Anesthesia Case of the Month

Bonnie J. Gatson Department of Comparative, Diagnostic, and Population Medicine, College of Veterinary Medicine, University of Florida, Gainesville, FL 32608.

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Marta Romano Department of Comparative, Diagnostic, and Population Medicine, College of Veterinary Medicine, University of Florida, Gainesville, FL 32608.

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History

A 7-year-old 4.1-kg (9.2-lb) neutered male domestic shorthair cat with a 5-year history of intermittent inappetence, lethargy, and hypersalivation was evaluated at the University of Florida Primary Care and Dentistry Service for severe, chronic gingivostomatitis. The cat had previously received conservative medical treatment, including combinations of oral administration of clindamycin, metronidazole, and tramadol and transmucosal administration of buprenorphine, by several referring veterinarians; however, despite treatment, there was no change in the cat's condition, and full mouth extractions were recommended. Initial physical examination revealed a grade 2/6 heart murmur, and a limited oral examination revealed severe, chronic gingivostomatitis and tonsillitis with generalized moderate dental tartar. Hematologic and serum biochemical analyses performed the day before the anesthetic procedure revealed moderate leukocytosis (WBC count, 25.8 × 103 WBCs/μL; reference range, 5.4 × 103 WBCs/μL to 15.4 × 103 WBCs/μL), characterized by neutrophilia (19 × 103 neutrophils/μL; reference range, 2.3 × 103 neutrophils/μL to 9.8 × 103 neutrophils/μL) and moderate presence of Döhle bodies observed on manual review of a blood smear; hyperproteinemia (9.5 g/dL; reference range, 6.2 to 8.1 g/dL), which was attributed to moderate hyperglobulinemia (7.5 g/dL; reference range, 3.4 to 5.4 g/dL); and mild hypoalbuminemia (2.0 g/dL; reference range, 2.2 to 3.4 g/dL). The owner declined the recommended preanesthetic thoracic radiographic and echocardiographic examinations.

Food, but not water, was withheld from the cat for approximately 8 hours overnight before the anesthetic procedure. The following morning, the cat was premedicated with hydromorphone (0.08 mg/kg [0.04 mg/lb], IM) and alfaxalone (2.50 mg/kg [1.13 mg/lb], IM) to facilitate aseptic placement of a 22-gauge, 1-inch catheter into the cat's right cephalic vein. Prior to use, the anesthesia machine and nonrebreathing circuit passed a retrograde fill test, and the endotracheal tube cuff passed a leak test. After the cat was preoxygenated by face mask for 5 minutes, general anesthesia was induced with alfaxalone (2.90 mg/kg [1.31 mg/lb], IV) administered over 60 seconds. Orotracheal intubation was attempted; however, copious blood-tinged saliva blocked visualization of the arytenoid cartilages, necessitating gentle suctioning of the oropharyngeal region. In addition, 2% lidocaine hydrochloride (0.50 mg/kg [0.23 mg/lb]) was sprayed onto the glottis to reduce laryngeal spasms. Marked swelling in the oropharyngeal region made orotracheal intubation challenging; yet, after several attempts, intubation was achieved with a 3.5-mm-internal-diameter cuffed Murphy-type endotracheal tube. Following intubation, the cat was connected to a standard anesthesia machine by way of a nonrebreathing circuit attached to the anesthetic machine end of the endotracheal tube. The position of the endotracheal tube was checked by measuring the distance from the anesthetic machine end of the endotracheal tube to the incisors when the endotracheal tube was seated at the level of the thoracic inlet. A syringe attached to the pilot balloon of the endotracheal tube was used to inflate the cuff of the endotracheal tube with air until a pressure of 20 cm H2O could be maintained in the nonrebreathing circuit during manual ventilation. The endotracheal tube was secured to the cat with a length of clean plastic tubing from an IV administration set that was tied around the endotracheal tube and, with the free ends beyond this knot extended bilaterally out of the cat's mouth, also tied behind the cats' ears. The cat was then placed in dorsal recumbency for the dental procedure.

General anesthesia was maintained with 1% to 2% isoflurane (vaporizer setting) in oxygen (2 L/min), and the cat was allowed to breathe spontaneously. Throughout the anesthetic procedure, a continuous infusion of lactated Ringer solution (3 mL/kg/h [1.4 mL/lb/h], IV) was administered through the IV catheter placed earlier in the cat's right cephalic vein. In addition, heart rate and rhythm (measured with a lead II ECG), oxygen saturation (measured with pulse oximetry), and end-tidal CO2 concentration and respiratory rate (measured with capnography) were monitored with a multiparameter monitora throughout the anesthetic procedure. Indirect arterial blood pressure was measured with a Doppler ultrasonic flow detector combined with a size 2 cuff attached to a sphygmomanometer, and the cat's rectal temperature was monitored with a digital thermometer. The cat's heart rate, respiratory rate, and blood pressure remained unremarkable throughout the procedure.

Following oral radiography and a thorough dental examination, bilateral maxillary and inferior alveolar nerve blocks were performed with administration of mepivacaine hydrochloride (2.0 mg/nerve block; 2.00-mg/kg [0.91-mg/lb] total dosage for this cat) 15 minutes before dental extractions were started. Also before extractions, a 2-cm2 foam pad was placed in the oropharyngeal region at the level of the glottis to help collect blood and debris anticipated during the procedure and thereby further prevent such material from being aspirated or swallowed or otherwise posing risks. Once extractions began, the cat's heart rate steadily rose to 150 beats/min noted 105 minutes after the time of premedication; thus, a second dose of hydromorphone (0.05 mg/kg [0.02 mg/lb], IV) was administered to help with pain control. Afterward, the cat's cardiorespiratory parameters remained within acceptable ranges for the remainder of the anesthetic procedure. After 1 hour of general anesthesia, the cat's rectal temperature was 36.1°C (97.0°F; reference range, 38.1° to 39.2°C [100.5° to 102.5°F]); therefore, a forced-air warming deviceb was placed on the cat. The cat's rectal temperature then steadily increased to 37°C (98.6°F) by the end of the procedure.

Dental extractions were completed 227 minutes after general anesthesia was induced. Isoflurane and fluid therapy were discontinued at that time, and the patient was placed in sternal recumbency for recovery. All patient-monitoring equipment, except for pulse oximetry and capnography, were removed, and the foam pad that had been placed in the oropharyngeal region earlier was also removed. The cat was maintained on 100% oxygen until its palpebral reflex returned, at which time the endotracheal tube was disconnected from the anesthesia machine and the cat was allowed to breathe room air. Twenty minutes after isoflurane administration was discontinued, the cat was observed to have swallowed several times. Therefore, a syringe attached to the 1-way valve on the pilot balloon of the endotracheal tube was aspirated until the pilot balloon appeared deflated. Extubation was attempted; however, marked resistance precluded removal of the endotracheal tube. After it was clear that extubation was not feasible without potential harm to the cat's trachea, alfaxalone (1.0 mg/kg [0.45 mg/lb], IV) was administered to reestablish general anesthesia.

The pilot balloon of the endotracheal tube was confirmed to be completely deflated, and no additional air could be removed through its 1-way valve. Oropharyngeal examination performed with a laryngoscope revealed oropharyngeal swelling similar to that observed before the anesthetic procedure, but no clinically important laryngeal edema. Lubricant gel was placed around the endotracheal tube at the level of the glottis; however, the endotracheal tube still could not be retracted. A small lubricated polypropylene urinary catheter was gently introduced between the inside of the tracheal wall and the outside of the endotracheal tube in an attempt to dislodge the tube. However, the catheter could only be advanced a few centimeters caudal to the level of the glottis before it was obstructed from passing further. During this time, additional boluses of alfaxalone (2.0 mg/kg [0.91 mg/lb] total, IV) were administered to maintain general anesthesia, and 100% oxygen supplementation was provided by reconnecting the nonrebreathing circuit to the anesthetic machine end of the endotracheal tube. The catheter was removed from the airway, and a single right lateral thoracic radiographic image was obtained. The radiographic image indicated that the endotracheal tube cuff was still inflated in the cat's trachea (Figure 1).

Figure 1—
Figure 1—

Right lateral thoracic radiographic view of an anesthetized 4.1-kg (9.2-lb) 7-year-old neutered male domestic shorthair cat after multiple failed attempts at extubation during recovery from full-mouth dental extractions. Notice the distended cuff (arrow) of the endotracheal tube in the lumen of the trachea.

Citation: Journal of the American Veterinary Medical Association 254, 1; 10.2460/javma.254.1.75

Question

What was the most likely reason that the endotracheal tube cuff remained fully distended in the cat's trachea despite complete deflation of the pilot balloon?

Answer

The inability to deflate the cuff of the endotracheal tube and extubate the cat was caused by a kink in the cuff inflation tubing at its insertion into the wall of the endotracheal tube. We believed that the tie used to secure the endotracheal tube slipped slightly during the procedure and kinked the inflation tubing. At the end of the procedure, this kink prevented complete deflation of the endotracheal tube cuff, despite the collapsed appearance of the pilot balloon (Figure 2). This hypothesis was confirmed when the knot tied around the endotracheal tube to secure it in place was manually slid toward the anesthetic machine end of the endotracheal tube (away from the cat), and the pilot balloon immediately inflated. With the cuff inflation tubing no longer kinked, deflation of the cuff followed by extubation of the cat was achieved. Later examination of the endotracheal tube confirmed that the cuff could be inflated and deflated normally when the inflation tubing was uninhibited (Figure 3).

Figure 2—
Figure 2—

Photograph of the endotracheal tube extubated from the cat in Figure 1. The cuff (arrowhead) has been reinflated to depict how a tie used to secure the endotracheal tube in place in the cat could cause a kink (arrow) in the cuff inflation tubing and prevent deflation of the cuff even when the pilot balloon (asterisk) is deflated.

Citation: Journal of the American Veterinary Medical Association 254, 1; 10.2460/javma.254.1.75

Figure 3—
Figure 3—

Photographs of the endotracheal tube extubated from the cat in Figure 1 showing that the cuff (arrowhead) and pilot balloon (asterisk) could be deflated (A) and inflated (B) properly when the inflation tubing was uninhibited by the plastic tie that had been used to secure the endotracheal tube in place in the cat.

Citation: Journal of the American Veterinary Medical Association 254, 1; 10.2460/javma.254.1.75

Discussion

The most ubiquitous type of endotracheal tube used in small animals is a cuffed Murphy-type tube made from translucent flexible nontoxic plastic.1 The cuff is an inflatable plastic sleeve overlying the circumference of the endotracheal tube and can be distended and collapsed by way of an inflation device consisting of a pilot balloon and inflation tubing. The cuff is connected to the pilot balloon by a thin hollow inflation tubing, the external portion of which connects to the pilot balloon at least 3 cm in length before entering into the wall of the endotracheal tube, where it then runs throughout the length of the tube until it connects to the cuff.2 The cuff can be expanded and collapsed by connecting a syringe into the 1-way valve seated in the pilot balloon. The pressure of gases sealed in the pilot balloon is transmitted to the cuff through the inflation tubing. The cuff and the external inflation tubing are typically made from the same flexible translucent plastic material as the endotracheal tube.2

Cuffed endotracheal tubes are routinely used to secure airways of adult domestic small animals because cuffed endotracheal tubes offer several advantages over those that are not cuffed. Such advantages include a decreased risk of patients aspirating foreign materials into their airways, the ability to use lower fresh gas flow rates and inspiratory pressures for adequate ventilation of patients, a decreased risk of personnel exposure to anesthetic gases, and an improved accuracy in monitoring airway dynamics and end-tidal concentrations of respiratory gases.2 Further, intubation of cats with cuffed endotracheal tubes is standard of care for dental and oral surgery to prevent surgical flush, blood, and debris from entering into the lower airways.3

Although complications associated with endotracheal tube insertion and long-term use have been documented in cats,4–6 scarcely has difficulty extubating veterinary patients been reported.7–9 Extubation difficulties are described in humans as rare but extremely disconcerting, and the complication is most commonly caused by a failure of the cuff to deflate.10 Inability to deflate the cuff is reported to occur commonly from failure of the deflating mechanism, and inadvertent kinking and occlusion of inflation tubing secondary to adhesive tape, artery forceps, and a retaining bandage have all been reported previously in humans.11–16 During these occurrences, the pilot balloon gives no indication regarding the state of cuff inflation. However, when extubation cannot be performed despite complete integrity of the inflation tubing and pilot balloon, underlying causes could include laryngeal swelling, forceful intubation with an inappropriately large endotracheal tube, folding of the plastic cuff on itself during deflation leading to a larger external diameter of the tube, and inadvertent surgical fixation of the endotracheal tube to the tracheal lumen.7,9,17–20 In addition, formation of an annulus (a clustering of the plastic material of the cuff that results in a circumferential ridge around the cuff) at the distal end of the cuff following complete deflation has been reported in dogs and cats as a possible impedance to routine extubation.7,9

Multiple measures have been suggested in the literature to overcome difficulty during extubation in instances when the cuff cannot be deflated through the inflation device. Initially, examination of the pilot balloon and inflation tubing should be performed to identify any source of kink or obstruction.10 In addition, a needle attached to a syringe can be inserted into the inflation tubing proximal to the occlusion as a corrective measure to deflate the cuff17; however, this may be difficult to accomplish depending on the location of the obstruction. Alternatively and with possible risk of injury to the patient or personnel, the endotracheal tube can be gently pulled until the cuff is visualized at the level of the arytenoid cartilages and then punctured either orally or through the cricothyroid membrane.10 If the decision is made to detach the pilot balloon and valve assembly from the inflation tubing, care should be taken because this procedure will only deflate the cuff when the malfunction is at the level of the pilot balloon, and forcefully pulling the pilot balloon may stretch the plastic of the inflation tubing thin to the point of occlusion from the mechanical stress.17

When extubation cannot be achieved despite proper function of the cuff inflation device, considering and resolving other causes is important. For instance, the most common cause of difficult extubation in small animal patients is intubation with an endotracheal tube that with a deflated cuff has an external diameter that may be too large for easy passage across the arytenoid cartilages because improper folding of the cuff or formation of an annulus could prevent the deflated cuff from lying flat along the walls of the endotracheal tube.7,8 When this scenario is suspected, the endotracheal tube can be returned to proper position in the trachea so that the cuff can be reinflated to smooth out possible problematic folds, then deflated again before reattempting extubation.7,10 Additionally, an oropharyngeal examination should be performed to determine whether airway edema could be the underlying cause of an extubation difficulty, and if edema is the cause, lubrication (eg, with a small amount of water-soluble nontoxic lubricant) of the endotracheal tube at the level of the glottis along with gentle traction may result in successful extubation, although trauma to the airway is a concern with this method.9 Further, when a procedure involves suture placement near an endotracheal tube, direct visualization or endoscopic examination of the endotracheal tube to check for tethering structures should be performed before extubation.18 Alternatively, use of a supraglottic airway device, instead of an endotracheal tube, could be considered for patients undergoing procedures performed around the larynx or extrathoracic trachea; however, use of a supraglottic airway device also has advantages and disadvantages.2

Given the severe, chronic gingivostomatitis of the cat in present report and the repeated attempts at intubation, the cause of the difficult extubation in this cat was originally believed to have been the cat's excessive pharyngeal swelling. Because the anesthetists assumed that deflation of the pilot balloon indicated complete collapse of the endotracheal tube cuff, the external portion of the inflation tubing was not inspected initially for a kink or other obstruction. Once the anesthetists realized that the catheter inserted into the airway to dislodge the endotracheal tube was likely obstructed by the tube's inflated cuff, radiography was performed, and results confirmed that the cuff was inflated. Immediately afterward, the anesthetists closely inspected the inflation tubing, then found and corrected the kink. The misconception that the degree of pilot balloon inflation directly indicates the degree of cuff inflation can prevent appropriate identification of the underlying cause of a difficult extubation; therefore, the inflation tubing should always be inspected regardless of the state of the pilot balloon.

The present report illustrated that a plastic tie used to secure an endotracheal tube in place in a patient could impede normal extubation and that equipment malfunction caused by human error could lead to potentially serious complications for animals undergoing general anesthetic procedures. Relatedly, studies21,22 evaluating critical incidents during anesthetic procedures in people have emphasized that human error is the dominant cause of anesthetic mishaps, resulting in up to 293 of 359 (82%) preventable incidents, whereas direct equipment failure caused only 50 of 359 (14%) preventable critical incidents and was implicated in 3 of 70 (4%) critical incidents resulting in substantial negative outcomes. Despite the relative infrequency of anesthetic equipment failure occurring during general anesthesia in people, user mismanagement of equipment is reported to cause between 4% and 25% of total equipment failures, most of which resulted in no harm to the patients.23,24 Although the most common sources of human error involve problems with drug administration or misuse of the anesthesia machine, airway mismanagement constitutes an important source of preventable human error.21–23,25 Regarding endotracheal tubes specifically, a study26 shows that 189 of 2,000 (9%) anesthetic complications in people were associated with endotracheal tubes and that, among the 189 incidences, common sources of human error associated with endotracheal tube use included endobronchial intubation (42%), esophageal intubation (18%), accidental disconnections from the anesthesia machine (7%), leakage around the endotracheal tube (7%), and inappropriate choice of endotracheal tube size (3%). That same study26 also shows that user inability to deflate the cuff of the endotracheal tube is a rare problem in humans on the basis of the problem being reported in only 0.5% of all reported endotracheal tube complications considered. The incidence of anesthetic equipment failure resulting in patient morbidity in veterinary medicine has not been thoroughly investigated. However, recognizing potential sources of error and implementing practices that mitigate the chance of replicating observed errors may substantially decrease the total number of anesthetic mishaps associated with operator mistakes during clinical management of veterinary patients.27

In a study21 examining predisposing circumstances associated with critical incidences for humans undergoing general anesthesia, inadequate experience of personnel was the most commonly cited factor leading to human error. Additional factors have also been reported,21,22 including inadequate familiarity with equipment, poor communication, haste or carelessness, inattention, operator fatigue, and failure to perform routine equipment checks. Further, human patients with an American Society of Anesthesiologist physical status classification28 of 1 or 2 (on a scale of 1 [healthy] to 6 [brain dead]) are more likely to experience human error with the anesthetic component of their procedure than are patients with a physical status classification ≥ 3; however, the impact of human error on patient outcome is more severe in critically ill patients.22

The present report exemplified an equipment-related technical error that was not immediately recognized, likely because of operator bias. Given the cat's condition of severe, chronic gingivostomatitis, the anesthetists incorrectly assumed that the inability to extubate the cat was a result of pharyngeal swelling. This assumption prevented the anesthetists from following the correct diagnostic steps in a logical order, impaired troubleshooting the problem effectively, and delayed identifying the underlying cause of extubation difficulty. The present report also highlighted a source of error that could occur during seemingly routine anesthetic procedures and underscored the need to ensure that the inflation tubing and pilot balloon are unimpeded when securing cuffed endotracheal tubes to patients so that problems with inflation or deflation of the cuff are prevented.

Footnotes

a.

IntelliVue MP50, Philips Healthcare, Andover, Mass.

b.

Bair Hugger warming unit, model 505, 3M Corp, Maplewood, Minn.

References

  • 1. Mosley CA. Anesthesia equipment. In: Grimm KA, Lamont LA, Tranquilli WJ, et al, eds. Lumb & Jones veterinary anesthesia and analgesia. 5th ed. Ames, Iowa: Wiley-Blackwell, 2015;2385.

    • Search Google Scholar
    • Export Citation
  • 2. Dorsch JA, Dorsch SE. Tracheal tubes and associated equipment. In: Dorsch JA, Dorsch SE, eds. Understanding anesthesia equipment. 5th ed. Philadelphia: Lippincott Williams & Wilkins, 2008;561632.

    • Search Google Scholar
    • Export Citation
  • 3. de Vries M, Putter G. Perioperative anaesthetic care of the cat undergoing dental and oral procedures: key considerations. J Feline Med Surg 2015;17:2336.

  • 4. Brodbelt DC, Pfeiffer DU, Young LE, et al. Risk factors for anaesthetic-related death in cats: results from the confidential enquiry into perioperative small animal fatalities (CEPSAF). Br J Anaesth 2007;99:617623.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 5. Mitchell SL, McCarty R, Rudloff E, et al. Tracheal rupture associated with intubation in cats: 20 cases (1996–1998). J Am Vet Med Assoc 2000;216:15921595.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 6. Hofmeister EH, Trim CM, Kley S, et al. Traumatic endotracheal intubation in the cat. Vet Anaesth Analg 2007;34:213216.

  • 7. Sanchis Mora S, Seymour C. An unusual complication of endotracheal intubation. Vet Anaesth Analg 2011;38:158159.

  • 8. Niimura del Barrio MC, Espadas I, Hughes JML. Breakage of two silicone endotracheal tubes during extubation. J Small Anim Pract 2015;56:530532.

  • 9. Norgate D, Palacios Jimenez C. A rare complication associated with the endotracheal tube during extubation in a cat. Vet Anaesth Analg 2017;44:14011403.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 10. Hartley M, Vaughan RS. Problems associated with tracheal extubation. Br J Anaesth 1993;71:561568.

  • 11. Adhikary SD, George SP, Korula G. Failure of endotracheal cuff deflation. Acta Anaesthesiol Scand 2005;49:590.

  • 12. Gleich SJ, Mauermann WJ, Torres NE. An unusual case of difficult extubation. Respir Care 2008;53:376.

  • 13. Hulme J, Agarwal S. Failure to deflate an endotracheal cuff. Acta Anaesthesiol Scand 2008;52:719.

  • 14. Nag DS, Samadder DP. Inappropriate fixation of an endotracheal tube causing cuff malfunction resulting in difficult extubation. Braz J Anesthesiol 2016;66:536538.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 15. Suman S, Ganjoo P, Tandon MS. Difficult extubation due to failure of an endotracheal tube cuff deflation. J Anaesthesiol Clin Pharmacol 2011;27:141142.

  • 16. Tanski J, James RH. Difficult extubation due to a kinked pilot tube. Anaesthesia 1986;41:1060.

  • 17. Brock-Utne JG, Jaffe RA, Robins B, et al. Difficulty in extubation. A cause for concern. Anaesthesia 1992;47:229230.

  • 18. Gotoh S, Sugiyama D, Imai E, et al. Difficulty with cuff deflation of reinforced tracheal tube caused by inflation line occlusion with silk thread ligation and fixation. BMJ Case Rep 2015;2015:bcr-2014207933.

    • Search Google Scholar
    • Export Citation
  • 19. Mishra P, Scott DL. Difficulty at extubation of the trachea. Anaesthesia 1983;38:811.

  • 20. Tashayod M, Oskoui B. A case of difficult extubation. Anesthesiology 1973;39:337.

  • 21. Cooper JB, Newbower RS, Long CD, et al. Preventable anesthesia mishaps: a study of human factors. Qual Saf Health Care 2002;11:277282.

  • 22. Cooper JB, Newbower RS, Kitz RJ. An analysis of major errors and equipment failures in anesthesia management: considerations for prevention and detection. Anesthesiology 1984;60:3442.

    • Crossref
    • PubMed
    • Search Google Scholar
    • Export Citation
  • 23. Fasting S, Gisvold SE. Equipment problems during anesthesia—are they a quality problem? Br J Anaesth 2002;89:825831.

  • 24. Cassidy CJ, Smith A, Arnot-Smith J. Critical incident reports concerning anaesthetic equipment: analysis of the UK National Reporting and Learning System (NRLS) data from 2006–2008. Anaesthesia 2011;66:879888.

    • PubMed
    • Search Google Scholar
    • Export Citation
  • 25. McIntyre JW. Anesthesia equipment malfunction: origins and clinical recognition. Can Med Assoc J 1979;120:931934.

  • 26. Szekely SM, Webb RK, Williamson JA, et al. The Australian incident monitoring study. Problems related to the endotracheal tube: an analysis of 2000 incident reports. Anaesth Intensive Care 1993;21:611616.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 27. Hofmeister EH, Quandt J, Braun C, et al. Development, implementation and impact of simple patient safety interventions in a university teaching hospital. Vet Anaesth Analg 2014;41:243248.

    • Crossref
    • Search Google Scholar
    • Export Citation
  • 28. American Society of Anesthesiologists. ASA physical status classification system. Available at: www.asahq.org/resources/clinical-information/asa-physical-status-classification-system. Accessed Sep 18, 2018.

    • Search Google Scholar
    • Export Citation
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