Abstract

Inhalant anesthesia in reptilian species presents many challenges including prolonged inductions and recoveries. Additional challenges can be encountered during the maintenance period, especially in patients intubated with small internal diameter endotracheal tubes. More specifically, endotracheal tubes with a small internal diameter can become occluded with mucous, which has the potential to create a one-way valve, allowing air into the lungs, but not back out.¹ This can result in hyperinflation of the lung and subsequent morbidity or mortality. Additionally, small internal diameter endotracheal tubes limit the volume of gas that can be adequately delivered to and exhaled by the patient. These concerns can be addressed by modification of a standard non-rebreathing system that provides constant fresh gas inflow and at the same time reduces airway dead space.

Non-rebreathing systems are indicated for use in reptiles weighing less than 10 kg.² At the San Diego Zoo a Mapleson non-rebreathing system with a Norman mask elbow is used for most reptile procedures requiring anesthesia. Using this unmodified system, gas flow follows the path of least resistance and is delivered to the patient during inspiration and towards the reservoir during expiration.¹ We have modified this system by firmly inserting a red rubber catheter into the Norman elbow, advancing the red rubber catheter into the tracheal lumen, and placing a standard induction mask over the patients head to create an airtight seal. It must be noted that when using this system, the external diameter of the red rubber catheter must be smaller than the tracheal lumen to allow for gas to flow from the patient to the reservoir and an airtight seal between the patient and induction mask is essential. This modification creates unidirectional gas flow and offers many advantages over a traditional non-rebreathing system. Trauma to the glottis and tracheal mucosal damage is minimized due to the small size of the catheter relative to the lumen of the trachea and increased pliability of the red rubber catheter when compared to more commonly used endotracheal tubes. Additionally, the concern for mucous accumulation at the end of the endotracheal tube is minimized and because the gas flow is unidirectional, the necessity to overcome pressure limitations created by small internal diameter endotracheal tubes is greatly mitigated. When desired, positive pressure ventilation can be provided by increasing the fresh gas flow rate or temporarily occluding the exhaust gas airflow. When using this modified system, effective respiration, adequate oxygenation and elimination of CO₂, appears to take place independent of spontaneous respiration or intermittent positive pressure ventilation.

We have successfully used this system in a variety chelonian and squamata species including venomous snakes and recognize this system has limitations including; incomplete airway protection, potential for pulmonary distension if exhaust gas outflow is occluded, and concerns for patient cooling and airway drying secondary to constant unidirectional gas flow.

The description of this technique should serve as a foundation for more detailed studies and future directions should include; blood gas analysis in anesthetized patients to better characterize ventilation, post-mortem examination of the pulmonary system to determine if this technique results in adverse changes to the respiratory epithelium, and investigation of the utility in avian and mammalian species.

Literature Cited

1.  Hartsfield, SM. 2007. Anesthesia machines and breathing systems. In: Tranquilli, W.J., J.C. Thurmon, and K.A. Grimm (eds). Lumb and Jones Veterinary Anesthesia and Analgesia. Ames, Iowa. 453–495.

2.  Schmacher J. and Y. Yelen. 2006. Anesthesia and analgesia. In: Mader, D.R. (ed). Reptile Medicine and Surgery. St. Louis, Missouri. 442–452.