A new airway device for small laboratory animals

Transcription

1 A new airway device for small laboratory animals A. Imai 1*, P. H. Eisele 2 & E. P. Steffey 1 1 Department of Surgical & Radiological Sciences, and 2 Center for Laboratory Animal Science, School of Veterinary Medicine, University of California, Davis, CA 95616, USA Summary There is a need for a device for improved management of the airway of small laboratory animals during general anaesthesia. This report introduces such a device, referred to here as the airway device (AD). The AD has some similarity to the laryngeal mask airway (LMA) developed for human patients, but the mask portion of the device is specifically designed for small laboratory animals. In addition, the device has an oesophageal extension and unlike the LMA does not have a cuff associated with the mask. This report also shares experience of tests of one prototype AD with six New Zealand white rabbits. The AD was used for administering isoflurane and its effectiveness was evaluated during conditions of spontaneous and controlled intermittent positive pressure ventilation. The results provide encouragement for further development of the AD for airway management of small laboratory animals. Keywords Rabbit; airway management; laryngeal mask; rabbit anaesthesia Airway obstruction and the resultant inadequate ventilation and oxygenation occur frequently in unconscious animals. Accordingly, careful airway management is an essential part of safe anaesthetic practice regardless of the species. In unconscious human patients, airway management is usually accomplished by inserting a tube into the trachea (i.e. tracheal intubation), but at times a face mask or laryngeal mask airway (LMA) is used (Brain 1983). While similar techniques are applied to animal patients and subjects involved in biomedical investigations, problems of size diminish their use in very small animals such as rabbits and rats. For example, the technique of applying a mask to the surface of the nose *Present address: B32-32, Hiwa-machi Kanazawa, Ishikawa , Japan Correspondence to: E. P. Steffey epsteffey@ucdavis.edu and mouth, while facilitating oxygen delivery, may not provide a patent airway because secretions or the tongue and other structures inside the oropharynx or nasopharynx may still serve to obstruct the airway. An endotracheal tube (ETT), on the other hand, most commonly facilitates a patent airway and permits effective positive pressure ventilation. However, performing endotracheal intubation in small laboratory animals is difficult and often traumatic to the animal because of their small size. In addition, an unacceptable limit to the size of airway orifice may occur. The LMA is a relatively new device developed especially for human patients. It consists of a tube that opens into an elliptical mask with an inflatable rim that is designed to provide and maintain a seal around the laryngeal inlet. The LMA is available in sizes to fit humans from neonates to large adults. The seal around the laryngeal opening permits Accepted 1 June 2004 Laboratory Animals Ltd.,

2 112 Imai, Eisele & Steffey spontaneous or mechanical ventilation with gas of controlled composition. This device provides more direct connection to the trachea than does a face mask, and is less invasive than the ETT technique. Unfortunately, presently available LMA sizes are not suitable for small laboratory animals and the inflatable cuffed mask itself may obstruct the airway if used in this group of animals. To resolve some of these difficulties and in turn extend our ability to reliably secure a patent airway during general anaesthesia particularly in small laboratory animals, we constructed a prototype of an airway device (AD) suitable for these species. This report summarizes the results of this development and our experiences of initial testing with anaesthetized rabbits. Materials and methods The airway device Anatomical definitions of the oral cavity, pharynx, larynx, trachea, and oesophagus were moulded from post-mortem rabbits, ferrets, rats, and mice and assessed for device design. A prototype was constructed according to the design shown in Fig 1. The prototype was composed of three parts (connected in series) a tube, a mask and a balloon. The device is constructed of pliable silicone. The tube portion contains a continuous spiral wire within its wall to better insure patency of the tube lumen. The balloon portion is designed to lie in the Fig 1 Prototype of an airway device for small laboratory animals proximal oesophagus. The mask surrounds and seals the larynx. The tube serves as a conduit for respiratory gases to and from the trachea. A prototype AD was evaluated in anaesthetized New Zealand white rabbits. The size of the mask for the adult rabbit used was cm and the length of the tube was 12 cm. The external diameter of the tube was 6.5 mm and its internal diameter, 5 mm. The balloon part was 3 cm in length. A polyethylene tube (ID 1 mm) was housed inside this particular prototype for inspired and expired gas sampling. The end of the gas sampling tube was opened at the base of the mask part. Animals, anaesthesia protocol, and data analyses Animals Six adult female New Zealand white rabbits were used. Mean body weight ( SD) of the rabbits was kg. Rabbits were obtained from a commercial supplier (Charles River Laboratories, Wilmington, MA, USA). All of them were specific pathogen free. They were housed in the AAALAC-accredited facilities at the Center for Laboratory Animal Science at the University of California, Davis. Each rabbit was housed in a wire cage (size: cm) in a temperature ( C) and light-controlled building. Rabbits were fed a commercial pelleted diet (Rabbit Chow Purina Mills, LLC, St Louis, MO, USA) and water ad libitum. Animal care and use were approved by the Campus Animal Use and Care Administrative Advisory Committee of the University of California, Davis. Anaesthesia All anaesthetics were performed during daylight hours of 08:00 h to 17:00 h. Unmedicated rabbits, not fasted for food or water, were weighed and placed in an acrylic chamber. Oxygen was provided to the chamber at a flowrate of 4 l/min for 5 min before anaesthetic induction. Anaesthesia was then induced by delivering 5% isoflurane in 4 l/min oxygen to the chamber. Following recumbency, the rabbit was taken from the box and anaesthesia was

3 A new airway device 113 deepened using a face mask connected to a Bain circuit with similar conditions of fresh gas delivery. After establishing an adequate depth of anaesthesia by checking the usual clinical signs for the rabbit (Imai et al. 1999), the AD was positioned. Device insertion was performed with the rabbit positioned in lateral recumbency and with its neck flexed ventrally. The balloon part of the device was then inflated with air (e.g ml) and the tube part of the device was connected to a Bain circuit. Anaesthesia was maintained with isoflurane in oxygen (2 l/min) provided via the device. The rabbits breathed spontaneously. Respiratory rate (f ), inspired and end-tidal isoflurane concentrations (I Iso and ET Iso), inspired and endtidal carbon dioxide (I CO 2 and ET CO 2 ), and inspired and end-tidal oxygen concentration (I O 2 and ET O 2 ) were monitored by a calibrated anaesthesia monitoring apparatus (Datex Capnomac Ultima, Datex Medical Instrumentation Inc., Tewksbury, MA, USA) via the gas sampling tube and were recorded. Pulse oximetry technology was used to monitor pulse rate and arterial oxygen saturation (O 2 Sat a ) throughout the anaesthesia. The pulse oximeter probe was positioned on the shaved base of the tail. Rectal temperature of the animal was frequently checked by thermister and maintained normothermic as necessary by externally applied heat. The influence of changing anaesthetic concentration was monitored during spontaneous ventilation. Mechanical ventilation was performed during periods of anaesthesia by connecting the breathing circuit to a Bird ventilator. Intermittent positive pressure ventilation (IPPV) was initially performed at a peak airway pressure (P aw ) of 10 cmh 2 O with a respiratory frequency of per min. Then the P aw was increased to 12 cmh 2 O and 15 cmh 2 O. The circumference of the abdomen of the rabbit was measured before and after IPPV to provide insight to changes in gastrointestinal gas volume that might accompany IPPV. After IPPV and return of spontaneous ventilation, the AD was removed and the status of the oral cavity and airway was quickly evaluated under direct visualization using a laryngoscope. A 3.0 mm ID cuffed ETT (Hudson RCI, Sheridan/CF Hudson Respiratory Care, Inc., Temecula, CA, USA) was then inserted and the anaesthetized rabbit was evaluated during spontaneous ventilation and IPPV conditions similar to those during the preceding use of the AD. Following these observations, the ETT was removed and the AD reintroduced and the ET Iso re-stabilized at 3.1% for 5 min. The I Iso concentration was then abruptly reduced to zero and conditions associated with awakening and AD removal were evaluated. Data analyses All the numerical data were grouped and are reported as mean SD. The ET Iso, I Iso, ET CO 2, I CO 2, ET O 2, I O 2 and f at the end of ET Iso conditions of 2.1%, 2.5%, and 3.1% during spontaneous and mechanical ventilation were statistically analysed by use of repeated measures ANOVA followed by use of the Tukey test when indicated. The results from the uses of the AD and ETT were compared by the Bonferonni test when indicated by results of repeated measures ANOVA. Significance was set at the 5% level. Results Intubation status Anaesthetic depth sufficient to insert the AD was judged adequate when the rabbit s eye was positioned centrally and jaw tone was relaxed enough to pass the tube into the mouth. During these conditions, the device was inserted blindly in one to three attempts without difficulty. Some attempts were not successful on the first try because either the anaesthesia depth was too light or the head positioning of the animal was inappropriate. The correct positioning of the device was checked by a variety of techniques such as ausculting for airflow noise in the throat using a stethoscope, and observing altered capnographic wave forms. When the mask part of the device was rotated to facilitate known airway leakage characteristics of leakage included: airflow sounds in the throat, detection of isoflurane

4 114 Imai, Eisele & Steffey at the nose, and alterations in the normal shape of the capnograph. Performance of AD and ETT Conditions of general anaesthesia were unremarkable regardless of the mode of airway maintenance. The f (breaths/min) and ET CO 2 (mmhg) at ET Iso 2.1%, 2.5% and 3.1% with AD were and 35 4, 47 6 and 39 5, and and 36 5, respectively. The f and ET CO 2 with ETT at the same dose of ET Iso were 42 3 and 38 2, 42 6 and 39 4, and 44 6 and 39 5, respectively, and did not differ from results with the AD. Positive pressure ventilation was accomplished with AD and ETT in all rabbits without obvious gas leakage. Mild localized redness (n 2) and mild swelling (n 1) in the pharyngeal region were seen after IPPV with the AD. The circumference of the abdomen before and after IPPV with the AD did not change. Recovery from anaesthesia was calm and uneventful in all rabbits. All rabbits lifted their head at the time of extubation. Three of the rabbits moved their mouth and tongue before extubation. No rabbits chewed the tube before or at the time of extubation. Averaged ET Iso recorded just before extubation was % (range: %). Discussion Securing the airway is essential to maintain adequate ventilation when an animal is anaesthetized. However orotracheal intubation is difficult and often traumatic in small species and as a result of the small size of the tube used may actually confound breathing mechanics. The AD described in this report was designed to offer advantages of tracheal intubation without some of the disadvantages specific to small laboratory animals. The AD has some similarities to the LAM used in human patients and in some animals. The commercially available sizes of LMA have been applied successfully to spontaneously breathing cats (Fujita et al. 1991, Asai et al. 1998), dogs (Braz et al. 1999), pigs (Patil et al. 1997), and ferrets (Brietzke & Mair 2001), and IPPV was successfully performed with this technique in cats, pigs, and ferrets. However, in some of these trials, gastric content aspiration and gastric distention occurred. The AD we developed has important differences from the LAM. These include: (a) smaller size, (b) the absence of an inflatable cuff surrounding the mask (this allows for a greater airway lumen, which is an advantage with such a small tube), and (c) the presence of a balloon projection intended to be positioned in the oesophagus for stabilization, to minimize the likelihood for air passage to the stomach, and to prevent regurgitation of lower oesophageal and stomach contents. These features enable the AD to provide improved airway management in small animal species. The work described was intended to be preliminary since only one species of small laboratory animal is represented and the number of rabbits used was small (n 6). Accordingly, firm conclusions are not proposed or are possible. Nonetheless, present results encourage further development. The device was easily inserted blindly in an adequately anaesthetized rabbit, and was well tolerated without clinicallysignificant complications. The device has many attributes. A patent airway was easily and simply attained, and was successfully maintained during both spontaneous and mechanically-controlled ventilation. Usually the airway was established on the first attempt, with no more than three attempts being necessary for successful insertion of the AD. Presumably with practice, adequate anaesthetic depth, and proper positioning of the animal, the number of insertion attempts can be minimized. Conversely, even with trained personnel, endotracheal intubation of rabbits is usually challenging, with multiple attempts often being necessary. The number of attempts before successful tracheal intubation and the amount of associated trauma often increase with decreasing animal size. The device offers many advantages of endotracheal intubation and seemingly

5 A new airway device 115 improves on this technique. It provides direct access to the trachea for controlled delivery and removal of inhaled anaesthetics while also providing for a larger airway diameter and, in turn, potentially improves breathing mechanics. Further, in the very small animal species, its ease of insertion without need for a laryngoscope or similar device, reduces complications associated with the need to visualize and force open the vocal cords, and penetrate the laryngeal opening with an ETT. Conversely, some negative features are undeniably present. Airway obstruction with a portion of the AD may occur when the tube is incorrectly positioned or dislocated. Application of a properly sized AD with proper placement and fixation are essential. Compared to the ETT, this AD, like the LMA, is less desirable for the animal who is at risk of regurgitation and/or active vomiting of gastric contents, or has blood or foreign matter in the upper airway. Likewise, despite our favourable results in the present limited studies, the AD may not produce an airtight seal around the larynx, especially under conditions of high inspiratory pressures during IPPV. However, we used pressures of up to 15 cmh 2 O without notable air leaks or measurable distension of the gastrointestinal tract and abdomen. The complete absence of local trauma cannot be avoided, although with careful use this complication should be trivial and should be more than balanced by the opportunities afforded. The mild redness and local tissue swelling that was seen in the pharyngeal region of three rabbits after IPPV were most likely caused by local friction between the mask portion of the AD and the pharyngeal tissue. The tube material (silicone) and local motion of the AD during IPPV may have also contributed to the incidence of the localized signs. Despite these minor observations, further consideration in this regard is warranted. Nonetheless, these features should be considered in the light of individualized needs and relative to other techniques available. References Asai T, Murano K, Katoh T, Shingu K (1998) Use of the laryngeal mask airway in laboratory cats. Anesthesiology 88, Brain AIJ (1983) The laryngeal mask. A new concept in airway management. British Journal of Anaesthesia 55, Braz JRC, Martins RHG, Mori AR, Luna SPL (1999) Investigation into the use of the laryngeal mask airway in pentobarbital anesthetized dogs. Veterinary Surgery 28, Brietzke SE, Mair EA (2001) Laryngeal mask versus endotracheal tube in a ferret mode. Annals of Otology, Rhinology and Laryngology 110, Fujita M, Orima H, Shimizu M, Motoyoshi A, Katayama M, Miyasaka K (1991) Use of laryngeal mask airway in small animals. Journal of Veterinary Medical Sciences 53, Imai A, Steffey EP, Ilkiw JE, Farver TB (1999) Comparison of clinical signs and hemodynamic variables used to monitor rabbits during halothaneand isoflurane-induced anesthesia. American Journal of Veterinary Research 60, Patil VU, Fairbrother CR, Dunham BM (1997) Use of the laryngeal mask airway for emergency or elective airway management situations in pigs. Contemporary Topics 36, 47 9