programmable pulse generator. The nuclei that are targeted are deep and small in size, requiring a variety of methods to increase the accuracy of targeting. This includes the use of frame-based imaging to visualize brain structures and to establish coordinates, electrophysiologic guidance with MER, and macrostimulation testing of an awake patient. The whole procedure may be completed on the same day or as a 2-staged procedure with the internalization of the electrode(s) and generator on a different day, typically 3 days to 2 weeks after the procedure. Currently, there is no evidence favoring the best timing of the second stage. The timing depends on many factors including duration of the procedure and patient cooperation. Another reason for delaying of the internalization is the “microlesion” effect caused by edema around the freshly implanted electrode. This effect may cause improvement of the patient's symptoms without any stimulation, and this impairs the ability to check for stimulation-induced benefits.17

The procedure begins with the placement of a rigid head frame to the patient's skull and magnetic resonance imaging (MRI) to visualize brain structures and to establish references to external coordinates for accurate insertion of the electrode into the target areas for stimulation. Depending on the institutional preference, different head frames are used. In a survey of North American centers performing DBS operations, the Cosman-Roberts-Wells frame was the most frequently used frame followed by the Leksell G.18 Both of these frames may restrict access to the patient's airway in varying degrees. There are also reports of the use of frameless navigation systems for a DBS.19 After imaging, the patient is transferred to the operating room where he or she is positioned in a supine or semisitting position on the operating room table with the stereotactic frame fixed to the bed. A bur hole is made in the cranium for electrode insertion. To localize the target area for stimulation (STN, GPi, and Vim), the neurophysiology team will obtain MERs that are used to detect and amplify the activity of individual neurons. The electrode is usually inserted 10 to 15 mm above the target site and is advanced 0.5 to 1 mm along its trajectory toward the target nuclei while spontaneous neuronal discharges are recorded. The neurophysiologists use the variations in spontaneous firing rates between specific nuclei (GPi and globus pallidus external) and movement-related changes in the firing rates to localize the specific brain target (Fig 2). Macrostimulation, which involves the clinical testing of the patient's movements, is then used to verify that the stimulation of the electrode at this location will improve their symptoms and not cause any side effects. After radiologic confirmation, the electrode is secured, and the wound is closed. If bilateral DBS insertions are planned, a second incision will be made on the other side and the procedure is repeated. The second stage, the internalization, is performed by tunneling the electrode(s) and connecting the extension cable through the scalp and subcutaneously on the side of the neck to an infraclavicular area where it is connected to the generator pacemaker.

Figure 2

Figure 2

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The Effects of Anesthetic Drugs on MERs

It is not completely known to what extent anesthetic drugs influence the MER, because the effects of anesthetic drugs are not homogeneous across different regions of the brain, and there are only limited studies.20 When sedation or general anesthesia is used during DBS insertion, propofol is the most frequently used anesthetic. Successful MERs have been made from different target sites (GPi, STN, and Vim) using propofol.4,21 However, differences in the characteristics of neuronal activity among individual target sites and also within the same target site with different disease processes have been reported with the use of propofol. Hutchison et al.21 showed that the firing rates in the GPi nucleus were substantially decreased, and long pauses were present in patients with dystonia under general anesthesia with propofol compared with patients mapped under local anesthesia. This finding was consistent with animal studies that showed enhancement of GABAergic striatal and globus pallidus external afferents to GPi with propofol.22 Another study by Steigerwald et al.23 reported that neither the estimated plasma concentration of propofol nor the level of consciousness of the patient influenced the neuronal firing rates in patients with dystonia. This is probably because of the overall decreased GPi neuronal firing rates in dystonia. The effects of general anesthesia with desflurane were studied by Sanghera et al.24 in 11 patients with dystonia and 6 patients with Parkinson disease. There were no differences between awake and anesthetized patients with respect to GPi nuclei firing rates for the dystonia group, but there was a significant decrease in GPi nuclei firing rates in the Parkinson group. Thus, there were differences in GPi neuronal firing rates between the patients with dystonia and Parkinson disease, and the effect of anesthesia may be more pronounced in Parkinson disease.

The ability to obtain MERs from the STN nuclei during anesthesia has been more successful.4,2529 The anesthetic techniques used have varied from conscious sedation with propofol and dexmedetomidine with no airway manipulation to general anesthesia with endotracheal intubation with either IV or inhaled techniques. The differences in the anesthetic effects on the various target nuclei (STN versus GPi) may be explained by the amount of their GABA input. The GPi neurons have a higher GABA input compared with the STN neurons and, therefore, are more suppressed by most anesthetic drugs.30 There is limited information regarding the effects of anesthetics on the Vim nuclei.

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Effect of Anesthetic Drugs on Macrostimulation Testing

Clinical stimulation testing of the patient is an important aspect of DBS insertion to observe the clinical benefits and adverse effects of the DBS. Essentially, this requires an awake and cooperative patient. The effects of conscious sedation may be minimized if short-acting drugs are used and they are stopped well before testing. General anesthesia interferes with the evaluation of the clinical benefits of DBS by the suppression of clinical symptoms such as tremors and rigidity.31,32 Also, the patient cannot report subjective effects such as paresthesia or abnormal motor activity associated with stimulation of adjacent structures (internal capsule and medial lemniscus). The examination of the optic track with flash visual evoked potentials in patients undergoing stimulation of the GPi nuclei under general anesthesia has been shown to be possible.33 Also, higher intensity stimulation has been used for the activation of the internal capsule during general anesthesia.34 Some centers that routinely perform DBS insertion under general anesthesia do not conduct any form of stimulation testing in the operating room but confirm the location of the implanted DBS electrode with MRI.35

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ANESTHETIC MANAGEMENT

Perioperative Evaluation and Preparation

Successful treatment with a DBS depends on proper patient selection. A multidisciplinary team consisting of neurologists, neurosurgeons, neurophysiologists, neuropsychologists, and psychiatrists initially evaluates the patient. Selection of an ideal patient includes an overall assessment of the patient with respect to diagnosis, cognitive and psychiatric status, access to care, and expectations by the patient, and the patient's response to medical treatment.36,37

There are specific challenges and considerations in the anesthetic management of patients undergoing DBS insertion (Table 1). Hence, all patients scheduled for DBS insertion should also be seen by the anesthesiology team before surgery. In addition to routine perioperative assessment and preparation, these patients require additional considerations because they may present with many comorbidities related to the disease processes for which the DBS is indicated3844 (Table 2). For patients who are awake for all or part of the procedure, a thorough airway assessment is important, and options of securing the airway at any stage of the procedure should be planned in advance. A history of obstructive sleep apnea should be elicited in all patients and planned for appropriately. Patients with psychiatric disorders will also have their own set of challenges, and patients with chronic pain will need special consideration for the management of their pain medications perioperatively. All patients who will be awake for any part of the procedure need to be given instructions for what to expect and good psychological preparation for all events. Increased anxiety preoperatively and during the procedure may lead to an increase of the patient's arterial blood pressure, posing a risk for intracerebral bleeding. Instructions for the continuation or discontinuation of disease-specific drugs should be provided in conjunction with the neurosurgical team, because some patients need to be in a “drug-off ”state to facilitate intraoperative mapping and clinical testing. This poses additional challenges to the perioperative care because the “drug-off” state may worsen the patient's symptoms, especially in Parkinson disease and dystonia (Table 2). If the symptoms are severe, a reduced dose of the patient's regular medication can be administered after discussion with the neurosurgical team.

Table 1

Table 1

Table 2

Table 2

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Anesthetic Techniques

To facilitate the intraoperative neurophysiologic mapping (MER) and/or clinical testing of the patient, most DBS procedures are performed with local anesthesia and monitored care or with conscious sedation during parts of the procedure when testing is not performed. However, general anesthesia may be needed for specific groups of patients who have an irrational fear of awake surgery, chronic pain syndromes, severe “off-medication” movements, severe dystonia or choreoathetosis, and the young pediatric population. The second stage of the procedure (internalization) is usually performed with general anesthesia. The airway may be secured with an endotracheal tube, because access to the airway is restricted with tunneling of the cable on the side of the neck. During this procedure, there are no concerns with specific anesthetic drugs because no testing is performed.

The stereotactic frame is usually placed on the patient's skull with the use of local anesthesia for the pin sites, which is followed by MRI for localization and tabulating of the variables for DBS insertion. Some patients may require sedation for frame placement and/or for MRI. If conscious sedation or general anesthesia is to be started in the radiology suite, the anesthesiologist must have adequate equipment and support to care for the patient in this potentially “remote” site. Also, if anesthesia is required for MRI, all safety concerns for anesthesia in an MRI suite must be adhered to.

Ideally, airway management (endotracheal tube and laryngeal mask airway [LMA]) should precede placement of the stereotactic frame. However, if general anesthesia needs to be induced with the frame in place, conventional laryngoscopy may be difficult. Other options of securing the airway such as fiberoptic endotracheal intubation or an LMA may be used. If an LMA is used, one needs to be aware of the increased risk of regurgitation and aspiration, especially in patients with Parkinson disease.

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Monitored Anesthesia Care

Local anesthesia is used as a subcutaneous infiltration at the pin sites and at the site of incision(s) for the bur hole(s) for electrode insertion. Supraorbital and greater occipital nerve blocks are an alternative because they have been shown to be less painful than subcutaneous infiltration, although they did not result in any difference in pain at the time of pin placement or during surgery.45 The local anesthetic drugs frequently used include bupivacaine, ropivacaine, and lidocaine with and without epinephrine.46 Complications of local anesthesia may include toxic blood levels resulting in seizures and respiratory and cardiac arrest. If the procedure has been long, additional infiltration may be required for closure.

Standard anesthesia monitors include an electrocardiogram, noninvasive arterial blood pressure, oxygen saturation, and end-tidal CO2. Invasive blood pressure monitoring may be indicated for blood pressure control. Monitoring may be technically difficult in some patients with severe movement disorders and spasticity. Omission of the urinary catheter will be more comfortable for the awake patient, but fluid administration needs to be monitored carefully to avoid hypovolemia. Supplemental oxygen is delivered through nasal prongs or a mask with an outlet for end-tidal CO2 and respiratory rate monitoring. Proper positioning of patients on the operating table is an important step to ensure maximal comfort and cooperativeness, especially for awake patients. The head and neck should be positioned with some degree of flexion at the lower cervical spine and extension at the atlantooccipital junction. This helps to make the patient's airway patent and make it possible for the anesthesiologist to secure the airway in an emergency. The legs should be flexed and supported under the knees to maintain stability when the head and back are elevated to a sitting position. To aid in positioning, special treatment modalities have been used such as physiotherapy, small doses of levodopa, and intrathecal hydromorphone.47,48 Patients with obstructive sleep apnea may need continuous positive airway pressure therapy intraoperatively. In these patients, the facemask needs to be placed before the head frame, and the patient's continuous positive airway pressure machine should be readily available. The use of clear plastic drapes will make it easy for the anesthesiologist to maintain verbal and eye contact with the patient throughout the case.

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Conscious Sedation

In some institutions and/or for some patients, conscious sedation is used for DBS insertion, especially during the opening and closure of the procedure. Frequently used drugs include midazolam, propofol, opioids such as fentanyl or remifentanil, and dexmedetomidine.57,25,26,49,50 However, there are concerns with the use of all these drugs as discussed above. The advantages and disadvantages of various drugs used for conscious sedation are shown in Table 3. Generally, the use of benzodiazepines is discouraged.51 Propofol has been widely used, most frequently as a continuous infusion, alone, or combined with remifentanil. The mean infusion rates of propofol reported in the literature are approximately 50 μg/kg/min.7,48,52 However, one needs to be aware, if using target-controlled infusion devices, that the pharmacokinetic behavior of propofol in patients with Parkinson disease may differ from the general population for which the model was developed.53 Dexmedetomidine with low-dose infusion rates (0.3–0.6 μg/kg/h) may be a better choice because of its non–GABA-mediated mechanism of action allowing for MER, hemodynamic stability, and analgesic properties.6,25,26 Optimal conditions for MER or stimulation testing can be obtained with the use of conscious sedation as long as short-acting drugs are used and stopped before the recordings and testing.

Table 3

Table 3

The use of depth of anesthesia monitors to titrate sedation and the state of arousal during DBS insertion would be ideal; however, studies have shown conflicting results. The reliability of bispectral index (BIS) monitoring during MER is questionable because the effects of anesthetics are heterogeneous across the different regions of the brain, and there is dissociation between the neocortical and subcortical effects of IV and inhaled drugs.20 Schulz et al.54 found that the use of BIS did not offer any advantages regarding the time to arousal, total propofol consumption, and cardiopulmonary stability. However, they did not study the effect of the anesthetics on MER. A study by Elias et al.26 showed a positive result for the use of BIS monitoring for titrating dexmedetomidine sedation. They found that the subthalamic MER signals were equivalent to the awake state when sedation was titrated to an easily arousable state (BIS value >80) in patients with Parkinson disease. However, deep sedation (BIS <80) suppressed MER signals.

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General Anesthesia

General anesthesia may provide a higher level of acceptance for DBS surgery by some patients and a possible enlargement of the group of patients that can be treated. Intraoperative mapping and stimulation testing will be difficult under general anesthesia. There are no prospective, randomized, blinded studies to compare the clinical outcome with that of an awake technique. There are few reports in the literature on the use of general anesthesia for DBS insertion.4,27,28,29,55 Yamada et al.27 found that general anesthesia in 15 patients with Parkinson disease did not adversely affect postoperative improvements in motor and daily activity scores, except for “off-medication” bradykinesia, when compared with 10 patients under local anesthesia. In another study with 10 patients, Lin et al.29 found that desflurane anesthesia allowed for adequate MERs for successful DBS insertion. Thus, DBS insertion under general anesthesia is possible with careful titration of anesthetics and with the use of limited electrophysiologic mapping. Randomized controlled studies are needed to compare the long-term clinical benefits of patients undergoing DBS insertion under general anesthesia with that of local anesthesia.

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COMPLICATIONS

The insertion of a DBS is not without the potential for perioperative complications, which demands vigilance in rapid recognition and treatment of these events by the anesthesiologist. Overall, intraprocedure complications have been reported to occur in 12% to 16% of patients.5,7 Intraoperative respiratory complications are of great concern, occurring in 1.6% to 2.2% of patients.5,7 In the awake patient, they may result from oversedation or intracranial events such as seizures or hemorrhage leading to a decreased level of consciousness. Acute airway obstruction may occur in a restless awake patient as the body shifts but the head remains fixed to the bed.5 All appropriate airway equipment should be readily available because managing the airway in a patient with a rigid head frame poses a great challenge. The frame restricts neck movement and covers some or all the patient's mouth and nose. Ideally, if possible, one should attempt to secure the airway without the removal of the patient's head frame, so the surgery could be continued if indicated. Releasing the frame from the table may take time, and in an emergency securing, the airway with an LMA may be the most appropriate option.

Other respiratory complications relate to the patients’ diseases. Patients with Parkinson disease may have restrictive pulmonary dysfunction from poor respiratory muscle function resulting in reduced forced vital capacity and reduced baseline arterial oxygen saturation, upper airway obstruction, dysarthria, and obstructive sleep apnea.7,38,51,56,57 Respiratory insufficiency caused by the absence of anti-Parkinson medications in the postoperative period may also occur.

Cardiovascular complications can lead to devastating outcomes. Hypertension has been associated with increased risk of intracerebral hemorrhages.58,59 This may be more problematic in the awake patient who becomes agitated and anxious. Arterial blood pressure must be controlled before the insertion of the electrode to prevent intracranial hemorrhages. Frequently used drugs include labetalol, hydralazine, nitroglycerine, sodium nitroprusside, and esmolol. The optimal level of blood pressure is not well defined; one may use a systolic blood pressure of <140 mm Hg or a 20% increase of the patient's usual range.59 Orthostatic hypotension may result from anti-Parkinson medications or might be further aggravated by the vasodilating effects of anesthetics, perioperative hypovolemia, and autonomic dysfunction. Glossop and Dobbs60 reported on 2 patients who experienced chest pain, tachycardia, hypertension, and oxygen desaturation during insertion of a DBS under local anesthesia. This was accompanied with ST segment changes and increased troponins, although further testing showed normal coronary arteries. They attributed the symptoms to abnormal vasoactive responses resulting in coronary vasospasm. Animal studies have shown that stimulation of the paraventricular region in the hypothalamus can cause either hypertension or hypotension.61

Other less common complications include venous air embolism.6265 Semisitting position in a hypovolemic patient increases the risk. During creation of the bur hole in awake patients, sudden vigorous coughing may be a sign of venous air embolism. Other signs are unexplained hypoxia and hypotension. Early detection may be possible with precordial Doppler monitoring. The incidence of venous air embolism as detected by a precordial Doppler ultrasound has been reported to be 4.5%. Hooper et al.65 in their small study of 21 patients noted 1 venous air embolism (1 of 22 lead insertions), and the important predictors were patient positioning and the occurrence of coughing. There are no reports of whether the precordial Doppler interferes with MER. Tension pneumocephalus has also been reported during DBS insertion.66

Neurologic complications may occur during or after the procedure.67,68 Focal deficits such as extremity weakness or confusion may not require any acute treatment by the anesthesiologist. Seizures have been reported to occur in 0.8% to 4.5% of patients and often occur during stimulation testing.5,7 Most seizures were focal and did not require treatment. Small doses of midazolam and/or propofol may initially be used, and the procedure resumed after control of the seizure. A sudden loss of consciousness resulting from an intracranial bleed or neurologic injury will require rapid and more aggressive treatment.

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CONCLUSION

The use of a DBS has and will continue to increase in popularity for the treatment of many functional neurologic disorders. This is especially true for an increasing elderly population ratio within the rapidly changing population demographics worldwide. DBS use has been shown to be safe, and new indications will continue to emerge. The role of the anesthesiologist in the care of these patients will also continue to evolve. New developments in surgical and imaging technology and a better understanding of the effects of drugs on the MER will lessen the difficulties and complications of these procedures. Even though the anesthetic techniques will continue to differ among various centers and may include monitored anesthesia care, conscious sedation, and general anesthesia, the general principles of anesthesia care remain the same. The anesthesiologist needs to be aware of the unique requirements of these patients and of these procedures. Continuous monitoring and extreme vigilance are vital to early diagnosis and rapid treatment of complications.

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