Use of laryngeal mask airway compared to endotracheal tube with positive-pressure ventilation in anesthetized swine

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1 Veterinary Anaesthesia and Analgesia, 2007, 34, doi: /j x RESEARCH PAPER Use of laryngeal mask airway compared to endotracheal tube with positive-pressure ventilation in anesthetized swine Pamela J Fulkerson DVM, Diplomate ACVA & Scott B Gustafson DVM, MS, Diplomate ACVS Magruder Hall, College of Veterinary Medicine, Oregon State University, Corvallis, OR, USA Correspondence: Pamela J Fulkerson, Magruder Hall, College of Veterinary Medicine, Oregon State University, Corvallis, OR 97331, USA. pam.fulkerson@oregonstate.edu Abstract Objective To compare the ease of placement and ventilatory parameters of a laryngeal mask airway (LMA) with an endotracheal tube (ETT) in anesthetized swine during positive-pressure ventilation (PPV). Study design Prospective, randomized, experimental trial. Animals Nine young domestic swine, weighing between 40 and 49 kg (mean 45.1 kg), being used for a separate terminal surgical study. Methods The pigs were immobilized with tiletamine/ zolazepam, mg kg )1, intramuscularly, followed by isoflurane in oxygen delivered by facemask. The lungs were mechanically ventilated through an ETT or an LMA, in random order, during the anesthetic period. Positive-pressure ventilation was adjusted to maintain end-tidal CO 2 (PE CO 2 ) between 35 and 45 mmhg, with peak inspiratory pressure (P insp ) of cmh 2 O. Buprenorphine, 0.3 mg intramuscularly, was given to each pig after instrumentation. Isoflurane vaporizer settings were adjusted to maintain a surgical plane of anesthesia. Respiratory rate (RR), tidal volume (V T ), minute volume ( _V E ), and PE CO 2 were measured and recorded at 5-minute intervals. After the collection of 1 hour of data, the alternate airway was placed. Swine were given at least 30 minutes to stabilize and another hour of data were recorded. At the time of airway placement, the ease of placement was assessed based on time and the number of personnel required. Data were analyzed using paired Student s t-test or Wilcoxon signed rank test where appropriate. Results Laryngeal mask airways were significantly easier to place than ETT. Values for V T and _V E were not significantly different between treatments. Peak inspiratory pressures were higher in ETT-ventilated swine. Conclusions and clinical relevance An LMA may be used as an alternative to an ETT in mechanicallyventilated anesthetized swine. Use of an LMA may reduce time and personnel required for placement of an airway. Keywords airway, controlled ventilation, endotracheal intubation, laryngeal mask, swine. Introduction Swine are frequently used as experimental models, due to their anatomic and physiologic similarities with humans. Swine have a long mouth, which does not open widely, and the larynx of pigs slopes ventrally which makes visualization of the larynx and endotracheal intubation relatively difficult (Hartsfield 1996). Pigs are prone to developing laryngospasm (Greene & Benson 2002), which can further complicate the placement of an endotracheal tube (ETT). Successful intubation of anesthetized swine is time consuming, and requires trained personnel and 284

2 special equipment (Wemyss-Holden et al. 1999; Greene & Benson 2002). The laryngeal mask airway (LMA) was developed for use in human anesthesia and was first described in 1983 (Brain 1983). Laryngeal mask airways have been used to manage millions of human cases since their commercial introduction in 1988 (Brimacombe & Berry 1993; Maltby 1994). The LMA is used with positive-pressure ventilation (PPV) in adults and children (Gursoy et al. 1996; Verghese & Brimacombe 1996; Brimacombe 1998). There are unique considerations when using the LMA with PPV. The mean airway pressure at which oropharyngeal leak develops around the airway (seal pressure) is lower with LMA compared with ETT. The reported seal pressure of the LMA ranges from 16 to 21 cmh 2 O (Gursoy et al. 1996; Cook et al. 2002, 2003). There is an audible leak at cmh 2 O when an LMA is initially placed which often disappears with time (Pennant & White 1993). The LMA Instruction Manual states as a contraindication to the use of LMA Classic (The Laryngeal Mask Company Limited, San Diego, CA, USA), used in this study, if peak airway pressures are expected to exceed 20 cmh 2 O. The trachea is well protected from oropharyngeal fluids and contamination by the use of LMA (Brimacombe & Berry 1993). Although the reported rate of aspiration pneumonia in humans is similar whether an LMA or ETT is used (Verghese & Brimacombe 1996), there may be a risk of tracheal contamination from gastroesophageal reflux and regurgitation with an LMA. The use of the LMA has been described in cats (Fujita et al. 1991; Asai et al. 1998; Cassu et al. 2004), dogs (Braz et al. 1999), pigs (Patil et al. 1997; Wemyss-Holden et al. 1999), and rabbits (Cruz et al. 2000; Bateman et al. 2005; Smith & Robertson 2004). In swine, the advantages of the LMA compared with ETT include ease of placement and the lighter depth of anesthesia necessary for placement (Wemyss-Holden et al. 1999). In cats (Fujita et al. 1991; Asai et al. 1998; Cassu et al. 2004) and rabbits (Bateman et al. 2005), successful mechanical ventilation has been described with the LMA. No published reports exist comparing the use of LMA with ETT in mechanically ventilated swine. Materials and methods Nine immature domestic swine, weighing between 40 and 49 kg (mean 45.1 kg), were anesthetized for an unrelated terminal surgical study. The study protocol was approved by the Institutional Animal Care and Use Committee of Oregon State University. Each pig was evaluated with both ETT and LMA during the same anesthetic procedure. Pigs were immobilized with tiletamine/zolazepam (Telazol; Fort Dodge, Fort Dodge, IA, USA), mg kg )1 intramuscularly. Anesthesia was induced with isoflurane in oxygen delivered by facemask. Buprenorphine [Buprenex; Reckitt Benckiser Healthcare (UK) Limited, Hull, UK], 0.3 mg intramuscularly, was given for additional analgesia and sedation. Two pigs were studied each day and the order of airway placement in the first pig was determined by the toss of a coin. Airways were placed in the second pig in reverse order from the first pig. Endotracheal tubes, 7 8 mm inner diameter, were placed with swine in lateral or dorsal recumbency, using a laryngoscope and lidocaine spray on the larynx, by personnel trained and familiar with the technique. Placement of a flexible stylet through the glottis was visualized using a long-bladed laryngoscope then the tube was passed over the stylet. Laryngeal masks (LMA Classic; The Laryngeal Mask Company Limited), size 4, were placed with swine in dorsal recumbency by the following method. The cuff of the LMA was deflated so the leading edge was smooth and firm. Sterile lubricating gel was placed on the back of the cuff. After removal of the facemask from the pig, the back of the cuff was placed against the hard palate. Confirmation of LMA placement with the tip of the cuff flattened against the palate was verified by palpation. The tongue was immobilized with one hand, while the other hand pressed either the tube or the cuff of the LMA against the palate as the LMA was advanced. The LMA was advanced until a definite point of resistance was met. The cuff was inflated with less than 30 ml air, as recommended in the instruction manual. During cuff inflation, correct LMA placement was verified by an outward movement of the tube (approximately 1 cm) and the presence of a slight bulge at the level of the larynx. After initial airway placement, all pigs were positioned in dorsal recumbency and remained in that position for the duration of the study. Ease of airway placement was graded based on the following scale: 1 ¼ less than 30 seconds, requiring only 1 person; 2 ¼ less than 30 seconds with two people required; 3 ¼ more than one attempt required, lasting more than 30 seconds, with two people involved; 4 ¼ multiple attempts required lasting more than 2 minutes with Ó 2007 The Authors. Journal compilation Ó 2007 Association of Veterinary Anaesthetists, 34,

3 two people involved. Following placement of the airway, venous and arterial catheters were placed for fluid administration and arterial blood pressure monitoring, and controlled IPPV was started. Tidal volume was initially set at ml kg )1, respiratory rate was breaths minute )1, and peak inspiratory pressures were kept between 15 and 23 cmh 2 O, as measured by the airway pressure manometer on the anesthesia machine. Tidal volume, as measured by spirometry (7800 Ventilator; Ohmeda, Madison, WI, USA; Vitalert 2000/Narkomed 3; North American Drager, Telford, PA, USA) and respiratory rate were adjusted as needed to maintain end-tidal CO 2 (PE CO 2 ) (Capnomac Ultima; Datex, Helsinki, Finland) at mmhg ( kpa). Ventilation was considered to be successful if there was no audible gas escape, chest excursion appeared normal, and PE CO 2 was maintained at mmhg. Vaporizer settings were adjusted to maintain a surgical plane of anesthesia. Swine were stabilized for at least 30 minutes prior to recording data. Respiratory rate, tidal volume (V T ), minute ventilation ð _V E Þ, and PE CO 2 were recorded every 5 minutes. Peak inspiratory pressures (P I ) were recorded every 5 minutes in eight of nine pigs (P I values were not collected in one pig). Following collection of 1 hour of data with the initial airway type, the second airway type was placed as described above. After a minimum of 30 minutes with the second airway in place, data were collected for 1 hour as described above. The second airway was left in place for the duration of the surgical experiment. Differences in mean V T (ml), VE _ (L), and P insp (cmh 2 O) were compared using the paired t-test. Ease of airway placement was compared with the Wilcoxon signed rank test. Results Inspiratory pressure ranged from 15 to 23 cmh 2 O in eight of the nine pigs in which it was recorded, and respiratory rate was breaths minute )1. There were no significant differences between V T (p ¼ 0.22) or _V E (p ¼ 0.053) when using ETT or LMA, although minute ventilation tended toward significance with a higher minute volume in the LMA treatment. Peak inspiratory pressure was slightly but significantly lower (p ¼ 0.048) with LMA compared with ETT (Table 1). Laryngeal mask airway placement score in all pigs was 1. Endotracheal tube placement scores were 2 in five pigs, 3 in two pigs, and 4 in two pigs. Table 1 Tidal volume (V T ), minute volume ( _V E ) and peak inspiratory pressure (P I ) in anesthetized swine under controlled ventilation with a laryngeal mask (LMA) or endotracheal tube (ETT). Mean values given for values compared with paired t-tests Pig ID Airway placement was significantly easier with the LMA compared with ETT (p ¼ 0.004). Discussion V T (ml) _ VE (L) P I (cmh 2 O) LMA ETT LMA ETT LMA ETT Mean * 20*, Data not collected. *Statistically significant differences (p < 0.05). Placement of an LMA in swine was significantly easier than ETT placement. The range of difficulty for ETT placement was between 2 and 4 on the scale. The scores were somewhat dependent on the experience of the person or people attempting the intubation. However, in all nine pigs, placement of the LMA required only one person, regardless of experience, and was always completed in less than 15 seconds from removal of the face mask or ETT. In humans, laryngeal masks are used in cases of difficult intubation (Pennant & White 1993; Maltby 1994) and inexperienced paramedical personnel are able to place them more accurately and faster than ETTs (Davies et al. 1990). In animal studies where ease of placement was evaluated, LMA were consistently easier to place even when the researchers had extensive experience with endotracheal intubation (Fujita et al. 1991; Wemyss-Holden et al. 1999; Smith & Robertson 2004). Mechanical ventilation was successfully performed for 1 hour in all nine pigs with either the LMA or the ETT. The pigs were in dorsal recumbency for the surgical study. In two of the pigs with the LMA, the neck had to be slightly ventroflexed 286 Ó 2007 The Authors. Journal compilation Ó 2007 Association of Veterinary Anaesthetists, 34,

4 so that the snout was approximately degrees off horizontal, to prevent gas leakage from around the LMA. It was also noted that if the LMA was not securely tied into place, and if it rotated, gas leakage was more likely to occur Although minute volume was not significantly higher in the LMA compared with the ETT treatment, there was a trend toward statistical significance. Larger numbers of pigs or IPPV over a longer period of time would be useful in further evaluating this trend. Power analysis using a p-value of 0.8 and an alpha value of 0.05 estimates that statistical significance would be demonstrated with a sample size of 16. The LMA and the trachea have a larger diameter, and therefore volume, than the ETT (Pennant & White 1993). These volume differences may account for the trend in minute volume difference. Gastric dilation and tympany have been reported in humans (Weiler et al. 1997) and rabbits (Bateman et al. 2005) when PPV is performed with an LMA. The incidence of clinically detectable gastric insufflation increases with increasing peak airway pressures (Devitt et al. 1994). In humans, gastric insufflation is considered unlikely with peak inspiratory pressures less than 20 cmh 2 O and tidal volumes less than 8 ml kg )1 (Brimacombe 1997). In cats, gastric insufflation was present at peak airway pressures of 18.9 ± 6 cmh 2 O (Fujita et al. 1991). The peak inspiratory pressures in two pigs with LMA exceeded 20 cmh 2 O, and tidal volumes in eight of nine pigs with LMA were slightly above 8mLkg )1 (range ml kg )1 ). It is possible that the difference in volumes observed between LMA and ETT treatments was being forced into the esophagus and stomach. However, there was no evidence of clinically significant gastric dilation based on the absence of abdominal distension and regurgitation observed in these pigs. The LMA treatment had a statistically significant lower peak airway pressure. This can be at least partially explained by the larger diameter of the LMA compared with the ETT. The mean difference in peak airway pressure was 1.5 cmh 2 O. In both groups, the peak airway pressures were within acceptable limits for positive pressure ventilation (Hartsfield 1996). The airway pressure difference was not clinically significant. Placement of the LMA is rapid and easy. Although we did not verify exact placement with a fiberoptic endoscope, clinically the airway was clear and patent, and ventilation was successful. In humans, exact placement is not critical for a clinically acceptable patent airway (Brimacombe 1998). Although not a part of the data collection period, all five pigs with the LMA as the second airway were successfully ventilated for hours. We have subsequently used the LMA as standard protocol for airway management in all pigs used in surgical laboratory studies, including two retired breeding Hanford miniature swine, weighing approximately 130 and 185 kg. Visualization of the larynx was extremely difficult in these pigs, endotracheal intubation was not attempted and both were ventilated successfully for several hours with a size 5 LMA. The LMA was successfully used with mechanical ventilation in kg swine for 1 hour. Securing a patent airway with an LMA was faster and technically easier than with an ETT. Based on this study and our subsequent experience, we consider the LMA an appropriate and effective tool for airway management in swine. Acknowledgements The authors wish to thank Lisa Aguilera, MS, AHT and Natalie Mintz, CVT for their technical assistance. References Asai T, Murao K, Katoh T et al. (1998) Use of the laryngeal mask airway in laboratory cats. Anesthesiology 88, Bateman L, Ludders JW, Gleed RD, et al. (2005) Comparison between facemask and laryngeal mask airway in rabbits during isoflurane anesthesia. Vet Anaesth Analg 32, Brain AIJ (1983) The laryngeal mask a new concept in airway management. Br J Anaesth 55, Braz, JRC, Martins RHG, Mori AR et al. (1999) Investigation into the use of the laryngeal mask airway in pentobarbital anesthetized dogs. Vet Surg 28, Brimacombe J (1997) Positive pressure ventilation with the size 5 laryngeal mask. J Clin Anesth 9, Brimacombe J (1998) Problems with the laryngeal mask airway: prevention and management. Int Anesthesiol Clin 36, Brimacombe J, Berry A (1993) The laryngeal mask airway the first ten years. Anaesth Intensive Care 21, Cassu RN, Luna SPL, Neto FJT et al. (2004) Evaluation of laryngeal mask as an alternative to endotracheal intubation in cats anesthetized under spontaneous or controlled ventilation. Vet Anaesth Analg 31, Ó 2007 The Authors. Journal compilation Ó 2007 Association of Veterinary Anaesthetists, 34,

5 Cook TM, Nolan JP, Verghese C et al. (2002) Randomized crossover comparison of the ProSeal with the classic laryngeal mask airway in unparalysed anaesthetized patients. Br J Anaesth 88, Cook TM, McCormick B, Asai T (2003) Randomized comparison of laryngeal tube with classic laryngeal mask airway for anaesthesia with controlled ventilation. Br J Anaesth 91, Cruz ML, Sacchi T, Luna SPL et al. (2000) Use of a laryngeal mask for airway maintenance during inhalation anaesthesia in rabbits. Vet Anaesth Analg 27, (Abstract). Davies PR, Tighe SQ, Greenslade GL et al. (1990) Laryngeal mask airway and tracheal tube insertion by unskilled personnel. Lancet 336, Devitt JH, Wenstone R, Noel AG et al. (1994) The laryngeal mask airway and positive-pressure ventilation. Anesthesiology 80, Fujita M, Hiromitsu O, Motoko S et al. (1991) Use of laryngeal mask airway in small animals. J Vet Med Sci 53, Greene SA, Benson GJ (2002) Porcine anesthesia. In: Veterinary Anesthesia and Pain Management Secrets (1st edn). Greene SA (ed.). Hanley & Belfus, Inc., Philadelphia, PA, USA, pp Gursoy F, Algren JT, Skjonsby BS (1996) Positive pressure ventilation with the laryngeal mask airway in children. Anesth Analg 82, Hartsfield SM (1996) Airway management and ventilation. In: Lumb & Jones Veterinary Anesthesia (3rd edn). Thurmon JC, Tranquilli WJ, Benson GJ (eds). Williams & Wilkins, Baltimore, MD, USA, pp Maltby JR (1994) The laryngeal mask airway in anesthesia. Can J Anaesth 41, Patil VU, Fairbrother CR, Dunham BM (1997) Use of the laryngeal mask airway for emergency or elective airway management situations in pigs. Contemp Top Lab Anim Sci 36, Pennant JH, White PF (1993) The laryngeal mask airway: its uses in anesthesiology. Anesthesiology 79, Smith JC, Robertson LD (2004) Endotracheal tubes versus laryngeal mask airways in rabbit inhalation anesthesia: ease of use and waste gas emissions. Contemp Top Lab Anim Sci 43, Verghese C, Brimacombe JR (1996) Survey of laryngeal mask airway usage in 11,910 patients: safety and efficacy for conventional and nonconventional usage. Anesth Analg 82, Weiler N, Latorre F, Eberle B et al. (1997) Respiratory mechanics, gastric insufflation pressure, and air leakage of the laryngeal mask airway. Anesth Analg 84, Wemyss-Holden SA, Porter KJ, Baxter P et al. (1999) The laryngeal mask airway in experimental pig anaesthesia. Lab Anim 33, Received 28 September 2005; accepted 31 December Ó 2007 The Authors. Journal compilation Ó 2007 Association of Veterinary Anaesthetists, 34,