Download 2002 - reptile respiration and controlled ventilation during anesthesia

REPTILE RESPIRATION AND CONTROLLED VENTILATION DURING ANESTHESIA
Stephen J. Hernandez-Divers 1*, BVetMed, MRCVS, Dipl ReVS Zoological
edicine,
5
ecialist in Zoo & Wildli
dicine (Reptiles), Matt R. Read 2 ,
V , VSc, Sonia M. Hernandez-Oivers3, V ,and Anneliese Strunk4 , DVM,
1,3,4 Exotic Animal, Wildlife, and Zoo Animal Medicine, Department of Small Animal
Medicine, College of Veterinary Medicine, University of Georgia, Athens GA 30602,
USA; 2Anesthesiology, Department of Small Animal Medicine, College of Veterinary
Medicine, University of Georgia, Athens GA 30602, USA
Abstract: Reptile respiratory anatomy and physiology are significantly different from
mammals (Wood and Lenfant, 1976; Perry, 1998; Wallach, 1998; Wang, at ai, 1998).
Reptile respiration can be conveniently divided into three clinically important aspects:
breathing frequency, depth of breathing (tidal volume), and the duration of nonventilatory periods (conscious apnea) (Wang, et aI, 1998). Functional lung (tidal)
volumes of various reptiles have been studied and vary tremendously, e.g. 12.5 ml/kg in
Boa species, and 45 ml/kg in Trachemys species (Wood and Lenfant, 1976; Perry,
1998; Wallach, 1998; Wang, 1998). Total lung capacities are much greater and may
exceed 300 ml/kg, suggesting a large functional reserve capacity (Perry, 1998).
Respiration is largely controlled in response to PaC02, Pa02, acid-base balance, and
lung stretch receptors. Changes in PaC0 2 and
2 are detected by arterial and
pulmonary chemoreceptors. Respiratory responses include changes in tidal volume,
br thi
frequency and non-ventilatory periods. In general, hypercapnia causes
considerable increases in tidal volume by suppressing lung stretch receptors, whereas
poxia primarUy increases breathing frequency by reducing or eliminating nonventilatory periods (Wang, et ai, 1998). These effects are more pronounced at higher
temperatures. These
ts help explain why reptiles remain apneic when maintained on
pure oxygen and why careful attention to body temperature is important for all
anesthetized reptiles.
Minute volumes of 30 -- 80 ml/kg/min appear to be a good estimate for most squamates.
example, observations an undisturbed 1.6 kg savannah monitor (Varanus
exanthematicus) documented two breaths per minute and a tidal volume of 35 ml
(Wood and Lenfant, 1976). As a consequence, calculated minute volume = 44
ml/kg/min. Minute volumes for aquatic reptiles may be greater; however, the use of pure
oxygen during anesthesia is unlikely to lead to hypoxia in turtles or crocodilians (Perry,
1998). Reptile respiratory physiology is further complicated by ventilation - perfusion
(VP) mismatch, and right to left pulmonary shunts (Perry, 1998). VP mismatch occur
naturally in reptiles, especially in aquatic species; however, they can be greater in
anesthetized reptiles positioned in lateral or dorsal recumbency. Voluminous or heavy
coelomic contents (e.g. ingesta, eggs) can further exacerbate shunting and VP
mismatch. Significant right - left pulmonary shunts probably complicate the use of
arterial blood gas evaluations in anesthetized reptiles.
2002 Proceedings • Association of Reptilian and Amphibian Veterinarians 145
Anesthetized reptiles become apneic during anesthesia due to the combined effects of
high pulmonary and carotid oxygen tensions, the absence of a muscular diaphragm and
the reliance on skeletal muscle movement for ventilation (Bennett, 1996; Perry, 1998). It
is therefore recommended that reptilian patients be intubated and artificially ventilated
during anesthesia. Traditionally this is accomplished by temporarily occluding the
outflow from a T-piece or similar circuit, or by manually compressing a rebreathing bag,
resulting in positive pressure ventilation of the patient. Depending on the extent of this
intervention, these manual techniques can result in significant variations in breathing
frequency, tidal volume, gas exchange, and have the potential to alter anesthetic depth.
The proper use of controlled ventilation with a mechanical ventilator can result in these
variations being reduced, and can improve the stability, control, and safety of
anesthesia. In our clinical practice, the Small Animal Ventilator (Vetronics, Bioanalytical
Systems, West Lafayette, IN, USA) has proven to be a reliable device well-suited for
reptiles ranging from 50 9 to 25 kg. This device acts as a typical non-rebreathing Tpiece circuit until the intermittent positive pressure ventilation (IPPV) switch is activatede
At that time, the system acts as a pressure-cycled ventilator. Once activated, the
outflow valve in the circuit is closed, so that the fresh gas flow of anesthetic gas and
oxygen are now directed through the endotracheal tube into the animal. Gas continues
to be directed into the patient until the airway pressure reaches a maximum inspired
pressure ( IP) set point. The MIP can be set between 1 - 40 cmH 20 (0.1 - 4.0 kPa).
Clinically, most reptiles are maintained between 4-10 cmH20. Once this MIP is reached,
the outflow valve opens, and both fresh and inspired gas flow preferentially exits the
animal and the circuit into the scavenging device. The breathing frequency can be
varied between 1 and 60 breaths per minute by adjusting the expiration length (EL) from
1 to 45 seconds. It is important to remember that with this ventilator, the rate of lung
inflation (inspiratory time) is dependent on the rate of fresh gas flow, since this
determines the rate of flow of gases into the patient and how quickly the set MIP will be
attained. As with all pressure-cycled ventilators, other factors may also affect how
quickly the MIP is reached, such as external pressure on the lungs, and resistance and
compliance of the breathing circuit and the patient's respiratory system (Hartsfield,
1996). Due to these effects, even though the MIP is reached, it is possible that the
minute ventilation of the patient is not being met, since the ventilator may be inactivated
prior to the adequate tidal volume being deliverede It is therefore important to correlate
ventilator activity with coelomic breathing movements.
The procedure for setting up the ventilator is as follows: 1) the reptile is intubated and
connected to the ventilator with the IPPV switch set to off. This enables a spontaneously
breathing reptile to inspire the anesthetic mixture while the system acts as an open Tpiece circuit; 2) the MIP is initially set lower than would be required (e.g. 2 - 3 cmH20)
to avoid inadvertent initial over-inflation of the lungs when the ventilator is turned on; 3)
the EL is initially set to 2 - 3 seconds to enable rapid evaluation and adjustment of
ventilation over several respiratory cycles; 4) the ventilator IPPV switch is turned on,
and the depth and rate of inspiration are observed. The MIP and fresh gas flow rate are
adjusted until normal coelomic excursions are observed. Higher MIP values will result in
greater degrees of lung inflation, and higher fresh gas flow rates will result in more rapid
inflation of the lungs (shorter inspiratory time); 5) once the desired breathing
146 2002 Proceedings • Association of Reptilian and Amphibian Veterinarians
characteristics are achieved the EL is set for the anticipated maintenance requirements
of the patient, usually between 10 to 30 seconds (giving a respiratory rate of 2 to 6
breaths/minute). At the end of the procedure, the inhalant anesthetic is discontinued,
and the ventilator settings are adjusted. 'The MIP is reduced and the EL increased in an
attempt to hypoventilate the patient, allowing PaC02 to rise to a point that it will
stimulate the return of spontaneous ventilation.
The principles of ventilation during anesthesia in reptiles are similar whether manual or
mechanical ventilation is used. The advantages of using this ventilator are its relative
simplicity, reliability, and versatility across a range of animal sizes. In addition,
ventilation rate and frequency, and therefore anesthesia can be more consistently
maintained and controlled. Perhaps most importantly, no manipulation of the breathing
circuit is required when changing the patient from spontaneous ventilation to controlled
ventilation and back again, since it functions as a non-rebreathing T-piece circuit until
the IPPV switch is activated. This characteristic is particularly useful in a clinical setting.
Key
ords: reptile, respiration, ventilation, IPPV, anesthesia
References:
Bennett RA. 1996. Anesthesia. In Mader DR (ed): Reptile Medicine and Surgery. WB
Saunders, Philadelphia, PA: 241-247.
Hartsfield SM. 1996. Airway Management and Ventilation. In Thurmon Je, Tranquilli WJ
and Benson GJ (eds): Lumb and Jones' Veterinary Anesthesia, 3rd edition, Williams and
Wilkins, Baltimore, MD: 515-556.
Perry SF. 1998. Lungs: Comparative anatomy, functional morphology, and evolution. In
Gans C, Gaunt AS (eds): Biology of the Reptilia, Volume 19, Morphology G. Society for
the Study of Amphibians and Reptiles, St Louis, MO: 1-92.
Wallach V. 1998. The lungs of snakesa In Gans C, Gaunt AS (eds): Biology of the
Reptilia, Volume 19, Morphology G. Society for the Study of Amphibians and Reptiles,
St louis, MO: 93-295.
Wang T, Smits AW, Burggren VVW. 1998. Pulmonary function in reptiles. In Gans C,
Gaunt AS (ads): Biology of the Reptilia, Volume 19, Morphology G. Society for the
Study of Amphibians and Reptiles, St Louis, MO: 297-374.
Wood se, Lenfant CJM. 1976. Respiration: Mechanics, control and gas exchange. In
Gans C, Dawson WR (eds): Biology of the Reptilia, Volume 5, Physiology A. Academic
Press, London, England: 225-273.
2002 Proceedings • Association of Reptilian and Amphibian Veterinarians 147