Efficacious and safe orotracheal intubation for laboratory mice using slim torqueable guidewire-based technique: comparisons between a modified and a conventional method

Tracheal intubation of laboratory mice before proceeding to open-chest surgery remains
an essential yet challenging procedure for investigators due to anatomical characteristics
that are not favorable for basic research 3]–5]. The present study demonstrates that this difficult and time-consuming procedure
can be quickly and safely completed using a novel method employing easily acquired
and inexpensive tools including a halogen light, tissue forceps, a 0.014-inch guide
wire and a 22-G IV catheter, without causing any short- or long-term complications.
Heightened ethical concerns for animals used in research have focused on the preservation
of life as the pivotal issue 18], 19], and the unique value of genetically modified mice underscores the need to protect
them from inadvertent harm during experimental processes 20]. Our results demonstrate that the crucial yet potentially life-threatening tracheal
intubation procedure can be performed more efficaciously and safely without the loss
of animal life, thus suggesting the application of this novel method in laboratories
requiring intubation of mice for subsequent experiments.

Comparison of our orotracheal intubation methods with other methods

Orotracheal intubation of laboratory mice was first reported by Ho and Furst in 1973
21]. Since then, a number of modifications in the procedure and instruments used have
been proposed to improve the success rate and to reduce unnecessary loss of animal
life. However, these reports have either not provided adequate details for researchers
who want to replicate the protocols 22], or have indicated that the procedure can only be performed with complex and expensive
equipment 6], 9], 11], 12] such as fiber-optic arthroscopes and endoscopes, or requiring operators with extensive
training (three students and one physician needed to accomplish 20 consecutive intubations
successfully within 60 s before performing subsequent intubations) 9]. Importantly, only a few studies have reported overcoming the two key difficulties
in tracheal intubation, i.e., clear visualization of the tracheal orifice 9], 12], 23] and easy advancement of the endotracheal tube into the trachea. 6], 9], 10] Tools meant to fully open the oral cavity such as a metal laryngoscope blade 13] or a custom-made laryngoscope 5] have the disadvantages of obscuring visualization of the vocal cords and triggering
pharyngeal-laryngeal reflexes, which can easily cause esophageal disposition and laryngeal
injury. Instruments designed to reveal the location of the tracheal entrance such
as expensive endoscopic instruments 11], 12], 24] including a modified arthroscope with affiliated light source plus video camera 6], 9] and an optic-fiber cable 11], 12] can easily induce a visual mismatch between the video images and the real oral cavity,
thus needing a much longer learning curve to master.

On the other hand, for both of our methods, the mice were placed in the dorsal recumbent
position at a 45-60° tilt with the upper incisors fixed by a silk band, thereby maximizing
the opening of the oral cavity and aligning the operator’s line of sight with the
epiglottis. Additional expansion of the operative field by tissue forceps to lift
the epiglottis anterosuperiorly along with transillumination of the trachea from the
ventral side of the mouse’s neck by a light source further helped localize the vocal
cords and trachea visually. The tissue forceps used in this study (forceps 2) functioned
as a miniature human laryngoscope with the curvature of its arms similar to the blade
of the laryngoscope, and it also provided good view to allow successful endotracheal
intubation. Importantly, they were slim enough not to obscure localization of the
vocal cords and trachea, thus avoiding inadvertent esophageal intubation during advancement
of the endotracheal tube. Our methods do not need expensive devices or extensive training,
as with most of the other methods. Most importantly, the instruments we used in the
Modified method to facilitate tracheal tube advancement, i.e., the floppy, slippery,
thin guide wire (only 0.36 mm in diameter) and a torque device, could be delicately
manipulated atraumatically to wherever the operator intended it to go, just as in
a human coronary artery which is smaller than the trachea of a mouse. In fact, utilization
of a guide wire to facilitate orotracheal intubation in mice has been described previously
6], 9], 10], 23], including Vergari et al 6], 9] who used a 0.4-mm guide wire, Hamacher et al 10] who used a 2F (0.67 mm in diameter) guide wire, and Thomas et al 23] who also used a straight 2F (0.67 mm in diameter) guide wire in their studies. The
main difference between their methods and ours is that the guide wires we used in
our Modified Group were smaller in diameter (only 0.36 mm) and designed specifically
for human coronary artery interventions, which can be well controlled by a torque
device to smoothly and accurately enter the trachea in the same manner as they are
manipulated in a coronary artery without causing injury to the endothelium they touch.
The results of the present study demonstrate the efficacy and safety of this novel
modified procedure for the orotracheal intubation of laboratory mice. This tool is
at least not inferior to other devices used for the same purpose, as reflected by
the 100 % procedural success rate and no immediate or late complications.

Comparison between the Modified and Conventional methods of tracheal intubation

Orotracheal intubation could be completed in all of our mice with either of our Conventional
or Modified method, however the relatively longer time and greater number of attempts
needed, along with significantly more difficulties encountered with the Conventional
method indicate that the 0.024-inch (0.61-mm) stiff stylet used in the Conventional
method was a less efficient instrument than the 0.014-inch (0.36 mm) floppy guide
wire used in the Modified method as an introducer for the tracheal tube. The significantly
higher incidence rates of procedure-related complications and short- and long-term
tracheal tissue damage further suggests the potential harm that can be caused by a
stiff stylet made from a needle with the end blunted. In contrast, the better performance
of the 0.014-inch guide wire used in the Modified method in terms of ease of intubation
and freedom from procedure-related complications or tracheal injury indicates that
it is both an efficacious and atraumatic method to enter the extremely narrow tracheal
opening and cross the rapidly vibrating vocal cords without triggering vigorous gag/salivary
reflexes or damaging the tracheal epithelial lining. In addition, the torque device
used for precise human percutaneous coronary interventions enhances the maneuverability
of the guide wire to accurately enter the trachea, thus reducing esophageal disposition.
Moreover, for experiments in which animals are kept alive for a period of time or
are intubated repeatedly 11] such as for radiological imaging 24], 25], pulmonary ventilation or agonist challenging studies 1], 13], 26], the absence of short- and long-term tracheal injuries in the Modified method can
the primary experiments, thus making it a more suitable method than others for a wider
range of in vivo research.

In summary, tracheal intubation of laboratory mice for subsequent experiments can
be accomplished quickly, safely and reliably with our proposed novel method employing
a slanted platform, an external light source, tissue forceps, a slim and soft 0.014-inch
guide wire, a torque device, and a modified tracheal tube, all of which can be readily
acquired at low cost and easily assembled in a typical animal laboratory dedicated
to cardiovascular research. This method can be considered as a good choice for relevant
experiments, especially for those in which repeated tracheal intubation is necessary
26].